×
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

Percent Gravity Load for Overturn / Lateral Bracing

Percent Gravity Load for Overturn / Lateral Bracing

Percent Gravity Load for Overturn / Lateral Bracing

(OP)
I have a 6.75" x 33.00" Glulam.

The Glulam will support, with face-mount hangers:
- 28” deep trusses at 24” o.c. on one side
- 11.88” TJI Joists at 16” o.c. on the other side.
- All top elevations are equal.

I’m concerned that the 28” trusses may cause the glulam to overturn, at bottom, toward the TJI side.

What percent of uniform gravity shall I provide of lateral overturn bracing?

I think it is either 10% or 20%... or is it 5%?

I should know this, but, I can neither recall nor find the answer in my library, files, etc.

Thank you!

RE: Percent Gravity Load for Overturn / Lateral Bracing

Simple span girder or continuous?

I'd just calculate the actual torsion on the girder due to the load imbalance and brace for that. A strap connecting the bottom of every fourth truss to the side of the glulam or something like that. To be honest, I think that relatively small eccentricities like this typically just get ignored.

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: Percent Gravity Load for Overturn / Lateral Bracing

(OP)
Simple

Koot... i like your suggestion about the metal strap. I may use that.

Do you know the answer to the main question? Where can i find the answer? I know it's a common scenario. I just can't locate the answer.

Thank you!

RE: Percent Gravity Load for Overturn / Lateral Bracing

An old rule for steel bracing used to be 2% of flange force.. not sure if that's what you're referring to.

RE: Percent Gravity Load for Overturn / Lateral Bracing

I'm afraid that I don't know the answer to the main question BVSD. I've heard of rules of thumb for lateral torsional bracing but never for just torsion bracing. Really, it's a bit of a misnomer to even call this bracing as you're resisting an applied load rather than restraining buckling. You're basically making roll beams out of whatever members you're tying back to.

When your girder attempts to roll, the points of load application in the hangers will shift so as to create a restoring torsion. It's a sketchy thing but, in combination with rotational restraint at the ends of girders, is probably part of what keeps these things upright when discrete bracing is absent. You can also develop a resisting couple between your diaphragm attachment at the top of the girder and the compression chords of the supported framing after a bit of movement.

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: Percent Gravity Load for Overturn / Lateral Bracing

(OP)
Lomarandil... I believe that is what I was thinking of. However, isn't that for columns?

Koot... this appears to be a torsional situation is it not?

RE: Percent Gravity Load for Overturn / Lateral Bracing

Quote (BSVBD)

Koot... this appears to be a torsional situation is it not?

This is a torsion thing but not necessarily a lateral torsional bucking thing. Your sheathing fastening at the top of the girder should be providing your lateral torsional buckling bracing per NDS requirements etc. Sometimes I use the AISC requiements because they feel a lot more rigorous.

Regarding the 2% rule, it was developed for columns but applies in a lot of situations where a thing - like a beam flange - is in compression. AISC's seismic design manual has requirements of a similar format for the lateral torsional buckling of beams. Just higher numbers for beams needing to go seriously plastic.

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.

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