AISC Available Moment vs. Unbraced Length Tables and LTB Equations

[Colfax]

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.

 

[Stenbrook]

Well, the charts (I assume you are referencing Table 3-10) are just a visual representation of the equations provided in Chapter F. If you plotted out the results for every beam for different lengths using the equations in chapter F, you will get Table 3-10. I would argue that if the equation provides you with a capacity that is higher than your required load, than the beam is sufficient, regardless of where the chart stops. It is not like the beam all of a sudden has 0 capacity just because it is 1 foot longer than where the chart stops. Also, the charts may stop for a particular beam because it may not be the most economical choice. Either way, in your case, the PE doesn't seem comfortable with that so you might have to just go with what he wants.

 

[Colfax]

I forgot to add that the loading case for this project has Cb= 1.34. I know the plots are just the equations plotted with Cb=1. Thank you for the quick response. I know what he says goes, I just wanted to know/understand his reasoning for this. He also said that I can use Cb up to the 24' where the plot stops and I searched and searched and couldn't find any reasoning for all of this. But you pointing out that it comes down to whether he is comfortable or not is enough justification. Thanks a lot!!

 

[Stenbrook]

The capacity in the charts can be multiplied by your Cb of 1.34 to obtain your actual capacity (to a certain extent). Most engineers that I know use Cb of 1.0 to remain a little conservative unless absolutely needed. I don't know of any rule that says you can't use Cb for greater than 24'-0" spans. If you look at section F2 in the 14th edition of the AISC manual, Cb is only used in equation F2-2. Once the beam becomes slender (Lb>Lr) then the moment capacity equation no longer includes that variable. Perhaps that is why he said that?

 

[Colfax]

The moment equation, F2-1 includes Fcr (F2-4) which incorporates Cb. The reasoning he gave me is that after the plot stops the beam twists too much. That's where I became really confused because I thought that is what the LTB equations considered. He is a super smart guy and I have the utmost respect for him, I just wanted to understand his reasoning. I'm sure you are correct that he wanted to be as conservative as possible. Thanks!!

 

[nutte]

He is absolutely incorrect. The tables are design aids. The tail is wagging the dog.

 

[JoshPlumSE]

I don't think it is unreasonable for the PE to not want to use the long slender beam and to prefer another. I just don't think he is doing a good job explaining his lack of comfort.

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).

 

[Colfax]

The deflection is << than L/450 and is even less than L/600.

 

[JoshPlumSE]

That sounds low, but what is the allowed deflection of the crane? And, could it potentially affect the operation of the crane? I recall seeing some cranes that had very strict deflection tolerances for proper operation.

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.

 

[Colfax]

The difference between the beam he wants to use vs. the beam I selected is his is a combination W+C shape and mine is just a W. All the calculations check out. The issue is the added labor to weld the cap and added cost of materials. I'm not trying to see who is wright and who is wrong, I was just seeing if there was a code or standard I was missing. I'm new to the structural side of things.

 

[Stenbrook]

Refer to Table 1-19 for Wide flange shapes with cap channels. I would assume that is what he is using for his beam and channel designations. ( I could be wrong though, hopefully someone will correct me if I am)

 

[nutte]

The deflection comments are irrelevant to the original question, which was can you go outside of the tables. Of course you can. Maybe the PE has a good reason for not wanting to, but his stated reason is flat out wrong.

 

[SJBombero]

I can not comment directly on the reasoning of the consultant PE. It did seem wrong to me to be using design aids to interpret the code. However, I can understand his decision if he has any experience in the crane industry. Capped WF have been a standard in the crane industry forever. On a monorail it is to try to limit the deflection without adding depth to the section. On a bridge crane, a capped beam is going to prevent the horizontal inertia forces from causing torsion in the beam. It would also make it stiffer in the horizontal plane. Every time you start or stop the bridge crane, there will be an inertia force on the wheels of the trolley.

 

[NOLAENG]

I'm with nutte. His reason doesn't make sense. If he's not comfortable, then he needs a different reason other than the graphs.
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.

 

[Colfax]

Thanks everyone!