Bridge Design

3Actions and Structural Safety

3.1 Types of Actions and Limit State Design

In bridge design, a variety of design actions should be considered at the construction stage or at the final stage (final static system). Construction loads are associated with equipment like formwork launching girders in span by span construction or moving scaffoldings in cantilever construction, as explained in Chapter 4. Under service conditions, the bridge is subjected to permanent actions (dead loads (DL), creep and shrinkage effects) and variable actions, like traffic loads, thermal, wind and earthquake actions, friction at the bearings or settlements of the foundations. Accidental actions may be considered at the final stage of the bridge or at the construction stages.

Nowadays, bridge design verifications are usually based on a semi‐probabilistic approach yielding a limit state design format. This is the approach adopted by most design codes, namely the Eurocodes and specifically Parts 2 of Eurocode 2 (denoted by EC2‐2 [1]) for concrete bridges, Eurocode 3 (denoted by EC3‐2 [2]) for steel bridges, and Eurocode 4 (denoted by EC4‐2 [3]) for steel‐concrete composite bridges. Loads are quantified by characteristic values, except for accidental actions usually defined on the basis of nominal values. A characteristic value is a value with a certain probability of not being exceeded; say 5%, as illustrated in Figure 3.1. Actions are classified as permanent actions, variable actions and accidental actions ...