Introduction - Structural and Stress Analysis - page 2

retaining walls prevent the slippage of embankments and offshore structures carry
drilling rigs, accommodation for their crews, helicopter pads and resist the action
of the sea and the elements. Harbour docks and jetties carry cranes for unloading
cargo and must resist the impact of docking ships. Petroleum and gas storage tanks
must be able to resist internal pressure and, at the same time, possess the strength
and stability to carry wind and snow loads. Television transmitting masts are usually
extremely tall and placed in elevated positions where wind and snow loads are the
major factors. Other structures, such as ships, aircraft, space vehicles, cars, etc. carry
equally complex loading systems but fall outside the realm of structural engineering.
However, no matter how simple or how complex a structure may be or whether the
structure is intended to carry loads or merely act as a protective covering, there will
be one load which it will always carry, its own weight.
1.2 L OADS
Generally, loads on civil engineering structures fall into two categories. Dead loads
are loads that act on a structure all the time and include its self-weight, fixtures, such
as service ducts and light fittings, suspended ceilings, cladding and floor finishes, etc.
Interestingly, machinery and computing equipment are assumed to be movable even
though they may be fixed into position. Live or imposed loads are movable or actually
moving loads; these include vehicles crossing a bridge, snow, people, temporary par-
titions and so on. Wind loads are live loads but their effects are considered separately
because they are affected by the location, size and shape of a structure. Soil or hydro-
static pressure and dynamic effects produced, for example, by vibrating machinery,
wind gusts, wave action or even earthquake action in some parts of the world, are the
other types of load.
In most cases Codes of Practice specify values of the above loads which must be used
in design. These values, however, are usually multiplied by a factor of safety to allow
for uncertainties; generally the factors of safety used for live loads tend to be greater
than those applied to dead loads because live loads are more difficult to determine
accurately.
1.3 S TRUCTURAL S YSTEMS
The decision as to which type of structural system to use rests with the structural
designer whose choice will depend on the purpose for which the structure is required,
the materials to be used and any aesthetic considerations that may apply. It is possible
that more than one structural system will satisfy the requirements of the problem; the
designer must then rely on experience and skill to choose the best solution. On the
other hand there may be scope for a new and novel structure which provides savings
in cost and improvements in appearance.