Strengthening of buildings for storey extension Master of Science Thesis in the Master’s Programme Structural Engineering and Building Technology BJÖRN JOHANSSON MARCUS THYMAN Department of Civil and Environmental Engineering Division of Structural Engineering CHALMERS UNIVERSITY OF TECHNOLOGY Göteborg, Sweden 2013 Master’s Thesis 2013:113 MASTER’S THESIS 2013:113 Strengthening of buildings for storey extension Master of Science Thesis in the Master’s Programme Structural Engineering and Building Technology BJÖRN JOHANSSON MARCUS THYMAN Department of Civil and Environmental Engineering Division of Structural Engineering CHALMERS UNIVERSITY OF TECHNOLOGY Göteborg, Sweden 2013 Strengthening of buildings for storey extension Master of Science Thesis in the Master’s Programme Structural Engineering and Building Technology BJÖRN JOHANSSON MARCUS THYMAN © BJÖRN JOHANSSON, MARCUS THYMAN, 2013 Examensarbete / Institutionen för bygg- och miljöteknik, Chalmers tekniska högskola 2013:113 Department of Civil and Environmental Engineering Division of Structural Engineering Chalmers University of Technology SE-412 96 Göteborg Sweden Telephone: + 46 (0)31-772 1000 Cover: Illustration of members that may be critical in storey extension projects. Department of Civil and Environmental Engineering Göteborg, Sweden 2013 CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 i Strengthening of buildings for storey extension Master of Science Thesis in the Master’s Programme Structural Engineering and Building Technology BJÖRN JOHANSSON MARCUS THYMAN Department of Civil and Environmental Engineering Division of Structural Engineering Chalmers University of Technology ABSTRACT Storey extensions are an increasingly popular way to densify cities. One problem may however be that designers sometimes lack experience and knowledge concerning the specific issues that arise during storey extension projects and an accompanying strengthening of the superstructure. The aim of this project was to ease the work for the designer by highlighting critical questions and possible solutions. The information was mainly gathered through interviews with persons actively involved in storey extension projects. The interviews gave, among other things, much input concerning experiences about how strengthening methods can be performed for varying boundary conditions. The knowledge collected from the interviews was thereafter complemented with fundamental information about strengthening methods through literature studies. Some focus was also put on older structures and the common considerations that follow building projects where existing buildings and their users are affected. Thereafter, the different strengthening methods were compared and some of them were also further evaluated through supplementary calculations. The results of the project show that there are many aspects to consider in storey extension projects, but also that many solutions are available. It is important to properly assess the building early to detect any critical members or unused capacities etc. It is also of importance to select the best suited strengthening method for the specific situation. Sometimes, the apparent solution may not be the most appropriate. Key words: storey extension, strengthening of concrete structures, structural systems, early design phase. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 ii Förstärkning av byggnader för våningspåbyggnad Examensarbete inom Structural Engineering and Building Technology BJÖRN JOHANSSON MARCUS THYMAN Institutionen för bygg- och miljöteknik Avdelningen för konstruktionsteknik Chalmers tekniska högskola SAMMANFATTNING Våningspåbyggnad blir allt vanligare i storstäder där en förtätning ofta eftersträvas, som till exempel i Göteborg. Ett problem kan dock vara att konstruktörer ibland saknar erfarenheter och kunskap om de speciella frågeställningar som kan uppstå vid påbyggnadsprojekt med eventuella stomförstärkningar. Detta projekt syftade till att underlätta konstruktörens arbete genom att belysa viktiga problem och möjliga lösningar. Informationen insamlades främst genom intervjuer med yrkesaktiva som varit inblandade i påbyggnadsprojekt. Intervjuerna gav bland annat många bra erfarenheter om hur förstärkningar etc. kan utföras vid olika förutsättningar. Kunskapen från intervjuerna kompletterades därefter via litteraturstudier med mer grundläggande information om olika förstärkningsmetoder. Viss fokus lades även på olika äldre stomsystem samt de särskilda frågeställningar som medföljer ett byggnadsprojekt där befintliga byggnader och användare berörs. Därefter jämfördes de olika förstärkningsmetoderna och några utvärderades även med kompletterande beräkningar. Projektets resultat visar att det finns många aspekter som måste beaktas i påbyggnadsprojekt, men även att det finns många bra lösningar. Det är viktigt att inventera byggnaden tidigt för att lokalisera kritiska element och outnyttjade kapaciteter etc. Därefter är det viktigt att välja rätt förstärkningsmetod till rätt situation. Möjligheten finns att någon annan förstärkningsmetod lämpar sig bättre i den specifika situationen än den för konstruktören mest uppenbara. Nyckelord: våningspåbyggnad, förstärkning av betongkonstruktioner, tidig dimensionering. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 iii Contents ABSTRACT I SAMMANFATTNING II CONTENTS III PREFACE VII 1 INTRODUCTION 1 1.1 Background 1 1.2 Purpose and objective 2 1.3 Scope and limitations 2 1.4 Method 3 1.5 Thesis outline 3 2 CONDITIONS FOR STOREY EXTENSIONS 5 2.1 Densification of the city by storey extension 5 2.1.1 Building in urban environment on top of an existing building 5 2.1.2 Increased need for parking, storages etc. 6 2.2 Geological conditions in Göteborg 6 2.3 Typical existing structures in Sweden and Göteborg 7 2.3.1 Residential buildings 7 2.3.2 Office buildings 10 2.3.3 Hotel buildings 12 2.3.4 Parking garages 12 2.3.5 Different kinds of foundations 13 3 EXPERIENCES FROM PREVIOUSLY EXECUTED PROJECTS 15 3.1 The studied projects 15 3.1.1 Hotel – Gothia Central Tower 15 3.1.2 Hotel – Scandic Opalen 16 3.1.3 Hotel – Scandic Rubinen 17 3.1.4 Office building etc. – Bonnier’s Art Gallery 18 3.1.5 Office building – HK60 19 3.1.6 Residential building – Apelsinen 19 3.1.7 Residential buildings – Backa Röd 20 3.1.8 Residential buildings – Glasmästaregatan 21 3.1.9 Residential building on garage – KaverösPorten 22 3.1.10 Residential buildings on garage – Studio 57 22 3.1.11 Student housing – Emilsborg 23 3.1.12 Student housing etc. – Odin 24 3.2 Experiences about suitability of existing structures and extensions 25 3.2.1 Experiences about accessibility 26 3.2.2 Experiences about economy 26 CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 iv 3.2.3 Experiences about extensions 27 3.3 Experiences about the Eurocodes and older design codes 27 3.4 Experiences about critical members and strengthening 28 3.5 Experiences about the construction work at the building site 30 3.5.1 Experiences about logistics 30 3.5.2 Experiences about weather protection 31 3.5.3 Experiences about residents and other affected persons 31 4 CONSIDERATION FOR THE EXTENSION 33 4.1 Height of extensions 33 4.1.1 Height allowed by zoning 33 4.1.2 Height nature of the surroundings 33 4.1.3 Consequences for fire regulations due to increased height 34 4.2 Type of superstructure for the extension 35 4.2.1 Self-weight of the extension 36 4.2.2 Fire protection 36 5 GENERAL APPROACHES FOR STRENGTHENING OF STRUCTURAL MEMBERS 38 5.1 Sectional enlargement with additional reinforced or plain concrete 38 5.1.1 Shear resistance at interfaces between old and new concrete 38 5.1.2 Strengthening with shotcrete 39 5.2 Strengthening with externally mounted steel 41 5.2.1 Strengthening members with prestressing steel 41 5.3 Strengthening with fibre reinforced polymers 43 5.3.1 Surface mounted FRP 47 5.3.2 Near-surface mounted FRP 49 5.3.3 Mechanically fastened FRP 51 6 STRENGTHENING OF STRUCTURAL MEMBERS 52 6.1 Strengthening of columns 52 6.1.1 Strengthening against crushing and buckling of columns by section enlargement 52 6.1.2 Strengthening against crushing and buckling of columns by adding steel profiles on the sides 54 6.1.3 Strengthening against crushing of columns by wrapping with CFRP 56 6.2 Strengthening of load-bearing walls 58 6.2.1 Strengthening against crushing and buckling of walls by section enlargement 58 6.2.2 Strengthening against crushing and buckling of walls by external struts 59 6.2.3 Strengthening against buckling of walls by vertical CFRP 59 6.3 Strengthening of beams 59 6.3.1 Strengthening of flexural capacity of beams by section enlargement 61 6.3.2 Strengthening of flexural capacity of beams by glued CFRP 61 CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 v 6.3.3 Strengthening of flexural capacity of beams by external prestressing 62 6.3.4 Strengthening of flexural capacity of beams by adding external steel profiles 62 6.3.5 Strengthening of shear capacity of beams by glued CFRP 63 6.3.6 Strengthening of shear capacity of beams by vertical post-tensioned steel rods 65 6.4 Strengthening of slabs 65 6.4.1 Strengthening of flexural capacity of slabs by section enlargement 66 6.4.2 Strengthening of flexural and shear capacity of hollow core slabs by filling the cores 67 6.4.3 Strengthening of flexural capacity of slabs by adding prestressing steel reinforcement 67 6.4.4 Strengthening of flexural capacity of slabs with glued CFRP 68 6.4.5 Strengthening of shear capacity of slabs by vertical post-tensioned bolts 69 6.4.6 Strengthening of shear capacity of slabs by vertical CFRP bars or strips 70 6.5 Strengthening of foundations 70 6.5.1 Strengthening with steel tube piles 71 6.5.2 Strengthening with steel core piles 72 6.5.3 Strengthening with winged steel piles 73 6.5.4 Strengthening with soil injection 73 7 COMPARISON OF SOME STRENGTHENING METHODS BY CALCULATIONS 74 7.1 Strengthening the axial capacity of columns 74 7.1.1 The studied columns 74 7.1.2 Strengthening with load-bearing steel profiles on the sides of the column 76 7.1.3 Strengthening with vertically mounted steel plates 78 7.1.4 Strengthening with vertically mounted CFRP laminates 79 7.1.5 Strengthening with section enlargement 80 7.1.6 Strengthening with CFRP wrapping 82 7.1.7 Summary and conclusions 82 7.2 Strengthening the flexural capacity of simply supported slab 84 7.2.1 The studied slab 84 7.2.2 Strengthening with surface mounted CFRP laminates 85 7.2.3 Strengthening with near-surface mounted CFRP bars 87 7.2.4 Strengthening with steel beams on top of the slab 88 7.2.5 Strengthening with post-tensioned steel strands 89 7.2.6 Strengthening with section enlargement on the compressive side 91 7.2.7 Summary and conclusions 92 8 GUIDELINES FOR THE DESIGN PROCESS 95 8.1 General considerations before the project has started 95 8.2 Considerations in the early state – pros and cons for existing structures 98 CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 vi 8.2.1 Critical members and excess capacity for various types of existing buildings 100 8.2.2 Typical damages in various existing buildings 100 8.2.3 Evacuation of various existing buildings 100 8.2.4 Layout of different existing buildings 101 8.3 Inspecting the state of the existing structure 102 8.4 Evaluation of the structure and the extension 104 8.5 Choice of strengthening methods 105 8.5.1 Lack of axial capacity of columns 105 8.5.2 Lack of compressive capacity in walls 110 8.5.3 Lack of flexural capacity in beams 111 8.5.4 Lack of shear capacity in beams 113 8.5.5 Lack of flexural capacity in slabs 114 8.5.6 Lack of shear capacity in slabs 119 9 CONCLUSIONS 121 9.1 Comments on the result 121 9.2 Importance of the project and key results 122 9.3 The method used in the project 122 9.4 Further studies and development 123 10 REFERENCES 125 CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 vii Preface This report is the product of a Master’s thesis project conducted at Chalmers University of Technology in the spring semester of 2013. It was carried out to conclude our Master’s degree in the field of Civil Engineering. The work done in this project has been evenly divided between us and we feel that we have enjoyed the time spent on it. We have been working with close cooperation and have both contributed to all parts of the project. If some minor distinction should be made, Marcus Thyman has spent more time on literature studies while Björn Johansson has focused a bit more on calculations. However, all steps in the process have been taken based on discussions. We could not have done this work without help and would like to thank everyone who has been involved. We want to direct special appreciation to our supervisor and mentor Lukas Jacobsson at VBK for his counselling and encouragement. We also like to thank our examiner, professor and supervisor at Chalmers, Björn Engström, for his assistance and useful advices on how to structure our work and proceed with our thesis. The consultant office VBK has provided us with workspaces, counsel and breakfast every morning along with good company and interesting lunch discussions. We want to thank them for their support and patronage. Emelie Eneland and Lina Mållberg, our opponents, who have shared workspace with us during the entire semester, also deserve many thanks. They have helped us develop ideas through feedback and given us many moments to cherish. This project would not have been the same without them. We also want to thank all the other persons who have been involved during the development of this thesis, persons who have been available for interviews, helped us with queries and guided us towards the end result. Last but not least, we want to thank our families and friends, for never-ending support and encouragement during all our years of education. Göteborg, June 2013 Björn Johansson and Marcus Thyman CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 1 1 Introduction The project presented in this report has treated different methods of strengthening existing buildings for storey extension. It is meant to help the designer in the early stages of a storey extension project. 1.1 Background Göteborg is presently the second largest city of Sweden with more than 500 000 inhabitants in 2011, Statistiska Centralbyrån (A) (2012). However, the population density is quite low in Göteborg compared to other larger Swedish cities such as Stockholm and Malmö, Statistiska Centralbyrån (B) (2012). There might be several reasons for this, but for certain is that the city of Göteborg has great possibilities to become a more densely inhabited city. This potential suits well with the intention of the City Council of Göteborg, who wishes to densify the central parts of the city, Stadsbyggnadskontoret (2009). Densification of the central parts enables the use of already established infrastructure, recreational facilities and similar. In this way existing neighbourhoods may also progress and evolve in new directions. Developing already attractive areas can therefore motivate a higher construction cost than a building on a less desired site. Densification can be performed in several manners, e.g. erecting new buildings on unused land, filling empty areas between existing buildings, replacing existing buildings with higher or denser ones, changing building functions or internal apartment arrangements to enable more people to live in already built structures, or vertically extend already existing buildings. The latter approach is the one that has treated in this project. There are many issues that may prove problematic during the different stages of storey extension projects. When new floors are added, the building will be subjected to higher loads both vertically and horizontally. These must in some way safely be transferred downwards through the structure to the foundation. In many cases there is an excess capacity of the existing building and its foundation, but this can vary a lot depending on where, when and how the structure was built. If the capacity is too low, it might sometimes be necessary to strengthen the existing structure or its foundation. Strengthening of existing structures has been performed many times before, but the experiences are neither very well documented nor treated thoroughly during the education in civil engineering. Therefore, it is relevant to research the field to create proper design handbooks or guidelines to aid the designer. When treating extensions and strengthening of existing buildings, each project may seem unique and case specific, but there are common aspects and considerations that make it possible to draw conclusions on when different approaches most often are suitable. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 2 1.2 Purpose and objective The purpose of this project was to develop strategies for how the designer should handle storey extension projects. The main focus was on how to identify a lack of capacity in the existing structural system and how to perform the needed strengthening in a good way. To fulfil this purpose, guidelines that can be used in design of building extensions were created. These guidelines are meant to be used as an aid when determining if and how to strengthen an existing structure. The guidelines should take different situations and boundary conditions into consideration. The user should be enlightened about important steps in the design process and alerted on critical issues. 1.3 Scope and limitations The main focus of this project was on strengthening of existing buildings in Göteborg. This choice was based upon the fact that the project was carried out with support from VBK, a structural design company located in Göteborg. The main part of the targeted audience is also active in the city. Conditions such as geology and building standards etc. are therefore influenced by the situations in Göteborg and Sweden. However, the results may also be applicable to buildings in other cities as long as the user is aware of the differences. Even if the methods discussed here are meant to be applicable mainly to storey extension projects, it should also be possible to apply the results of this project to other types of situations where strengthening is needed. It should however be noted that the choice to focus on strengthening for storey extension may limit the number of investigated strengthening methods. Since the soil conditions may have a large effect on the capacity of buildings, especially in Göteborg, evaluations and possible foundation improvements were also treated to some extent. Strengthening of the structure above ground was however treated more thoroughly. The choice of structure for the extension itself was also treated, since it largely affects the need for strengthening of the existing structure. Issues other than the load-bearing system, such as accessibility and need of fireproofing etc., were handled in a simplified manner. Furthermore, the type of buildings investigated was limited to concrete structures, mostly since this material is very common in Göteborg and Sweden. However, other building materials were treated when it comes to strengthening and the extension itself. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 3 1.4 Method The purpose of this project could have been reached in several ways. One possible way would have been to perform a case study where an existing structure is vertically extended. In this way, different strengthening methods could have been evaluated for the specific case. However, since the subject is very extensive and a vast variation of existing structures can come in question for storey extensions, another approach was chosen. The chosen approach is very dependent on information from previously executed projects, since these experiences are valuable for future projects. The chosen approach consisted of two parts. The first part aimed to investigate methods for storey extension and strengthening. Apart from literature studies, where strengthening methods were investigated, emphasis was put on interviews. To cover a wider range of possible situations, persons involved in twelve different projects were interviewed. Some of the interviews were carried out during meetings, while others were conducted via telephone or email correspondence. Among the studied projects were examples of extensions on top of residential buildings, hotels, office buildings and garages. This approach was chosen to identify differences in the issues that can come in question for the various situations. The main focus in the interviews was on the key aspects that the designer and/or site manager had to consider in the specific project, i.e. the main differences between the project at hand and a more regular design project. The questions asked in these interviews are presented in Appendix A. Emphasis was put on how the designer solved the problem with the increased load on the existing structure, but other important considerations such as new elevators and how to handle the current residents and tenants were also discussed. The next step in the project was to evaluate and organise the information about strengthening and the studied projects. To supplement the information at hand, some experts in the fields of geotechnical engineering and fibre reinforced polymers were also contacted and interviewed. Thereafter, critical issues were connected to specific conditions and potential solutions. In other words, it was stated under which conditions a lack of capacity in a structural member often occurs and how this problem can be solved. These solutions were then investigated further and compared to each other with the ambition to find advantages and disadvantages. This comparison included calculations in which some of the most important structural members were strengthened according to different methods. It also included a discussion where the suitability of the methods in different situations was considered. Based upon the gathered information, guidelines were created which should aid the designer to find a possible design. These guidelines should primarily pinpoint important steps and issues that can come in question during the process. 1.5 Thesis outline The main result of this project, the guidelines, can be found in Chapter 8. This part of the thesis is therefore the one that will be of most use for the designer. However, the results presented here are based upon the information provided in the rest of the CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 4 thesis. Chapter 8 is structured to follow the design process, from choice of building to design of structural members that should be strengthened. The strengthening methods are put into a context and their applicability in storey extension projects is discussed. Chapter 2 contains background information about the conditions in the city, mostly concerning geology and common existing structures that may come in question for storey extension projects. Chapter 3 is another important part and contains the information that has been gathered from the interviews, i.e. collected experiences from executed projects. Considerations about the extension itself are treated in Chapter 4. A big part of the report is located in Chapters 5, 6 and 7, where possible strengthening methods are explained and discussed. Chapter 6 contains the main facts about the methods and is organised after type of structural member so that the designer easily can find methods that are relevant. To get general information about how to use different materials to strengthen the members, the designer is instead referred to Chapter 5. This division is used to minimise the number of repetitions. In Chapter 7 some of the treated methods are evaluated further through calculations. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 5 2 Conditions for storey extensions In this chapter information is given about the conditions for storey extensions in Göteborg concerning the intent of the city council, the geological conditions and the typical existing structures that can come in question in a storey extension project. 2.1 Densification of the city by storey extension Development of cities occurs continuously and whether this progress is in the right or wrong direction may differ from case to case and perspective of opinions. For a city to be able to advance and expand, it has to account for its current surroundings. The City Council of Göteborg has a desire to further utilise already existing infrastructure and public transportation systems, Fritiofsson et al. (2008). The fulfilment of this ambition can be achieved in various ways, but some sort of city densification is the common approach. Göteborg also wishes to have an integrated society within local regions where people with different backgrounds and in different stages of life are living and working. This can be achieved through various types of leisure activities, but a range of various available apartments and offices etc. may also create a more diverse community. New and more modern apartments will for example attract different types of residents than older ones. The difference in price range may of course be a contributing factor to this. However, the layout and size of the apartments etc. may also be used as another tool to further attract a targeted tenant group. Repairing and upgrading existing structures is in many cases less expensive than erecting new structures, Täljsten et al. (2011). Improving existing structures also consumes fewer resources than tearing down and rebuilding, making it more environmentally friendly. A more rapid construction process can be expected as well, while the building simultaneously remains usable. 2.1.1 Building in urban environment on top of an existing building There are several benefits when building a new structure on top of an older, such as already disposable services and no need to build new connecting roads. Construction work in an urban environment can however also have its drawbacks. It is important to adapt the site to the current surrounding and its traffic flow, while also considering the people that are living and working within the area. Difficulties to find storage space for the building material close to the site may also put higher demands on logistics and planning of the construction process. The size of the structural members and whether or not to use some kind of modules are to be decided from case to case. However, the vertical transport of structural members through a weather protection onto the existing building may also prove problematic and needs to be considered. Using for example large wall elements may CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 6 give a quite fast result, but there can also be advantages to build with smaller parts which can be transported mainly through elevators and stairwells. There can also be regulations for noise and vibration in certain regions that limit the use of specific equipments and working methods entirely or at certain hours. One way to reduce the number of disturbed persons might be to use the top floor of the existing building as offices for the contractor, Samuelsson, E. (2013-01-24). This enables the workers to be closer to the site, but the storey also acts as a barrier towards other parts of the building. To use the existing building as location for the office might however not always be feasible, since this require evacuation of an entire floor for quite some time. 2.1.2 Increased need for parking, storages etc. Another issue that needs to be handled during densification is the increased need for parking spaces and facilities such as laundry rooms, storage rooms and waste disposal. In many cases the latter might be solved by implementing the facilities into the existing structure or by placing these in a detached shed. Parking spaces however require a large area and this issue may not be solved as easily. In some cases a new parking garage might even be necessary. However, in the central parts of Göteborg, the norm for available parking spaces per household has decreased quite drastically during the last decades. In some districts a decrease from two cars per household to only 0.5 or 0.6 might be possible, Östling (2013-02-06). The number of parking spaces per household in an area is dependent on its location and distance from the central areas, so such a reduction is not applicable everywhere. 2.2 Geological conditions in Göteborg Göteborg is located by the mouth of Göta River and has therefore quite complicated geological properties with regard to structural engineering. The most common soil profile in Göteborg is topsoil above clay, followed by friction material and finally bedrock, Alén (2013-02-25). Some areas might be dominated with an almost 100 m deep layer of clay, while bedrock is visible directly at the surface in other areas. The intermediate situations may include different thicknesses of clay where the depth to bedrock may vary considerable under the very same building. Constructing heavier buildings on this kind of soil may result in unwanted effects, such as uneven settlements, which ultimately may end in failure. There are however ways to manage and overcome this undesirable effect. In Göteborg piling is the most common solution. Early piling was limited to the length of available tree trunks, which also limited the possible weight and height of the buildings, Alén (2013-02-25). This is one of the reasons why Göteborg is a rather sparsely populated city. However, with improved knowledge of piling and soil improvement, an increasing amount of heavier and taller buildings have been erected during the last decades. Different kinds of foundations are treated further in Section 2.3.5. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 7 The design process is not as straightforward when it comes to storey extensions as for new buildings, since a load increase must safely be transferred to the bedrock without causing damages on the original structure or its foundation. However, there might be cases where existing buildings have unutilised capacity, which enables the structure to carry additional loading without experiencing damages. Possible methods to strengthen the foundation beneath a structure are presented and discussed in Section 6.5. 2.3 Typical existing structures in Sweden and Göteborg Since the scope of this project limits the types of investigated structures to those that are made of concrete, other kinds of structures were not treated at all. This excludes steel and timber structures as well as the many old buildings that were built with load- bearing masonry walls. The choice to only consider concrete structures still allows a wide range of different structures to be studied, since the material has been frequently used during the last century. The ability to change the properties of concrete by altering the components in the mixture together with the ability to cast very free shapes has made the material popular. The desire to be able to design buildings for different kinds of activities and to shorten the erection time has in combination with increased knowledge about the material resulted in a variety of structural systems. Even though the history of concrete dates back over two thousand years, the first Swedish building with a concrete structure was built in the 1910s, Carlsson (1965). Concrete slabs became increasingly popular during the ‘20s, while the use of load- bearing concrete walls developed during the ‘40s. However, the big breakthrough for the structural material came in the early ‘50s, when it quickly took over the market from structural masonry, Björk et al. (2003). The improved construction methods contributed to a reduced construction cost for the superstructure. In 1930 the superstructure represented 72 % of the total expenses in a building project, while the corresponding figure in 1960 had decreased to 38 %, Carlsson (1965). 2.3.1 Residential buildings There are many ways to design a residential building with a load-carrying structure made of concrete and the methods have varied and developed throughout the years. In this section common existing residential buildings and their basic characteristics are described. The information is not primarily based on the situation in Göteborg due to the lack of statistics about the buildings in the city. Instead, the examples represent common residential buildings in Sweden. One way to categorise apartment buildings is according to their primary shape. Figure 2.1 shows simplified sketches of the three basic appearances that symbolise the most common residential buildings in Sweden, which are long and narrow lower buildings, square-shaped tower blocks and long and narrow taller buildings. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 8 Figure 2.1 Different common shapes of existing residential building, a) long and narrow lower building, b) square-shaped tower block, c) long and narrow taller building. 2.3.1.1 Long and narrow lower buildings The most common residential building in Sweden has a rectangular shape where the length is considerably longer than the depth, Björk et al. (2003). Many of these are about three to four storeys high since buildings of this height for a long time were permitted to be built without elevators. In southern Sweden four storeys without elevators were allowed to be built until 1960, while three storeys could be built in this way until 1977. Thereafter, residential buildings with more than two storeys needed elevators. The stairwells (and possible elevators) most often only serve the adjacent apartments without the use of corridors. This can induce problems in storey extension projects, since the extension requires either many elevators or the use of access balconies. In the end of the ‘40s the use of regular masonry bricks in the load-bearing walls started to be replaced by use of blocks made of concrete, Björk et al. (2003). However, the old approach to use load-carrying façades together with load-bearing spine walls, the central wall illustrated in Figure 2.2a, still remained. Lightweight concrete blocks were sometimes placed in the façade, but the interior walls were often made up by regular concrete blocks due to sound demands. The slabs were often made of in-situ cast concrete. Load-bearing walls of in-situ cast concrete became increasingly popular during the end of the ‘50s and the new method also brought a big change in the load-carrying structure, Björk et al. (2003). Load-carrying façades and spine walls were replaced by the cross-wall system with load-bearing transversal interior walls and gables. This cross-wall system is illustrated in Figure 2.2b. One big benefit in storey extension projects with the cross-wall system is that the transversal walls often have an excess capacity, since they have been designed with regard to sound demands. These buildings are also very stable in the transverse direction. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 9 Figure 2.2 Different structural systems, a) load-bearing spine wall (dark) and façades, b) the cross-wall system that became popular during the ‘50s. To be able to utilise the expensive elevators better, residential buildings with access balconies became quite popular during the ‘60s. The elevators were placed apart from the house itself and access to many apartments was gained without the need of interior corridors, see Figure 2.3. As before, the cross-wall system was most often used. An advantage with this type of building in storey extension projects is that it is easier to use a similar layout in the extension without needing to install many elevators. Figure 2.3 Building with access balconies. The ‘70s brought a rapid increase for prefabricated concrete elements in the load- carrying structure, since the construction time then could be reduced. Slabs were often prestressed and the cross-wall system was often used. According to Stenberg (2012) the prefabricated residential buildings from the ‘70s are often very robust. One advantage for storey extensions with prefabricated buildings might be that many elements have the same size, which should simplify a possible use of prefabricated elements in the extension. As mentioned earlier all residential buildings with more than two storeys that have been built after 1977 have elevators. The mass-production of large housing complexes subsided at this time and the buildings from the ‘80s and onward are more adapted for sites near the city centre rather than the suburbs. These buildings are often more CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 10 unique, even if the knowledge from the ‘70s, e.g. the cross-wall system and prefabrication, in many cases was used. 2.3.1.2 Square-shaped tower blocks The second category of residential buildings has a more square-shaped layout with stairwells located in the centre of the building. According to Willén (2013-02-06) this attribute can be advantageous for storey extensions, since only one elevator needs to be installed. In a similar way as for the long and narrow buildings at that time, lightweight and ordinary concrete blocks were during the ‘40s used in the square-shaped buildings, Björk et al. (2003). Both the façades and the apartment-dividing walls are load- bearing and mainly arranged to meet the demands concerning the apartment layout, which means that their placing can be irregular. Square-shaped buildings from the ‘50s and ‘60s are often higher, e.g. about eight to ten storeys, Björk et al. (2003). These buildings often have in-situ cast exterior load- bearing walls, sometimes with an outer insulating layer of lightweight concrete blocks. The walls on the highest storeys might however consist of only the lightweight blocks, since the load is lower in this part. This property can be unfavourable in a storey extension project. 2.3.1.3 Long and narrow taller buildings Significantly taller, and often longer, versions of the long and narrow buildings also exist. An important difference that comes from the height is that they always have elevators. These tall and long buildings gained popularity during the ‘60s and were in the beginning often cast in-situ and built according to the cross-wall system. Non load-bearing façades could be made either by lightweight concrete blocks or prefabricated sandwich elements. In the end of the ‘60s and during the ‘70s, prefabricated elements were often used. 2.3.2 Office buildings The functionality demands on office buildings have throughout the years resulted in a wide range of structural systems, Carlsson (1965). Some general considerations can however be noted, when it comes to the specifics about office buildings. One of the main differences when compared to residential buildings is that office buildings most often are designed to be adaptable to future changes. Since the activities in the building can alter many times during the service life of the building, structural systems that prevent changes in the layout are avoided. This desire has through the years often led to the use of structural systems with columns instead of load-bearing walls. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 11 In the beginning of the 20 th century, the office buildings were often built in steel, but during and after the Second World War, the steel price rose drastically, Carlsson (1965). Therefore, almost no structural steel was used in the ‘40s and ‘50s. The high steel price instead promoted the use of concrete. The concrete was in-situ cast in the beginning, but during the years, prefabricated elements were used more and more often. The structural systems often consist of columns with flat slabs or various combinations of columns and beams. An elevator shaft is also often used for stabilisation. In the end of the ‘60s, more and more structural steel was used again, according to Carlsson (1965). Steel columns, steel beams and concrete floors are today very common in office buildings, Skelander (2013-12-12). When it comes to the general layout in office buildings, two main variants can be seen as the most important. The first one is referred to as the European way by Carlsson (1965). The main idea with this method is to use internal corridors that let the employees access their separated offices. When compared to dwellings, this kind of office building only permits windows in one direction, which is sufficient for offices. Figure 2.4 shows three different basic layout alternatives that have been used for office buildings with internal corridors. Figure 2.4 Different layouts of office buildings, after Carlsson (1965). The second common layout contains big open plans in which the employees sit together. This kind of layout is by Carlsson (1965) referred to as the American way. It can easily be understood that this kind of layout demands another structural system than the alternative with separated offices. In many cases the slabs span the whole width of the building. When considering the suitability for storey extension projects, office buildings show several important differences from residential buildings. The most prominent might be the lack of load-bearing walls designed with regard to sound restrictions. Since columns often have less excess capacity than load-bearing walls, see Section 3.4, this should mean that office buildings more often utilise a higher rate of their load-bearing capacity. Therefore, strengthening should be required more often for office buildings. Another disadvantage with structural systems that contain columns is the lack of extra stabilisation that comes with load-bearing walls. Even if the existing structure is braced by elevator shafts or steel trusses, it is not likely that it has any excess capacity. However, one advantage with office buildings compared to housings is the internal layout. Regardless of whether internal corridors or open halls are used, it should be fairly easy to avoid many elevators or access balconies for the extension. Another advantage may be that the entire building can be rented by a few or even one single CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 12 company. If these move to new offices, an opportunity to renovate the building can occur. 2.3.3 Hotel buildings Hotel buildings can in many ways be seen as something in-between residential and office buildings. The storeys that contain the hotel rooms often have transversal load- bearing walls that are designed with regard to sound demands. This can, in the same way as for residential buildings, give a robust structure that has an excess capacity. The main difference from residential buildings is however the big open spaces at the entrance floor and possible restaurants etc. Both Scandic Opalen and Gothia Central Tower are examples of hotels where the structural system on the entrance floor was found to be critical during the storey extension, see Section 3.1. One advantage with hotels, in the same way as for office buildings, is that the rooms often are one-sided with access via internal corridors. This property may reduce the need of new elevators. 2.3.4 Parking garages Unlike for other types of buildings specific statistics about the parking garages in Göteborg are more available. According to Nilsson (1991) there were 92 parking garages with room for more than 30 cars in central Göteborg in 1990. Out of these, 80 garages were made of cast in-situ concrete, two consisted of prefabricated concrete and four were built with a combination of cast in-situ and prefabricated concrete. 43 of the garages were categorised as free standing by Nilsson, while two other garages were placed on the roof of existing buildings and two were built-together with adjacent buildings. The rest of the garages were placed beneath existing structures. Even if this information is relatively old, it gives a good indication about most of the existing parking garages in Göteborg. When it comes to the structural system of garages, the most characteristic feature is the need of big open spaces. According to Jones and Stål (2007) most of the parking garages have structural systems that consist either of slabs and beams on columns or flat slabs directly on columns. The systems with beams can be designed either with the beams in the longitudinal or transversal direction. A combination can be used as well so that a system of crossing beams is created. A construction method that utilises interaction between prefabricated prestressed beams and in-situ cast slabs has also been used in the city during the last years. Since parking garages in Sweden and Göteborg are subjected to relatively severe exposure conditions, mostly due to the wet climate and de-icing salts that the cars bring into the garage, many of the structures show signs of damage. One example is the parking garage at Tunnlandsgatan in Göteborg, which was vertically extended and renamed to Kaverösporten, see Section 3.1.9. The garage was built in 1965 and consists of columns, beams and slabs of in-situ cast concrete, Nilsson (1991). Before the extension was made, the garage showed several signs of damage. According to CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 13 Nilsson some of these were visible reinforcement bars in the façades, local damages at the top surface of the slabs (beneath tires), signs of reinforcement corrosion through thin concrete covers in columns and walls and water puddles on the floor close to columns. Even if these damages were found in one specific garage, they are examples of damages that often need to be handled when parking garages are extended. The fact that parking garages often are in bad shape makes it even more important to inspect the existing structure carefully before the extension is decided and designed. Possible decay can result in a load-bearing capacity that seriously falls below the originally designed value. The extensive need of renovation in many parking garages may however imply that strengthening due to storey extension can be considered. If for example the columns need to be strengthened to take the already existing load, it can be motivated and cost effective to strengthen them a bit extra at the same time. The simplicity of the structural system together with the low use of insulation and installations etc. in many self-standing parking garages are things that might facilitate a storey extension. When compared to residential buildings, it can be fairly easy to place installations etc. through the lower structure without major disturbances. It can even be reasonably simple to drill holes in the decks and place new columns down through the garage. 2.3.5 Different kinds of foundations As mentioned in Section 2.2 the ground in Göteborg is dominated by bedrock and clay. This clay has complicated the construction process over the years and continues to do so even today. In this section it is described how the problem with the soil has been solved throughout the years and how this affects possible storey extensions. Methods to strengthen the foundations are instead found in Section 6.5. 2.3.5.1 Foundations on solid rock In the beginning of the 20 th century foundations on solid rock were simply realised by casting a concrete wall straight down to the rock, Björk et al. (2003). However, during the mid ‘50s a new method started to become popular. The bedrock was levelled into terraces and the blasted bits of rock were spread out to even out the surface. A reinforced slab was then cast on top of it, where thicker dimension were commonly used directly beneath the load-bearing walls. On the other hand, plinths have also been used in many cases throughout the years. If the building is founded directly on bedrock, there are normally no problems with the foundation when increasing the load, Alén (2013-02-25). An inclined bedrock surface may require some extra attention, but Alén claimed that the increased frictional resistance that can be derived from the additional load most often is enough to avoid strengthening. Consequently, buildings founded on bedrock are very suitable for storey extensions. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 14 2.3.5.2 Foundations on firm, semi-firm and soft soil Until 1960 foundations on firm to semi-firm soil were made with concrete walls standing on narrow footings that were localised beneath the wall itself, Björk et al. (2003). As with the foundations on solid rock, the method could be replaced by a whole bottom slab with thickenings below the load-bearing walls. In the beginning of the 20 th century, foundations on fairly soft soil were designed as fascine works, similar to a raft made out of timber. At the time when the concrete buildings became increasingly popular, the method had however been replaced with the same type of foundation as presented in the previous sections, namely the cast slab with thickenings beneath load-carrying walls. The thickness of these slabs might however be greater than those on more solid ground. For deeper layers of soft frictional soil, end-bearing piles or friction piles have been used. As the names suggest, the end-bearing piles rest on more solid soil or bedrock while the forces from the friction piles are transferred between pile shaft and soil through friction. 2.3.5.3 Foundations on very soft soil (clay) Clay is common in the Göteborg region and has over the years often required piling. As with frictional soil, end-bearing piles can be used if the distance to bedrock or firm soil is not too far. Otherwise, cohesion piles can be used where the forces are transferred through cohesion between the pile and the soil. Before 1930, timber piles were the only choice when buildings on clay were constructed. The use of concrete piles developed during the ‘30s, but the real popularity for the method came after the Second World War, Alén (2013-02-25). Timber piles are however still used in some situations and it is not uncommon with piles that combine timber and concrete. In these cases, the lower part (the part that is constantly beneath the ground water level) consists of a timber trunk, while the upper part is made of concrete. The surrounding groundwater helps to preserve the timber, while the overlying concrete is located within the transition area that can be quite severe for timber. Driven concrete piles are very common in Sweden and Göteborg. In fact, the prefabricated pile elements that are spliced together were invented in western Sweden, Alén (2013-02-25). The elements are normally 13 m high, but due to the splicing, piling in Göteborg has reached about 80-90 m down into the soil. According to Alén (2013-02-25) the design codes for the piles have changed throughout the years so that piles from e.g. the ‘50s today can take more load than they were originally designed for. This can be advantageous in a storey extension project. On the other hand, the geotechnical capacity is not treated in the same way. This means that there can be situations where the piles themselves can take the increased load, but at the same time, the ability to transfer the load to the soil is too low. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 15 3 Experiences from previously executed projects To be able to make use of existing knowledge concerning storey extensions, several reference projects have been studied. Information has been collected through research and interviews with persons involved in storey extension projects. A more thorough overview of the gathered information is available in Appendix B, while key aspects are described in this chapter. The main questions that were asked during the interviews can be found in Appendix A. 3.1 The studied projects The majority of the studied projects are situated in the Göteborg region, but two of them are located in Stockholm. It was desired to find different types of projects that represent various types of structures. In this way, specific critical issues could be identified for each type. 3.1.1 Hotel – Gothia Central Tower Gothia Central Tower, located in central Göteborg, was built in 1984 and initially reached 62 m above the ground with its 18 storeys. It consists mainly of in-situ cast concrete with a big core in the middle of the tower for stability, Samuelsson, E. (2013-01-24). Load-bearing walls between hotel rooms go downwards through the building except at the lower entrance and conference floors, where columns are used instead. This is illustrated in Figure 3.1. The building is mainly founded on footings on top of the bedrock, but short end-bearing piles have been used in some places. Six new storeys are being added at the time of writing, giving the building a new height of 83 m. Even more storeys were sought, but the columns on the lower floors had too low capacity, Samuelsson, E. (2013-01-24). Unlike the original building, the structural system in the extension mainly consists of VKR-columns, HSQ-beams and hollow core slabs, see Figure 3.1c. Some slabs and beams in the upper storeys of the original building have been strengthened with carbon fibre reinforced polymers. The slabs were strengthened with regard to bending moment and the beams were strengthened to be able to spread the high concentrated loads from the new steel columns that were placed on top of the beams near the edge. The anchorage of the new part was achieved by attaching post-tensioned steel plates to the upper core. These plates extend several storeys downwards where they are anchored into the existing core, see Figure 3.2. Careful surveying of the existing building showed that the building was vertically straighter than initially calculated, which meant that the design value of the horizontal load due to unintended inclination could be decreased. Another contributing factor to the decrease of the horizontal loads was that a more favourable terrain category with regard to wind load could be chosen than when the original building was designed. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 16 Figure 3.1 Structtural system of Gothia Central Tower, a) entrance floor, b) upper storey in old building, c) storey in the extension. Figure 3.2 Principle for anchorage of the new core in Gothia Central Tower. 3.1.2 Hotel – Scandic Opalen The hotel Scandic Opalen, located in central Göteborg, was built in the beginning of the ’60s. The original building has eleven floors and consists of in-situ cast concrete, Samuelsson, E. (2013-01-24). Transversal walls between the hotel rooms take the load in the upper part of the hotel, see Figure 3.3a. On the two lower storeys, the CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 17 layout differs so that more open spaces are created. The horizontal loads are transported downward by the gable walls and the elevator shafts. The foundation consists of end-bearing piles. Five extra storeys were added in 2009. As displayed in Figure 3.3b, the structural system consists of steel columns and beams. On top of the beams hollow core slabs are supported. To make the extension possible strengthening was performed in terms of increasing the bracing capacity of the gables, installing new columns through the old installation room and driving new piles into the clay along one of the gables, Samuelsson, E. (2013-01-24). Figure 3.3 Plans in Scandic Opalen, a) storey in original building, b) storey in extension. 3.1.3 Hotel – Scandic Rubinen Hotel Scandic Rubinen is located at Kungsportsavenyn in central Göteborg. The original building was built in the ‘60s and consists mainly of in-situ cast columns and beams, Jarlén (2013-03-13). On top of the beams prefabricated TT-slabs are supported. The height of the original building varies and the lower part contains three storeys above ground plus one basement. At the time of writing a storey extension is being built on the lower part of the hotel. As can be seen in Figure 3.4 five new storeys are added so that the extended part will reach the same height as the left part in Figure 3.4. The new structure consists of steel columns and HSQ-beams with hollow core slabs, Jarlén (2013-03-13). To minimise the height of the beams, the steel columns stand with a spacing of 4 m, which can be compared with 12 m for the columns in the original structure. This difference in spacing is solved by storey-high trusses (number 3 in Figure 3.4) that shift the load to the concrete columns. Among other things, the project also includes strengthening of rectangular concrete columns by additional steel profiles on the sides of the columns. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 18 Figure 3.4 Section through Scandic Rubinen. 3.1.4 Office building etc. – Bonnier’s Art Gallery Bonnier’s Art Gallery is located in central Stockholm and was built upon an existing three-storey building. The original superstructure consists of columns, walls and slabs of in-situ cast concrete founded on footings on bedrock, Skelander (2013-02-12). The old building lies in a steep slope which means that all three storeys are visible at one side of the building while the road on the other side of the building is in level with the roof of the old structure. Five new storeys were added in 2006. The first two floors contain an art gallery, while the remaining levels hold offices. Many of the original columns were too weak for the extension and needed to be strengthened, see number 6 in Figure 3.5. This was achieved by section enlargement, Skelander (2013-02-12). Stability issues were solved by a new stabilising stairwell in prefabricated concrete (number 2 in Figure 3.5) and a new concrete wall that was installed at one gable, ELU (2013). The wall was prefabricated in the added part while the continuation of this wall in the old building was strengthened through section enlargement (number 3 and 4 in Figure 3.5). Drilled steel core piles were used to anchor the stabilising wall. Figure 3.5 Structural system of Bonnier's Art Gallery, a) section, b) plan of new part and c) plan of old part. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 19 3.1.5 Office building – HK60 HK60 is an office building in Sickla, Stockholm. The original building contained eight storeys and was constructed in 1962. The whole building was cast in-situ. The external walls in the longitudinal direction are load-bearing and inside the building there are two rows of columns with beams, see Figure 3.6. The storey extension project, finished in 2013, included removal of the old roof and parts of the walls on the top floor, which earlier had been used for installations, Bågenvik (2013-03-14). Thereafter, four storeys with steel columns, HSQ-beams and hollow core slabs were added. The lower floors were renovated at the same time and, since a more open layout was desired, every second of the concrete columns were removed. To make up for this decrease in capacity, strengthening of the remaining columns was required. This was achieved by section enlargement, where 10-15 cm concrete was added on one side of the columns. According to Jonsson (2013-04-17), the choice to only strengthen one side of the columns was based on the fact that the added load was greater on that side. Self-compacting concrete was used and new stirrups were installed in the column to anchor the new layer. Interaction at the interface between the concrete layers was however neglected. Apart from the columns Bågenvik stated that the structure was strong enough to avoid strengthening. Figure 3.6 Structural system of HK60, a) section, b) plan of new part and c) plan of old part. 3.1.6 Residential building – Apelsinen A storey extension is planned on a four-storey building located in Kungsbacka, 30 km south of Göteborg. The original structure was built in 1976 with a load-carrying system of concrete walls mainly oriented in the transverse direction of the building, CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 20 Johansson (2013-01-31). The building is located on varying thickness of clay above bedrock and two thirds of the structure is founded on end-bearing piles, while the other end is founded on footings due to a shorter distance to the bedrock. A two-storey extension is yet to be carried out along with renovation of the apartments in the building. The structural design for the extension and its accompanying strengthening was carried out according to the Eurocodes, since the magnitude of reconstruction was quite extensive, Kilersjö (2013-02-05). No real weaknesses were detected in the building, but as with the residential building at Glasmästaregatan (Section 3.1.8), beams must be placed upon the roof slab to shift the load to the walls. Strengthening of the foundation will also be required and extra piles are to be added and connected to the load-bearing walls through lintels in the same way as described in Section 6.5.1. 3.1.7 Residential buildings – Backa Röd The five residential buildings in Backa (northern Göteborg), each with four storeys, were built in 1971 and are parts of a large residential complex. The buildings are low square-shaped tower blocks with a stairwell in the centre providing direct access to the apartments. The tower blocks are in need of renovation, which is to be carried out in association with a storey extension, Gerle (2013-02-12). Both internal and exterior walls are load-bearing and consist of prefabricated concrete elements, Carlsson (2013- 03-28). Figure 3.7a shows the layout of the load-bearing walls in the original building. The buildings are founded on end-bearing piles due to a deep layer of clay. In the future two storeys are to be added on each building. The load-bearing walls in the extension will instead consist of timber studs, Carlsson (2013-03-28). Figure 3.7b shows that, even if the extension contains six apartments per floor instead of four, the timber stud walls can be placed above the old walls. Additional glulam beams will however be needed above some openings in the original structure. The calculations for the foundation are not finished at the time of writing, but Carlsson estimated that the extension only will add about 5-10 % additional weight to the piles. However, additional piling will probably be needed beneath the new elevator. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 21 Figure 3.7 Load-bearing walls in the residential buildings in Backa Röd, a) storey in original building, b) storey in extension. 3.1.8 Residential buildings – Glasmästaregatan This project includes two buildings built in 1965 and situated in Krokslätt in southern Göteborg. Both are residential buildings, mainly four storeys high. The structures are typical Swedish residential buildings where each stairwell only serves the adjacent apartments. The load-carrying internal concrete walls were cast in-situ along with the slabs, Carlsson (2013-02-06). A few prefabricated columns are located along the façade. The building is placed directly on bedrock. Two new floors are being added at the time of writing. Most of the original structure is very robust and therefore not in need of any strengthening, Carlsson (2013-02-06). However, since the load-bearing walls of the new part do not coincide with the original walls, the roof slab needs to be strengthened with longitudinal steel beams that shift the loads to the walls. This is illustrated in a simplified way in Figure 3.8. It was decided to carry out renovations of the old apartments along with the storey extension, Östling (2013-02-06). To reduce the need for elevators and thereby the costs, internal corridors are being built to access the new apartments. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 22 Figure 3.8 Buildings at Glasmästaregatan, a) the two buildings viewed from above with load-bearing transversal walls, b) load-bearing system for the extension. 3.1.9 Residential building on garage – KaverösPorten KaverösPorten is situated in Kaverös in Göteborg and was originally a parking garage built in 1965. The garage, situated on bedrock, is three storeys high and consists of in- situ cast columns, beams and slabs, Nilsson (1991). The structure was in very bad state before the project started. This is described further in Section 2.3.4. In 2009, the garage was renovated and an extension with two to three floors with apartments was added, Östling (2013-03-04). In addition to the renovation of the concrete members, a new system of beams was added on top of the old roof slab to be able to transfer the new loads to the columns. When compared to the project at Glasmästaregatan, Östling also said that it was a large benefit that the original building had no residents to consider. KaverösPorten has not been investigated as thoroughly as the other projects and is therefore not treated in Appendix B. 3.1.10 Residential buildings on garage – Studio 57 Studio 57 is situated in Eriksberg on the north side of the river in Göteborg and consists of three residential buildings built on top of a parking garage. The garage was built during the ‘90s at which time a deeper knowledge about how to design with regard to resistance against de-icing salts etc. had developed. The building was therefore in a very good state. The structure consists of columns, beams and slabs that were all cast in-situ, Wibom (2013-04-05). Both the beams and the slabs were post- tensioned, which resulted in a tight structure that prevents cracking. The slabs were cast on top of a corrugated steel plate so that a composite slab was created. The foundation consists of end-bearing piles that go through an about 15 m deep clay layer to an inclined bedrock surface. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 23 The extension was finished in 2009 and consists of three residential buildings with three to four storeys. Mostly due to a very tight schedule it was decided not to strengthen the original structure, but instead let the new residential buildings rest on big precast concrete beams that shift the load to new columns that go through the garage and down to new pile groups, Wibom (2013-04-05). This is illustrated in Figure 3.9. Wibom claimed that if a solution with strengthening of the old foundation had been chosen, it would have been hard to ensure that the added load would go to the new piles. For the case with end-bearing piles, the old piles must deform more before the new piles are loaded (if they are not prestressed). Figure 3.9 contains simplified sketches of the building. c) Figure 3.9 Structural system of Studio 57, a) overview from above where 1 shows the old garage and 2 shows the extensions, b) section in longitudinal direction, c) photo of the building with the new columns that support the extension. 3.1.11 Student housing – Emilsborg The student housing built in the early ‘60s is a five to six storeys high building (excluding the basement) with a curved banana shape and internal corridors, Bergstrand (2013-03-01). This layout is rather similar to the one used in Scandic Opalen, see Section 3.1.2. The entire structure was cast in-situ on foundation walls, but since the underlying bedrock is inclined, concrete footings were also used in some places. A two-storey extension was completed in 2012 in connection with a renovation of the existing apartments. The load-bearing walls of the original building had in general an excess capacity due to sound regulations, Bergstrand (2013-03-01). Strengthening of the roof slab was achieved by casting an additional layer of concrete. One of the most critical parts was the connection between the load-bearing walls and the foundation walls. Strengthening of the foundation was required beneath the new elevators. The CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 24 building had very good stability in the transverse direction, but needed some extra attention in the longitudinal direction in form of cross bracings. 3.1.12 Student housing etc. – Odin The building called Odin was erected in 1940 near the central station in Göteborg and a major reconstruction was performed in 2002, when six storeys were added. Today, the building contains student apartments, offices, a supermarket, a hotel and a restaurant. It also has a parking garage in the basement. The original structure has an in-situ cast column-beam system of rather poor quality concrete, with a strength class corresponding to around C15-C20, Wibom (2013-04-12). The building is located on deep thicknesses of clay and the original foundation was therefore performed with 18 m long timber trunks as cohesion piles. An illustration of a section through the building can be seen in Figure 3.10. During the design of the extension the soil and foundation were analysed with a FEM- software. From this it was found out that the piles and soil could take the increased load, but the pile caps were too weak. To shift the new load from the pile cap, it was decided to strengthen the foundation with winged steel piles, Wibom (2013-04-12). This is illustrated in Figure 3.11. The VKR-profiles were prestressed to ensure that the winged steel piles were loaded immediately. Many of the columns also needed some extra attention and it was decided to increase their capacity by section enlargement with self-compacting concrete to ensure proper filling. The choice to use section enlargement instead of for example steel profiles or CFRP wrapping was primarily made to reduce the risk for punching shear. The increased area of the column reduces the local shear force per unit with on the pile cap. Figure 3.12 contains some illustrations of different section enlargements that were performed. Figure 3.10 Structural system of Odin. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 25 Figure 3.11 Illustration of how the foundation at Odin was strengthened. Figure 3.12 Strengthening of some of the columns at Odin. 3.2 Experiences about suitability of existing structures and extensions To choose a structure with prospects for storey extension needs careful consideration. The same goes for the choice of superstructure for the extension, which can be very dependent on the existing structure. In this section the collected experiences concerning these decisions are discussed. Buildings with load-bearing internal walls are very common in building structures in Göteborg. If buildings with such walls are used in residential buildings or hotels, sound demands can make the walls thicker than needed for the structural capacity for buildings with limited height. This often results in very robust structures, which are suitable for storey extensions. Another type of buildings that, according to Östling (2013-03-04), generally are suitable for storey extensions are parking garages. Östling has experiences from CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 26 storey extensions both on residential buildings and a parking garage, and argued that it was very beneficial to avoid having to consider people living in the original building during the construction. On the other hand, garages are often in a bad shape, which can result in extensive need of renovation. However, if columns etc. already are in need of strengthening, it can be advantageous to take the opportunity and strengthen them for storey extension as well. 3.2.1 Experiences about accessibility Older buildings have often been built according to other accessibility demands than today, meaning that the inside measurements and lack of elevators differ from the current requirements. The rules of today must be fulfilled in the extension. However, an increase in accessibility can also be achieved for the existing apartments. Due to the geometrical properties of the existing building and layout of apartments etc., it may however not always be economically defendable to install elevators that are accessed from every apartment. At Glasmästaregatan in Göteborg the accessibility was increased from 38% to 77% for the existing apartments. Even though the city planning office would prefer 100% accessibility, this was not possible to motivate economically, Östling (2013-02-06). It should however be noted that an installed elevator is considered as an increase in the standards of living for the residents and therefore motivates an increased rent. It is therefore important to consider the shape of the building when selecting a potential building for storey extension. A more favourable layout in terms of elevators is when the existing stairwells already serve a large amount of the apartments, e.g. in form of corridors. This was the case at Emilsborg, where only three new elevators gave full accessibility to both old and new apartments, Bergstrand (2013-03-01). The placing of the elevators is decisive for how the new floors are to be designed. For square shaped tower blocks a good solution can sometimes be to incorporate the elevator shaft into the existing structure. This method can be possible since the apartments often are placed around one single stairwell, which gives accessibility to all flats. For long and narrow buildings, as with the project on Glasmästaregatan mentioned in the previous paragraph, several elevators are often needed to achieve accessibility to all new apartments. A way to reduce the number of elevators might however be to build internal or external passways to which a small number of elevators are connected. Another aspect with the accessibility demands of today is how they affect the layout of apartments. The project in Backa Röd (Section 3.1.7) is one example where the demands on open spaces in bathrooms and kitchens prevented the use of similar layout in the extension as in the original building. 3.2.2 Experiences about economy Bostads AB Poseidon, a housing company located in Göteborg, is in general very positive to storey extensions, Gerle (2013-02-12). However, according to Östling CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 27 (2013-02-06) it might sometimes be difficult to financially motivate a storey extension unless certain conditions are met. Poseidon intends to perform storey extensions of some tower blocks in Backa Röd, see Section 3.1.7, and has already completed a test project where they simultaneously renovated existing buildings and lowered the energy consumptions, Gerle (2013-02-12). According to Gerle, simultaneous renovation and storey extension makes the project more justifiable than just renovation. One general consideration from several of the studied objects is that the choice of existing structure and extension often is made so that strengthening is limited or avoided, especially when it comes to residential buildings. The estimated rent is often what limits the price for the project, since a high construction cost ultimately leads to a higher rent for the residents. According to Östling (2013-02-06), some persons from the city council searched through Göteborg in the early 2000s to identify buildings that were suitable for storey extension projects. Apart from criteria concerning the surroundings and location of the building, they also searched for robust structures situated on bedrock. By choosing such structures the economic aspects are according to Östling optimised. However the more important and popular a location is, the more money might be motivated to spend on strengthening. As an example, when Gothia Central Tower was extended, rather large strengthening measures were taken. 3.2.3 Experiences about extensions When it comes to the extension itself, the studied examples are very different and the interviewed persons have various opinions of what kind of structure is best suited. At Glasmästaregatan, Willén (2013-02-06) argued that a timber stud structure is good, since it is light-weight and reduces the number of times that the protecting tent needs to be opened. Kilersjö (2013-02-05) however mentioned that a timber alternative was rejected for the project at Apelsinen (Section 3.1.6) due to sound requirements. During the extension of Emilsborg (Section 3.1.10) a semi-prefabricated concrete solution was used instead, mostly due to the genuine and solid appearance that follows with the choice of a concrete structure. It was also desired to obtain a structure that corresponded with the rest of the building, Bergstrand (2013-03-01). Yet another solution that is frequently used is a column-beam system in steel with concrete hollow core slabs. This method is mainly chosen when open spaces or an adjustable layout is desired. 3.3 Experiences about the Eurocodes and older design codes The Eurocodes were recently established as the governing design code, and since 2011 all new structures must be designed according to the Eurocodes. However, the scope of the Eurocodes is rather limited when it comes to redesign and strengthening of existing structures. Blanksvärd (2013-04-08) said that a code for treating existing structures was to be complemented and that Blanksvärd himself would contribute to it. He estimated that this part was to be finished sometimes between 2015 and 2020. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 28 Until then the designer is forced to interpret the codes without certain guidelines. The Eurocodes must be applied for the extended part, but it is more unclear what rules are applicable for the old part. Wibom (2013-04-12) said that his consulting firm based their design on logical deductions and calculations, claiming that the upcoming design code would arrive to a similar result. He mentioned an example with a pile foundation being verified according to the old design code and said that it would be illogical to reverify it with new codes. The partial safety factors are in the Eurocodes applied differently compared to the old Swedish codes. According to Skelander (2013-02-12) it is possible to apply the new standards to older existing buildings, but he pointed out that it is important to distinguish the loads from each other. It is also important to remember that the Eurocodes are harsher than their predecessors. The project at Gothia Central Tower was started earlier, just to be able to follow the old design code, Samuelsson, E. (2013-01-24). Samuelsson believed that it would not have been possible to continue with the project without reducing the number of added storeys if the Eurocodes were to be followed. 3.4 Experiences about critical members and strengthening The structural member that came up to discussion the most times during the interviews was the roof slab. This slab is in general not designed to be loaded by a new structure with its imposed loads. If it is not possible to place the new members directly on the existing load-carrying members, some kind of strengthening of the slab is in general necessary. At Glasmästaregatan and KaverösPorten longitudinal steel beams were placed upon the roof slab to shift the load to the primary wall members as illustrated in Figure 3.8, Carlsson (2013-02-06) and Östling (2013-03-04). A similar method will be used at the residential building Apelsinen according to Johansson (2013-01-31). At Emilsborg the roof slab was instead strengthened by an additional layer of concrete that was applied after cleaning and wetting the already rough top surface, Bergstrand (2013-03-01). Yet another solution to cope with the new loads on the roof slab was used at Scandic Opalen, where new columns were placed in the installation room that is situated on the top floor of the old building, Samuelsson, E. (2013-01-24). In the projects Odin and HK60 the original roof slab was completely removed and replaced instead of strengthened. Another part of the structure that was critical in some projects was the foundation. This was especially the case for buildings situated on clay. At Scandic Opalen, Odin and Apelsinen new piles were needed beneath some of the load-bearing walls. At Backa Röd additional piles will probably be needed to transfer the load from the new elevators, Carlsson (2013-03-28). When Bonnier’s Art Gallery was built, drilled steel core piles were installed and anchored to take the tension from the new bracing trusses, Skelander (2013-02-12). At Studio 57 end-bearing piles were used for the existing building, Wibom (2013-04-05). This type of piles complicates the strengthening of the foundation, since it might be difficult to increase the load without failure in the piles. This is due to the fact that the new piles need to deform before contributing to the global resistance. This effect can however be avoided if the added piles are prestressed. For this specific project new pile groups were instead used. New columns transfer the loads downwards to the new piles independently of the existing CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 29 structure. Strengthening of the foundation can be expensive and Östling (2013-02-06) claimed that a foundation on bedrock is almost a prerequisite when performing storey extensions for residential buildings. This was however not the case for all the studied projects. Load-bearing walls are seldom the most critical members for low buildings. The capacity of columns was on the other hand crucial in several of the treated projects. In both Gothia Central Tower and Scandic Opalen the capacity of the columns on the lower floors restricted the number of added floors, Samuelsson, E. (2013-01-24). In the project with Bonnier’s Art Gallery a large number of columns needed to be strengthened by an additional layer of concrete, Skelander (2013-02-12). However, the load increase in this project was very large, so strengthening of the load-bearing elements was quite expected. The residential building at Glasmästaregatan also had a few load-bearing columns that probably would have needed to be strengthened, if more storeys would have been added, Carlsson (2013-02-06). Every second of the columns in the office building HK60 were removed in order to achieve a freer internal layout, Bågenvik (2013-03-14). However, this led to that the remaining columns had to be strengthened. Here a one-sided section enlargement was chosen. For Hotel Scandic Rubinen rectangular columns were strengthened by applying steel HEB- profiles at the two opposing sides, which increased the capacity with regard to buckling and crushing, Jarlén (2013-03-13). Another project where the columns were critical was Odin, where they were strengthened on several storeys. At this site various shapes of section enlargements were applied, mainly due the fact that the columns needed an increased area to reduce the risk of punching shear failure. Some of the interviewed persons mentioned problems with too high compressive forces at the connections between load-bearing members, e.g. when concentrated forces from columns should spread out into larger members or when narrow members are placed upon each other with a perpendicular orientation. Samuelsson, E. (2013- 01-24) described that strengthening of several beams was required at Gothia Towers due to the small cross-sectional area of the new columns that were placed upon the beams. Here the strengthening of the beams was performed by use of carbon fibre reinforced polymers, CFRP. Another example is Emilsborg where one problem was that the connection between the foundation walls and the walls that rested on them in some places was rather small, Bergstrand (2013-03-01). The walls were not strengthened, but the capacity was limiting for the increased load. How to take the horizontal forces from the wind and unintended inclination can also be a problem, especially for high rise buildings. For Scandic Opalen the ability to take the tilting moment was increased by the use of steel plates that were attached to the gables, Samuelsson, E. (2013-01-24). For Gothia Central Tower the global stability could instead be accounted for by utilising the fact that the existing building stands straighter than assumed in the initial design and that a better terrain category could be adopted. Bonnier’s Art Gallery also had critical stability issues, which were solved by a new shear wall and an additional staircase acting as a core. The many transversal load-bearing walls in the studied residential buildings make them very stable in the direction that otherwise would seem to be the critical one. A lack of load-bearing walls in the longitudinal direction may however be a problem. Bergstrand (2013-03-01) explained that new bracing steel trusses were needed at Emilsborg to stabilise the building in the longitudinal direction. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 30 Even if the building has enough capacity to transfer the tilting moment from the original structure down to the foundation, problems can occur when the tensile forces should be transferred from the extension to the original building. In several of the projects this was simply solved by overlapping steel ties. At Emilsborg, that has an extension with a rather high density compared to the other buildings, the main purpose of the added steel bars was however to fixate the semi-prefabricated wall elements before casting, Bergstrand (2013-03-01). However, at both Hotel Scandic Opalen and Hotel Gothia Central Tower, the two highest buildings that were investigated, it was critical to transfer the tensile forces from the new structure to the old building. A rather similar solution was chosen for both structures. In Gothia Central Tower steel plates were attached to the new core and stretched several storeys down where they were anchored. The steel plates were pretensioned to ensure that they are activated directly when elongated. 3.5 Experiences about the construction work at the building site The interviews resulted in many important aspects about how a storey extension project differs from erection of new buildings. Both Kilersjö (2013-02-05) and Östling (2013-02-06) explained that the communication between commissioner, designer, contractor and other participants is even more important in storey extension projects than in normal building projects. Samuelsson, E. (2013-01-24), among others, also claimed that the designer must be engaged very early to ensure a superstructure for the extension that is adapted and fits properly to the existing load-bearing system. According to Samuelsson it is crucial to survey the building in an early state and compare it to the old documentation. He and several others of the interviewed persons explained that the measurements in the original drawings don’t always correspond exactly to reality. If this is not observed and trust is put into the drawings, it can give severe consequences, especially if prefabricated elements are used in the extension. Problems with unwanted load effects due to eccentricity can also occur, if the precise location of columns and load-bearing walls is unknown. 3.5.1 Experiences about logistics The logistical problems of a storey extension project can be vast since the building often is situated in the middle of a built environment. It can therefore be difficult to find available space for building site offices and storages etc. Willén (2013-02-06) explained that this was a dilemma at Glasmästaregatan. One way to improve the situation might be to move the site offices inside the existing building, as was done at Gothia Central Tower, Samuelsson, E. (2013-01-24). However, for this to be possible an evacuation of the users is required. This may not always be appropriate and possible in residential buildings. It is also motivated to plan deliveries so that they occur just before the material is needed at the site to reduce the need for storage. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 31 Another logistical issue that differs from when new buildings are erected is that all material must be lifted to the roof in some way. Kilersjö (2013-02-05) explained that this fact must be considered in an early state, when the economic aspects are treated. 3.5.2 Experiences about weather protection When erecting a new structure on an already existing building, the old roof often needs to be removed. Since the roof is important for the weather protection, some temporary cover might become necessary. It is possible to erect the extension and make a tight building before the original slabs are torn down. Another approach is to erect a weather protective tent in which the extension then is built. If a timber solution is chosen, the use of a tent is often required to protect the timber anyway. A tent like this was for example used at Glastmästaregatan, where it also was desired to limit the number of times the tent was opened, Willén (2013-02-06). This intent even affected the choice of structural system so that an alternative with timber studs was selected. Johnsson (2013-02-12) at Lindbäcks Bygg said that when their prefabricated timber modules are assembled, they start with the weather protecting roof. Each morning, if the weather is favourable, they start by lifting off the roof from the building to be able to mount the modules. By the end of the day the roof is put in place again to protect the building during the night. More information about these modules is provided in Section 4.2. 3.5.3 Experiences about residents and other affected persons Throughout the different interviews the persons that live or work in the existing building came up for discussion many times. Several of the interviewed persons explained that it can be hard to satisfy the residents who often think that the project only causes them trouble without directly improving their situation. One important issue is to encourage good cooperation with affected persons through the entire project, Östling (2013-02-06). At Glasmästaregatan this was achieved by forming a group with volunteers who discussed the project and provided suggestions for improvements. In that project the commissioner also chose to dedicate one person to keep the residents updated with information about the project and being available to answer question. At Backa Röd a pilot project was performed on one of the buildings, Gerle (2013-02-12). By doing so the commissioner could evaluate possibilities at the same time as residents in the upcoming buildings could view the results and benefits of the project. During this pilot project, several things that could be improved were discovered. Another possible way to improve the popularity of the project is to make sure that the original residents benefit from it as well. This can for example be done by giving them access to new elevators or taking the opportunity to renovate the old building at the same time. It is however very important to consider the consequences of an increased rent carefully. Several tenants may choose to look for other alternatives, if an increased rent is forced upon them. It is of importance that these people are CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 32 properly informed in advance and, if possible, helped towards new accommodations. If it is decided to increase the rent in the old part of the building, it is however very important that the raise only comes from the renovations and the access to new services, Östling (2013-02-06). The extension itself should never affect the economy for the original residents. Concerning evacuation it was decided in many of the investigated projects to let the tenants stay in the building as long as possible, due to economical reasons. In several of the projects, the accompanying renovation of the original building however resulted in part-time evacuation. The extension of the office building HK60 was however performed after the old tenant had moved out. The purpose of this redesign was to improve the appearance of the building and attract new tenants, Bågenvik (2013-03-14). In this way, the construction work inside the old building was simplified. One way to remove the disturbance in the most affected part of the old building is to evacuate the top floor and possibly use it as building site office, Samuelsson, E. (2013-01-24). As discussed in Section 3.5.1 this can also simplify the logistical situation. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 33 4 Consideration for the extension In this chapter important considerations that should be regarded in an early stage of a storey extension project are presented. Focus is here put on the extension. 4.1 Height of extensions There are many things that together limit the number of storeys that can or should be added to the building. Besides resistance and stability of the structure and its foundation, there are some other parameters that also should be considered. These limitations are presented in this section together with some of the effects that a higher building may generate. 4.1.1 Height allowed by zoning The first and most significant limit is the regulations in the zoning documents. Sometimes, the allowed height has not been fully utilised by the existing building, which means that it is easier to get permission for a smaller extension. However, if changes in the zoning restrictions are needed, as is often the case, it is important to be prepared for a rather long processing time. The City Council of Göteborg will normally not make changes in the restrictions unless they are in alignment with their general plan of the area. In general, a processing time of 30 months is to be expected, Swan (2013-02-22). However, the processing time might be reduced if the plan is complementing a need in a certain area or is in agreement with the council’s aim to produce 3500 residencies each year. 4.1.2 Height nature of the surroundings One thing that affects the height of the extension is how well it fits into the height nature of the surrounding built environment, Bergenudd (1981). This can according to Bergenudd be treated in several different ways and some of these are illustrated in Figure 4.1. Sometimes it is preferred to keep the buildings on an equal level by adding the same number of floors to all of the buildings. In other cases storeys can be added to lower buildings to create a more homogeneous height nature and in yet other situations it could be better to make an accentuation by adding a high extension to one building only. CHALMERS, Civil and Environmental Engineering, Master’s Thesis 2013:113 34 Figure 4.1 Different ways of treating the height nature and impression of the built environment, after Bergenudd (1981). 4.1.3 Consequences for fire regulations due to increased height The fire regulations are highly dependent on the number of storeys in a building. If extra floors are added, the regulations for the entire structure may change. Therefore, this section contains an overview of parameters that change at the different heights. The information is based on the Swedish building code Boverkets byggregler (BBR 2012), Chapter 5 Brandskydd (fire protection) and has been arranged in a list presented in Appendix C to better illustrate the distinction between the changes that accompany the different heights. However, it is highly recommended to read the whole text at Boverket’s homepage since the information in Appendix C does not cover the regulations that are independent of the number of storeys. Figure 4.2 shows a summary of the list in Appendix C by indicating at which storeys the fire regulations are changed. Since some of the demands are based on the height of the building rather than the number of storeys, a second row in the figure has been added. Observe that some of the limits have a greater influence than others. Figure 4.2 Critical heights concerning fire regulations. Figure 4.2 indicates that, apart from the number of added storeys, the total number of storeys can have a great influence as well. As an example, it can be better to increase the number from six to eight than from seven to nine, thus avoi