In concrete box girder bridge design, diaphragms serve a crucial role in transferring loads from the bridge deck to the bearings. Given their typical dimensions, where depth often surpasses width twofold, these structural elements are commonly engineered as deep beams. However, their necessity can be questioned in scenarios where bridge bearings are positioned directly beneath the webs. According to research by Jorg Schlaich & Hartmut Scheef (1982), in such configurations, loads from the deck can be transmitted straight to the bearings, bypassing the need for diaphragms.
While this alternative arrangement offers a potential simplification, it’s not without its drawbacks. Notably, it poses challenges for future maintenance operations, making bearing replacement a more complex undertaking due to limited accessibility.
Beyond their load-transfer function, diaphragms also play a significant role in enhancing the torsional rigidity of the bridge deck. This additional stiffness is essential for resisting twisting forces induced by traffic, wind, and other external loads. By providing torsional restraint, diaphragms contribute to the overall structural stability and performance of the bridge.
Therefore, the decision to incorporate diaphragms or opt for direct web-to-bearing load transfer involves a trade-off between constructional simplicity and long-term maintenance considerations, along with a careful assessment of the bridge’s torsional stiffness requirements.