Applied Loads and Stability of Steel and Steel-Concrete Composite Bridges - Finite Element Analysis and Design of Steel and Steel-Concrete Composite Bridges - page 160

different actions considered in each group. It should be noted that where
groups of loads are not taken into account, railway traffic actions shall be
combined in accordance with EC0 [ 3.4 ] .
3.7 DESIGN APPROACHES
3.7.1 General
The design of steel and steel-concrete composite bridges should fulfill the
basic requirements of design, wherein, over its intended life, the bridges
should sustain the applied loads and remain fit for use. Therefore, the current
codes of practice proposed design rules and guides to ensure that the bridges,
like other structures, must have a specified strength, perform in an acceptable
manner, and be durable over the intended life. The design rules and guides
were based on two main design approaches, which are briefly highlighted
in this section. The design approaches are commonly known as allowable (per-
missible) stress design approach and limit state design approach .
3.7.2 Allowable Stress Design Approach
Earlier design rules specified in the current codes of practice were based on
allowable (permissible) stress design approach. In this design approach, a fac-
tor of safety was adopted to account for the uncertainty in the loading, in
material properties, in empirical design equations, and in the construction
process. The allowable stresses were predicted by dividing the failure stress
by the factor of safety. The failure stress may be taken as the yield stress or the
proportional limit stress of the material of construction. In this design
approach, a structural analysis could be performed to evaluate the stresses
at the specified combination of loads, which were then checked against
the specified allowable stresses. The allowable stress design approach was
commonly used in the past owing to its simplicity and safety. Because stres-
ses, and hence deformations/deflections, were kept at low levels, nonlinear-
ity of material and/or structural behavior could be neglected and working
stresses were calculated from linear elastic theories. In performing the struc-
tural analyses, stresses from various loads could be added together. However,
this design approach has some disadvantages, which are mainly due to the
use of a single factor of safety with different applied loads, and the analysis
of the structure under working loads may not provide a realistic assessment
of the behavior of the structure at failure. It should be noted that structures
designed adopting the allowable stress design approach have moderate