U Frame Analysis to BS 5400 and BD56_96

The classic U-Frame action example can be found in “half-through” railway
bridges[1],[2] or pedestrian bridges. The key concept of the U-frame action is
illustrated in Figure 1a. The two longitudinal girders are subjected to a sagging
bending moment, which causes compression in the top flanges. At certain
locations along the bridge span, a continuous U-shaped frame is formed from
the horizontal deck beams and vertical elements in the main girders – usually
full depth stiffeners welded to the web.