AISC LRFD 1.pdf

190 FRAME STABILITY [Comm. C2.hensive analytical procedures. Once a trial selection of framing members has beenmade, the use of the alignment chart in Figures C-C2.2a and b affords a fairly rapidmethod for determining adequate K values. However, it should be noted that thisalignment chart is based upon assumptions of idealized conditions which seldomexist in real structures (ASCE Task Committee on Effective Length, 1997). Theseassumptions are as follows:(1) Behavior is purely elastic.(2) All members have constant cross section.(3) All joints are rigid.(4) For braced frames, rotations at opposite ends of beams are equal in magnitude,producing single-curvature bending.(5) For unbraced frames, rotations at opposite ends of the restraining beams areequal in magnitude, producing reverse-curvature bending (inflection point atthe beam midspan from lateral loading only).(6) The stiffness parameter L P/ EI of all columns are equal.(7) Joint restraint is distributed to the column above and below the joint in proportionto I / L of the two columns.(8) All columns buckle simultaneously.(9) No significant axial compression force exists in the girders.The alignment chart for sidesway uninhibited shown in Figure C-C2.2b is based onthe following equation:2G AGB( π/ K)−36( π/K)− = 0(C-C2-1)6( G A + GB)tan ( π / K)Fig. C-C2.1. Column effective length.LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION