STRUCTURAL DESIGN FOR ARCHITECTURE

building will be required to carry out both of
these functions and it must be designed
accordingly, and suitably attached to all of the
walls. Reinforced concrete floors provide the
best horizontal structural elements for these
purposes. Where timber elements are used,
their ability to act collectively as a horizontal
diaphragm must always be considered. In
cases where they cannot do so, a horizontal
bracing girder must be incorporated into the
plane of a roof or floor to give it the ability to
act as horizontal-plane bracing (Fig. 5.31).
In low-rise buildings of up to four storeys
the magnitudes of the lateral loads due to
wind are not high and the resistance of these
is rarely a critical factor in the design. The
building will be satisfactory so long as it
contains some walls in two orthogonal direc-
tions and any single-plane walls which are
present are connected, by an adequate
horizontal system, to other wall groups which
are capable of bracing them. The exact disposi-
tion of the loadbearing and bracing walls is not
critical although a symmetrical arrangement of
bracing walls will produce the most satisfac-
tory structure for the resistance of lateral load.
In tall multi-storey buildings of five storeys
or more, the resistance of wind load can,
however, be a critical consideration. The
additional factors which affect their planning
are reviewed in Section 5.3.4.

5.3.3.3 Resistance of individual walls to out-of-
plane forces
Wind pressure is an out-of-plane load on the
external walls of buildings. It produces
bending-type internal forces in walls and can
result in the generation of levels of tensile
stress which the masonry cannot resist (Fig.
5.30b). The task for the designer is to contain
the tensile stress which develops. This can be
done by adopting a structural form which either
limits bending action or which neutralises,
using axial compressive stress, any tensile
bending stress which does develop. Bending
action is countered by minimising span. The
external walls of masonry buildings span either
vertically, between floors, or horizontally,
between buttressing walls (Fig. 5.34). If the

Masonry structures

Fig. 5.34 In response to out-of-plane wind loads, the

external walls of loadbearing wall buildings span either

vertically between horizontal-plane bracing elements or

horizontally between buttressing walls.

strategy being adopted to limit bending action

is that of providing buttressing walls, then

these have to be spaced closely. This obviously

affects the planning of the building.

If the strategy for limiting the bending stress

caused by out-of-plane wind pressure on exter-

nal walls is one of neutralising the tensile

stress with compressive stress, the walls must

be arranged to span vertically between floors

in response to wind loading. This ensures that

any tensile stress acts on the horizontal cross-

sectional planes where it can be neutralised by

the compressive stress produced by the gravi-

tational loads. The magnitude of the tensile

bending stress can be minimised by ensuring

8 See Macdonald, Structure and Architecture Appendix 2 for

an explanation of this term. 173

(a)

(b)