Spandrel Beams

[StructuralMike]

I have a very typical situation but cannot find any good practical information on design. I have a spandrel beam at a stair tower that must support a brick facade which is eccentric to the beam. With the opening from the stairwell there is no place to install bracing to counter the torsion. So I have no other option than to design the beam for torsion. However the loads are pretty high and the eccentricity is over 12" which makes a wide flange not very efficient. So I want to either use a tube or weld plates on the side of the beam to make a tube which is more efficient. But I cannot find any (practical) design methods for designing the tube. AISC Design Guide 9 makes discussion on tubes but typically Cw is zero which makes "a" zero and makes the tables useless. Is there any practical solution for such a typical problem?

 

[Galambos]

Mike,

look further into section 4.4 for closed sections. if you still need help, let me know.

ted

 

[UcfSE]

Did you see the notes in sections 4.4 and 4.6.2 of the design guide, and section H.3 of the 2005 AISC spec? They don't have examples but there is some explanation of closed section response to torsion.

 

[StructuralMike]

I have reviewed section 4.4 and 4.6. The equations seem to be for concentrated torsion. The example in the Design Guide 9 is also for concentrated torsion. I contacted AISC-Solutions requesting some examples on uniform torsion but they seem to come back and say the guide was made for wide flanges and not tubes and are pretty much leaving me hanging. They have sent me some papers but none address the problem.

 

[Galambos]

mike,

these sections are written for the general case of torsion and torsional stress. notice Tu is used to find the shear stress at a point, just like we find the bending stress for a given value of Mu along the length of a beam.

 

[Galambos]

mike,

also, refer to your mechanics of materials textbooks.

 

[samdamon]

Mike- There are a variety of ways to carry the torsion. Trying to do it just with a bare wf is usually inefficient. One way I have used is design a vertically oriented plate to be welded to the top and bott flange of the beam. You are essentially turning the wf into a tube. The closed section is very efficient at resisting torsion. Leave the plate short a few inches at each end of the spandrel and restrain the top and bot flanges of the wf with clip angles and finger tight bolts, slotted so they dont act as a major axis moment connection.

For addtnl data on torsion design suggest "Design of Welded Structures" by Blodgett.

 

[J1D]

Mike
If you make the spandrel beams to be closed sections, the calculation is very simple. In addition to other stresses as a beam, you just add the torsional shear stress as per Eq. (4.4) of the design guide. A critical place is at the ends, you need to make the closed section all the way into the columns.

Other than spandrel beams with eccentric load, I don’t know if you can have separate beams for the facades, for example, channels under the wall and laterally ties to the spandrel beams.

Another thought: can you make the spandrel beam to be offset as well to suit the facade?

 

[StructuralMike]

Samdamon,

I read the Blodgett book over the weekend and it has the exact case I have. Its approach is a little different then the AISC design guide but it works and is very practical.

Thanks all of the good info.