×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Rectangular HSS beam with AXIAL load and slab on top flange -- what is Ly?

Rectangular HSS beam with AXIAL load and slab on top flange -- what is Ly?

Rectangular HSS beam with AXIAL load and slab on top flange -- what is Ly?

(OP)
Hi,

I'm designing a 12-ft long rectangular HSS beam (say, HSS 12x4) with axial load and the beam is also connected to a slab through shear studs on the top flange. So you can see the top flange is well restrained.

For axial load check, do I consider the HSS unbraced in the weak direction? Or should I assume the slab on top flange is fully bracing the HSS in weak direction?

Any reference to AISC literature is welcome.

RE: Rectangular HSS beam with AXIAL load and slab on top flange -- what is Ly?

I would consider it fully braced.

BA

RE: Rectangular HSS beam with AXIAL load and slab on top flange -- what is Ly?

I would consider it unbraced,

or alternatively, include a torsion calc that checks the HSS for torsion induced by the buckling load being applied at the centroid and the resistance at the edge (in your example it would be an eccentricity of 6").

The easiest way in my eyes would be to calculate the Cr as unbraced. Calculate the Mr as braced. Then use the standard combined loading checks.

RE: Rectangular HSS beam with AXIAL load and slab on top flange -- what is Ly?

(OP)
jayrod12, thanks for your reply.

When you said "Cr", did you mean Fcr, or something else?

As for Mr, I believe you mean the required flexural strength per Ch. H of AISC. So that is clear, unless you say otherwise.

RE: Rectangular HSS beam with AXIAL load and slab on top flange -- what is Ly?

I'm with BA on this one. You're absolutely braced for weak axis buckling. The only question is whether or not you're braced for torsional buckling. And, given the torsional restraint provided by the floor deck and the fact that HSS sections are great in torsion, I'm willing to be bet that you're braced for torsional buckling too.

The most technically correct way to investigate this would be as constrained axis buckling (pic below). The results of such an investigation are generally pretty favorable with respect to strength. It's a fair bit of work however. In common situations where there are supported beams tying in at regular intervals, designers typically check unbraced weak axis buckling between beams instead of torsional/LTB buckling. This is done under the correct assumption that the procedure is conservative but not excessively so. For a beam that has no supported members tying in at regular intervals, there may be real economy to be gained by evaluating the situation more carefully.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Rectangular HSS beam with AXIAL load and slab on top flange -- what is Ly?

I would assume it is braced for moment, unbraced for axial load.

DaveAtkins

RE: Rectangular HSS beam with AXIAL load and slab on top flange -- what is Ly?

Quote (YCLJ)

When you said "Cr", did you mean Fcr, or something else?

As for Mr, I believe you mean the required flexural strength per Ch. H of AISC. So that is clear, unless you say otherwise.

I'm not familiar with AISC (Canada Eh) but I mean Cr as in column capacity, and Mr I mean moment resistance for an HSS that is considered braced top flange.

RE: Rectangular HSS beam with AXIAL load and slab on top flange -- what is Ly?

Perhaps it depends on the relative magnitude of the moment versus the axial load. The HSS beam is assumed to be a simple span so that the top is in compression and the bottom is in tension from the simple span moment. To some extent, bending compensates for axial load as it reduces compressive stress in the bottom.

Treating the member as braced for moment and unbraced for axial load would certainly be a conservative approach and, unless a more detailed analysis was warranted, would be a sensible assumption.

BA

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources