The Raith Bridge, carrying the M74 over the River Clyde, needed an in- depth assessment after Scott Wilson Scotland identified potential problems. The consultant had found that the pier and abutment diaphragms of the box girders did not comply with the geometric requirements of the bridge assessment code.
Analysis was carried out using Lusas Bridge. The structure has two three- span twin steel box girders, with a concrete deck.
A finite element model was built. Two spans were modelled in detail with shells representing the steel box and concrete slab. Beam elements represented the contribution from the longitudinal stringer beam, transverse beams and vertical and transverse stiffeners.
A finer mesh was used for a pier and abutment. The third span was included as beam elements.
Stress plots were produced for specific load cases and output to spreadsheets for assessment.
Results showed that the diaphragms were vulnerable and may require further analysis to prove the capacity.