Cm coefficient calculation for B1 multiplier
Cm coefficient calculation for B1 multiplier
(OP)
Hi,
According to Appendix 8, cm = 1.0 when columns are subjected to transverse loading between supports. What is this transverse loading? What causes transverse loads? Is this from lateral loads (wind/seismic)?
According to Appendix 8, cm = 1.0 when columns are subjected to transverse loading between supports. What is this transverse loading? What causes transverse loads? Is this from lateral loads (wind/seismic)?






RE: Cm coefficient calculation for B1 multiplier
RE: Cm coefficient calculation for B1 multiplier
RE: Cm coefficient calculation for B1 multiplier
RE: Cm coefficient calculation for B1 multiplier
RE: Cm coefficient calculation for B1 multiplier
Gotta be careful with precise definitions. Lateral loads delivered to columns by a typical moment frame beam would not trigger Cm = 1.0. Wind loads delivered to a column by a girt on the other hand would.
Your best litmus test for Cm is as follows:
If your column shear diagram has a discrete step in it, Cm = 1.0. Otherwise, Cm is per the equation.
The trick here is to recognize that there's an important difference between
a) a case where the lateral load is just a load and;
b) a case where the lateral load is both a load and a lateral restraint.
I've tried to illustrate the difference below. With your truss / moment connection situation, the lateral load is also a lateral restraint (case B). As such you, can calculate cm (and your euler buckling loads) as the worst of these two cases:
1) a column spanning from the base plate to the truss bottom chord with Cm < 1.0 per the equation and a moment at the top.
2) a column spanning from the truss bottom chord to the truss top chord with Cm < 1.0 per the equation and a moment at the bottom.
This should improve things substantially. Naturally, it's business as usual when checking the other column axis.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Cm coefficient calculation for B1 multiplier