Single-Angle Member in Compression by AISC

[J1D]

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?

 

[blake989]

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.