Structural Design for Architecture
greater than 27. In practice, however, it is
usually desirable to achieve a value which is
considerably smaller than this (a maximum of
around 20).
Given that the range of wall widths which is
used in masonry is limited by the available
dimensions of the masonry units, it is the
effective lengths and effective heights of walls
which must be adjusted by the designer of the
building in order to control their slendemess.
These are the distances between the horizontal
and vertical lines along which the walls are
restrained against lateral movement
(movement out of their own plane (Fig. 5.28)).
In the case of height the distance between
lateral restraints will normally be the same as
the storey height of the building because the
roof and intermediate floor structures can
normally be regarded as being capable of
providing effective lateral restraint. The effec-
tive length of a wall is the distance between
lateral support provided either by walls,
bonded at right angles to the wall in question,
or by substantial piers.
In most buildings, the storey height will be
such that the slenderness ratio, calculated
from the effective height, will be within the
required limit. Where this is not the case it will
not normally be possible to adjust the slender-
ness ratio by altering the storey height because
many factors besides those concerned with
structural performance influence the choice of
storey height. In these cases the required
slenderness ratio must be achieved by altering
the plan so as to reduce the effective length of
the wall, either by inserting buttresses or by
altering the positions of walls which are
perpendicular, on plan, to the wall in question.
It is important that the structural elements
which are considered to provide lateral support
for a wall are in fact capable of doing so. Two
factors affect this, the strength of the support-
ing elements and the way in which they are
attached to the wall. Buttressing walls and
concrete floors and roofs will normally have
adequate strength. The ability of timber floors
or roofs which are sheathed with boarding to
act as horizontal diaphragms may be in doubt,170 however, but where this occurs, they can be
Fig. 5.29 A bracing girder is located at eaves level and
acts in conjunction with the gable walls to provide lateral
support for the top of the side wall.strengthened either by sheathing in structural
plywood, so as to form a strong horizontal box
beam, or by incorporating into the floor or roof
structure a system of diagonal bracing (Fig.
5.29).5.3.2.4 Summary of the form-determining
factors which are derived from the need to
resist gravitational load
The need to resist gravitational load exerts two
major influences on the plan-forms which are
adopted for loadbearing masonry buildings.
Firstly, the requirement to provide support for
all areas of floor and roof favours the use of
certain plan geometries; secondly, the need to
prevent buckling requires that the slenderness
ratios of walls should not be unduly large. To
satisfy these requirements the following
general rules are observed when planning
masonry buildings.1 The arrangement of supporting walls is
tailored to the type of roof or floor system
which they will support. For one-way-
spanning systems parallel sets of walls are
used and placed at approximately equal
distances apart. For two-way-spanning
systems a cellular arrangement is adopted inExtra members are introduced into the ceiling plane to
form a horizontal-plane girder which provides lateral
support for the top of the wall