A3: Overall stability
It is essential for timber construction to interact with the other elements of a building to provide a stable completed structure. Most timber floors are required not only to act as diaphragms transmitting horizontal loads to shear walls, but also as lateral restraints to vertical walls.
Technical Guidance Document Part A of the Building Regulations 1997 gives guidelines on the requirements for strapping for masonry houses as follows:
(a) A wall in each storey of a building should be extended to the full height of that storey and have horizontal lateral supports to restrict movement at right angles to its plane.
(b) Floors and roofs should -
- act to transfer lateral forces from walls to buttressing walls, piers or chimneys, and
- be secured to the supported wall by connections specified in paragraphs (c) and (d).
(c) Walls should be strapped to floors at first floor level, at intervals not exceeding 2m by stainless steel straps or galvanised mild steel straps which have a minimum 30x5mm section as shown in fig A 3.1 (a) and (b). Straps need not be provided -
- in the longitudinal direction of joists, if the joists are not more than 800mm centres and have at least 90mm bearing on the supported walls, or
- where the joists are carried on the supported wall by joist hangers of the restraint type described in IS 325 and shown in fig A 3.1 (c), at not more than 800mm centres;
- where floors are at or about the same level on each side of a supported wall as shown in fig A 3.1 (d) and contact between floors and wall is continuous or at intervals not exceeding 2m. Where contact is intermittent, the point of contact should be in line or nearly in line.
(d) Walls should be strapped to roofs as shown in fig A 3.2 (a) and (b) by stainless steel straps or galvanised mild steel straps which have a minimum 30x5mm section. Vertical strapping should be provided at eaves level at intervals not exceeding 2m as shown in A 3.2 (c). Additional vertical straps may be necessary where the roof -
- has a pitch of less than 15°, and
- is in wind zone C as designated in TGD A.
- the length of opening should not exceed 3m measured parallel to the supported wall, and
- where a connection is provided other than by an anchor, this should be provided throughout the length of each portion of the wall situated on each side of the opening, and
- where connection is provided other than by mild steel or stainless steel anchors or by packs, these should be spaced closer than 2m. on each side of the opening to provide the same number as if there were no opening, and
- no other interruption of lateral support is permissible.
In cross wall construction, the floor and roof elements of the structure are supported by the load bearing walls which usually run perpendicular to the front/rear walls, (Fig. A 3.3 (a)). Alternatively, the horizontal elements may span from front to rear with internal load bearing walls also supporting the load (Fig. A 3.3 (b)).
Fig. A 3.3 Vertical Load (152Kb)
In addition to carrying vertical loads, wall panels may be required to withstand wind forces resulting in shear or racking forces.
Walls subject to horizontal shear or racking forces may need to be restrained from sliding and held down (see fig. A 3.4).
Fig. A 3.4 Horizontal loads (14Kb)
Floor, roof and ceiling systems may act as horizontal diaphragms to take loads in their own plane and transfer loads into shear walls.
Figure A 3.5 illustrates such horizontal diaphragms resisting wind loads on a gable wall.