single-angle member in compression by aisc
 

single-angle member in compression by aisc
i wonder which radius of gyration of cross-section, rx, ry, or rz, should be used in the calculation per aisc, lrfd for single-angle members?
the question is not answered from the specification. from the commentary c4 of the specification, it seems to me that since the flexural-torsional buckling strength calculation is approximated by the reduction factor q, the calculation of lamda in eq. 4-1 and 4-2 is from either rx or ry, not rz. however, in example e4.3 of aisc manual of steel construction (lrfd), rz was explicitly used, which resulted in a design strength of 28.7 kips.
according to the commentary c4 again, for more conservative values, the strength can be calculated as per appendix e of aisc lrfd, which directly considers the flexural-torsional buckling (but is more mathematically difficult). funny thing is that i obtained higher strength from the supposed-to-be-conservative approach. for the single angle in the example e4.3, the strength i got is then 42 kips.
any comments?
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here is how i handle angles, you are right, neither rx or ry are used. we must not have the same lrfd editions, but when i work with single angle compression members i calculate an rw and rz. major axis w-w, and minor axis z-z. in lrfd second edition i use example 3-8, in asd nineth edition example 10 p. 3-55.