Why Wood is Not Used to a Greater Extent in the Construction of Multi-story Buildings?

Transcription

1 Why Wood is Not Used to a Greater Extent in the Construction of Multi-story Buildings? Lars ELIASSON and Thomas THÖRNQVIST School of Engineering Forest and Wood Technology Linnaeus University, Växjö, Sweden In connection with Sweden s entrance into the EU, the building of housing higher than two floors with wood frames became permissible. This became possible due to changes in building regulations from being material dependent to function based. The changed regulations have meant that today s housing is constructed with more than two floors in wood, but not to the extent that would be possible. Investigations have shown that those who live in multi-story housing with wood frames are very happy, and that the acoustics in these houses are considered just as good as in housing with concrete frames. Still, why are more wood frame housing not built? There are indications that it would be cheaper to building with wood than concrete. The reasons why have been speculated, everything from expensive maintenance to the loss of a wood building system. To get a more profound picture of the actual conditions, a questionnaire was conducted among SABO (Swedish Association of Municipal Housing) companies in Sweden that have more than 1,000 flats in its possession. In total, 88 housing companies participated in the evaluation, of which 22 state that they already have residential properties built in wood. A majority of the companies included in the study consider that the maintenance costs of the façades would be high compared to other alternatives and that buildings with wood frames would not meet the acoustics requirements. An overwhelming majority state that housing with wood frames meets the requirements for durability and bending joists very well. Key words wood in construction, maintenance cost, multi-story buildings Paper BC-3 1 of 8

2 Background Almost all single-dwelling housing in Sweden is built with wood frames. The most common frame material in multi-story housing is concrete (Bengtsson, 2003). The use of wood in loadbearing frames has not been used in the building of large high housing since the 1800s. And the usage of wood as an alternative in façades has not been widespread. Still, areas using wood are roof construction, internal panels, floors and carpentry work, e.g. windows. However, the share of large buildings where wood was chosen as the building material has increased in recent years. Major research has been conducted both nationally and internationally to attain a more effective wood building system. Several building projects have been conducted in Sweden, illustrating that it is possible to carry out large projects with the use of wood as a building material in frames and façades. As more projects with multistory wood housing are being planned, the way to produce these houses has developed concerning both the prefabrication and assembly of the building site. Large efforts have been made to reduce the effect of precipitation. Table 1 shows a summary of numerous wood building projects from 1995 to During this period, the houses have become higher and the level of prefabrication has increased. Table 1. Comparison compilation of numerous wood building projects in Sweden 1995 to 2009 (Stehn, et. al, 2008; Serrano, 2009) Välludden Växjö 1995 Lotsen Skellefteå 1998 Inre Hamnen Sundsvall Älvsbacka strand Skellefteå 2008 Limnologen Växjö Usage Residence Office Residence Residence Residence Number of floors 4 and Building method Building site Prefabrication Prefabrication Prefabrication Prefabrication Level of prefabrication Floor element Installations Building site Pre-installed with connections on site Moisture assurance preproduction Moisture assurance assembly Factory in the field Under open sky Indoors in the factory Floor element with installations Ventilation pipe built into the element in small scale. Heating and sprinkler built on site. Indoors in the factory Under open sky Under weather protection Floor and volume element with installations Sprinkler and ventilation pipes etc. built into the element, only supplemental work on site Indoors in the factory Under weather protection with integrated crane Floor element with installations Sprinkler and ventilation pipes built etc. in longitudinally into the beams - supplemental work on site Indoors in the factory Under weather protection with integrated crane After the city fires that took place in Sweden at the end of the 1800s, houses higher than two floors in wood frames were not built until During the same period, this lead to the cessation of knowledge development regarding the building of high and large wood buildings. (Vessby, 2008; Brandskyddsföreningen, 1927). Paper BC-3 2 of 8

3 The building codes were changed in 1995 from being material dependent to function-based (Boverket, 2006; Vessby, 2008). With the changed building codes, interest from the wood industry increased, with expectations of an increase in wood in the building sector (Nord, 2005). As commissioned by the government, general outlines were drawn to advance the usage of wood in the building industry (Anon 2004) and Sweden adopted a national wood building strategy from 2005 to The aim of the national wood building strategy was to turn wood into an obvious building material alternative, even in large building projects. During the preliminary work for this strategy, the historical debt was also mentioned, i.e. the knowledge lost within the area of wood structures due to the ban to use wood as the material in large buildings. Studies are being conducted concerning the inhabitants experiences from a specific wood housing area and how they experience wood as a building material. The inhabitants, who have purchased flats in the actual area, experience wood very positively, better than they expected (Schauerte, 2008). Jarnehammar et al (2008) shows how the inhabitants in the Välludden quarter, the first multi-story wood housing in Sweden, experience this indoor environment. Questions that the inhabitants have had to answer concern, among others, indoor temperature, noise from ventilation and other sources, air humidity, and if the floor is difficult to clean. Roos et. al (2008) present a study of how building engineers and architects experience wood in a building context and in these cases, not only to build blocks of flats, but also schools and commercial buildings. Architects and engineers are occasionally hesitant to suggest wood due to a lack of experience and knowledge. In North American, Kozak & Cohen (1999) have observed that the marketing of wood from architects and designers is unsatisfactory. In another study, O'Connor et. al (2004) have identified a number of barriers against the usage of wood as a building material in large houses. Aim and Method The aim of this study is to find out the views of municipal housing companies to wood in the production of new rental properties. Further, the aim is to study what understanding municipal housing companies think that their architects, designers, planning manager and entrepreneur have of wood. A form was sent out to all rental companies in Sweden with more than 1,000 flats in their possession. The form is comprised of 32 statements, each answered with Right, Wrong, or No Opinion, attachment 1. Results The questionnaire was sent to 168 housing companies and the study was conducted during the spring One printed form was sent together with an addressed and postage-paid return envelope to the managing director in each respective company. The rate of return was 52% (89 returned answers). One company answered no opinion on all statements. This questionnaire was therefore not included in the analysis; hence, 88 questionnaires are included in the processed results. Of the answers included in the analysis, 24% stated that they manage wood housing, i.e. around one-fourth of the companies who responded to the questionnaire have experience with Paper BC-3 3 of 8

4 wood as a building material and have wood housing. The share of companies that do not have wood housing in its possession is thus 76%. The housing companies about the architects A position was taken on how the architects were perceived regarding their desire and knowledge to design wood and if they are material neutral. Several of the companies think that the architects are not unwilling, but state that they lack sufficient knowledge to design wood. The companies also think that their architects are material neutral. The housing companies about the designers Most of the companies think that the designers used to have a conviction and confidence to wood as a building material. At the companies that lack their own experience of wood housing, the perception is that the designers lack a confidence to wood that is somewhat higher than those companies with their own wood housing. The same applies to the companies perception of the designer s knowledge. The housing companies about the planning managers Of the companies with multi-story buildings, most realise that the planning managers are convinced about wood as a material and they believe that their planning managers have knowledge about wood. The companies without their own wood housing expressed no opinion about the planning managers conviction or knowledge about wood. The housing companies about the entrepreneurs Of the companies with multi-story wood housing, several answered that there is a trust and a belief in wood amongst the entrepreneurs. Among the companies lacking experience from wood housing, most expressed no opinion about the entrepreneurs. The housing companies on moisture during the building period Of the companies with experience in multi-story wood housing, one company answered that moisture during the building period was a problem and therefore did not risk using wood on additional housing. Still, a majority state that moisture is not a problem. Companies without wood housing have a different perception, and one-fifth state that the danger for moisture damage during the building period is so great that you do not risk building with wood. The housing companies and the health of the inhabitants None of the companies think that there is a health danger to live in wood housing, but that among the companies lacking experience, a number had an opinion on actual health risks. The housing companies and the effects on the environment Several of the companies consider it positive to the environment to build wood housing. From both the companies with and without experience of wood housing, some companies had no opinion on how a wood house affects the environment. The housing companies on wood as a proven material Of the companies lacking experience with wood housing, some expressed concern at investing in building with wood, i.e. about one-fifth are hesitant, but more than half state that they feel no hesitation to wood due to it being unproven. Paper BC-3 4 of 8

5 The housing companies and the risk for fire Regarding the statement of trust to wood, even in a fire situation, a majority of companies answered that they have no concerns about the risk for fire. Among the companies lacking experience, the trust is not as large versus the group with wood housing experience. The housing companies and the acoustic requirements A majority of companies state that frames and beams in wood pass the acoustic requirements. Among the companies lacking their own wood housing, the same view is not expressed as those with experience of wood housing. This applies to both those that trust wood to pass the acoustic requirements and those without any opinion. The housing companies and bending beams None of the housing companies that have built and owned wood housing think that large bending of the beams is a problem. More than half of the group that do not have their own wood housing think that bending beams are not a serious problem, but among those companies, some consider it a reason why multi-story flats are not built in wood. The housing companies and the maintenance cost for frames and façades Of all the companies, less than one-fifth think that wood frames give high maintenance costs. Regarding the maintenance costs for façades, both the companies with and without experience of wood housing think that the cost will be too high. Of the companies that think the costs will be too high, almost one-fourth have their own wood housing. The housing companies and the durability requirements of façades Barely half of the companies that answered the questionnaire state that wood in façades does not meet durability requirements. This group also contains several housing companies with their own wood housing. Among the companies with experience of wood housing, more than half think that façades in wood fulfil the durability requirements. The housing companies perception of what the tenants think about wood About three-fourths of the companies think that the tenants would choose to live in a house with wood frames or wood façades. The housing companies about how the building culture affects the choice of frame and façades The last two statements in the questionnaire concern how tradition and culture can affect the choice of material. Almost half of the companies think that the choice of frame and façades are affected by the existing tradition. Both the companies with and without wood housing expressed the same conclusion. Discussion When answering a questionnaire with a number of statements, the results will be affected by past experiences, and the experience here with wood is assumed to have just this effect. The first statement We own/manage/have built multi-story wood housing give the possibility to analyse how one s own past experience affects the perception of wood as a building material by dividing the companies into two groups, one with wood experience and one without. For a number of statements, no large difference is found between both groups. However, for Paper BC-3 5 of 8

6 numerous statements, there are large differences in what the companies think. Examples of this are the statements about wood as an unproven alternative or how the problem with moisture during building is considered. The statements without any great difference between both groups concern the architects neutrality to the material wood and desire to design in wood. The use of wood in the building of large houses has increase since 1995, from nothing to today s approximate 20% of the total. Numerous municipalities in Sweden have, however, declared that wood shall be an equivalent alternative among other building materials in all building, with the aim to get more wood into building. In some cases, those who have answered the questionnaire have also written their own comments, many of which concern swaying confidence to wood façades. Other comments regard moisture damage during usage or how to repair after a fire. In certain cases, even the problems with acoustics are commented upon, where some experienced that wood frames do not fulfil the requirements laid out. Several of the companies see wood as an unproven material compared to steel and concrete and that this leads to uncertainty, which in turn leads to not risking or having the opportunity to choose wood as an alternative. Moisture during the building period noted in several building projects and in some cases, this has been disadvantageous to wood. Great efforts have been made to ensure that precipitation during the building does not negatively affect the building itself. In recent building projects in Sweden, the houses have been mounted indoors in a tent with even an integrated crane in the tent stand to load in material. No building deals well with moisture during the building period, regardless of material. Knowledge about the perception that housing companies have of various actors and the tenants view of wood cannot be used to explain why wood is not used to greater extent. Swedish municipalities own almost all of the housing companies included in the study. The choice of building material is affected by a large number of parameters, of which the political decisions taken locally and municipally are affected by. Decisions taken of an environmental nature imply an advantage for wood. An example of this is the Municipality of Växjö, which has decided that wood should always be evaluated as a material for various building projects. References Anon Mer trä i byggandet. Underlag för en nationell strategi att främja användningen av trä i byggandet. (In Swedish) Ds 2004:1, Stockholm. Näringsdepartementet, Regeringskansliet, Stockholm. (in Swedish) Bengtsson, A Framing technological development in a concrete context The use of wood in the Swedish construction industry. Doctoral Thesis no. 99. Department of business Studies, Uppsala University. Boverket, Regelsamling för byggande: Boverkets byggregler, BBR, BFS, 1993:57 med ändringar till och med 2006:22 (in Swedish) 2 ed Karlskrona: Boverket 235. Brandskyddsföreningen, Svenska Stadsbränder. (in Swedish) Stockholm Jarnehammar, A. Nilsson, I. Englund, F Trästaden ett uthålligt koncept. Erfarenheter från 10 års drift av Välludden. IVL Svenska Miljöinstitutet AB, Stockholm (In Swedish) Paper BC-3 6 of 8

7 Kozak, R.A. Cohen, D.H Architects and structural engineers: An examination of wood design and use in non-residential construction. Forest Products Journal 49: Nord, T Structure and developments in the solid wood value chain. Dominant saw milling strategies and industrialized housing. Licentiate Thesis 2005:57. Luleå University of Technology. O'Connor, J. Kozak, R. Gaston, C. Fell, D Wood use in nonresidential buildings: Opportunities and barriers. Forest Products Journal 54 (3): Roos, A. Woxblom, L, McCluskey, D.R Architects, and Building Engineers,and Stakeholders Perceptions to Wood in Construction Results from a Qualitative study. p in Bergseng, E., Delbeck, G. Hoen, H.-F. (red.), Proceedings of the Biennial Meeting of the Scandinavian Society of Forest Economics, Norway April, Scandinavian Forest Economics No. 42, Schauerte, T Desired and perceived customer value are the Limnologen endusers satisfied? p in Serrano, E. (eds.). Documentation of the Limnologen Project Overview and Summaries of Sub Projects Results. Växjö University, School of Technology and Design, Report no 56. Serrano, E Background and overview, p 1-10 in Serrano, E. (eds.). Documentation of the Limnologen Project Overview and Summaries of Sub Projects Results. Växjö University, School of Technology and Design, Report no 56. Stehn, L. Rask, L-O. Nygren, I. Östman, B Byggandet av flervåningshus i trä - Erfarenheter efter tre års observation av träbyggandets utveckling. School of Technology and Design. Report, No 47. Växjö University. (in Swedish) Vessby, J Shear walls for multy-storey timber buildings. Thesis for the degree of licentiate of Engineering. Växjö University, School of Technology and Design. Report no 44. Bilaga 1 Enkät Vänligen besvara nedanstående frågor med ett kryss för det alternativ Du tycker är rätt. Skriv gärna ned kommentarer, synpunkter och andra orsaker, som inte tagits upp här, till att trä inte används i större utsträckning än det görs i dag i flervånings bostadshus. Fråga Rätt Fel Ingen åsikt 1 Vi äger/förvaltar/har byggt flervånings trähus 2 Våra arkitekter vill inte rita flervånings bostadshus i trä 3 Våra arkitekter kan inte trä och därför kommer inte trä i fråga för flervånings bostadshus 4 Våra arkitekter är materialneutrala 5 Våra konstruktörer tror inte på trä i flervånings bostadshus 6 Våra konstruktörer kan inte trä och därför kommer inte trä i fråga för flervånings bostadshus i trä 7 Våra projektörer tror inte på trä i flervånings bostadshus 8 Våra projektörer kan inte trä och därför kommer inte trä i fråga för flervånings bostadshus i trä 9 Våra entreprenörer tror inte på trä i flervånings bostadshus 10 Våra entreprenörer kan inte trä och därför kommer inte trä i fråga för Paper BC-3 7 of 8

8 flervånings bostadshus i trä 11 Våra entreprenörer vågar inte ge garantier för flervånings bostads hus i trä, därför vågar vi inte bygga med trä 12 Det finns inga bra byggsystem för flervånings bostadshus i trä och därför kommer inte trä i fråga 13 Fukten under byggtiden är ett problem för trä, vilket gör att vi inte vågar bygga flervånings bostadshus i trä 14 Ett trähus som skyddas mot fukt under byggtiden kan man flytta in i direkt efter att det är färdigt 15 Fukten i betong kan leda till problem med bokmalen i nya betonghus. Detta problem undviker man om man bygger i trä 16 Trä kan angripas av röta och andra svampar, vilket gör att vi inte vågar bygga flervånings bostadshus i trä 17 Vi tror att trä rör sig för mycket för att det skall vara ett materialalternativ i flervånings bostadshus 18 Vi tror att det kan vara hälsovådligt att bo i trähus 19 Vi tror att det är positivt för miljön att bygga flervånings bostadshus i trä 20 Vi vågar inte satsa på trä i flervånings bostadshus för att det är ett oprövat alternativ 21 Vi litar inte på trä i stomme och utvändig beklädnad ur brandsäkerhetsskäl 22 Vi tror inte trä i stomme och bjälklag klarar våra krav på akustik 23 Vi tror att träbjälklag sviktar för mycket 24 Vi tror inte att trä klarar hållfasthetskraven som måste ställas på flervånings bostadshus 25 Vi tror att underhållskostnaderna för trästommen kommer att bli för höga 26 Vi tror att underhållskostnaderna för den utvändiga beklädnaden kommer att bli för höga 27 Vi tror inte att trä i stommen klarar av våra beständighetskrav 28 Vi tror inte att trä i den utvändiga beklädnaden klarar av våra beständighetskrav 29 Vi tror inte att våra hyresgäster vill bo i hus med trästomme 30 Vi tror inte att våra hyresgäster vill bo i hus med utvändig beklädnad i trä 31 Byggkulturen i vårt verksamhetsområde avgör materialvalet i husens stomme 32 Byggkulturen i vårt verksamhetsområde avgör materialvalet i husens utvändiga beklädnad Paper BC-3 8 of 8