Introduction to beams

Introduction

In order to design a beam in accordance with the AISC code for steel design, 6 limit states must be considered. Theseare yielding, Lateral-Torsional Buckling, Web Local Buckling, Flange Local Buckling, Shear Capacity, andServiceability. Only when a beam satisfies these limit states can it be considered safe for public use.

The six limit states

• Yielding is the most common limit state and the first to address. It refers to the strength of thebeam to resist the largest possible moment that can be applied to the beam. Basically, it limits the beam frombending. Yielding depends on the load, the supports, the span of the beam, and the strength of the steel.
• Lateral-Torsional Buckling , the second limit state refers to the beam's ability to hold up againsttorsion, or twisting in the lateral direction. This limit state compares the lateral bracing to a maximum allowablebracing length. With adequate bracing, the beam will not twist into failure.
• The third limit state, Web Local Buckling refers to the strength of the web of a member in a beam to resist failure. Basically, the width and thickness of theweb must be large enough to withstand the loading conditions. This means the width-thickness ratio mustfall between certain limits so the web does not collapse or fail.
• The fourth limit state, Flange Local Buckling , is just like Web Local Buckling, except the limits are for the flanges of a member in a beam. Itrefers to the strength of the flange of a member to resist failure. The width and thickness of the flange must belarge enough to withstand the loading conditions. This means the width-thickness ratio must fall between certainlimits so the flange does not collapse or fail.
• Shear Capacity , the fifth limit state, usually is not the controlling limit state, except forbeams with very small lateral spans. The shear in the web of a beam must be limited so it does not exceed themaximum allowable shear.
• Serviceability , the final limit state, refers to the beam's deflection. The beam must be serviceable andnot deflect so much that vibrations can be a problem and should not deflect to a noticeable angle that people candetect and feel uncomfortable with.