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a design guide
Rob Grantham and Vahik Enjily
BRE Centre for Timber Technology and ConstructionContributing authors
Martin Milner – Chiltern Clarke Bond
Mostyn Bullock – Chiltern International Fire
Geoff Pitts – The Palmer Partnership
Prices for all available BRE publications can be obtained from: BRE Bookshop 151 Rosebery Avenue London EC1R 4GB Tel: 020 7505 6622 Fax: 020 7505 6606 email: [email protected] BR 454 ISBN 1 86081 605 3 © Copyright BRE 2003 First published 2003 BRE is committed to providing impartial and authoritative information on all aspects of the built environment for clients, designers, contractors, engineers, manufacturers, occupants, etc. We make every effort to ensure the accuracy and quality of information and guidance when it is first published. However, we can take no responsibility for the subsequent use of this information, nor for any errors or omissions it may contain. Published by BRE Bookshop by permission of Building Research Establishment Ltd and TRADA Technology Ltd
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Cover photographs:
front: a seven-storey timber frame building on Brook Street, Nottingham.
Structural frame by Prestoplan Purposebuilt. back: TF2000 building at Cardington.
Preface iv Scope iv Readership iv Acknowledgements iv Foreword vi Chapter 1 Introduction 1
1.1 The feasibility study 1 1.2 The TF2000 project 1 1.3 The TF2000 building 2 1.4 Multi-storey timber frame market 3
Chapter 2 Structural stability and robustness 5
2.1 Introduction 5
2.2 Building layouts and structural stability 6 2.3 Minimum stability requirements: for general strength and stiffness 6 2.4 Minimum stability requirements: for robustness 8 2.5 Building Regulation requirements for accidental damage 9 2.6 Timber frame design against disproportionate collapse 9 2.7 Lessons learnt from TF2000 test building 13
Chapter 3 Fire safety 16
3.1 Introduction 16
3.2 TF2000 compartment fire test 17 3.3 TF2000 stair fire test 17
Chapter 4 Differential movement 19
4.1 Introduction 19
4.2 Regulations, codes and standards 19 4.3 Sources of movement 20 4.4 Research conducted on the TF2000 building 22 4.5 Reduction in movement by design 23 4.6 Provision for movement 24
Chapter 5 Achieving performance 26
5.1 Introduction 26
5.2 Timber frame benefits in multi-storey construction 26 5.3 Benchmarking at TF2000 27 5.4 Construction process lessons from TF2000 29 5.5 Build tolerances 30 5.6 Safe construction procedures 32
Appendix A Regulatory guidance for fire safety 33
Introduction 33
Means of escape 34
Internal fire spread (linings) 36 Internal fire spread (structure) 38
References and further reading 48
Scope
Although this publication will be of general interest to those concerned with the design, construction and performance of timber frame structures the text focuses on aspects specific to multi-storey buildings of platform frame type construction. Design and best practice guidance is provided on:
❐ Structural stability and robustness (disproportionate collapse) ❐ Fire safety
❐ Differential movement ❐ Construction benchmarking
❐ Construction process and building tolerances
These subjects, which are of particular importance for multi-storey timber frame structures, were investigated as part of the TF2000 project, a
collaborative R&D venture lead by BRE and TRADA Technology with joint funding from the Government and industry. Lessons from the TF2000 project are covered in the guidance to provide context to the recommendations and principles of best practice provided. Guidance is also closely linked to codes, standards and building standards for England and Wales, Scotland and Northern Ireland. General principles of timber frame design and construction are not included in this document unless in support of the issues outlined above. Common specifications for the construction and design of timber frame buildings are given in the TRADA publication Timber Frame Construction.
Readership
This book will be of interest to all building professionals responsible for the design and construction of multi-storey timber frame buildings. Building control, local authorities and insurance companies will also benefit from the normative guidance provided for timber frame buildings.
Since this publication documents the results and lessons learnt from research conducted on the world’s first six-storey timber building using the platform frame technique of construction, the information contained within will also have global appeal for regulators and code writers.
Acknowledgements
The TF2000 partnership is unique; international competitors throughout the supply chain have united to create a peerless example of the 90’s approach to innovation, pooling state-of-the-art knowledge on technology, product and process development. This project would not have proceeded without the support of the following organisations acknowledged as Partners and Associates to the TF2000 project. This recognises the greater contributions received from TF2000 Partners. The Building Research Establishment Ltd (BRE) and TRADA Technology Limited (TTL) would like to thank them for their financial support and technical co-operation without which this design guidance could not have been provided.
Two major contributors have been jointly in charge of the project from the onset: Vahik Enjily of BRE Centre for Timber Technology and Construction (CTTC), and Simon Palmer of Palmer Partnership (previously TRADA Technology Ltd).
TF2000 Partners
❐ Office of the Deputy Prime Minister, previously ❐ Century Homes
Department of Transport and Local Regions (DTLR) ❐ Prestoplan Purpose Built ❐ P J Steer Consulting Structural Engineer ❐ Walker Timber
❐ Stewart Milne Timber Systems
TF2000 Associates
❐ Andy Collett Associates ❐ Brick Development Association ❐ British Gypsum ❐ British Gypsum Isover
❐ Chiltern Clarke Bond ❐ Chiltern International Fire ❐ Crown Timber ❐ Cullen BP
❐ David Carr Consulting Engineer ❐ Department of the Environment for Northern Ireland ❐ Devon Fire & Rescue Service ❐ Forestry Commission
❐ F R Shadbolt & Sons Ltd ❐ Hanson Brick
❐ Hazlin Doors Ltd ❐ HM Fire Service Inspectorate
❐ Highfield Consultancy ❐ Luton & Bedfordshire Fire & Rescue Service ❐ ITW Paslode ❐ Marley Building Materials
❐ Nexfor Ltd ❐ NHBC
❐ PACE ❐ Palmer Partnership ❐ Panel Agency ❐ Pinewood Structures ❐ Rugby Joinery UK ❐ Scottish Executive ❐ Simpson Strong-tie ❐ SODRA Timber ❐ The A Proctor Group ❐ TRADA ❐ Trus Joist MacMillan ❐ TTF (UKTGC)
❐ UKFPA ❐ UKTFA
❐ Woodbridge Timber
Design Guide steering group
Production of this design guidance has been scrutinised by a steering group of leading industry experts on timber frame construction; many of them have been involved actively in the TF2000 project. The contribution of the following group members is gratefully acknowledged:
❐ Steve Ashton. Prestoplan Purpose Built ❐ Barbara Bedding. TRADA Technology
❐ Mostyn Bullock. Chiltern International Fire ❐ Antony Burd. Fire Building Regulations, ODPM ❐ Andy Collett. Andy Collett Associates ❐ Charles Grant. Walker Timber, UKTFA
❐ Paul Graver. BRE Bookshop ❐ Peter Grimsdale. Peter Grimsdale Associates ❐ Geoff Harding. ODPM ❐ John Haynes. NHBC
❐ Phillip Key. PACE ❐ James Lavender. Chiltern International Fire ❐ Steve Limb. British Gypsum ❐ Ian Loughnane. Prestoplan Purpose Built ❐ Hugh Mackay. Stewart Milne Timber Systems ❐ Jonathan MacMullen. Greenframe ❐ Paul Marsh. UKTGC ❐ Jim McBride. Century Homes ❐ Seamus McCrystal. Building regs NI ❐ Martin Milner. Chiltern Clarkebond ❐ Simon Palmer. The Palmer Partnership ❐ Geoff Pitts. The Palmer Partnership ❐ Bob Selmes. Forestry Commission ❐ Peter Steer. P J Steer Consulting Engineer ❐ Paul Stollard. Building Standards, Scotland ❐ David Sulman. UKFPA
❐ Rab Taylor. Wren & Bell ❐ Mark Wilson. Panel Agency ❐ Bryan Woodley. UKTFA
The construction industry has undergone a major step change in its bid to improve the quality of buildings and reduce their environmental impact.
One of the leading protagonists in this area has been the timber construction industry. The Timber Frame 2000 (TF2000) project is a shining example of the
commitment by industry and Government to the technological progress of timber frame buildings. Situated in the Building Research Establishment’s (BRE) large building test facility at Cardington, the TF2000 six-storey experimental building has been subjected to a rigorous programme to test the performance of both UK and overseas multi-storey timber frame constructions. Issues such as construction process benchmarking, stability, differential movements, disproportionate collapse, compartmental fire and timber stair performance have all been assessed. Test results have proven that timber frames are well suited to multi-storey construction.
The TF2000 building was a world-first and is already having a huge impact on the advancement of multi-storey timber frame construction. The Government’s commitment to sustainable development places great importance on the use of construction materials with low environmental impact, such as timber, and further encourages the use of sustainable resources of timber.
This guidance document gives the industry the necessary tools for designing and constructing timber frame buildings in a safe and economic manner and also provides industry with a new and lucrative market.
As sustainability moves from being a ‘buzz-word’ to reality, I hope that timber frame buildings will play an increasing role in buildings of the future.
Brian Wilson MP
Minister of State for Energy and Construction
Modern timber buildings constructed using the platform frame technique were first introduced to the UK from Sweden in the 1920s. Since then, their popularity has grown and so has our knowledge and experience of their performance in service. It was the government’s confidence in their performance that lead to a change in the fire safety regulations in 1991, allowing for the first time the number of storeys to potentially reach eight (in England and Wales) without any additional fire resistance requirements other than those existing for many four-storey buildings.
This created new opportunities for the timber frame industry to expand its market in private and social housing, compartmented flats and commercial projects for hotels, nursing homes, student accommodation and other commercial buildings. Four-storey timber frame buildings have been constructed extensively since the mid-eighties for various occupancy classes. British Standards, meanwhile, have been or are being updated or revised, where appropriate, to cover such practices. However, in certain respects current code recommendations are extrapolated. For this reason, the need of guidance for buildings with five or more storeys has recently been recognised as vital. Consequently, a co-operative project was set up in October 1995 by the Office of the Deputy Prime Minister (ODPM) then Department of the Environment, Transport and the Regions (DETR), the UK timber frame industry, BRE and TTL, entitled Timber Frame 2000 (TF2000). It followed a feasibility study on such buildings, involving close consultation with the construction industry, ODPM and building professions [1].
1.1 The feasibility study
The aim of the feasibility study was to review the design issues relating to safety, and to examine the construction requirements for multi-storey timber frame buildings. Case studies involving seven real projects were investigated during 1994 and 1995. The feasibility report [1]reviewed and summarised
design and construction requirements. Much emphasis was placed upon the economical and technical issues and potential of taller timber frame
buildings (four to eight storeys) as a desirable method of construction. The research requirements and design options for full-scale tests on at least a five-storey test building were identified.
1.2 The TF2000 project
Consequent to the feasibility study's findings and conclusions, the TF2000 project started in October 1995, preparing the way for proposed tests on the first six-storey timber frame building of its kind in the world. Two main committees were set up to deal with managerial and technical issues of the project [2]. The UK timber frame industry and ODPM, together with
representatives from TTL and BRE, constituted the Management and
Chapter 1
Introduction
Technical Committees of TF2000. Operational Task Groups were set up to deal with specific areas of investigation. The test building (see left) was a six-storey platform frame type of residential construction, with four flats per floor around a service core built in BRE’s Cardington hangar. Brick cladding was selected and C16 timber was used. To assess productivity targets,
prefabrication and in-situ construction methods were used for comparison.
The programme of research work is defined in the table below. The main responsibility of the Management and Technical committees were:
❐Schematic architectural and engineering design of full-scale test-building.
❐Definition of the test-building to enable costing and tender action.
❐Definition of test programmes
❐The design and construction of the six-storey timber frame test-building [3].
❐Conduct identified test programmes [4][5].
❐To bring all aspects of construction together, from Regulations to
Research to Design to Construction and to include whole Building Evaluation.
❐Regulatory harmonisation between English/Welsh, Scottish and Northern Ireland Building Regulations.
❐Produce authoritative guidance documents.
1.3 The TF2000 building
The TF2000 project team concluded the following, in relation to the broad principles of the test-building:
❐Concentration upon multi-occupancy residential construction.
❐The test building should simulate typical compartmental flats.
❐Platform-frame construction and cellular layout, along with the use of communal lifts and stairways should be used.
❐Building height considerations and Fire Regulations in England, Wales, Scotland and Northern Ireland should be harmonised.
❐The proposed test building should be six storeys and should be clad with clay brick.
❐The test-building should be a real-life building incorporating all the current design and construction practices.
❐Fire performance of communal stairways and lift shafts made of timber should constitute one of the major assessments in the test building. Three alternative architectural schemes were considered from which a ‘commercial’ brief was derived [8]. The scheme was then reviewed,
storey-by-storey, to remove elements not required to meet the needs of a prototype building and the core research programme. The building comprised:
Construction of the TF2000 building at BRE Cardington
● Value engineering and process benchmarking ● Differential movement.
● Structural performance: whole building stability (racking stiffness). ● Fire: stairs and compartmentation performance.
● Acoustics: walls and floors [6][7]. ● Disproportionate collapse. ● Guidance documents.
❐ six storeys;
❐ four flats per storey, each with two bedrooms, kitchen, bathroom, living room and hall;
❐ a plan-aspect ratio of approximately 2:1;
❐ platform-type timber frame;
❐ timber stair and lift shaft;
❐ single timber stair;
❐ brick cladding.
The following general specifications for key elements were agreed:
❐ The ground floor is notionally a concrete slab-on-ground.
❐ External walls consist of two layers of 12.5 mm standard (Type I) plasterboard with a vapour control layer and 89 x 38 mm C16 homegrown timber studs with glass wool insulation in between. The sheathing is 9 mm Oriented Strand Board (OSB/3). The cavity is 60 mm with single-leaf
1/
2-brick cladding connected with stainless steel ties to the timber frame.
❐ Internal loadbearing walls consist of C16 homegrown timber stud groups at 600 mm centres with two layers of 12.5 mm plasterboard and 9 mm OSB/3 sheathing to one side, where needed for wind bracing. The internal non-loadbearing walls consist of C16 homegrown timber studs with one layer of 12.5 mm plasterboard to each side. The compartment walls are twin-leaf using C16 timber studs with glass wool insulation in between and OSB Type 3 sheathing is used where wind resistance is required.
❐ Compartment floors consist of two layers of plasterboard ceiling (19 mm Plank 12.5 mm plasterboard) on joists with mineral wool in between. OSB Type 3 is used as a floor deck. Floating floors contain proprietary resilient battens with plasterboard and Type P5 chipboard. Timber joists with low moisture content of 12% were specified for floors 1 to 4; the fifth floor comprises timber I-beams and metal webbed beams.
❐ The roof comprises trussed rafters with hipped ends supporting concrete interlocking tiles on felt and battens.
1.4 Multi-storey timber frame market
The UK construction market, in common with most construction markets world-wide, has come under increasing pressure to reduce costs and enhance client value, at the same time maintaining or improving quality and performance standards. Maximising land usage, particularly on brownfield
(inner city) sites, is also dictating increased building heights for residential buildings, from two to three storeys to four to eight storeys. In the five to eight-storey markets, research showed that a significant market existed for five and six storeys but there was a much lower demand for seven and eight storeys at the time (see left).
The construction processes involved in adopting steel and concrete framed buildings are well known to the
construction supply chain. The drive for market demands of improved efficiency, better quality and performance, faster construction and better-cost control has
7 or 8 6 5 4 3 Number of storeys 5% 15% 33% 45% 2%
Market potential for timber frame buildings in 1996
led to the questioning of how we build. Other pressures to change the way we build have come from increased concerns about the impact of the construction process on the environment and local communities. This has forced responsible and leading clients to take stock of the sustainability impacts of how we build, as well as the operation and maintenance of buildings. The market response has been to investigate the use of timber frame as the solution to these demands.
The considerable savings that can be gained from off or on-site
prefabrication and the use of efficient erection techniques make multi-storey timber frame very attractive. The professional designers and surveyors, and large sections of the contracting and building industry, still have to be made aware of timber frame's potential and be convinced of its advantages. This has and continues to be achieved through Timber Frame 2000, which has brought enormous publicity for timber frame buildings.
The UK construction demands for residential accommodation, from housing, student accommodation and hotels is significant. For housing alone the Government predictions suggest that more than 3.5 million new dwellings will be required from 2002 to 2016. Currently, the market for buildings of four storeys and above is estimated to be 25% and it is predicted that the market will increase to as high as 45%. For the student accommodation and hotel markets, the trends are already evident for five-storey inner city high-density units.
This presents a significant challenge to the UK construction supply chain with its diminishing labour force and increased business performance demands. Furthermore, client requirements for higher building standards and the increasing demand from regulatory improvements, particularly in thermal insulation, acoustic performance and health and safety issues are pushing the industry to reconsider on-site methods of construction and to investigate other ways of building homes.
2.1 Introduction
This chapter provides guidance for timber frame engineers on the stability design issues that are not covered in current British Standards or other guidance documents. Structural stability guidance is supported by tests conducted on the TF2000 building [9]and is limited in scope to currently
publicised timber platform frame construction up to seven storeys in height. Advice is provided for three different stability design conditions that the timber frame structural engineer considers during the engineering of a multi-storey building:
❐ Strength and stiffness design
❐ Robustness design based on good practice but not case specific calculations
❐ Robustness design based on case-specific calculations
The approach to the first stability check is covered in BS 5268 which, with the exception of timber frame racking resistance, is not restricted to the storey height of a building. The second and third stability design conditions can be dependent on the storey heights.
The second stability condition is achieved through good practice and is applicable to all building types. For most timber platform frame layouts, the robust detailing is inherent in the method of construction and the engineer’s duty is to ensure that the layouts are not vulnerable from poor construction or minor accidents that would cause failure disproportionate to the cause. For multi-storey frames, it may be necessary to check assumptions of diaphragm action and general tying forces to ensure that the building design is appropriate for anticipated load and construction conditions. General principles of timber frame design, construction and detailing are given in Timber Frame Construction [10]although some caution is required as the
details presented in that document relate to buildings up to four storeys. Specific design at each level is required to transfer forces.
The last robust design condition is typically called ‘disproportionate collapse design’. The Building Regulations cover the type of buildings that are to be checked to ensure that disproportionate collapse does not occur when subjected to a defined set of load and support conditions.
Guidance given in this chapter also refers to the responsibility of the timber frame engineer for informing the building design team of the implications of frame stability as it can influence the design of claddings, services and supporting structures.
Chapter 2
Structural stability and robustness
Ion building courtesy Chiltern Clarke Bond
2.2 Building layouts and structural stability
Structural layout and form of a multi-storey building will influence the stability of framing elements under normal vertical and horizontal loading. The layout will also influence the overall building robustness. Based on guidance presented in the BS 5268 and Eurocodes, the following section presents general principles on how timber frame can provide robust design solutions.
2.3 Minimum stability requirements
For general strength and stiffness
The timber codes, BS 5268 and Eurocode 5, provide appropriate guidance for strength and stiffness of the components that make up the multi-storey timber frame. However, clarification is needed on how horizontal resistance, referred to as racking resistance in timber frame, can be calculated using the current codes BS5268: Parts 6.1 and 6.2 as they currently state that they are limited to four storeys only.
This limit has not been given for strength reasons (the wind forces can be increased through location as much as height) but related to the lack of information on robustness and experience of the use of platform timber frame above four storeys.
There have been concerns that the deflection of timber frame under horizontal forces requires additional limits for buildings above four storeys. The current limits suggested by BS 5268: Parts 6.1 and 6.2 take a stiffness limit of the panel height divided by 300 as being acceptable. This value is clearly theoretical as such movements are likely to lead to cracking of finishes that have not been experienced in timber frame. The TF2000 tests demonstrated that the actual stiffness of a timber frame building is
significantly higher than the code limitations. The TF2000 findings were:
Eurocode general principles on building robustness
Select a structural form which has low sensitivity to the hazards considered. Select a structural form and design that can survive adequately the accidental removal of an individual element or a limited part of a structure, or reasonable localised damage.
Avoid as far as possible structural systems which may collapse without warning. Provide structural forms that can be tied together.
Ensure that layouts and plan arrangements provide returns and intersecting walls and floors.
Adopt compatible materials used in the structure and ensure adequate interaction.
Timber frame response
Timber platform frame techniques are inherently robust through the interconnectivity of walls and floor panels. Backed by over 30 years of experience and further tested and demonstrated on the TF2000 building through full-scale tests [11][12].
Trussed rafter roofs have proven robustness against damage [13].
TF2000 demonstrated the integrity of platform frame construction and the natural robustness of the structural form given by the conventional
connectivity of walls and floors.
Timber frame buildings typically use compatible materials materials that are easily fastened together. Care is needed to understand the interaction with other parts of the building such as cladding.
❐ The addition of plasterboard lining to a sheathed timber frame building increased the lateral stiffness by a factor of 3.3.
❐ After applying masonry cladding, the building stiffness was recorded as 17.7 times that of the bare timber frame with sheathing.
While these test results are specific to TF2000 and must not be used in any other layout, the TF2000 building has provided evidence that BS 5268 racking design principles are adequate for strength and stiffness for buildings higher than four storeys and that there is no reason for normal cellular platform frame to have additional deflection limits imposed. It is considered, therefore, that the use of BS 5268: Parts 6.1 and 6.2 can be extended to the design of platform frame timber buildings up to eight storeys.
Cellular layouts
Examples of building layouts and the influence on a building stability
The open plan nature with no transverse structure is not appropriate for platform timber frame and additional structure is required for stability design.
The introduction of portal frame elements can provide solutions to open plan layouts but attention is needed to stiffness limits and connectivity of the framing types as well as differential movement of different materials.
Cellular layouts are best suited to multi-storey platform timber frame. Internal load bearing walls may need to be strengthened to carry horizontal forces.
Open-plan layouts
Where party walls separate the structure into separate units, the engineer should ensure that the horizontal forces can be taken by each unit or transferred across the
party walls. For reasons of acoustic performance, there is a limit to the quantity of structural ties permitted across a party wall as shown in Timber Frame Construction[10].
For structures or layouts that are unable to provide sufficient racking resistance, the use of structural engineering calculations and bespoke designs solutions, such as portal frames, can be considered. The deflection limits should be appropriate for the structure and finishes and justified on a case-by-case basis. In these situations, the deflection limit may be at least height / 500 for the framing to keep to the same stiffness as the timber frame elements. Consideration of differential movement is also required – see Chapter 4.
Multi-storey buildings are more likely to include tall panels for ground floors, such as restaurants and reception areas at hotels, or top floor rooms, such as penthouse apartments or plant rooms. Since BS 5268: Part 6.1 is limited to 2.7 m height panels, BS5268: Part 6.2 guidance shall be used for panels above 2.7 m (but limited to 4.8 m) on all forms of building.
In any horizontal load condition, the wind is likely to be the dominant load. However, any building should have sufficient horizontal strength and stiffness for resisting a minimum horizontal long-term force equivalent to 2.5% of the vertical dead + live load.
Transfer structures
It is common to have transfer structures below multi-storey timber frame construction, such as basement car parks or open plan reception areas. The substructures, sometimes made from different materials, such as concrete and steel, are to be designed for adequate support to the timber frame. Consultation with the timber frame engineer and substructure designer is needed to ensure appropriate support conditions.
Design for the construction period
Caution is required in timber frame multi-storey construction where the racking resistance requires plasterboard or vertical load from the roof to contribute to the stiffness and strength. During construction the building can be exposed to wind loads before vertical loads are applied or any
plasterboard is fixed. It is acceptable to reduce the wind load in
accordance with BS 6399 for the construction period. The stability of the building during construction shall be considered as a design requirement. It is good practice to ensure that at least the lower half of the framing has adequate racking resistance without contributions of plasterboard, as reliance on temporary bracing in large multi-storey construction has proven to be more complex than low-rise projects.
A design check may be required on the vertical elements, such as studs in party walls where the design of the stud assumes lateral stability is achieved through the fixity of the plasterboard. On multi-storey, the construction process period can be increased and construction loading should be considered prior to the plasterboard being fixed. It is also possible that significant vertical loads can occur during construction through storage of construction materials, such as plasterboard packs, on the building. This aspect shall be considered in the design process with temporary restraint or additional members added as appropriate.
2.4 Minimum stability requirements
For robustness
For general robustness of timber frame it is accepted through experience and case histories that timber platform frame robustness is achieved through the application of standard detailing that has become established
Broadway Plaza courtesy
over the last 30 years and more. Timber Frame Construction [10] provides
typical detailing and material sizes to achieve proven robust solutions. For multi-storey timber frame, checks on the nailing schedules are required to ensure that appropriate transfer of the horizontal and shear design loads can be achieved at each platform level.
Specific design checks for accidental damage require the engineer to determine whether the building form is not unduly sensitive to damage, caused accidentally or otherwise, such that collapse or partial collapse is not disproportionate to the original cause. The principle for design against accidental damage is primarily based on ultimate strength and not
serviceability. In addition, as with all construction materials, there is no defined period for the stability of the building subject to accidental damage. Increased design factors for accidental damage conditions are provided in BS 5268: Part 2.
The extent of the damage to be considered is not material dependent but one of agreed philosophy. Specific design checks may be required under the Building Regulations, commonly referred to as design against disproportionate collapse.
2.5 Building Regulation requirements for
accidental damage
Background and compliance
Regulations exist for accidental damage design limitation based on the principle that in the event of damage occurring to a building partial collapse is acceptable providing it is proportionate to the cause. The UK Building Regulations, (Part A for England and Wales, Part C for Scotland and Part D for Northern Ireland), cover this aspect under the heading of
Disproportionate Collapse.
At the time of the TF2000 project, and writing of this publication, the UK Building Regulations require that only buildings with a height above four storeys are to include a check in the design process to ensure that any collapse is proportionate to the cause. Revised Building Regulations may alter this approach but the principles presented here will cover all building heights if applicable.
While timber platform frame construction has a history of excellent performance against instability and there are no known examples of accidental damage disproportionate to the cause, it is the duty of the engineer to ensure that the structure being designed is not unduly sensitive to minor accidental damage or misuse.
2.6 Timber frame design against disproportionate collapse
In common with guidance documents for steel and concrete it is neither practicable nor necessary to provide a definition of the cause of the event that could lead to disproportionate collapse. As stipulated by Approved Document A, the structural engineer’s approach to achieve robustness design for timber frame buildings compliant with Building Regulations is to check the structure against either the notional removal of a defined length or specific member of the structure or to design key elements to resist a force of 34 kN/m2over that element.
Option and compliance
1 Provide vertical and horizontal ties to
resist defined forces
2 Consider notional removal of a support
member, one at a time in each storey in turn, to check that upon its removal the rest of the structure would bridge over the resulting lack of support, albeit in a substantially deformed condition, or that the risk of collapse of the remaining structure due to the removal of the member is limited to 15% of the area of the storey or 70m2within the storey or
immediately adjacent storeys, whichever is less.
3 If it is not possible to bridge over a
missing member or to limit the area at risk, the member should be designed as a protected element. The protected members (sometimes referred to as ‘key elements’) should be designed for a load of 34 kN/m2applied in any direction.
Timber frame option
Timber platform frame can be considered as a structural form of close centred posts and beams with nominal tie resistance provided at each joist to wall stud junction. The regulations suggest high tie forces to take account of the typical wide spacing of post and beam structures using steel or concrete materials. While these tie forces could be designed to spread through the timber frame elements it is not considered a practical option, although not impossible.
This option provides the appropriate route for platform timber frame structures. Guidance on the length of notional member to be removed is provided in the following sections.
Can be adopted for timber frame structures when required.
Options for disproportionate collapse design
Loadbearing element type Beam Column External wall Internal wall Floor/roof Definition of element
Primary structural support member acting alone Primary structural support member acting alone
All loadbearing walls that form the perimeter and external face of the building but not party walls
All loadbearing walls within the building including party walls Structural beam and decking: joists, rafters etc.
Extent of structure
Clear span between supports
Clear height between lateral restraints Length between intersecting walls (party walls, return walls, internal room dividers), or between key element columns. Minimum length of wall to be considered 2.4 m. There is no maximum length of wall.
Length limited to between intersecting walls or key element columns or 2.25H, where H is the clear height between lateral supports (eg floor-to-floor).
2.4 m width of the floor/roof for the clear span between designed supports.
Compliance Notional removal or protected element Notional removal or protected element Notional removal Notional removal Notional removal
Definitions of the extent of structure being considered under the robustness design
Intersecting walls
Typical intersecting wall framing detail
Support walls
Removing floors and roof panels
Key elements: load paths
Trussed rafter roof
Minimum length of intersecting wall to be 1200 mm in total (framed openings can be permitted but are in addition to the 1200 mm length). All intersecting walls may be either racking walls or non-loadbearing walls.
Substantial non-loadbearing partitions can be used where the resultant load paths can be proven and where the walls are not to be removed.
Additional consideration should be given to check that if a roof or floor panel is removed, wall panels adjacent to the removed elements are stable.
The resultant horizontal force from a key element is to be designed and detailed to be carried by the remaining structure.
Trussed rafter roofs have proven robustness against damage [13]. Clarification of structural elements used for robustness compliance
External wall Intersecting wall
Compatibility of other materials used in the structure
The UK Building Regulations requirements for robustness refer to the whole building and not just the structural frame or loadbearing elements.
Therefore, the check for robustness shall include the effect on elements supported or restrained by the timber framing.
In considering cladding, the area of cladding to be checked for
disproportionate collapse should be treated the same as the timber frame. This applies to brickwork as demonstrated by the TF2000 test programme. Special consideration is required for cladding, particularly the effect of window openings and movement joints that form natural breaks in brickwork.
Other issues:
Where the lower storeys are constructed from other structural materials (concrete, masonry or steel frame), the effect of loss of support from the underlying structure on the timber frame shall also be considered. Co-ordination of the removal or deflection of support structure must be allowed for in the timber frame design check. Where two engineers are responsible for different sections of the building, such as concrete frame engineer for the ground floor and timber frame engineer for the upper floors, co-ordination is needed to ensure that each party is aware of the impact of their robustness stability design on each other’s structure.
Stability robustness designs should consider the following structural loads applied simultaneously:
❐ Full dead load (including any finishes or fixed M&E plant).
❐ 1/3 imposed floor load without floor reductions due to number of storeys.
❐ 1/3 imposed roof load.
❐ 1/3 wind: typically not needed in a timber frame check due to method of resisting wind forces through the racking resistance of walls. The removal of members each in turn is unlikely to result in the loss of more than 1/3rd of the racking resistance being removed.
Element Floor/wall
Protected element
Support member within a wall,
eg studs
Failure limit
Deflections > L/30 or when deflection of the floor under consideration prevents safe egress from the building, whichever is smaller.
Member over-stressed under accidental load conditions from applied load of 34 kN/m2in any direction.
Member over-stressed owing to additional load following the notional removal of one adjacent member.
Comments
For timber members the element will be overstressed before deflection limits are reached.
The common approach is to use engineered wood products such as LVL or Glulam.
Additional studs may be required for short-term strength purposes.
Definition of failure limits for the timber frame elements
2.7 Lessons learnt from the TF2000 test building
Testing for robustness
The TF2000 test building provided proof of the inherent robustness and availability of secondary load paths in platform timber frame. Two
loadbearing walls were removed, one internal and one external, to check the structural integrity of the building when required to span over a missing supporting member. The results [11]demonstrated the potential of standard
platform frame timber buildings to span with only small movement over gaps formed by the removal of loadbearing supports.
The following conclusions have been drawn from the TF2000 tests.
Wall panels: standard platform frame 4.2 m-long wall panels as tested on
the TF2000 building were found to have sufficient strength created by the plasterboard/timber board sheathing to span unsupported.
Conclusions drawn from this are that sheathed walls with no openings designed to BS 5268: Part 6.1 can be regarded as deep beams with vertical shear taken in the panel-to-panel connections and tension taken out through the sheathing material in continuation with any timber framework across panel junctions.
Sheathed walls 2.4 m high and 4.2 m long can be assumed satisfactory without calculation.
Timber design check Member strength
Serviceability checks Connections
Strength factor
Duration of load factor from BS 5268 can be taken as K3= 2.00
For all members mean values
Duration of load factor on basic loads for nailed joints can be taken as
K3= 2.00, ie all long-term design values can be multiplied by 2.
Comments
Applicable to bending, shear and bearing.
Applicable to nails, screws and bolts for timber element to timber element. Not to be used on the racking resistance calculations. If BS 5268: Parts 6.1 and 6.2 are to be used under accidental load checks, an additional factor of 1.2 can be used for strength design checks.
Timber design modification factors for accidental damage checks
4 3 2 1 Panels removed
Panel above the gap redistributes forces as a cantilever Outline of external
wall panels Panel above the gap
redistributes forces as a 'deep beam'
The following diagrams summarise other observations and conclusions drawn from the tests.
Diagrammatic elevations on panels spanning over a gap for a typical five-storey timber frame
Floor panels: the standard platform frame detail is to build the floor such
that the floor is supported on all sides, even though it is designed to span from one wall to another. The design of floors ensures that there is sufficient strength and stiffness in the direction of the span. TF2000 tests
demonstrated that the floor has additional strength through the transverse capacity of the floor that is supported on the walls parallel to the span. The TF2000 test was to remove the designed loadbearing support walls. For the removal of an internal loadbearing wall, the floors spanning 3.6 m and 4.2 m wide deflected at the unsupported edge by up to 24 mm. For the removal of an external wall, the unsupported floor deflected by 4 mm. Both test results demonstrated the inherent robustness of platform timber frame.
External wall panel Removal of support wall Support wall Section A-A
Floor decking built into external wall panel
A A Floor span Support wall Support wall Floor built into wall Floor decking acts as a plate element Nailing transfers forces Plan on floor As initially designed Plan on floor
Loadbearing wall removed
Diagrammatic plan of the TF2000 test floor behaviour
Using rim beams to increase resistance
On the TF2000 building, external wall panels had an additional support system of rim beams that were continuous around the building perimeter at each floor level. These can be designed to support the floor and wall above as part of the robustness design. Support conditions were detailed such that removal of the panel below does not remove the beam’s support.
For most buildings, a rim beam will be required to provide support of the structure when notional loadbearing panels are removed. Alternatives are to have floor systems such as on the TF2000 where encastré support was demonstrated, or to use continuous or cantilever floor frame systems over loadbearing walls.
Intersecting wall Intersecting wall
Wall to be removed
Plan on wall panels
Plan on rim beam Load from floor and panel
Header joist/rim beam spans across gap supported by adjacent/transverse panels or designed key elements support columns
Elevation on rim beam with the notional removal of wall below
Plan on rim beam and supporting structure below
Exploded view – support from the notional removal of walls provided by intersecting walls
3.1 Introduction
To provide evidence to support multi-storey timber frame dwellings, a series of full-scale fire tests was conducted on the TF2000 building [14][15]:
❐to demonstrate compliance with the functional fire safety requirements as defined by the relevant regulations and technical standards;
❐to provide evidence in order to support the harmonisation of the different regulations and technical standards.
Appendix A gives a comprehensive guide to the appropriate sections of the relevant statutory guidance. Evidence in support of this harmonisation is provided within the Appendix by the cross comparison of the different regulations and technical standards. Two critical legislation harmonisations that resulted from the TF2000 compartment fire test were as follows:
❐The use of combustible materials in separating walls now permitted up to 18 m rather that just 11 m, based on the Timber Frame 2000 tests.*
❐The use of combustible materials in external walls within 1 m of the boundary now permitted up to 18 m rather than just 11 m, based on the Timber Frame 2000 tests.*
As an outcome of the testing programme, several essential generic building practices were identified as requiring attention, a summary of these
practices is highlighted below:
❐As with any construction project, care must be taken to ensure that the plasterboard is attached using all the required fixings. Especially when more than one layer of board is being utilised, each layer must be independently fixed. Failure to ensure the correct fixing of plasterboard would result in reduced performance in a real fire scenario.
❐Building Regulations Approved Document B states: ‘The external envelope of a building should not provide a medium for fire spread if this is likely to be a risk to health and safety.’ Correct location of cavity barriers and fire stopping is, therefore, important to maintain the integrity of the structure wherever the cavity of a building provides a medium for fire spread.
❐With the increasing number and variety of different construction methods, not only specifically related to timber frame but to all construction
technologies, the training of fire brigades to instil an understanding of construction methods, fire performance and risk would reduce the risk of service personnel being injured.
❐As with all types of construction, the current common design practices for buildings do not address the issue of vertical flame spread from floor to floor via the windows and/or other external openings.
❐All construction methods are sensitive to the quality of workmanship. Workmanship in relation to the success of fire safety provisions in any building is of vital importance with all efforts being made to ensure and maintain these provisions.
Chapter 3
Fire safety
* Extracts taken from the Consultation
Document on the Sixth Amendment to the Building Standards (Scotland) Regulations 1990, proposals to Amend Parts D & E of the Technical Standards, July 2000, Scottish Executive Development Department. Both pieces of legislation are now in force.
3.2 TF2000 compartment fire test
A detailed summary of the test is contained in a BRE report [16].
The primary objective of the TF2000 compartment fire test was to evaluate the fire performance of a medium-rise six-storey timber frame building subject to a severe natural fire exposure. Specific fire resistance issues of structural integrity and compartmentation were assessed during the test. Fire loads in the compartment, comprising dried timber battens, provided a
realistic assessment of fire spread during the test. This enabled a quantitative appraisal of the true performance of this form of construction tested to the Fire Resistance test methods. The compartment fire test demonstrated that this construction can meet the functional requirements of the Building Regulations for England and Wales and the Building Standards for Scotland and Building Regulations for Northern Ireland for such buildings. The compartment fire test met the stated objectives of the programme. The following conclusions were drawn from an analysis of the data and from observations during and after the test.
❐ Derived values of time equivalence demonstrated that the performance of a complete timber frame building subject to a real fire is at least equivalent to that obtained from standard fire tests on individual elements.
❐ Results indicated that fire conditions in the living room of the flat represented an exposure approximately 10% more severe than a standard 60-minute fire resistance test.
❐ The test demonstrated that timber frame construction can meet the functional requirements of the Building Regulations for England and Wales, the Building Standards for Scotland and Building Regulations for Northern Ireland in terms of limiting internal fire spread and maintaining structural integrity.
3.3 TF2000 stair fire test
A BRE report [17]gives a summary report for this fire test series.
The TF2000 building was fitted with a single stair of timber construction that was located in a stair shaft, the walls of which were of timber frame construction.
The intention was that the results of the stair tests would provide data that would assist regulators in the United Kingdom to consider changes leading to a possible harmonisation of the technical guidance in support of their Building Regulations. It was necessary to define at an early stage the fire performance objectives for a stair in such a residential building during a fire situation. In meeting the fire safety requirements of the Building Regulations, the fundamental consideration for the stair is as follows:
The stair has to remain usable for firefighting after initial evacuation of occupants immediately at risk and for subsequent evacuation by the other occupants of the flats who are initially advised to remain in their dwellings.
TF2000 Building
Post flashover fire in the living room of the fire flat
In the event of a fire in the TF2000 building, the stair will be used by the fire brigade, upon attendance at the scene, to gain rapid access to the building to remove any people who are immediately at risk from the fire. Upon completion of this duty, the stair will provide the access for the fire brigade to fight the fire from inside the building. Once the fire has been brought under control or extinguished the stair would then be used to complete the safe evacuation of other occupants.
The stair must remain usable during all these operational phases. It must continue to support its design load for the duration of the incident and must not itself contribute significantly to a state of fire development that would render the stairwell space inaccessible to firefighters.
In terms of fire, the most onerous situation was regarded as one where the fire is actually in the stair itself. A fire that starts and grows in the stair may arise due to materials being left or stored in the stairwell that are then either accidentally or purposefully ignited.
It was recognised that a large fire load in the stairwell could result in the development of untenable conditions for means of escape from heat, smoke and toxic fumes. This could happen at an early stage independent of the inherent fire load of the actual stair enclosure and stairs.
With these points in mind, it was proposed that the stair in the TF2000 building should demonstrate a significant resistance to becoming involved in a fire when subjected to an appropriately severe fire source that is in intimate proximity to exposed timber components of the stair. The resulting fire (including any contribution from the stair construction) should not cause the loadbearing capacity of the stair to be reduced below a serviceable level and should not cause a breach of the compartmenting elements of structure enclosing the stair.
The test demonstrated that the specific timber type and treatment used for the experiment together provided an appropriate level of fire performance to satisfy functional fire safety objectives for a stair in the residential building. More detailed information is given in reference [17].
As a consequence of this research project, regulatory authorities may wish to consider the use of an appropriately treated timber stair as adequate in terms of meeting the functional requirements of the UK Building Regulations, on a case-by-case basis.
4.1 Introduction
With all structural and non-structural components of a building, detailing should allow for any potential movement across connections. This movement between two connected parts of a building is known as differential movement. Allowance for
differential movement is of particular importance in a timber frame building where dry internal environments cause the timber to shrink across the grain and therefore reduce the overall height of the construction. When timber is delivered to site it may have a moisture content of up to 20% that reduces down to 12% and below after the building has been occupied for a few years [18].
Other building components, such as brick or stone cladding, may increase in height during the building’s design life, owing to both reversible and irreversible moisture related movement and thermal expansion. This differential movement between the cladding and timber frame, and even between the timber frame and internal stairwells of mixed construction, must be allowed for in design. Buildings less than four storeys height can be easily designed to
accommodate differential movement using the guidance and standard details given in other publications and standards [13][19]. Because differential
movement is cumulative with increasing height, accommodating this movement is much harder in multi-storey buildings. Designs for differential movement in timber frame buildings of four or more storeys should attempt to reduce the sources of movement as much as possible. This chapter provides a summary of the sources for movement in the timber frame and cladding and gives guidance on designs and detailing for multi-storey buildings. Sections 4.2 to 4.4 provide background material; the actual guidance for conducting designs is contained in sections 4.5 and 4.6.
4.2 Regulations, codes and standards
Designing for differential movement within the fabric of multi-storey timber frame buildings in the UK will normally be required for satisfying building control. Although differential movement may occur between many different types of connected parts within the building, both structural and non-structural, the main areas of concern will be movement between the timber frame and cladding, service shafts and structures of dissimilar movement such as lift shafts.
Failure to accommodate such differential movement in the building may tend to indicate non-compliance with different parts of the Building Regulations 2000, Building Standards (Scotland) Regulations 1990 or Building
Regulations (Northern Ireland) 2000. Only the Building Regulations (England
Chapter 4
Differential movement
and Wales) 2000 provide specific guidance on the problem of differential movement in Section 2 of Approved Document A9. This refers the reader to two British Standards, BS 8200: 1985 Design of non-loadbearing external vertical enclosures of buildings and BS 5628: Part 3: 2001 Code of practice for use of masonry – Materials and components, design and workmanship, which provide detailed information concerning the movement of different types of cladding but do not provide the level of detail required for assessing the vertical movement in a multi-storey timber frame building. The following sections expand upon the principles for movement of the timber frame and the differential movement that should be accommodated at various interfaces in multi-storey timber frame buildings.
4.3 Sources of movement
Whilst some movement within buildings is to be expected, it will greatly depend upon variations in the environmental conditions and applied loads. These conditions and loads may change during the construction and lifetime of the building and will need to be considered for individual elements of the building. It is important to gain a good estimation of the range of conditions and loads that different parts of the building will experience and understand the propensity for movement of different materials and constructions.
4.3.1 Moisture movements
Many of the materials used in the construction of buildings are hygroscopic. That is to say the amount of moisture contained in the material will be in equilibrium with the relative humidity of surrounding air in steady-state conditions. Changes in the air relative humidity will cause gradual changes in the moisture content of the material. Changes in the material moisture content are often accompanied by dimensional changes, but these will not necessarily occur equally in all three principal axes of the material. For example, the dimensional movement of timber with respect to changing moisture content is negligible along the grain but should be considered in both directions perpendicular to the grain, tangential and radial. Tangential movement is generally twice as large as radial but will vary from species to species. Normal cutting patterns for saw logs produce timber with varying grain orientation that could be purely tangential, radial or a mixture of both. The effects this can have on timber members is shown on left. Design calculations should consider the average of tangential and radial movement, which will provide a good approximation of the actual moisture movement Since bricks are more homogeneous, their expansion due to the uptake of water will occur to the same degree in all three axes, although this
expansion will not necessarily be fully reversible when conditions cause the moisture content to reduce.
Some manufactured materials, such as fired clay products and cementitious products exhibit irreversible (permanent) changes in size owing to the uptake of moisture [21]. This movement may occur over a period of months
or years. For clay bricks, water absorbed from the atmosphere as they cool after firing in the kiln causes the bricks to expand by about
0.2 to 1.5 mm/m over the life of the bricks; about half of this occurs in the first week[22]. Further movement of the bricks may also occur as seasonal
variations due to the cyclic wetting and drying. These additional movements are reversible and less dominant than the long-term permanent expansion.
Shrinkage of timber with tangential and radial grain orientation
4.3.2 Thermal movements
All materials are subject to some increase in size as their temperature rises, and vice versa. The amount by which they change in size for a given temperature difference varies widely, as does the range of temperatures to which different parts of the building fabric may be subjected.
To estimate the thermal movement of construction materials, the temperature range that the material will experience in service is needed. This is not necessarily the range of ambient temperatures expected over the life of the building since the material temperature will depend on its thermal characteristics (surface absorptivity and emissivity, conductivity, diffusivity, capacity) as well as the characteristics of its environment (air temperatures, radiant gains and losses, evaporative potential). These factors have been taken into account in other texts that provide a detailed description and design data for this type of movement [23][24][25].
When considering the thermal movement of hygroscopic materials, such as timber, it is important to consider the effect on other types of movement, such as moisture movements. Although an increase in temperature causes thermal expansion of the timber, it will also cause the moisture content to reduce and so induce shrinkage. The dominant effect of these two opposing movements when equilibrium is reached will be the moisture movement. This will be the movement type considered in design for internal timber members that are not subjected to rapid changes of temperature and moisture content. Bare external timber that is used for cladding without a painted finish may be subjected to rapidly changing temperature. This can cause thermal expansion before the more dominant shrinkage movement occurs.
4.3.3 Movements from induced stresses
Consideration should also be given to the movement occurring from induced stresses due to the material self-weight and any serviceability loads. Since timber frame buildings are generally constructed using a platform frame technique, vertical movements in the timber frame may be considered as unrestrained. The vertical movement of studs is negligible, even for multi-storey structures where axial shortening of studs may account for only 1 mm of total movement at eaves level. The stiffness and movement of timber loaded perpendicular to the grain requires in depth knowledge of the mechanics involved and is not covered in this section. The current draft of Eurocode 5 gives a more detailed account of the requirements for design. Although the elastic compression of a timber frame building can account for a significant proportion of the overall movement, delayed compression (creep) can be greater than the initial elastic deflection with time. To
estimate the delayed compression deformation, its value should be assumed to be equal to the elastic compressive movement for the total dead and imposed loads on the building.
4.3.4 Bedding in
Timber frame wall and floor panels are constructed to tight tolerances in factory conditions to produce a building with good dimensional accuracy. There will, however, be some variation in the cut length of studs, and hence the height of wall panels, which cause small gaps between the walls and floors. As the construction proceeds, these gaps are reduced owing to the self weight of the supported structure and it is commonly accepted that once the roof has been constructed, including the tile or slate covering, all of the bedding in movement will have occurred.
4.4 Research conducted on the TF2000 building
The movement that occurs in a timber frame is often referred to as shrinkage. Although this statement is a good approximation for low-rise timber frame structures, research conducted on the TF2000 building highlighted that there are other significant and sometimes more dominant movements that occur in multi-storey timber frame structures [26]. For
example, the compressive movement of the sole plate and bottom rail on the ground floor panels is greater than shrinkage of the same members in medium-size structures. Figure left shows the results of tests conducted on the TF2000 building that identified the
relationship between shrinkage and elastic compression for multi-storey structures. For lower storeys, the two types of movement are similar over the short-term although compression perpendicular to the grain can produce much higher movement over medium- and long-term time-scales when creep effects contribute.
For the timber frame, the majority of vertical movement comes from the floor zone above wall panels and rim beams on external walls (as described in Chapter 1). Figure left, below, shows the relative movement of structural members with measurements taken from the TF2000 building. The movement of joists would have been much greater if they had not been pre-conditioned to 12% moisture content when delivered to site.The dominant movement of the joist is mainly due to the depth of section when compared with other members.
The amount of differential movement may also be reduced through control of the site construction process. Construction of the TF2000 building ensured that the roof construction and tiling were completed before commencement of the brickwork cladding. Sequencing of the construction such that the dead weight of internal linings is already supported by the structure before construction of the brickwork cladding can also reduce the provision required for compressive movement of the timber frame. Admittedly the design sequence will not always be known at the design stage but timber frame buildings that adopt closed panel systems will benefit from additional dead weight early on in the construction sequence. 0 1 2 3 4 5 6 Movement St orey o f b u ildi ng Compressive movement Shrinkage movement
Relative movement due to shrinkage and elastic compression for the TF2000 building
Time: construction start to occupied building
Vertical movement of timber framing members
Summation of 1 to 4 (total movement)
4 Joists
2 Studding
1 Sole plate and bottom rail 3 Top rail - 7% of total - 4% of total - 64% of total - 25% of total 1 year
Movement of the timber frame on the ground floor of the TF2000 building
Cumulative downward movement at each floor level
4.5 Reduction in movement by design
Simply determining the amount of expected differential movement may not be enough for more stretching designs in multi-storey structures. The accumulation in differential movement on upper storeys of the building may be too great around openings and liftshafts and will need to be reduced to a tolerable level. When the design value for differential movement has been determined, this should be accommodated in the constructed building by providing adequate movement joints and suitable connections such as flexible wall ties and sliding wall ties.
Options for reducing the amount of differential movement are:
❐ Use timber joists and headers with a low target moisture content, typically 12% or lower so that there is very little shrinkage in service.
❐ Substitute timber joists and headers with engineered wood products with low moisture contents.
❐ Reduce the amount of timber loaded perpendicular to the grain, such as multiple-sole plates and header plates.
❐ Use clay bricks with low movement characteristics for cladding.
❐ Specify alternative claddings such as timber boarding or tiles. To demonstrate the effect some of these options have on reducing differential movement, predictions for a typical six-storey timber frame building are shown below. This clearly demonstrates the benefits of using materials with low movement characteristics for multi-storey buildings; taking full account of this in design can save expensive detailing requirements around openings and for connections such as wall ties. Movement tolerant wall ties should be used for multi-storey timber frame.
0 1 2 3 4 5 6 -30 -20 -10 0 10 20 30 40 50
Vertical downward movement for a typical six-storey building, mm
NORMAL JOISTS SUPER DRY JOISTS ENGINEERED JOISTS HIGH MOVEMENT BRICKWORK LOW MOVEMENT BRICKWORK
OPTIONS: Contraction of the timber frame Expansion of the brickwork Br ic kw or k mo v ement T im b er mo v ement Cavity
Example of differential movement for a multi-storey brick-clad building
The above movement values for the timber frame account for all shrinkage and compression (both elastic and creep) but do not allow for bedding in of the timber frame. The common construction sequence for timber frame ensures that the timber frame, prior to construction of the brick cladding, supports the majority of the design dead load including the roof load. If a different construction sequence is adopted, an additional 3 mm movement per storey should be allowed for in design.
The majority of the timber frame movement will be expected to have occurred during the first 36 months of the building’s occupation. Further small movements in the building will occur with seasonal variation in conditions but these will be minimal by comparison. If other target moisture contents are specified for timber components, the design allowance for movement will have to be adjusted.
4.6 Provision for movement
Designs suitable for differential movement of the external wall construction will inevitably require the provision of expansion gaps at eaves level between the cladding and around window openings. When the timber frame is connected to any dissimilar material that will move differentially in-service, the connectors or materials should accommodate such movement without transferring load. If mastic sealant or other compressible material is used to fill gaps provided for differential movement, the designer should ensure that there is sufficient room left for movement once the sealant is compressed. Designs often wrongly assume that such sealants compress to nothing. Weather sealants provided around vertical interfaces between openings and cladding should also accommodate movement or be maintained to ensure good performance – see figure on page 25.
Options for building components
Normal timber installed at 20% mc (1)
Super dry timber installed at 12% mc or below (1)
Engineered wood products installed at 12% mc or below (1)
Brickwork cladding
Hung tiles, slates and timber cladding Other claddings
Notes
(1) Design values assume that the timber moisture content (mc) is kept at or below the target value.
(2) The total depth of cross-grain timber should include all sole plates, bottom rails, top rails, joists and any other timber loaded perpendicular to the grain that adds to the total building height at eaves level.
This includes I-beams.
Allowance for movement
1.40 mm / 50 mm of cross-grain timber (2) 0.6 mm/ 50 mm of cross-grain timber (2) 0.4 mm/ 50 mm of cross-grain timber (2) 1 mm/ 1m height (3) Negligible Varies (4)
(3) A conservative value for use in the absence of manufacturer’s data.
(4) Other publications provide details [2][12] For design, these values can be assumed in the absence of more precise data
