AISC LRFD 1.pdf

198 STABILITY BRACING [Comm. C3.ing, etc. The brace force recommendations for frames, columns and beam lateralbracing are based on an assumed o = 0.002L, where L is the distance between adjacentbrace points. For torsional bracing of beams, an initial twist angle, o ,isassumed where o = 0.002L/h o , and h o is the distance between flange centroids. Forother o and o values, use direct proportion to modify the brace strength requirements,P br and M br . For cases where it is unlikely that all columns in a story areout-of-plumb in the same direction, Chen and Tong (1994) recommend an average∆ o = 0. 002L/ n o where n o columns, each with a random o , are to be stabilized bythe brace system. This reduced o would be appropriate when combining the stabilitybrace forces with wind and seismic forces.Brace connections, if they are flexible or can slip, should be considered in the evaluationof the bracing stiffness as follows:1 1 1= + (C-C3-2)βact βconn βbraceThe brace system stiffness, act , is less than the smaller of the connection stiffness, conn , or the stiffness of the brace, brace . Slip in connections with standard holes neednot be considered except when only a few bolts are used. When evaluating the bracingof rows of columns or beams, consideration must be given to the accumulationof the brace forces along the length of the brace that results in a different displacementat each beam or column location. In general, brace forces can be minimized byincreasing the number of braced bays and using stiff braces.3. ColumnsFor nodal column bracing, the critical stiffness is a function of the number of intermediatebraces (Winter, 1958 and 1960). For one intermediate brace, i =2P/L b ,and for many braces i =4P/L b . The relationship between the critical stiffness andthe number of braces, n, can be approximated (Yura, 1995) as i = N i P/L b , where N i=4–2/n. The most severe case (many braces) was adopted for the brace stiffnessrequirement, br =2 4P/L b . The brace stiffness, Equation C3-6, can be reduced bythe ratio, N i /4, to account for the actual number of braces.Fig. C-C3.4. Definitions.LRFD Specification for Structural Steel Buildings, December 27, 1999AMERICAN INSTITUTE OF STEEL CONSTRUCTION