Torsional analysis of a beam with web stiffeners 5

[jimrcasper]

I am trying to analyse a wide flange section with web stiffners to verify that it will be able to accept a torsional load. The W14X30 currently has 1/4" plate each side at 48" o.c. AISC Steel design guide 9 "Torsional Analysis of Structural Steel Members" does not address this situation. Does anyone know of a reference that does address situation? And/or software?

 

[1prsplmps1]

jimrcasper: I do not know how precise you wanted to be but SAP2000 is a good software. In addition, you can divide your beam+stiffener in several sections (x section wise) to calculate geometric properties of the whole system, stresses, angle of twist, etc. Any mechanics of material book can help you w/ the procedure..

1prsplmps1

 

[UcfSE]

I don't think you can account for the stiffeners with regular analysis. You would have to take all those equations and solve them with the condition that the cross section won't warp at the stiffener location but will warp everywhere else. That's why it isn't included in the design guide. Even though you stiffen the section somewhat it really won't help that much. Your other option is an intensive FEA. Stiffeners at 48" o.c. won't help much anyway. Your flanges will still bend laterally regardless of stiffeners. Just design it as in DG9.

 

[JAE]

Stiffeners don't add anything to the torsional stiffness or to the torsional capacity of a wide flange. That is why there isn't anything in the AISC design guide.

 

[1prsplmps1]

Actually stiffeners for steel WF lintels supporting eccentric siding loads had been used to increase the warp coefficient (@ the stiffeners location) and therefore to increase a bit the torsional capacity of the WF. ASCE have few journals on the subject..

 

[JAE]

corrected.....I should have said "Stiffeners don't add much at all..."

 

[VirtualEngineer]

Usually when I run into a torsional situation, I try to get two members resisting the load.

It becomes a simple exercise with vertical loads acting upon two separate beams.

Another solution is to weld a continuous plate up against the toes of the flanges thereby adding a second web parallel to the first. Then the torsion can be developed similarly to a HSS section.


Regards,


JPJ

 

[bonjoy]

There's a book titled "Design of Welded Structures" published by the James F. Lincol Arc Welding Foundation (Cleveland, Ohio) that deals with torsion.

Basically, the principles in there are:

1. Avoid torsion
2. If you can't avoid it, then use a closed shape (i.e. HSS) with rigid end connections

In other words, W, C, S, L shapes etc are all lousy at resisting torsion. Think of a paper towel tube with a cut along the length.

If all else fails, "stiffener plates" welded to the web on a 45 degree angle would help a little

JC

 

[BATman2]

The book bonjoy refers to is excellent and I agree with his post. Be sure to read the section about torsional rigidity, it is not the same as the sum of the moments of inertias,in fact the rigidity is much less. Good examples are shown in the book. (It used to be really cheap to buy)

NZ

 

[JAE]

bonjoy - good analogy to the paper tube. My Mechanics prof used a different analogy to visualize relative torsional stiffnesses.

If you look at the cross section of the member and visualize it as a cut opening in a flat sheet of plastic, then dip the flat sheet into a tub of soap, then induce a little air pressure on one side of the sheet, you will get a slightly bulging soap bubble.

The volume under the soap bubble is a measure of the relative torsional stiffness. So a pure pipe would have a circular cut-out in the sheet that leaves a full hole equal to the outside dia. of the pipe. The soap bubble would be large. If you cut a slit in the pipe lengthwise, you would only have a circular cut representing the 359 degrees of pipe wallFor a wide flange, you only have small little bulges along the individual plates and the volume is small.

 

[JAE]

Like these...

 

[VirtualEngineer]

"Design of Welded Structures" published by the James F. Lincoln Arc Welding Foundation is a great book.

Section 2.10 "Designing for Torsional Loading" covers the torsional subject extensively.

It is available from the Lincoln Arc Welding Foundation at give-it-away-for-almost-nothing prices. The cost is something like $5.00 for a printing cost and a small additional shipping cost.

It has something like 600 pages of heavy duty stuctural design concepts with examples shown.

Highly recommended for evey engineering library.

It even mentions the membrane concept JAE is talking about.


Regards to All,


JPJ

 

[JAE]

Here's a link discussing the real technical end of it...

http://ocw.mit.edu/NR/rdonlyres/Aer...39AA-752A-47C0-B037-8D379615F602/0/unit11.pdf

 

[AUCE98]

JAE,

The theory that you were discribing in the soap bubble analogy is refered to as Prandtl's Membrane Analogy, lots of fun.

Regards,

AUCE98

 

[1prsplmps1]

JAE: Great analogy and great link, thanks!!

 

[jimrcasper]

Thank you for all your help. I finally received the plans, and 14X30 member has existing 1/4" single diagonal bracing.I am trying to use "Design of Welded Structures" nomograph on page 2.10-7
In this instance is "b" the height of the memeber(14"), the diagonal lenght of the member(19") or the width of the member (6.75")? I understood it to be the last case.

The book also refers to the torsional rigidity as being 3.54I, what is the "I" in this stiuation? The plates are at 45 degrees so there is no bending, just tension or compression. Thank you in advance for all your assistance.

 

[UcfSE]

See Problem 9 on 2.10-20 for "I" explanation by the example. I think if you read further, like the whole section, you'll probably find your answers. Let us know if you don't and we will try to help out. Are there engineers in your office who can help with this design?