AISC LRFD 1.pdf

Comm. C2.] FRAME STABILITY 189Buckled shape of column isshown by dashed line.Table C-C2.1K Values for ColumnsTheoretical K value 0.5 0.7 1.0 1.0 2.0 2.0Recommended designvalue when ideal conditionsare approximated0.65 0.80 1.2 1.0 2.10 2.0End condition codeinfluence of foundations, even where these footings are designed only for verticalload, can be very substantial in the case of flat-ended column base details with ordinaryanchorage. For this condition, a design K value of 1.5 would generally be conservativein Case (f).While in some cases masonry walls provide enough lateral support for buildingframes to control lateral deflection, light curtain wall construction and wide columnspacing can create a situation where only the bending stiffness of the frame providesthis support. In this case the effective length factor K for an unbraced length ofcolumn L is dependent upon the bending stiffness provided by the other in-planemembers entering the joint at each end of the unbraced segment. If the combinedstiffness provided by the beams is sufficiently small, relative to that of the unbracedcolumn segments, KL could exceed two or more story heights (Bleich, 1952).Translation of the joints in the plane of a truss is inhibited and, due to end restraint,the effective length of compression members might be assumed to be less than thedistance between panel points. However, it is usual practice to take K as equal to 1.0(ASCE Task Committee on Effective Length, 1997). If all members of the trussreached their ultimate load capacity simultaneously, the restraints at the ends of thecompression members would be greatly reduced.Several rational methods are available to estimate the effective length of the columnsin an unbraced frame with sufficient accuracy for design. These range fromsimple interpolation between the idealized cases shown in Table C-C2.1 to compre-LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION