What is the typical role of diaphragms and stiffeners in concrete-encased sections or box girders?

• A. They provide lateral stability, transfer shear between walls/girders, and maintain alignment under loads.
• B. They support vertical loads only.
• C. They increase torsional flexibility.
• D. They are decorative elements.

Answer: (A)

Diaphragms and stiffeners act as internal bracing that ties the hollow cross-section together. Their main role is to provide lateral stability, transfer shear between opposite walls or girders, and keep the walls and webs aligned under loading. By linking the faces into a closed, stiffer box, they enhance torsional resistance and prevent local web buckling where compression or shear is high. They are not primarily for vertical load support, they do not increase torsional flexibility, and they are not decorative.