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!
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
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
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
RE: Percent Gravity Load for Overturn / Lateral Bracing
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
Koot... this appears to be a torsional situation is it not?
RE: Percent Gravity Load for Overturn / Lateral Bracing
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.