BS 5400 Part 3 Design of Steel Bridges

Click on the Clause No. for the commentary.


CLAUSE No.SUBJECT U-frame cross members subject to vertical loading.




Which loads need to be considered on the cross member?


It is useful to remember that we are considering the stability of the main girder compression flange and consequently the loading condition which produces the maximum compressive stress in this flange.This loading condition should also be considered when determining Fc.

The definition of θ is the difference in rotation between the cross member of the U-frame under consideration, and the mean of the rotations of the cross members of the adjacent U-frames on either side. The rotations are calculated in radians under the loading , at the junction of the relevant cross member with the main beam under consideration, assuming that the cross member is simply supported.

This suggests that uniform loads, such as dead load, which will produce the same θ at the end of each cross girder may be ignored.
Similarly, if the maximum stress in the main girder is produced from HA loading, the UDL effects may also be ignored and the KEL need only be considered. As the KEL will only be applied to one cross girder, then the average θ on the adjacent cross girders will be zero, therefore θ is due to the KEL only on the U-frame under consideration. This however gives very conservative results. A more accurate analysis of the U-frame can be achieved using a space frame model as described in Section 6.8 of the Designers' Guide to EN1993-2.


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