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*G = permanent, Q = variable tV = vertical, JH = horizontal (parallel to the motorway)

In accidental situations, the combination factors used in the equations given above are reduced slightly to account for the lower probability of occurrence of that situation. The total design action Fd is given by:

Fd = Y,Yg ,jGk ,j + Y ai Ak ,1 + Yq ,iWi,iqk ,1 +Y,Yq iWnqk j i>1

where Ak1 is the accidental action, is applied (instead of 1.0) to the main variable action, and is applied (instead of ^0) to the accompanying variable actions. In accidental situations, the values of the partial factors yg,

Extending the example of the motorway gantry to include an accidental action (in this case a traffic impact load shown in Figure 2.12) gives revised values for the design actions, as set out in the table below.

In this revised table, Combination 3 assumes that the imposed load is the main variable action (with ^ = 0.7) and wind is accompanying (^2 = 0); Combination 4 assumes that wind is the main variable action (^ = 0) and the imposed load is accompanying (^2 = 0.6). The design actions that result for Combination 3 are more onerous than for Combination 4, but less onerous than Combinations 1 and 2.

Combination of actions for accidental design situations

Combination 3 Combination 4
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