What is a stiffened girder?
A stiffened girder is a girder whose purpose is to give a structure sufficient stiffness and stability. In Dutch it is called a verstijvingsligger. It is often a steel girder, such as an I-section, that is itself additionally stiffened with welded plates. An ordinary I-section is strong for loads perpendicular to the flanges, but can deform under large or concentrated forces from other directions. Stiffeners prevent that.
Types of stiffener
- Vertical (transverse) stiffeners: a steel plate between the top and bottom flanges, perpendicular to the web. These prevent the thin web from buckling or crippling, especially at supports and under point loads such as a column standing on the girder.
- Horizontal or longitudinal stiffeners: a strip or angle welded lengthwise onto the web. These are used mainly on deep plate girders, where the web is tall and thin.
What does stiffening protect against?
- Web buckling due to shear forces or concentrated loads.
- Lateral-torsional buckling: the sideways deflection and twisting of a slender girder.
- Deflection and vibration of the structure as a whole, for example in a bridge deck. In suspension bridges, the girder in the deck that distributes the loads over the hangers is also called a stiffening girder.
Application
You see stiffened girders in steel structures of industrial halls and bridges, and in heavy steel beams in homes, for example a steel beam over an opening that carries a column or wall. The stiffening rib on a strip footing is also a stiffened girder, in that case in concrete.
Tips and points of attention
- Do not weld or drill a steel girder yourself without the structural engineer’s approval: a hole or weld in the wrong place can weaken the web.
- Look at the bearing: for a steel beam in a home, the structural engineer decides whether stiffeners are needed at the supports.
- Fire resistance and rust: like any steel girder, a stiffened girder must be given fire-resisting cladding where necessary and protected against corrosion.
Related terms
- Girder
- I-section
- Plate girder
- Lateral-torsional buckling
- Stiffening rib
Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.