Approximate methods for allocating sizes to
structural elements
Appendix 2
A2.1 Introduction
The methods which are outlined here for deter-
mining the sizes of structural elements are not
those of rigorous final structural design calcu-
lations. Rather they are quick methods which
allow the sizes of structural elements to be
determined approximately. They are prelim-
inary planning tools which are sufficiently
accurate, in most cases, to allow the feasibility
of proposed structural arrangements to be
assessed. They are applicable to the
mainstream forms of structure.
One of the main objectives of structural
design is to produce structures which are safe
and serviceable at reasonable cost. This
requires that the sizes of the cross-sections of
structural elements be sufficiently, but only
sufficiently, large to carry safely the loads
which are applied to them. The principal
mechanism by which this aspect of structural
design is controlled is through calculations.
A2.2 Structural analysis
The principal factor which determines the size
required for a structural element is the amount
of load which it carries and the element-sizing
part of structural calculations must normally
therefore be preceded by an assessment of
this. The process by which it is done is known
as the analysis of the structure. Even approxi-
mate element-sizing calculations require that
some form of rudimentary structural analysis
be carried out.
Structural analysis can be subdivided into
the three distinct processes of load assessment,
preliminary analysis and final analysis. Load
assessment involves the prediction of the
maximum load which will occur on the struc-
ture in its lifetime. In the case of architectural
structures there are three principal types of
load: these are dead load - the permanent gravi-
tational load caused by the weight of the
building and its fixtures; gravitational imposed load- variable load caused by the weights of the
occupants of the building, furniture and other
moveable items; and wind loading - non-gravita-
tional imposed load caused by the action of
wind pressure. In the case of most structural
elements the most unfavourable load grouping
is the combination of dead and imposed gravi-
tational load. The approximate element-sizing
calculations which are presented here will
therefore be based principally on this load
combination. The figures which are specified in
the current British Standard for dead and
imposed gravitational loads are given in Tables
A2.1 and A2.2.
The preliminary analysis of a structure is a
process in which the three-dimensional object
which is the structure is broken down into its
constituent elements so that the forces which
are imposed on the elements under the action
of the maximum load condition can be deter-
mined. It involves the tracing of the path which
is taken through the structure by the load from
the floor and roof surfaces, to which it is
applied initially, to the foundations, where it is
ultimately resisted. Some structural elements,
such as floor slabs, are acted on directly by the
loads. Others, such as columns, receive load
from the parts of the structure which they
support. The end product of the preliminary
analysis is a set of individual structural
diagrams (one for each element) on which the
forces which act on each element are marked. 239