Help with 2005 AISC Manual Chapter F
Help with 2005 AISC Manual Chapter F
(OP)
I want to design a built up beam...say a W24x104 with a W16x26 turned horizontally and welded to the top flange (compression flange). Since the W24x104 has both compact flange and web, should I use the equations in Section F4 of the Steel Manual and use the new section properties of the combined section?






RE: Help with 2005 AISC Manual Chapter F
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RE: Help with 2005 AISC Manual Chapter F
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Help with 2005 AISC Manual Chapter F
RE: Help with 2005 AISC Manual Chapter F
While the torsional resistance, and therefore LTB resistance, of the combined section is meaningfully improved by the addition of the two W16x26 flanges, it's still an open section from a torsion perspective and I don't think that you can necessarily assume that the section is fully braced. I think that the most technically correct way to evaluate the member is still as a singly symmetric section as JAE has suggested.
When I did this, it was late in the evening and I was short on noodling time. I took this shortcut which I still feel is valid:
1) I designed the main beam (W24 for you) such that it worked for strength for all failure modes other than LTB. I assumed full compression flange bracing for LTB.
2) I used the combined section properties for deflection. I had a section builder app so this was easy for me.
3) I figured out the axial force in the compression flange of the main beam assuming that the secondary member (W16 for you) was absent. I then checked the secondary member as a column for that same load. I treated that fictional column as though the weak axis and torsional buckling lengths were zero and the strong axis buckling length was the distance between lateral supports for the combined section (the entire beam length in my case).
The sketchy part of this poor man's LTB check is whether or not the fictional column can be considered to be continuously braced for the torsinal buckling mode. While it takes very little to provide that bracing, torsional buckling of the fictional column and LTB of the combined section are similar phenomena.
I find the weld requirements to be of interest in this situation. In particular:
1) Obviously, you have to satisfy VQ/I demands. Additionally, there's a weld demand created by the need for the members to act compositely when they try to LTB buckle together. I don't know how to put a number to that. I've seen methods proposed to deal with this for built up columns but I'm unsure how to translate that to this scenario. I'm confident that the demand would be pretty small compared to VQ/I requirement though.
2) After welding -- and even without welding -- the combined section has a residual stress pattern that doesn't really match anything that the code flexural capacity checks were developed for. I think that this is generally ignored.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Help with 2005 AISC Manual Chapter F
The application of this beam that I want to design is for pure flexure only, no torsion, applied axial force.
The moment using the ASD load combinations is 850 kip-ft. Per Table 3-10, a W27x161 is good for 702 kip-ft @ Lb=38’-0”. Unfortunately a W27x161 is not a shape that is readily available via warehouse at the project location so my project manager said try a lighter sections, slap a member on the top flange like a carrier girt and it call it fully braced. The reason for using a W24x104 is because this member fully brace get me a moment of about 720 kip-ft however I am not fully convinced by adding another wide flange turn horizontally on the compressing flange can make the top flange fully braced. Lp is a function of the radius of gyration of the compression flange plus one-third of the web area in compression.
What I am unsure about is how to check the LTB for this combined section for the relatively large moment it will see. All the other checks are pretty straight forward (i.e. yielding, FLB TFY, sidesway…blah blah blah).
In a nutshell, the contractor literally demo’d everything plus the kitchen sink, now they don’t have steel to hoist up heavy equipment. Now I need to design a beam with an Lb of 38’-0” for the contractor to use as a hoist beam.
RE: Help with 2005 AISC Manual Chapter F
RE: Help with 2005 AISC Manual Chapter F
Another approach would be to choose sections that result in a combined Iy greater than Ix. That would preclude LTB. Maybe a little truss on top.
Is there no way to get a rotational brace on this thing at mid span? Maybe a channel welded to the top flange and extended to the roof?
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Help with 2005 AISC Manual Chapter F
RE: Help with 2005 AISC Manual Chapter F
This is similar to what you are doing with a W16 instead of a channel cap so I would see this as somewhat applicable.
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RE: Help with 2005 AISC Manual Chapter F
RE: Help with 2005 AISC Manual Chapter F
Is the hoist load applied to the bottom flange of the W24? If so, then you should gain at least an additional 15% of flexural capacity based on the load being applied below the shear center.
RE: Help with 2005 AISC Manual Chapter F
The only ways that I'd be satisfied with the Appendix 6 checks would be:
1) If they were in addition to other checks that looked at the composite section or;
2) The connection between beams was detailed to permit longitudinal slip between them. Say, intermittent angles used as keepers.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.