stiffener
One of a series of angle bars, Z-bars, beams, rods, or other shapes, attached in order to provide them the necessary stiffness or rigidity.
stiffener: the reinforcement that stops things flexing
A stiffener is a structural member, usually a rolled steel shape like an angle bar, channel, or flat bar, welded or bolted to a primary structural component to increase its resistance to bending and lateral deflection. Where an unsupported steel beam or plate would sag or wobble under load, a stiffener prevents that movement by distributing stress over a larger area and increasing the effective moment of inertia.
Common stiffener profiles include equal or unequal angle sections, which are L-shaped in cross-section; vertical stiffeners (often called web stiffeners) welded perpendicular to beam webs to prevent buckling under compression; and horizontal stiffeners that control lateral-torsional buckling. Flat bar stiffeners work well for plate girders and column flanges. The choice depends on the direction of applied force, available space, and whether the stiffener must also carry shear load or merely provide geometric bracing.
In plate girder design, stiffeners are spaced at intervals along the web to prevent the thin steel plate from buckling outward when the beam is bent. For columns, stiffeners reinforce the web around connection points where concentrated loads would otherwise crush the steel or cause local buckling. Box columns and hollow sections use internal stiffeners to maintain shape under torsion and bending. The stiffener is typically welded directly to the primary member using fillet welds, and must be detailed to transfer its restraining force effectively.
Where stiffening fails
Stiffener inadequacy usually shows as local buckling in the web of an unstiffened plate girder, visible as wavy distortion. An undersized stiffener or one spaced too far apart cannot develop enough restraint. Poor weld quality at the connection between stiffener and primary member means the stiffener does not actually transfer force and becomes cosmetic. Stiffeners must be long enough to engage the full depth of the member they support, and positioned so that their geometry matches the applied loads.
The terminology reflects the function directly: a stiffener stiffens. In modern practice, finite element analysis helps engineers optimize stiffener size, spacing, and orientation to meet buckling criteria while minimizing weight and cost. Where fabrication tolerances are loose or material quality varies, more frequent or larger stiffeners are added as insurance against field problems.