torsion on cold formed steel built-up box members
torsion on cold formed steel built-up box members
(OP)
Does the AISI contain any provisions for designing for torsion loads on cold formed steel built-up box members?
My situation is a built-up window sill member. The sill supports a large two-story aluminum window. The sill receives vertical dead load and horizontal wind loads primarily from three window mullions, one near each end of the sill and one at mid-span of the sill. I have designed the sill for the vertical and horizontal loads, however, the architectural detail has the window framing outside the plane of the metal stud wall, supported on wood blocking that is in-turn attached to the outside face of the cold formed steel box member. I am concerned about torsion on the box member from the vertical dead load as a result of this loading eccentricity.
I am inclined to suggest a revision to the window detail to frame the window in the plane of the wall, so that the sill receives the vertical loads directly from the window without any (or with minimal) eccentricity, but I don't think that is going to fly with the architect. Otherwise I am inclined to suggest a different framing method for the sill (and probably the jambs), such as a hot rolled steel HSS member instead of cold formed steel box member.
Any thoughts or suggestions?
My situation is a built-up window sill member. The sill supports a large two-story aluminum window. The sill receives vertical dead load and horizontal wind loads primarily from three window mullions, one near each end of the sill and one at mid-span of the sill. I have designed the sill for the vertical and horizontal loads, however, the architectural detail has the window framing outside the plane of the metal stud wall, supported on wood blocking that is in-turn attached to the outside face of the cold formed steel box member. I am concerned about torsion on the box member from the vertical dead load as a result of this loading eccentricity.
I am inclined to suggest a revision to the window detail to frame the window in the plane of the wall, so that the sill receives the vertical loads directly from the window without any (or with minimal) eccentricity, but I don't think that is going to fly with the architect. Otherwise I am inclined to suggest a different framing method for the sill (and probably the jambs), such as a hot rolled steel HSS member instead of cold formed steel box member.
Any thoughts or suggestions?






RE: torsion on cold formed steel built-up box members
BA
RE: torsion on cold formed steel built-up box members
RE: torsion on cold formed steel built-up box members
The sill has short cripple studs below it. What do you think of this load path?: 1) Call for wood blocking to be at least as deep as the box member (in this case 6"). 2) Attach the blocking to the outside vertical web of the the box member with enough screws to transmit the vertical load to the web of the outside vertical member of the box via screw shear. 3) Assume the vertical load is then evenly distributed to the cripple studs in compression.
There would still be an eccentricity and thus a moment applied to the cripple studs since they would be loaded near their outside flanges only, but I could check for that and possibly design a stud to resist the moment.
I do not want to go this route, but it's a thought. Do you think it is a viable load path to transmit the vertical load to the cripple studs?
RE: torsion on cold formed steel built-up box members
BA
RE: torsion on cold formed steel built-up box members
A sketch showing all the loads, details and dimensions, might save you ten thousand words which don’t really get the picture across. And, it might get you some good ideas to help you solve your problem. Supporting a two story window with a middle mullion, which is supported by a built-up light gage steel window sill, which is cantilevered out over some cripple studs below, and is finally supported on wooden blocking btwn. the sill and the cripple studs... is that about right? Sounds about like supporting a brick wall on a 2" thick layer of sponges and wondering why the wall is moving and cracking.
RE: torsion on cold formed steel built-up box members
DaveAtkins
RE: torsion on cold formed steel built-up box members
RE: torsion on cold formed steel built-up box members
FWIW, I do a lot of CFS design and Arch's are always showing boxed beams where they should not be used. They also always come up with goof details like this as well.
How tall and wide is the window?
RE: torsion on cold formed steel built-up box members
DaveAtkins
RE: torsion on cold formed steel built-up box members
I was not intending to "flagpole" the cripples of the foundation wall (i agree, a bad idea). Rather, introduce the moment into the cripple and let it be resolved with a couple from the light gage sill (spanning between kings) and the floor connnection (in shear)
RE: torsion on cold formed steel built-up box members
Would you care to elaborate why torsion should not be combined with cold formed sections? And why hot rolled sections + torsion are OK ?
RE: torsion on cold formed steel built-up box members
p.s. Sorry, I tried to upload a sketch yesterday, but was not successful.