AISC LRFD 1.pdf

184Comm. CCHAPTER CFRAMES AND OTHER STRUCTURESC1. SECOND ORDER EFFECTSWhile resistance to wind and seismic loading can be provided in certain buildingsby means of shear walls, which also provide for overall frame stability at factoredgravity loading, other building frames must provide this resistance by frame action.This resistance can be achieved in several ways, e.g., by a system of bracing, by amoment-resisting frame, or by any combination of lateral force-resisting elements.For frames under combined gravity and lateral loads, drift (horizontal deflectioncaused by applied loads) occurs at the start of loading. At a given value of theapplied loads, the frame has a definite amount of drift . In unbraced frames, additionalsecondary bending moments, known as the P moments, may be developedin the columns and beams of the lateral load-resisting systems in each story. P is thetotal gravity load above the story and is the story drift. As the applied loadincreases, the P moments also increase. Therefore, the P effect must often beaccounted for in frame design. Similarly, in braced frames, increases in axial forcesoccur in the members of the bracing systems; however, such effects are usually lesssignificant. The designer should consider these effects for all types of frames anddetermine if they are significant. Since P effects can cause frame drifts to be largerthan those calculated by ignoring them, they should also be included in the serviceload drift analysis when they are significant.In unbraced frames designed by plastic analysis, the limit of 0.75 c P y on columnaxial loads has been retained to help ensure stability.The designer may use second-order elastic analysis to compute the maximum factoredforces and moments in a member. These represent the required strength.Alternatively, for structures designed on the basis of elastic analysis, the designermay use first order analysis and the amplification factors B 1 and B 2 .In the general case, a member may have first order moments not associated withsidesway which are multiplied by B 1 , and first order moments produced by forcescausing sidesway which are multiplied by B 2 .The factor B 2 applies only to moments caused by forces producing sidesway and iscalculated for an entire story. In building frames designed to limit oh / L to a predeterminedvalue, the factor B 2 may be found in advance of designing individualmembers.Drift limits may also be set for design of various categories of buildings so that theeffect of secondary bending can be insignificant (Kanchanalai and Lu, 1979, andATC, 1978). It is conservative to use the B 2 factor with the sum of the sway and theno-sway moments, i.e., with M lt + M nt .LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION