Long-span bridges that carry both road and rail traffic on a steel truss deck are subject to two very different service actions. The railway loads are transferred locally through a deep cross-section, while temperature acts on the whole structure through atmospheric exposure and solar radiation. Because the chords of a truss deck are far apart, both in depth and across the width, even small temperature differences between them cause differential elongations. Restrained by the suspension system and by the supports, these elongations produce axial forces in the chords and bending of the deck. Whether the resulting effects are comparable to those of the railway traffic is the question this thesis addresses, taking the Tsing Ma Bridge, a suspension bridge with a 1377 m main span that carries a dual three-lane highway and a twin-track railway on a steel truss deck, as a case study.