National building regulations in relation to multi-storey wooden buildings in Europe

Transcription

1 National building regulations in relation to multi-storey wooden buildings in Europe Birgit Östman and Bo Källsner SP Trätek and Växjö University 1

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3 Preface This report is part of a comprehensive project directed towards the development of energy efficient and environmentally adapted systems undertaken by the Växjö University within the EU project Concerto-SESAC. This is Report 5 dealing with the building regulatory conditions in several European countries. Other reports under the overall project deal with the environmental performance (Report 1), market potential (Report 2), economics (Report 3) and general conditions (Report 4) for such constructions. The task to produce Report 5 has been commissioned to SP Trätek. 3

4 Summary The national building regulations are generally being altered towards functional or performance criteria, rather than being prescriptive. This development was accelerated by the Construction Products Directive (CPD), which was adopted in 1988 within EU. The implementation of the CPD has opened up the European market for building products. It has also facilitated the use of multi-storey wooden buildings in many countries. CPD is detailed in so called interpretative documents and then further specified in European standards. Generally, the European standards deal with harmonised methods for verification. These standards exist on the technical level, but building safety is on the political level governed by national legislation. National building regulations will therefore remain, but the new European harmonisation of standards will hopefully provide means of achieving more common national regulations. However, this process is complicated and the speed of implementation in national building codes varies between countries. A broad variety of systems of technical requirements for buildings exist in the various European countries. The goals and topics are quite similar, and most countries call their regulations performance based, but a detailed study reveals considerable variety of functional requirements, performance requirements and specifications. Five building requirements in national building regulations influencing wood buildings have been identified and analysed: fire safety, acoustics and vibrations, stabilisation, seismic design and durability design. The main conclusion is that the regulatory limitations are most obvious in fire and acoustic performance requirements, while the other requirements are more indirect, but still limiting the use of wood in buildings. All these limitations need to be further addressed. Conclusions and recommendations are provided for the different building requirements analysed. 4

5 Content Preface 3 Summary 4 1. Introduction European system for building design Structural Eurocodes 6 2. Literature overviews on building regulations General overviews Overview on wooden buildings 8 3. Building requirements influencing wooden buildings Fire safety Limitations in building regulations European harmonization of performance classes Reaction to fire (visible wood) Fire resistance (structural design) Conclusions and recommendations on fire safety Acoustics and vibrations Limitations in building regulations European harmonization Conclusions and recommendations on acoustics and vibrations Stabilisation Limitations in building regulations European harmonization Conclusions and recommendations on stabilisation Seismic design Limitations in building regulations European harmonization Conclusions and recommendations on seismic design Durability design Limitations in building regulations European harmonization Conclusions and recommendations on durability design Other technical barriers Opportunities Environment and recycling Energy demand and energy efficiency in buildings General conclusions References 25 5

6 1. Introduction The aim of this report is to provide an overview of the regulatory systems in Europe, to analyze possible obstacles to multi-storey wooden buildings and suggest modifications. 1.1 European system for building design Building regulations are generally being altered towards functional or performance criteria, rather than being prescriptive. This development was accelerated by the Construction Products Directive (CPD), which was adopted in The CPD will be replaced by the Construction Products Regulation (CPR), currently under development, but the same trend is foreseen to be continued. CPD contains six essential requirements: 1. Mechanical resistance and stability 2. Safety in case of fire 3. Hygiene, health and the environment 4. Safety in use 5. Protection against noise 6. Energy economy and heat retention All essential requirements are detailed in so called interpretative documents and then further specified in European standards. The implementation of the CPD has opened up the European market for building products. A CE-marked product can be put on the market anywhere within the EU. The system with harmonised standards for testing and classification has lead to a common language for the regulators in each member state. However, this process is complicated and the speed of implementation in national building codes varies between countries. Generally, the European standards deal mainly with harmonised methods for verification. These standards exist on the technical level, but building safety is on the political level governed by national legislation. National regulations will therefore remain, but the new European harmonisation of standards will hopefully provide means of achieving more common national regulations. 1.2 Structural Eurocodes The structural Eurocodes are European design standards of components and structures for buildings and closer to the building regulations than most other standards. The Eurocodes aim to standardise design rules within Europe and allow the calculation and verification of load-bearing capacity of components and structures for different materials, based on the semi-probabilistic design concept with partial safety coefficients. The Eurocodes comprise ten parts: Eurocode 0 Basis of structural design EN 1990 Eurocode 1 Actions on structures EN 1991 Eurocode 2 Design of concrete structures EN 1992 Eurocode 3 Design of steel structures EN 1993 Eurocode 4 Design of composite steel and concrete structures EN 1994 Eurocode 5 Design of timber structures EN 1995 Eurocode 6 Design of masonry structures EN 1996 Eurocode 7 Geotechnical design EN 1997 Eurocode 8 Design of structures for earthquake resistance EN 1998 Eurocode 9 Design of aluminium structures EN 1999 All Eurocodes relating to materials, i.e. 2, 3, 4, 5, 6 and 9, have a Part 1-1, e.g. EN , which covers the design of civil engineering works and buildings, and another Part 1-2, e.g. EN , which deals with the structural fire design. The Eurocodes have to be implemented by the national standard committees in all European countries. National annexes with specific rules and values to maintain the level of safety prevailing in the respective countries have been developed. 6

7 2. Literature overviews on building regulations Building regulations provide a solid base for safe, healthy, energy efficient, environment friendly and comfortable buildings. A broad variety of systems of technical requirements for buildings exist in the various European countries, despite the existence of the CPD and the development of Eurocodes. The goals and topics are quite similar, and most countries call their regulations performance based, but a detailed study reveals considerable variety of functional requirements, performance requirements, and specifications, with inconsistency within the requirements of some countries. Research into the differences in formulations is a first and important step towards better mutual understanding of national sets of building regulations which is essential for the possibilities to further harmonise the systems in the various countries. Some overviews on differences in national building regulations are presented below. 2.1 General overviews A comparative study of building regulations and systems of building control in eight European countries [30] has shown a broad variety of organization models with a severe role for the private sector. A similar paper [22] is based on the findings of a comparative analysis of building regulations for housing in eight European countries, carried out on behalf of the Dutch Ministry of Housing and intended to locate the Dutch Building Decree within the spectrum of regulations in other European countries. The paper makes a detailed comparison of systems of building control, the formulation of the requirements, and the contents of selected subjects. Another paper [31] presents the results and conclusions of a comparative study of the building regulations in Belgium, Denmark, England, France, Germany, the Netherlands, Norway and Sweden. The systems and formulations of the requirements and the contents of some subjects of requirements (for houses) have been compared in detail: stairways and ramps, fire safety, noise, daylight, accessibility and dimensions of habitable space and habitable room. It is concluded that the broad spectrum of different systems forms a major barrier for further harmonisation of building regulations in Europe and even a barrier for the realisation of an internal European Market. One paper [15] describes the developments of European building regulatory systems and place them within the context of general trends in the regulatory sciences, particularly with reference to Europeanisation studies. Within the regulatory sciences there is broad consensus about the trend towards deregulation and privatisation in recent decades. Studies conducted under the heading `Europeanisation' analyse the effects of European policies on the policy frameworks of the member states in more detail. Are the systems converging or diverging? Although the history of this kind of research is short, most authors agree that European policies have had a profound impact on the policies of the member states, but that this impact has not been uniform. The focus is on three elements: the scope of the technical requirements, the building-permit procedures and the division of control and inspection responsibilities. The analyses reveal no evidence that the scope of technical requirements in European countries is diminishing. Through its directives, the European Union exercises a convergent influence on the contents, scope, and formulation of these technical requirements. In addition, all member countries are taking steps to streamline their administrative procedures. The importance of the role played by private organisations in checking and controlling regulations is increasing across the borders. Although they differ in pace and emphasis, these administrative deregulation and privatisation trends exhibit a number of parallel developments. 7

8 2.2 Overview on wooden buildings One study was concentrated on wooden buildings [1]. It concluded that there are no regulatory barriers to the use of wood or wood-based products in the construction of residential buildings. This is mainly due to that fact that governments, through their regulations, cannot be prejudiced towards any particular material. Despite this there are many limitations to the use of wood and wood-based products, which could act as barriers if not addressed in the future. The BRE study [1] identified barriers, mostly regulatory, to enhanced use of wood and wood-based products in residential buildings in European Union. The study is based on a questionnaire survey of construction sector professionals such as clients, planners (e.g. building authority), designers (architects, engineers), suppliers, contractors, and national correspondents of each of EU member states as listed in the CEI Bois database. The report cautioned that the survey results could be biased in favour of wood construction, since individuals familiar with such constructions participated in the survey. The study concluded that even if there are no direct regulatory barriers to use of wood for residential building construction in the European Union, there still are several regulatory limitations: The main regulatory limitations are perceived to be fire and acoustic performance, particularly in multi-storey dwellings. In some countries there are regional differences in building regulations. There is a lack of codes and standards for many wood products. Consequently, the certification procedures for technical performance of such products become costly and time-consuming. Familiarity with Eurocodes is increasing, but their use is still very limited. There is uncertainty and a lack of in-depth knowledge of building regulations relevant to the use of wood in construction. External use of wood and wood-based products is mainly limited by the height of the building and the distance between adjacent buildings related to the requirements of external spread of fire. The maximum number of storeys permitted varies between countries. The BRE study also showed that the lack of appropriate education, training and skills, not only in the wood-based industries, but also in key related occupations was the most important institutional barrier to increased use of wood for construction in the European society. Some further barriers to the enhanced use of wood are briefly summarized in [14]. The general conclusion is that regulatory requirements are functional and not prescriptive in almost all European countries and any material can be employed as long as the functional requirements can be met. However, there are many limitations to the use of wood and wood-based products, which need to be addressed and ultimately eliminated. One of the main regulatory limitations to the enhanced use of wood-based products in residential construction relate to the fire performance and sound insulation specifications, specifically when materials and building elements are used in multi-storey and/or multi-occupancy residential constructions. For single-family dwelling construction there are no substantial limitations to the use of wood and wood-based products. Overviews on one specific building function are included in chapter 3. 8

9 3. Building requirements influencing wooden buildings Five building requirements in national building regulations influencing wood buildings have been identified and analysed: Fire safety, Acoustics and vibrations, Stabilisation, Seismic design and Durability design. These are presented below together with Other technical barriers and Opportunities. 3.1 Fire safety Fire safety is widely considered as one of the most significant obstacles for increasing the use of wood in construction. Most fire regulations in Europe have traditionally been very prescriptive and based on experience from large city fires. Fire safety is an important contribution to feeling safe, and an important criterion for the choice of materials for buildings. The main precondition for increased use of timber for buildings is adequate fire safety. World-wide, several research projects on the fire behaviour of timber structures have been conducted over the past decades, aimed at providing basic data and information on the safe use of timber. Novel fire design concepts and models have been developed, based on extensive testing and modelling. The current improved knowledge in the area of fire design of timber structures, combined with technical measures, particularly sprinkler and smoke detection systems, and well-equipped fire services, allow the safe use of timber in a wide field of applications. As a result, many countries have started to revise their fire regulations, thus permitting greater use of timber Limitations in building regulations However, major differences between European countries have been identified, both in terms of the number of storeys permitted in timber structures, see figure 1, and of the types and/or amounts of visible wood surfaces in interior and exterior applications [32]. Several countries have no specific regulations, or do not limit the number of storeys in timber buildings. Figure 1. Restrictions of the use of timber structures for higher buildings, set by national prescriptive regulations, have been eased in Europe over the last decades [33]. A further increase in permitted use is expected. 5 storeys 2 storeys (incl. 0) 3-4 storeys No information 9

10 There are further limitations in the use of wood in multi-story buildings, mainly related to visible wooden surfaces, both for interior and exterior applications. One example is given in Figure 2 [32]. Wall and ceiling linings in flats without sprinklers Surface linings of ordinary wood 2006 Figure 2. Wall and ceiling linings of wood in multi-storey buildings are allowed in some countries and not in others due to prescriptive rules on reaction to fire class [32]. 5 storeys 3-4 storeys 2 storeys (incl. 0) No information Most countries have restrictions on the use of wooden facade claddings. Some countries have no restrictions, but, on the other hand also very limited experience of using wooden facades due to building traditions. Several countries allow wooden panelling in flats, but usually not in escape routes. Wooden floorings are permitted in flats in most countries and in some countries also in escape routes. Installation of active fire protection systems, e.g. residential sprinklers, may allow for higher buildings with timber structure or further use of visible wood in some countries [32]. However, these systems are still quite unknown in most countries European harmonization of performance classes The consequences of the move to performance-based requirements are especially pronounced for the fire regulations that traditionally have been prescriptive. The CPD requirements on fire safety are that structures must be designed and built such that, in the case of fire: load-bearing capacity can be assumed to be maintained for a specific period of time the generation and spread of fire and smoke is limited the spread of fire to neighbouring structures is limited occupants can leave the building or be rescued by other means the safety of rescue teams is taken into consideration. These essential requirements are implemented and detailed in European standards, see Figure 3. The two areas regulated in building codes, Fire resistance and Reaction to fire, are illustrated in Figure 4. CPD and its essential requirement on fire safety in the case of fire Interpretative documents (ID) with classes for fire performance Testing and classification CEN TC 127 Fire safety in buildings Calculation rules CEN TC 250 / SC 5 Structural Eurocodes / Timber Structures Products Figure 3. Systems for European fire standards for building products. 10

11 Reaction to fire (visible wood) Figure 4. The two important areas for fire regulations. The reaction to fire means the initial response of a material to fire exposure. The main product group is internal and external surfaces in buildings. The European system contains main classes A to F, and then sub classes s1-s3 for smoke production and d0-d2 for burning droplets. It has resulted in a system with very many specific classes and the different countries have chosen different options. The national building codes differ to a certain extent in this matter. In the Nordic countries, the harmonized fire classes were implemented in the national building regulations in the early 2000-ies, but with certain differences. The situation has then been analyzed and a common use of the fire classes proposed [25]. Nothing similar is known at the European level. An overview of the implementation in several European countries is presented in Table 1. Table 1. Implementation of European fire classes in national building regulations [33]. Country Officially implemented in national building regulations Only European classes applicable in building design from Austria 2008 Not defined yet Belgium Not defined yet Not defined yet Denmark 2004 Not defined yet Estonia Finland France 2003 Not defined yet Germany Not defined yet Not defined yet Italy 2005 Not defined yet Netherlands 2003 Not defined yet Norway 2003 Not defined yet Poland Spain Sweden UK (England and Wales) 2002 Not defined yet Fire resistance (structural design) Fire resistance means that structural elements, e.g. wall elements, must withstand a fully developed fire and fulfil certain performance requirements, load-bearing capacity (R), integrity (E) and insulation (I), see Figure 5. The building elements are expected to withstand the fire exposure for a specified period of time, e.g. 60 minutes. Timber structures can achieve high fire resistance, e. g. REI 60, REI 90 or even higher. 11

12 Figure 5. Performance criteria for fire resistance. They are used together with a time value, e.g. REI 60 for an element that maintains its load-bearing and separating functions for 60 minutes. The fire resistance can be either tested or calculated according to Eurocode 5, part 1-2. New and improved calculations methods have been published recently [33]. This new information will be potential input to the next revision of Eurocode 5. Figure 6. The very first Europe-wide guideline on the fire safe use of wood in buildings was published 2010 [33]. It is expected to facilitate the extended use of wood in buildings and support the general trend on sustainability in the construction sector Conclusions and recommendations on fire safety Main conclusions are - There are major differences between European countries for the use of wood products in buildings due to national fire regulations - The use of wood is restricted by impact of local and regional interpretation of fire regulations and lack of knowledge throughout the decision chain from regulator to designer - There is a need for exchange of experience - Further development and application of new technologies for fire safety engineering and performance based design will facilitate extended and fire safe use of timber in buildings. There are several new possibilities for a more advanced fire design of timber buildings. European guidelines and models for load bearing and non-load bearing structures are available [33]. New technologies for fire safety engineering and performance based design will provide further tools for extended use of timber in buildings. One example is risk index models [10], another is the extended use of active fire protection e.g. residential sprinklers for verifying alternative fire safety design [16]. Main recommendations are Educate and provide guidance documents for building chain: From supplier to the end user, including regulator, designers and architects Support performance-based design of timber in key, prioritised applications Participate actively in the international standardisation and co-operation Initiate work on harmonised national building regulations. 12

13 3.2 Acoustics and vibrations Acoustics is an important performance characteristic for building with wood and a prerequisite for the acceptance of wooden buildings by building industry, building owners and consumers. Acoustics concerns both sound and vibration, and for wooden constructions there are some important features that differ from those in concrete and other heavy constructions. A state of the art study has been presented recently [5]. The weight (mass per unit area) of a construction is a decisive parameter for the air-borne sound insulation properties, especially for the lower frequency range (in general Hz). This means that wooden constructions may have poor sound insulation at the lower frequencies. Different wood building systems may exhibit somewhat different behaviour. Impact sound from people walking is the most common sound insulation problem for lightweight floors, and the most severe one at low frequencies. An important difference between the sound of footsteps and other sources of noise is that footsteps produce a high degree of noise disturbance, even at low frequencies. In addition, the evaluation procedure is known to frequently fail to correlate measured impact sound insulation and perceived acoustic quality; people complain on impact noise even though the building has been classified as fulfilling higher than minimum demands according to the standardised procedure. When people walk, run or jump on a floor in a lightweight construction there is always a risk of annoying floor vibrations and springiness due to the human activities. Springiness is referred to the deflection that occurs from such activities. The deflection is often local and affects, besides the active person himself, only other occupants in the direct vicinity. Vibrations mean an oscillating motion of the floor which can be either transient or resonant. In order to improve the vibration properties there are mainly three different methods: to increase the mass, enhance the stiffness properties and to improve the damping. Noise from installations is in many cases dominated by low frequencies, whereby special consideration is needed for wooden constructions. Installation equipment may excite vibrations more easily in a wooden floor than in a corresponding concrete element. Prediction models are important tools for the design of new constructions and building projects. Existing models are best suited for concrete structures, or similar heavy and homogeneous constructions. The lack of prediction models for lightweight constructions constitutes a severe drawback for the building in wood. The costly process of using test buildings is common even though the obtained measurement results are not useful for application to slightly different building constructions. Hence, there is a need to develop prediction tools. A major project is ongoing [9] to solve some of the problems with light weight constructions. It has strong links to the European standardization within CEN and to ongoing COST Actions, mainly FP0702 Net-Acoustics for Timber based Lightweight Buildings and Elements and TU 0901 Integrating and Harmonizing Sound Insulation Aspects in Sustainable Urban Housing Constructions. Figure 7. Acoustics and vibrations [5]. 13

14 Estimated Equivalent L'n,w [db] Estimated Equivalent R'w [db] Limitations in building regulations The current acoustic (=sound and vibration) requirements in multi-storey family houses have their origin in those times when multi-storey wooden houses were not allowed. This imply a clear disadvantage in competitiveness for multi-storey family houses with lightweight frame systems the acoustic quality in a lightweight building structure is perceived differently as compared to a heavyweight building structure of the same acoustic class. In particular it is the low frequency impact and airborne sound as well as vibrations that might become very evident and disturbing in lightweight structures [19, 20]. When the first regulatory sound insulation requirements appeared more than 50 years ago, the frequency range Hz became traditional for requirements in Europe. However, in countries with a tradition for light-weight building practices, e.g. Sweden and Norway, the need to include lower frequencies (< 100 Hz) gradually became obvious. Since 1996, the frequency bands down to 50 Hz have been included in the regulatory minimum requirements in Sweden, but in no other countries. However, during the past decade low-frequency descriptors (down to 50 Hz) have been introduced in the criteria for the higher, voluntary quality classes in the classification schemes in all five Nordic countries and in Lithuania Multi-storey housing Row housing Austria Belgium Czech Rep. Denmark Estonia Finland France Germany Hungary Iceland Ireland Italy Latvia Lithuania Netherlands Norway Poland Portugal Slovakia Slovenia Spain Sweden Switzerland UK 40 Figure 8. Airborne sound insulation between dwellings Legal main requirements in 24 European countries 2008 Minimum values [18] Multi-storey housing Row housing Austria Belgium Czech Rep. Denmark Estonia Finland France Germany Hungary Iceland Ireland Italy Latvia Lithuania Netherlands Norway Poland Portugal Slovakia Slovenia Spain Sweden Switzerland UK 40 Figure 9. Impact sound insulation between dwellings Legal main requirements in 24 European countries 2008 Maximum values [18]. 14

15 The regulatory requirements for sound insulation between dwellings may be summarized as Airborne sound insulation - Several descriptors - Big difference between min/max, 5 db for multi-storey and 10 db for row housing - The strictest requirements are found in Austria - Low-frequency descriptors applied only in Sweden Impact sound insulation - Several descriptors - Huge difference between min/max, 17 db for multi-storey and 22 db for row housing - The strictest requirements are found in Austria - Low-frequency descriptors applied only in Sweden European harmonization In most countries in Europe, building regulations specify sound insulation requirements for dwellings. The requirements are expressed by descriptors defined in standards. Within building acoustics, ISO standards are implemented as European (EN) standards and national standards. Comparative studies of regulatory sound insulation requirements in 24 countries in Europe and sound classification schemes in 9 countries are described in [19, 20]. The findings included that requirements and classes are different in all countries and that a diversity of descriptors is applied in Europe. It is concluded that harmonization of descriptors is needed to facilitate exchange of data and experience between countries and to reduce trade barriers. In addition, modification of sound insulation requirements is important in several countries in order to adapt regulations better suiting to current construction trends and peoples' needs. The comparison shows considerable differences in terms of descriptors, frequency range and level of requirements. In Eurocode 5 a method for design of residential floors with respect to vibrations is given. The method can be applied on multi-storey buildings and includes a static and a dynamic criterion. The method includes two parameters which are open for national choice. Vibration of floors is one of the topics identified where the design rules of Eurocode 5 need to be improved. The intension is to have new design rules ready for the next version of Eurocode 5 within this area. A method based on classification of floors into five different vibration classes has been proposed [28]. Each class is defined by limit values given for six design quantities. Table 2. Current standardized field descriptors ISO 717:1996 [18] ISO 717 descriptors for evaluation of field sound insulation Basic descriptors (single-number quantities) Spectrum adaptation terms (listed according to intended main applications) Airborne sound insulation between rooms (ISO 717-1) R' w D n,w D nt,w None C C C C Airborne sound insulation of facades (ISO 717-1) C C C C R' w D n,w D nt,w None C tr C tr, C tr, C tr, Impact sound insulation between rooms (ISO 717-2) L' n,w L' nt,w None C I C I, Total number of descriptors 3 x 5 = 15 3 x 9 = 27 2 x 3 = 6 15

16 3.2.3 Conclusions and recommendations on acoustics and vibrations Regulatory sound insulation requirements need tightening in some countries. As a starting point for further discussion, suggestions for airborne and impact sound insulation criteria providing standard and increased comfort should be given. While tightening regulations implies a growing need for exchange of information and experience, the diversity in Europe creates difficulties for efficient cooperation, and harmonization of descriptors is needed. The benefits of harmonizing descriptors include facilitating the exchange of construction data, design details and development of design tools. Based on experience, legal requirements and classification criteria could be adjusted and optimized. It is proposed to establish cooperation in Europe and to prepare an acoustical housing directive with a related strategy paper Research for quieter European homes in 2020 in the same way as with European initiatives for environmental noise. The noise issue has also received increasing attention from WHO, World Health Organisation. In a large analysis of European housing coordinated by WHO, neighbour noise was identified as a health problem, and reduction of noise exposure in the home was included in the proposed main objectives for a housing policy [19, 20]. Conclusions on sound insulation requirements may be summarized as Several descriptors applied for both airborne and impact requirements Big differences in regulatory requirements, especially for impact sound For design of light-weight buildings, low frequencies are important Low-frequency requirements have been implemented only in Sweden Low-frequency spectrum adaptation terms applied in higher voluntary classes in classification schemes in the Nordic countries Harmonization of descriptors important to facilitate exchange of experience and to reduce trade barriers Projects are ongoing, but further actions are needed. Main recommendations are Participate actively in the international standardisation and co-operation Initiate and support work on harmonised national building requirements Use the descriptors D nt,w + C for airborne sound; and L' nt,w + C I, for impact sound Develop common criteria for floor vibrations. 16

17 3.3 Stabilisation In this section only stabilisation of wooden buildings with respect to wind loads is dealt with. In section 3.4 seismic design is discussed. Wooden buildings are in comparison with concrete buildings characterized by low self weight. As a consequence of this fact wooden buildings are sensitive to wind loads and need to be accurately anchored with respect to vertical uplift forces. Figure 10. Schematic transfer of external horizontal wind loads (bold arrows) via a floor diaphragm to vertically anchored gable walls. The dashed lines indicate deformations in the building. A number of different systems for stabilization of multi-storey buildings are available [29]. In the conventional system for wooden buildings; wall, floor and roof diaphragms are assembled from sheets fixed by mechanical fasteners to a timber frame. These wall diaphragms (shear walls) are characterised by relatively low stiffness and strength in shear. This means that such wall elements in contrast to wall elements of concrete can not be treated as rigid bodies when subjected to vertical point loads in the shear plane. In case of tall buildings the wall diaphragms can be replaced by solid wood elements (Cross Laminated Timber) in order to improve stiffness properties and load-carrying capacity. For a one-storey building the design procedure is focused on the load-carrying capacity of the structure. For multi-storey buildings the stiffness properties increase in importance and requirements on static deflections and vibration response may determine the dimensions of the structures. The knowledge in Europe within this area is limited and need to be expanded. Another area where more knowledge is needed concerns brittle failures in anchoring devices fastened to the substrate. Such failures may give raise to progressive failure Limitations in building regulations In Eurocode 5 part 1-1, common European rules for design of timber buildings are given. In the present version of the code two simplified methods, A and B, for design of wall diaphragms (shear walls) exist. The main difference between the two methods is that in method A the vertical studs must be fully anchored with respect to uplift forces while in method B the bottom rails have to be anchored. Both methods can be applied for one-storey buildings but only method A can be used for multi-storey buildings. The recommended method in Eurocode 5 is method A, but method B may be given as a national choice. In Eurocode 5 no rules for design of Cross Laminated Timber (CLT) elements are given. Within the Technical Committee CEN/TC124, Working Group 3 (WG 3), however, a working document for requirements on CLT products has been drafted. 17

18 3.3.2 European harmonization A research project has been conducted in Sweden with the aim to replace the two methods for design of wall diaphragms in Eurocode 5 by a unified plastic design method that is possible to apply on multistorey buildings. The new design method is simple and flexible and can be applied on complicated geometries, boundary conditions and load configurations. The 3-dimensional structural behaviour of the buildings can be considered resulting in more favourable force distributions with less need of anchoring devices. The research work has resulted in a Swedish handbook for design of wall diaphragms [12]. The plastic design method has been internationally discussed within CIB-W18 and the basic principles have been proposed to be included in the next version of Eurocode 5. v H 3 H 3 H 2 H 1 H 3 H 2 H 1 H 2 H 1 Figure 11. Separation into loading cases according to the Swedish handbook for plastic design of wall diaphragms Conclusions and recommendations on stabilisation The present rules in Eurocode 5 are not fully suited for multi-storey wooden buildings. More suitable methods have been developed and should be implemented in next version of Eurocode 5. 18

19 3.4 Seismic design Wooden buildings have a good reputation when subjected to seismic events. Experience from North America and Japan shows that wooden buildings can resist catastrophic earthquakes, while sustaining only minimal damage [2]. Many modern timber buildings have even survived showing no visible signs of damage. The advantage of wooden buildings is based on low self-weight, good dynamic properties (damping) and in general very regular building geometry Limitations in building regulations In many countries, especially in northern Europe, seismic design of buildings is not required or may be disregarded for light timber houses and houses in lower building classes European harmonization Based on previous experience, buildings designed by means of modern codes perform well for earthquakes. In the European region, Eurocode 5, design of timber structures, and Eurocode 8, design provisions for earthquake resistance of structures, are new design codes and these may be applied, for example, in the exportation of wooden buildings and building expertise to seismic areas. Guidance on the use of Eurocodes in the seismic design of wooden residential buildings has been published [26, 27]. Wooden buildings are usually regular, both in plane and in height, and in such cases, a simplified modal response spectrum analysis may be used. The body forces created by the ground acceleration on the building are converted to a base shear force imposed in both principal directions. EC 8 gives the methods to calculate this shear force. The structures resisting these lateral forces such as shear walls, floor diaphragms and anchorages are then designed against this base shear force Conclusions and recommendations on seismic design An effective way to design for lateral loads, including seismic loads, in residential wooden houses, is the use of plywood panels in shear walls. These shear walls have a high lateral force-resisting capacity and the joints are in general very ductile. The ductility of the joints is very critical as it also affects the level of shear force to which the wall is subjected. The high performance of plywood shear walls is based on the ductility and energy dissipative characteristics of nailed or screwed joints on plywood in shear walls. Figure 12. Seismic activity in Europe Magnitude > 3 [3]. 19

20 3.5 Durability design A key issue for the competitiveness of wood is the possibility to control durability, maintenance and life cycle costs for constructions and components. Traditionally, durability design of wooden components and structures is based on a mixture of experience and adherence to good building practice, sometimes formalized in terms of implicit prescriptive rules. Therefore, the expected performance cannot be specified in quantitative terms. The design cannot be optimized and any change will be associated with certain risks. A modern definition of durability is: The capacity of the structure to give a required performance during an intended service period under the influence of degradation mechanisms. Conventional durability design methods for wood do not correspond to this definition [4]. The development of performance-based design methods for durability requires that models are available to predict performance in a quantitative and probabilistic format. The relationship between product performance during testing and in service performance needs to be quantified in statistical terms and the resulting predictive models need to be calibrated to ensure that they provide a realistic measure of service life, including a defined risk level. Durability by design is a somewhat simplified term, intended to be understood as design for optimal durability. For building products of wood or wood-based materials it encompasses all variations of dimensions, shapes, joints, fasteners, and treatments including coatings, that may influence the service life, or the durability of the product. Durability by design also implies different actions or design features depending on the dominant hazard as well as the expected performance and the accompanying limit states. In a European perspective, decay fungi is the overriding hazard, and to counteract premature decay and ensure the desired design life of wooden building products or elements, the most important aim for durability by design is to keep the moisture content of the wood below critical levels. Durability by design is thus to a large extent a question of shaping the construction and the detailing so that an efficient water protection is attained. Alternatively, or as an additional beneficial design feature, a design is sought that allows for an efficient drying of wood components that have been wetted above critical levels of moisture. With an inverse perspective, it is of utmost importance to avoid creating the negative influence of water traps, in other words such design solutions that can be foreseen to cause extended periods of increased wood moisture contents and thereby decay. Given favourable conditions, even a very local colonisation of decay fungi will eventually spread and jeopardize the function of a wood construction on a larger scale. There is a growing demand for methods of calculating life cycle costs of construction works, and a prerequisite for such calculations or predictions is reliable and manageable methods for predictions of service lives. The best known and most advanced approach is from Australia [13] to develop performance based engineering design procedures applying a probabilistic approach. A first European guideline is underway within the WoodExter project [24]. Figure 13. Example of recommended horizontal joints in a wooden façade [17] 20

21 3.5.1 Limitations in building regulations There are generally no direct limitations in the building regulations. But in a sense, durability by design has always been employed in the planning and execution of construction works, even though it has often been used in a basic form which has been mostly intuitive and based on experience and tradition. Such an approach, being less refined and less well founded on scientific evidence, is not sufficient for contemporary building planning and engineering needs European harmonization Service life estimation methods for wood and wood-based products are an area of research that so far has been given little attention in Europe. Whilst the concept is not new the development has been very slow in comparison with competing materials such as concrete. Efforts have been made to proposing models for understanding the climatic impact at wood surfaces. One of the major driving forces is the CPD (Construction Products Directive, 89/106/EEC), for which an ongoing process is running with the aim of strengthening it and giving it more weight by its transformation into a European Regulation Conclusions and recommendations on durability design There is potential in future European building and construction for an increased use of wood as an environmentally friendly and renewable material providing the durability and linked service life issues are given appropriate attention and the research is brought closer to the needs of the building community [4]. The most critical elements in the detail design, the influence of moisture on wood and wood-based materials must be minimized in constructions, e g by: A. Minimizing moisture ingress into the material providing an efficient water shedding providing shelter by eaves or covering parts protecting end grain (by detailing) avoiding exposed butted end joints choosing dimensions and profiles that do not cause extensive swelling and shrinkage and that are tolerant to any dimensional changes that may still occur using fasteners (e.g. nails, screws) that give minimal stresses, leading to cracks applying the fasteners in a correct way, i.e. not too close to edges and ends avoiding short distances from e.g. low parts of cladding to ground or hard surfaces, which will add to the wetting by rain splash and lingering snow choosing appropriate surface coatings or other treatments, giving special attention to end grain, priming with oils and sealing with sealants and/or paint B. Ensuring rapid drying if the material takes up excessive moisture giving opportunities for ventilation of surfaces or back sides of panels that may become wetted using water vapour permeable surface coatings or other diffusion open treatments keeping contact areas between single wood elements small avoiding narrow gaps at joints The overall recommendation is to continue the development on performance based design for the durability design of wooden buildings in order to make this area more equivalent to other performance requirements for building design. 21

22 3.6 Other technical barriers Education, training and skills were identified to be of paramount importance to enhancing the use of wood and wood-based products in buildings in the BRE study mentioned before [1]. The shortage of professionals and their knowledge were also seen to be of greatest concern, together with technical backup, lack of interaction with other materials, approvals, construction process and availability. To a lesser extent, safety, networking, industrial standards and planning was mentioned. A study on possible harmonization of building regulations in the Nordic countries for wooden houses in order to make the Nordic countries a well-functioning market and an integrated region has been published [7]. The purpose was to strengthen the Nordic industry through similar and transparent requirements, so with a minimum of technical trade obstacles the market can increase with the intention to further develop the construction of wooden houses. The study started with a questionnaire to the industry clarifying some of the differences in requirements and the building codes were compared. Different areas were pointed out as most interesting to analyse: stairs, size and dimensions of specific rooms, energy, structural design and moisture protection in bathrooms. There are different types of requirements, formal and informal, that a manufacturer of wooden houses has to know about. Small differences can have a big impact. Most of the differences are not significant for the quality of the house, but most likely the end-user has to pay more for the house. The work showed how important good communication is for harmonization of building requirements and that there is a need for more cooperation between the countries in order to get rid of existing obstacles and to avoid new ones. A follow up study of major ongoing wood building projects in Sweden summarised the findings in a report [23]. The study was part of the Swedish national wood construction strategy. Specific issues of concern to wood construction projects include [6]: o Fire requirements have taken a long time to solve in the initiative project. The impression is that the authorities involved are not used to interpreting legislation as it applies to higher wood-framed residential buildings. Another impression is that the application of the law varies within countries. It may, for example, be extremely difficult to build with visible wood in apartments, even if the apartments are fitted with sprinkler systems o Sound proofing is another issue which requires further research to ensure a high-quality living environment o The cost of facade maintenance throughout the lifecycle in relation to the cost of investment requires further monitoring. There is a risk that short-term decisions will create additional costs in the long term o Installations should be integrated into the framework. A higher level of prefabrication has been requested to avoid extensive subsequent installation work at the construction site o Weather protection should be specified at the planning stage and quality assured with flexible protection. Experience shows that protection by tents with overhead travelling cranes is of great benefit not only for dry construction, but also to the work environment. The speed and ease with which the height can be increased is extremely important. 22