How does a steel girder bridge transfer load from the deck to the substructure and what is the role of diaphragms?

• A. Deck loads are carried by the deck slab directly to foundations; diaphragms transfer shear between supports.
• B. Deck loads are carried by girders; diaphragms transfer shear and distribute bending moments among girders, while bearings transfer reactions to the piers/abutments.
• C. Diaphragms only provide cosmetic support; girders don't carry loads.
• D. Girders transfer loads to the deck through rigid connections; diaphragms transmit vertical loads only.

Answer: (B)

The load path in a steel girder bridge starts with the deck loads being carried by the girders. The deck slab works with the girders to transfer vertical reactions into the girder members, which then bend and transfer those reactions to the end supports. The end supports have bearings that convey the vertical reactions into the substructure (piers and abutments) while allowing some movement.

Diaphragms provide transverse bracing between the girders. They transfer shear between girders and help distribute bending moments among them, so no single girder carries an excessive share of the load. This keeps the girders acting together as a stable, rigid system and helps resist lateral forces from wind or seismic, in addition to maintaining the correct spacing and alignment of the girders. The bearings at the ends still pass the resultant reactions into the substructure.

So, loads go into the girders, diaphragms share and spread those forces across the girder system, and bearings transmit the final reactions to the piers and abutments.