Civil Engineering Reference
In-Depth Information
Timber-framed, masonry-clad
walls
Differential movement between the
timber structure and the separate
brickwork is a potential source of
problems. This movement is mainly
the result of shrinkage across the
grain of horizontal timber members
such as floor joists, rails and plates.
Most of the shrinkage occurs after
construction and during initial
occupation, as the building dries out.
If insufficient allowance is made for
differential movement, the
components fixed to the timber frame
- such as windows, rafters and direct
claddings - can bear on the brick
cladding (Figure 3.38). The rule of
thumb recommendation is to provide
6 mm soft joints in the ground storey,
12 mm in the next storey, and so on.
Window sills, tilted or rotated if of
timber, or broken if of tile, are among
the problems associated with
differential movement (Figure 3.39).
Another typical movement related
Figure 3.39
Rotation of a sill caused by the shrinkage of
a timber frame
Figure 3.36
'Considerable movements could still take
place without threat of collapse.' A
farmhouse near Tenbury Wells
Figure 3.37
Here a complicated strap ties together
beam and external wall plate to the head of
a post (Photograph by permission of
G C R Hughes)
Where movements have occurred
in the past, for example where
bearings of bressumers have pulled
off their supports, it is not uncommon
to see blacksmith-made straps spiked
into the timber and bent over and
built into the offending wall.
Restoration of stability is of course
still feasible by similar, albeit ad hoc,
methods (Figure 3.37).
For coefficients of linear thermal
and moisture expansion etc, see
Chapter 1.2.
Figure 3.38
Differential movement between frame and cladding needs to be allowed for at a number of
points
