Plate girders are large beams typically used in bridges and buildings, composed of a web plate and flange plates. The web plate, while providing depth for bending resistance, is often relatively thin, making it susceptible to instability under load.

Purpose of Stiffeners in Plate Girders

To address the potential instability of the thin web, stiffeners are incorporated. These are structural elements attached to the web plate. The primary functions of these stiffeners relate to enhancing the stability and load-carrying capacity of the web.

Vertical Stiffeners

Vertical stiffeners, also known as transverse stiffeners, are welded to the web plate perpendicular to the longitudinal axis. Their main role is:

• Preventing Web Buckling: They significantly increase the resistance of the web against buckling caused by shear stresses and compressive stresses induced by bending. They act like intermediate supports for the thin web panel, dividing it into smaller, more stable panels.
• Load Transfer: They can also help transfer concentrated loads or reactions applied to the flange into the web, preventing local web failure modes like crippling, although buckling prevention is their primary stiffening function.

Horizontal Stiffeners

Horizontal stiffeners, often called longitudinal stiffeners, are welded along the length of the web plate. Their primary purpose is:

• Preventing Web Buckling: They increase the critical buckling stress of the web, particularly effective in reducing buckling due to bending stresses and improving the overall shear buckling resistance of deep webs.

Analysis of Options

• To prevent yielding: Yielding is a material failure mode related to exceeding the elastic limit. Stiffeners do not directly prevent the base material from yielding; yielding is primarily controlled by the cross-section's material properties and dimensions.
• To decrease the deflection of the girder: While stiffeners add some stiffness, their primary function is not to reduce the overall deflection of the girder. Deflection is mainly controlled by the moment of inertia ($I$) of the entire girder section (flanges and web) and the applied load.
• To prevent web buckling: This is the key function of both vertical and horizontal stiffeners. They enhance the stability of the thin web plate against buckling instabilities under shear and bending stresses.
• To prevent web crippling: Web crippling is a failure mode occurring under concentrated loads where the web collapses locally. While vertical stiffeners can help transfer these loads, their principal role as stiffeners is to prevent buckling instability across the web panel. Buckling and crippling are distinct failure modes, though stiffeners primarily address buckling.

Therefore, the most accurate and primary reason for using both vertical and horizontal stiffeners in plate girders is to prevent the web from buckling.