What is the role of diaphragms in a steel girder bridge?

• A. Diaphragms transmit vertical loads directly to the foundations.
• B. Diaphragms primarily resist wind loads on the deck.
• C. Diaphragms transfer shear and distribute bending moments among girders, while bearings transfer reactions to the piers/abutments.
• D. Diaphragms are used only for cosmetic spacing.

Answer: (C)

Diaphragms act as cross-bracing that ties the main girders together and forms a more rigid deck frame. Their main job is to transfer shear from the deck and distribute bending moments among the girders so that no single girder carries an excessive portion of the load. This collective action also helps resist lateral movement and torsion, keeping the girders aligned and stable under service loads and wind.

Bearings, on the other hand, handle the reactions that the bridge frame transfers to the supports (piers and abutments) and accommodate movements due to expansion, contraction, and settling. So the diaphragms’ primary role is to share and transfer internal forces between girders, while bearings carry reactions to the foundations.