Field Measurements of Diaphragm and Top Lateral Members of Three Trapezoidal Composite Box Girder Bridges

The results of field tests on three trapezoidal box girder bridges are presented. The study concentrated on the measurement of the strains in the external and internal diaphragms and the top laterals of the boxes. The strains were collected when the girders were loaded by a crane lifting up on the girders before placement of the deck, during the casting of the deck, and under truck loading after hardening of the deck. The measured results were compared with a three-dimensional finite element model of the bridge. The measured forces in the diaphragms were very low and generally much lower than predicted from the finite element analysis. This was partially due to the flexibility of the connections to the girder and the connection of the diaphragms to flanges in the model. The forces in the top laterals, which are used to produce a semi-closed section prior to hardening of the deck, were much smaller then predicted by the non-composite finite element model. The concrete deck hardened during the concrete placement and within hours after placement was acting compositely with the steel girder. The external diaphragms and the top lateral forces were very small during the truck load tests of the composite bridge. The stiff slab provided a closed section that replaced the structural function of the top laterals and provided a continuous connection between the girder that eliminated the structural function of the external diaphragms.