AISC Available Moment vs. Unbraced Length Tables and LTB Equations
AISC Available Moment vs. Unbraced Length Tables and LTB Equations
(OP)
I work for a crane manufacturer and the PE we work with and I are at odds over a recent project. The beam I selected for the project checks out with all the standards and he agrees. The problem is that the plot of the beams available moment vs. unbraced length stops at 24' in the AISC manual and the required length is 28'. He says that if the plot stops in the manual, the beam can't be used. Utilizing equations F1-1, F2-4, and F2-1 from the AISC manual, the available moment exceeds the required. The manual says the beam plots stop arbitrarily at a span to depth ration of 30 in most cases. My question is, which governs? Do the available moment vs unbraced length tables or the LTB equations? I am not trying to be disrespectful to the PE, I am just trying to get a better understanding of this.






RE: AISC Available Moment vs. Unbraced Length Tables and LTB Equations
RE: AISC Available Moment vs. Unbraced Length Tables and LTB Equations
RE: AISC Available Moment vs. Unbraced Length Tables and LTB Equations
RE: AISC Available Moment vs. Unbraced Length Tables and LTB Equations
RE: AISC Available Moment vs. Unbraced Length Tables and LTB Equations
RE: AISC Available Moment vs. Unbraced Length Tables and LTB Equations
At these much longer slenderness levels, you get elastic buckling, which is quicker and more sudden. Granted, I believe the formulas contain some extra safety factors for this. Also, he may be concerned about the slenderness for other serviceability related reasons. Deflection will likely be larger during service level loading and he might prefer to keep in down... because ultimately he is going to be blamed if there are deflection serviceability problems (or perceived problems).
RE: AISC Available Moment vs. Unbraced Length Tables and LTB Equations
RE: AISC Available Moment vs. Unbraced Length Tables and LTB Equations
What about the lateral deflection?
What about vibration?
Not trying to justify the engineers argument. Just saying that there could be a valid argument there.
Also, how much weight difference is there between the beam he wants to use and this one? If it's relatively close, then why argue? I remember working on jobs that were fast track jobs where they were spending hundreds of millions of dollars on mechanical equipment, then the structurals got leaned on to save a few thousand dollars. Didn't make much sense to me.
RE: AISC Available Moment vs. Unbraced Length Tables and LTB Equations
RE: AISC Available Moment vs. Unbraced Length Tables and LTB Equations
RE: AISC Available Moment vs. Unbraced Length Tables and LTB Equations
RE: AISC Available Moment vs. Unbraced Length Tables and LTB Equations
RE: AISC Available Moment vs. Unbraced Length Tables and LTB Equations
I've had to make my own spreadsheet to generate the graphs in order to check an existing beam that was past the graph limits.
For a side rant, I've also made the spreadsheet so I can actually see which line is which.
RE: AISC Available Moment vs. Unbraced Length Tables and LTB Equations