buckling-type failure is an important consider-
ation. This is dependent on slenderness. Often
reinforced concrete walls or columns are
relatively short, however, and the overall
dimensions which are adopted can be deter-
mined by the need to prevent the level of
compressive stress from becoming excessive
rather than the need to maintain the slender-
ness ratio low enough to maintain adequate
resistance to buckling.
The strength of a reinforced concrete
element is determined principally by the
overall size of its cross-section, by the strength
of the constituent concrete and by the quantity
and location within the cross-section of the
reinforcement which is positioned within it.
The strength of concrete can vary widely
(typically between 25 N/mm^2 and 70 N/mm^2 )
depending on the mix proportions and
water/cement ratio which are specified. It is
therefore possible, by manipulating the
strength of the concrete and the amount of
reinforcement which is present, to produce a
range of strengths within a given overall size of
cross-section. For this reason the overall
dimensions of elements tend to be determined
by other criteria than the need to provide
adequate strength. These are the need to limit
deflection in the case of bending-type
elements and the need to avoid excessive
slenderness in the case of axially loaded
elements. The 'rules of thumb' given in Table
4.1 provide reasonably realistic approximations
to the sizes which will be required for
reinforced concrete elements.
A2.6 Masonry structures
As with reinforced concrete, the basic strength
of masonry can vary over a fairly wide range
depending on the strengths of the constituent
bricks or blocks and the mix proportions of the
mortar which is used. For this reason the
overall thickness of walls and columns tends to
be determined by considerations of limiting
the slenderness ratio or constructional con-
venience rather than by the maximum load
which is carried. Basic masonry elements may
therefore be sized approximately from the data
given in Tables 5.1 and 5.3.
A2.7 Timber structures
A2.7.1 Introduction
Rigorous structural design calculations for
timber are complex because the carrying
capacity of timber elements can be affected by
a large number of factors. Included in these are
the duration of the load, the moisture content
of the timber, the extent to which load sharing
between elements is possible, the overall
dimensions of the element, the direction of the
load in relation to that of the grain, and a
number of other effects. These are taken into
account in rigorous element-sizing calculations
by adjusting the value which is used for the
permissible stress to suit the individual
circumstances of the structure. The permissible
stress which is used in a particular case is the
basic allowable stress for the species involved
multiplied by one or more stress-modification
factors. The exact procedure which is used to
evaluate the permissible stress in a particular
case depends on the type of element under
consideration (i.e. on whether it is solid or
laminated, etc.) and on the nature of the inter-
nal force which it will carry. These procedures
will not be explained in detail here.
The fact that so many factors can affect the
strength of a timber structure means that
simplified approximate sizing calculations give
a slightly less reliable indication of the final
sizes which will be required than is the case
with equivalent methods for other materials.
Particular care is required when the limits of
the spans in which a particular component is
normally used are approached. This is
especially the case with bolted trusses in
which the viability of a proposed arrangement
is likely to be determined by the feasibility of
the joints. Where the limits of normal practice
are approached, the feasibility of a proposal
can only be tested by carrying out rigorous
sizing calculations. 263
Appendix 2
