The Design of Modern Steel Bridges - TEDI

76 The Design of Modern Steel Bridges
(2) excessive local damage like cracking, splitting, spalling, yielding or slip,
affecting appearance, use or durability of the structure
(3) excessive vibration causing discomfort to pedestrians or drivers.
4.2 Permissible stress method
In the modern generation of structural design codes, the specific requirements
for the relevant limit states are stated explicitly. In the past, however, the codes
did not identify the various limit states separately; they were like a cooking
recipe which produced the desired end product, but the ingredients of which
were not specifically chosen for particular objectives.
The process of structural design is not an exact science, nor are the data on
which a design can be based accurate. There are uncertainties in the loading, in
the material properties, in the engineering analysis and in the construction
process. In the past, design codes allowed for these uncertainties by specifying
a permissible stress for the most adverse combination of working loads. The
permissible stress was obtained by applying a factor of safety on the stress
observed or calculated to occur at failure. The failure stress was generally
taken as the yield stress and the working loads were specified as those loads
that could be expected to act on the structure several times in its design life.
In this permissible stress or working load method a structural analysis was
made to evaluate the working stresses at the specified combination of working
loads, which were then checked against the specified permissible stress. Thus
X working stress4permissible stress
failure stress
i.e.4
safety factor
The main advantage of this method is simplicity. Because stresses, and
hence deformations/deflections, were kept low under working loads, nonlinearity
of material and/or structural behaviour could be neglected and working
stresses were calculated from linear elastic theories. Stresses from various
loads could thus be added together. The disadvantages of this method are:
(1) One global factor of safety cannot deal with the different variabilities of
different loads; for example, variations of dead load from the calculated
working value is usually small compared with the variation of extreme
wind or vehicle loads from their working values. This is particularly
serious when two loads of different variabilities counteract each other;
the safety factor used in this method may give a very false impression
of the danger that can be caused by a modest increase in one of the loads.
(2) The analysis of the structure under working loads may not provide a
realistic assessment of the behaviour of the structure at failure. If the