Design of Steel Beams Dr. Bashar Behnam

General When a beam bends, the compression region above the neutral axis is analogous to a column and it will buckle if the member is slender enough. The compression portion/region of the cross section is restrained by the tension portion. The outer deflection (flexural buckling) is accompanied by twisting (torsion)

Lateral-Torsional Buckling This form of instability is called lateral-torsional buckling (LTB). Lateral-Torsional buckling can be prevented by bracing the beam against twisting at sufficiently close intervals. This can be accomplished two different types of stability bracing.

Stability Bracing There are two types of stability bracing Lateral bracing: X-bracing Concrete deck Torsional bracing: Cross frame Diaphragm

Lateral bracing X-Bracing Concrete Deck

Torsional bracing Cross-Frame Diaphragm

Failure Modes There are four failure modes The cross section of the beam becomes fully plastic. Lateral-torsional buckling (LTB), either elastically or inelastically. Flange local buckling (FLB), either elastically or inelastically. Web local buckling (WLB), either elastically or inelastically.

Classification of Shapes AISC Manual (American institute of Steel Construction) classifies cross-sectional shapes as: Compact Noncompact Slender

The unbraced length The unbraced length Lb is the distance between points of lateral support, or bracing. The moment strength of compact shapes is function of the unbraced length, Lb.

Nominal Flexural Strength If the unbraced length, no longer than Lp, the beam is considered to have full lateral support and the nominal flexural strength can be calculated

Nominal Flexural Strength If Lb is greater than Lp but less than or equal to the parameter Lr, then the strength is based on inelastic LTB.

Nominal Flexural Strength If Lb is greater than Lr, the strength is based on elastic LTB.