Lu for pemb
Lu for pemb
(OP)
I'm trying to determine Lu for the beam in a rigid frame (tapered web), the top flange is braced laterally with purlins at every 2.5 feet and the bottom flange is braced with diagonal braces spaced at approx every 9 feet. For LTB Lu is 9 feet since the bottom flange is in compression; however it would seem to me that the inplane buckling strength for the beam should govern for the axial capacity and Lu would be much higher. The span for the rigid frame is 120 feet with splices located approx 35 feet from each end column. Am I missing something?






RE: Lu for pemb
Am I missing something?
RE: Lu for pemb
RE: Lu for pemb
Mike McCann
MMC Engineering
RE: Lu for pemb
RE: Lu for pemb
RE: Lu for pemb
RE: Lu for pemb
RE: Lu for pemb
RE: Lu for pemb
1) SA Buckling (and frame buckling): The Design Guide would have you use the full length of the member (with a K=1.0 assuming you are using the Direct Analysis Method). But, the calcualtion of the buckling load would be more complex since the properties vary along the length of the tapered member. Though if you have approximately uniform axial load, you can just assume an "equivalent" moment of inertia for buckling calcualtions.
One of the tricky things with tapered frames is that it should be easier for frame buckling to control over weak axis buckling. Therefore, the Direct Analysis Method becomes (IMHO) a more viable option than traditional hand calcs.
2) Beam Buckling (LTB): This should be calculated for each unbraced segment using the properties of the beam at the halfway point of the unbraced segment. This should be calculated separately for each flange which experiences compression. In your case, therefore, the 2.5 feet would be used when investigating top flange LTB buckling. The 9 feet would be used when investigation bottom flange LTB.
FWIW: I am not an expert in metal buildings or tapered frames. So, it's not like I speak from years of knowledge. My experience is relatively new as my company (RISA) is considering incorporating this design / code check procedure into our programs.