Rigid-frame bridge

A Rigid-Frame Bridge is a bridge in which the superstructure and substructure are rigidly connected to act as a continuous unit. Typically, the structure is cast monolithically, making the structure continuous from deck to foundation. The connections between members are rigid connections which transfer bending moment, axial forces, and shear forces. A bridge design consisting of a rigid frame can provide significant structural benefits, but can also be difficult to design and/or construct.

The use of rigid-frame bridges began in Germany in the early twentieth century and quickly spread to the Americas. Emílio Henrique Baumgart and Arthur G. Hayden, in particular, gained notoriety for their use of concrete rigid frames in the early 1920s. At the time, reinforced concrete was commonly used in bridge design but the superstructure was designed with bearings on the substructure. In concrete rigid-frame design, there are no bearings. Instead the superstructure is cast monolithically with the substructure and the entire bridge from deck to footing is continuous.

Engineers have found this type of design advantageous for many reasons. Moments at the center of the deck of a rigid-frame bridge are smaller than the corresponding moments in a simply supported deck. Therefore, a much shallower cross section at mid-span can be used. Additional benefits are that less space is required for the approaches and structural details for where the deck bears on the abutments are not necessary. Engineers have also noted some disadvantages of rigid frame bridges. The placement of steel reinforcing bars can be very difficult and the forming/placement of the concrete is complicated. Furthermore, rigid frames are statically indeterminate and the analysis is more challenging than that of simply supported structures.

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