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
Brick cladding on TF2000
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.
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
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
Brickwork movement Timber movement
Cavity
Example of differential movement for a multi-storey brick-clad building
storeys
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
If masonry is adopted for the cladding, the design of wall ties shall take account of the differential movement as well as the transfer of horizontal forces.
Common areas on timber frame buildings that require special detailing are:
❐ windows;
❐ doors;
❐ openings for services;
❐ soffits;
❐ cavity ties [27];
❐ battens across floor zones;
❐ junctions for mixed cladding designs;
❐ lift shafts and stair wells of mixed construction.
Many of these details are included in Timber Frame Construction [10]and other publications [19][28].
Good detailing at these intersections that accommodates the design differential movement can ensure that costly maintenance is avoided during the service life of the structure. Common failures occur through poor detailing, insufficient provision for movement or poor construction on site.
Good designs must be communicated correctly on site and checked to ensure that costly failures do not occur.
Vertical movement at joint as timber frame shrinks Window fixed to studs of frame
Differential movement around windows
5.1 Introduction
The TF2000 project recognised the need for market based facts as well as technical research. Part of the activities was to undertake a productivity benchmarking exercise on the test building [29].
Benchmarking can be defined as ‘the continuous process of measuring products, services and practices against the toughest competitors or those companies recognised as industry leaders’. Hence a study was undertaken on building construction performance using timber frame, concrete and steel framing techniques. Since full account of this study has been previously published [30]this chapter merely summarises the report and expands on the key issues for improving the performance of timber frame for the multi-storey residential based market.
The benchmarking investigations have proven that platform timber frame provides the construction industry reduced build times and high in service performance. These benefits can be realised through management of the processes involved within the project’s development. This chapter provides specific guidance relating to achieving good performance for the building.
The key issues are presented as:
❐Construction process lessons
❐Build tolerance
❐Safe Construction procedures
5.2 Timber frame benefits in multi-storey construction
The TF2000 building provided the first full scale test for buildings above two storeys and was the first 6-storey residential timber frame building. Since TF2000 there have been a number of six-storey buildings that have proven on a commercial scale the benefits identified on the TF 2000 project.