LTB if support is at the load?
LTB if support is at the load?
(OP)
I have a long, simply supported beam with a single load at the center (like a fixed gantry hoist). It seems like if I provide full lateral support at the ends and the load point, that I wouldn't have to make any concessions for lateral torsional buckling. Is there any justification for this in AISC?
RE: LTB if support is at the load?
RE: LTB if support is at the load?
You'd also have support for weak-axis loads from the brace-point
RE: LTB if support is at the load?
RE: LTB if support is at the load?
RE: LTB if support is at the load?
Your instincts aren't too far off with that. While your setup doesn't preclude LTB, it is generally quite favorable to have bracing at the point of load application. If you do the k = L/2 evaluation with with an accurate assessment of Cb, I expect that your results will reflect this.
Your setup relies on the load itself to, effectively, brace the beam bottom flange laterally at the point of load application. I suspect that's fine but I wouldn't mind running an FEM buckling analysis on it for confirmation just the same. Or just run a real brace down to the bottom flange for good measure.
RE: LTB if support is at the load?
RE: LTB if support is at the load?
Usually? It's kind of a tricky question to answer that robustly.
Imagine if your beam were really the top chord supported steel joist below. Here, we can agree that the joist would roll over catastrophically, right? The bottom chord would kick out laterally?
Next, ask yourself, what about the your real beam prevents this same outcome? I would say that it's two things:
1) The beam bottom flange acting like a girt to restrain itself laterally at the point of load application. Calling this beam torsional resistance is just another perspective on the same thing.
2) The load being applied below the top flange such that a couple is formed that would tend to resist the bottom flange kicking out laterally. This you've probably got in spades too if I'm envisioning your situation correctly.
Both of these mechanisms are real, no doubt. But neither usually gets evaluated explicitly and neither is as "positive" as positive lateral bracing to the bottom flange would be. That's why , for something that feels as though it might be pretty high stakes, I wouldn't mind some positive lateral bracing to the bottom flange.
RE: LTB if support is at the load?
RE: LTB if support is at the load?