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TIMBER FRAMED BUILDINGS AND NZS 3604 Roger Shelton1, Graeme Beattie2 ABSTRACT: This paper briefly charts the history of the New Zealand Timber Framing Standard (NZS 3604), gives some of the background to the recent revision, and presents the initial findings of a survey conducted to establish the performance of 3604 type buildings during the recent series of earthquakes in Canterbury. KEYWORDS: Light timber framed buildings, NZS 3604, Canterbury earthquake 1 INTRODUCTION 123 2 BACKGROUND TO NZS 3604 Stick framed timber buildings have been used for residential and low rise building structures in New Zealand for over 100 years. The essence of stick framing is the use of many, small section, timber elements of low structural capacity distributing the applied loads in conjunction with “non-structural” elements such as linings and claddings. This approach builds redundancy into the structure in two ways: Failure of one member allows re-distribution of loads through alternative load paths Practical framing arrangements provide more structure than is strictly necessary (ie walls which are ignored for bracing purposes). NZS 3604 “Timber framed buildings” [1] sets out the construction requirements in means of compliance format for light timber framed buildings in New Zealand. A companion document “BRANZ P21 Test method” [2] provides a versatile calculation basis and test method for lateral load bracing elements. In applying engineering rationale to a well developed, “sorted” building system, the standard was ground breaking when it was first published in 1978. By placing limits on the size and scope of buildings covered, safe but not unduly conservative solutions are provided for a wide range of low-rise timber buildings without the need for the involvement of structural engineer designers. Thus the cost of the inherent conservatism is balanced against the savings in structural design fees, where the simplicity of the design does not warrant such close attention. Those limits govern the building size, height and roof slope, floor loadings, and snow loadings. Wind and earthquake loads are also limited by restricting the zone or area in which the building may be situated. For buildings outside these limits, a specific structural design is required for each building. Both documents have been revised and updated several times since they were first written but remain at the core of tiber framed building construction in new Zealand. Over the years since 1978 most critical elements of the building structure covered in NZS 3604 have been verified by calculation where possible, or by test where appropriate. The latter is particularly true of fixings. As changes in building technology occur and new systems and techniques are developed, this testing and verification process continues. Redundancy goes a long way to providing resilient buildings which are able to cope well with This is in contrast to the philosophy behind typical “engineered” structures which use fewer, larger and stronger members, each carrying much heavier loads. The stick frame approach has evolved to suit New Zealand’s timber resource and the carpentry skills of its construction workforce. Thus, much of the development and fine tuning of the system over the years has evolved from the ground up. Timber framed buildings now represent about 95% of structural timber usage (by volume) in New Zealand. They also constitute about 95% of the existing residential building stock, although the proportion of new buildings is slightly lower as new systems (such as light gauge steel framing) are introduced. 1 Roger Shelton, BRANZ, 1222 Moonshine Road, Judgeford, New Zealand. Email: [email protected] 2 Graeme Beattie, BRANZ, 1222 Moonshine Road, Judgeford, New Zealand. Email: [email protected] 2.1 CHANGES IN 2011 The current revision of the Standard was conceived as a "Limited technical review", and focuses on five main areas: i. Updating to accommodate the change in the cited Loading Standard from NZS 4203 to the AS/NZS 1170 series [3]. ii. Migration of the cladding provisions and details from NZS 3604, into the New Zealand Building Code [4] clause E2/AS1, the Acceptable Solution for weathertightness. iii. General improvement of readability and clarification iv. Updating of durability requirements with other Standards v. Updating to reflect new building techniques and materials, in particular engineered timber products. The change of loading standard from NZS 4203 to AS/NZS 1170 meant revisiting all the selection tables in NZS 3604. However the net effect for users is generally quite minor. Two areas where changes were more significant were in earthquake and snow loads. Greater understanding of New Zealand’s seismicity and the effects of soil types resulted in quite major changes to the earthquake loading standard. These were treated in NZS 3604 by picking a default demand level (z = 0.46, and soil class E) and providing a table for other conditions. Changes to the snow loading standard prompted fine tuning of the treatment of snow loads in NZS 3604, and by adjusting the altitude boundaries of the snow zones and basic ground snow loads, much great coverage around New Zealand was possible with negligible penalty in terms of allowable member spans. More detailed information on these changes is described in The Engineering Basis of NZS 3604 [5]. The provisions for durability have changed to better align with other Standards, in particular the corrosion zone classifications used in ISO 9223 [6]. The aggressive nature of timber treatments high in copper towards galvanised fasteners has also been recognised, with greater requirements for stainless steel fixings in exposed situations. Use of engineered wood products (LVL and Glulam) and proprietary roof truss systems has been extended and clarified. Engineered wood products may be used either as direct substitutes for sawn timber, or as an equivalent proprietary system, provided the system has been engineered in accordance with NZBC B1/VM1, and the load reactions do not exceed 16 kN to avoid overloading the rest of the timber framed structure. The procedure for design checking and construction of trussed roofs were developed in conjunction with the Frame and Truss Manufacturers Association [7] and are consistent with their Code of Practice. Girder truss loads and reactions are also limited to 16 kN. 3 THE PLACE OF NZS 3604 IN THE NEW ZEALAND BUILDING INDUSTRY NZS 3604 is a technical document, written and developed by the industry to suit New Zealand’s building environment. It is aimed primarily at “designers” although there is a lot of information relevant to builders, particularly in the area of fixings and connections. With the introduction of the NZ Building Act in 1992 [8] and the NZ Building Code (NZBC) shortly after, NZS 3604 slotted neatly into the building control hierarchy as a series of pre-engineered solutions for the structural safety, durability and weathertightness aspects of timber framed building construction. It thus became referenced under the NZBC as an “Acceptable Solution” for timber framed buildings. This means that it is the preferred, but not the only, way of constructing timber buildings in New Zealand. Buildings, or parts of buildings, which fall outside its scope require specific structural engineering design to verify that they meet the intent of the NZBC. This has become common practise, with the basic building covered by NZS 3604 but an increasing number of specifically designed components such as engineered wood products, bracing panels, timber connectors and so on. NZS 3604 provides a framework for all participants of the building industry, for example the wind zones are used by component manufacturers (such as windows), and have also been translated into the Building Code compliance documents for weathertightness. The Standard is also used as a benchmark for suppliers and manufacturers to design and produce their own proprietary products and systems, which are designed and tailored to fit within its design philosophy. To aid this process, the background, engineering basis, and critical parameters are set out in [5]. Over the 34 years of NZS 3604’s existence, New Zealand building styles have changed dramatically, and this has proved to be NZS 3604’s greatest challenge. The 100 square metre rectangular box has disappeared and today’s houses are larger, have fewer walls, more windows, and are built in more exposed locations, thus placing much greater demands on the structure. There are continual requests from many quarters to include more situations, materials and systems, and as a result the document has increased in size from a modest A5 pamphlet, into a large ring binder with over 400 pages. 4 CANTERBURY EARTHQUAKES 4.1 INTRODUCION The revision was almost complete late in 2010 when the Standard was put its greatest test, the series of earthquakes in Canterbury lasting, with aftershocks, until the end of 2011. The majority of residential buildings in Christchurch are timber framed structures, and up to half of them would have been influenced, either wholly or in part, by NZS 3604. Thus, its provisions and design philosophy got a good road test. 4.2 SEPTEMBER 2010 The majority of building damage after the September 2010 event resulted from liquefaction and lateral spreading of the foundation sub-soils, as shown in Figure 1. This resulted in widespread consequential damage to timber superstructures as shown in the figure. However, buildings experiencing only shaking damage fared well in general, and no collapses of timber framed buildings occurred. Figure 1. House severely distorted by ground movement and lateral spreading. 4.3 FEBRUARY 2011 The February 2011 event produced ground shaking up to 2 times the ULS design levels, and damage to timber framed houses was much more widespread, particularly on the hillside suburbs near the epicentre to the southeast of the city. Houses on concrete floor slabs suffered again from liquefaction and lateral spreading of the foundations, although houses built on suspended timber floors with perimeter concrete foundations generally performed well. Damage that did occur was concentrated on brittle elements such as brick chimneys, veneers and roof tiles, and timber elements such as weatherboards fared particularly well. There was no loss of life in residential buildings, but there were a few building collapses, and the cost of repairs and rebuilding will be a significant drain on New Zealand’s economy for many years to come. 4.4 JUNE 2011 The effects of the major aftershocks in June were similar to those of the February event. In particular, many hillside houses in the Sumner/Redcliffs area were further damaged as this was very close to the epicentre. 4.5 BRANZ DAMAGE SURVEYS BRANZ undertook a comprehensive survey of the performance of residential buildings after both the September and February events. After the September event, the BRANZ surveyors accompanied the EQC Insurance Assessors, in an effort to minimise disturbance to the home owners, many of who were badly traumatised by the event. Unfortunately this gave little control over the properties visited and meant that the sample was badly skewed. This survey was almost completed when the 22 February 2011 earthquake struck. Following the February event, BRANZ undertook a another survey, this time of over 300 houses, randomly selected from within the boundaries of Christchurch city. The process involved randomly selecting a little more than 50 mesh blocks from the Statistics New Zealand database. Within each mesh block, six adjacent houses were selected for surveying at the southeast corner of each mesh block. Each property was visited by a team of two BRANZ representatives with a comprehensive survey form to gather observations about the site and its hazards (eg liquefied, rock-fall susceptible), house age, house style, construction materials and then estimates were made of the extent of damage sustained by the various elements of the structure. Of the 314 fully completed surveys 93 houses were identified as having been built during the period of existence of NZS 3604. The data collection process was designed to fully describe the property and its construction, and to quantify the damage experienced in the two earthquakes. 4.6 SUMMARY OF FINDINGS 4.6.1 Foundations The predominant types of foundations used in Christchurch houses were either a suspended timber floor on concrete piles with a concrete perimeter foundation, or in the more modern houses, a concrete floor slab. The effects of liquefaction and associated lateral spreading on the concrete floor slabs has been well documented. However if there was no ground movement on the property, the slab performed well. Timber suspended floors performed relatively well, even on soils affected by liquefaction. This performance can be attributed to the presence of the concrete perimeter foundation which provided a squat lateral force resisting element. In buildings where ground movement or lateral spreading did occur, at least the piled foundations allowed access beneath for repairs or re-levelling. Where problems did occur was where the foundation wall supporting the veneer and the piled floor foundation were separate or not well connected together. This detail was unable to resist ground movement, as can be seen in Figure 2. function and held the upper part of the house together. This can be seen in the severely deformed house in Figure 4. Valleys were one exception to this, and proved to be a weak point in many houses due to the loss of continuity (see Figure 5). The consequential damage can be seen in Figure 5. Figure 2. Independent wall and floor foundations have separated. 4.6.2 Walls Timber framed walls generally performed well where the site was not affected by liquefaction. Timber claddings such as weatherboards were virtually undamaged and because of their elemental nature didn’t show the effects of minor ground distortion. Sheet cladding such as plywood or fibre-cement were also relatively undamaged. They clearly provided a bracing or stiffening function to the building even if not specifically designated as bracing walls. Plasterboard was the predominant type of wall lining in the houses surveyed, with lath and plaster, and fibrous plaster the next most common systems. 85% of wall linings had some extent of cracking to at least the sheet joints. Diagonal cracking to sheets, usually emanating from internal corners (see Figure 3) was much less common, and complete sheet detachment was rare. Figure 4: The roof framing has held the upper part of the building together. This is a simple detail to rectify and has already received a positive response from practitioners spoken to by the authors. Figure 5 Ceiling damage consequent to failure at the valley Figure 3. Cracking of plasterboard frequently began at internal corners. An observation was made by several owners that their houses now seemed more “flexible” as a result of the earthquakes and the numerous aftershocks. This was evidenced by vibrations and creaking during strong winds. BRANZ is currently investigating the causes of this phenomenon and possible remedial measures. 4.6.3 Roofs Roof structures (either stick framed or nail plate trussed) were rarely damaged, even when the house was severely distorted. The continuity provided by purlins or tile battens and ceiling battens provided a diaphragm 4.6.4 General Taking an overview of the performance of light timber framed buildings in the Canterbury earthquakes, while it is true that they performed well in general, there are some lessons to be learnt from the experience. Structures on hillsides, complex shapes, and horizontal and vertical irregularity will always provide challenges for light timber framing systems. Multiple foundation levels can make dynamic behaviour unpredictable, and take the structure outside the simple dynamic models that both AS/NZS 1170.5 and NZS 3604 are based upon. Additionally, complex plans and elevations often result in connection details that are difficult to achieve in the context of light timber framing. Buildings with large window openings may result in structures with low local stiffness, and there were many examples in the hill suburbs in particular where lack of stiffness resulted in damage to windows and large glazing elements (see Figure 6). allowed surveyors access, at a time of real anguish and upheaval in their own lives. REFERENCES [1] [2] [3] [4] [5] Figure 6. The window was unable to accommodate the in-plane displacement of the timber structure. One result of the Canterbury earthquake series was that the seismic hazard factor, z, for Christchurch has been increased from 0.22 to 0.3 to account for the greater seismicity of the Canterbury region for at least for the next few years. Fortuitously, the seismic zone of NZS 3604 that encompasses Christchurch (zone 2) already included a z factor of 0.3 so no alteration was required. 5 CONCLUSIONS The initial conclusion from observations of the performance of light timber framed buildings in the Canterbury earthquakes was that they performed well if they were within the scope of NZS 3604, and there was no liquefaction at the site. This is in spite of the fact that many experienced levels of ground shaking that were beyond what they would have been designed to resist. However damage was very costly, and thought should be directed towards ways to minimise this for the future. There are already a number of lessons to be learnt from the experience which should improve performance in future events. The results of two surveys of timber framed building performance are being analysed at the moment, and more detailed findings will be available towards the end of the year. ACKNOWLDEGEMENTS The survey was funded by the Building Research Levy. The authors are grateful for assistance in conducting the surveys provided by contractors, and staff and students from Auckland and Canterbury Universities. They would also like to pay a special tribute to the people of Christchurch who willingly opened their homes and [6] [7] [8] Standards New Zealand. 2011. NZS 3604 Timber framed buildings. SNZ, Wellington, New Zealand. Shelton RH. 2011. A wall bracing test and evaluation procedure. BRANZ Technical Paper P21. BRANZ Ltd, Judgeford, New Zealand. Standards Australia/Standards New Zealand. 2002. AS/NZS 1170. Structural design actions, Parts 1 to 5. Standards Australia Sydney, Australia. New Zealand Government. The Building Regulations 1992. First Schedule, The Building Code. NZ Government, Wellington, New Zealand. Shelton RH. 2007. The engineering basis of NZS 3604. BRANZ Study Report 168. BRANZ Ltd, Judgeford, New Zealand. International Standards Organisation, 1992. Corrosion of metals and alloys – Corrosivity of Atmospheres – Classification. ISO 9223. ISO Geneva, Switzerland. Frame and Truss Manufacturers Association of New Zealand (FTMA). 2010, Code of Practice. FTMA, Wellington, New Zealand. New Zealand Government. The Building Act 1992. NZ Government, Wellington, New Zealand.