Torsion properties of Built-up W-sections
Torsion properties of Built-up W-sections
(OP)
A current project in our office has required us to size a very large W-section. A height restriction of 24" has been set by the architect to make the beam more "interesting" for us. The beam spans 45' and with its cont. and point. loads must resist a 154K-ft moment. We have just recently chosen to design a built-up beam that will consist of (2) W21x147 beams w/ a cont. 1/2" plate above and below. One of the point loads is 77k and must be set into the webs of one of the beams, thus generating a torsional moment on the beam. We have treated the built-up beam as an open and closed section while generating its properties. We have been looking for additional information on such built-up/composite sections as we have not been able to calculate what we feel is an acceptable F.O.S.
Does anyone have any experience with any beams of similar construction? Does anyone have additional concerns for the performance of the beam (welds, web stiffeners etc.)?
Thank you
Does anyone have any experience with any beams of similar construction? Does anyone have additional concerns for the performance of the beam (welds, web stiffeners etc.)?
Thank you





RE: Torsion properties of Built-up W-sections
RE: Torsion properties of Built-up W-sections
RE: Torsion properties of Built-up W-sections
Once you make sure that your connections can take the torsion out to the supporting structure, you're in business.
RE: Torsion properties of Built-up W-sections
RE: Torsion properties of Built-up W-sections
Calculate the J for the closed section JAE and JedClampett refer to, in absence of more exact way, assume a box.
The welds from the closing plates need pass the shear flow in code terms.
Then you will get your (approximate yet reasonably acceptable) safety factor.
Most likely your check is one with moment, anc eccentric shear, hence torsion. If you don't remember how to exactly proceed one of us might give further advice.
RE: Torsion properties of Built-up W-sections
I had placed a more detailed explanation of the situation on the following thread in the general structural area:
Thread507-69213 some of you may have a stand on using the total section (inluding the flanges) for the combined stress situation from bending and torsion. The welds and the webs are the critical sections as the shear flow carries through the weld and the webs are critical with the warping shear generated from the eccentric loads.
JAE-
great point with regard to the additional stresses in the plates. We currently have some web stiffeners detailed at the locations where a beam hangs into the box girder.
ALthough not a "HEAVY SECTION", do you guys have any welding concerns with the material at these locations for a W21 x147 ?
thanks again
ecastle
RE: Torsion properties of Built-up W-sections
RE: Torsion properties of Built-up W-sections
Please correct me if I'm wrong, cause I'm facin an analoge situation here.
" For the torsion strenght I would take in account only the section properties of an equivalent RHS formed by the two W's inner half flanges and their respective webs.. but this aproach depends on the welding arrangement utilized to join the W's and the cover plates. When dealing with open
shapes, the torion can be "broken down" into St. Venant torsion (pure shear stresses) and Warping torsion (both shear stresses and axial/bending stresses as result you avoid to get in a very complex issue."
You should check AISC's Design Guide 9 and also Salmon & Johnson here you can find a very good theory discussion on
torsional stresses.
See also SCI's advisory 249 Design of Members Subjected to Torsion