SMF Beam Bracing

[kgengr]

I have a question regarding SMF beam bracing. Per AISC 341-05, 9.8, both flanges of the beam shall be braced. The brace strength, Pbr, is from the 13th ed steel manual equation A-6-7. Now if I am bracing the top and bottom flange at the same location (via tab plate) – should I design the bracing and resistance [wood diaphragm] for twice the brace strength 2*Pb? This seems appropriate as if I was to brace each flange separately (say with conc over metal deck for the top and angle kickers for the bottom flange) I would design each brace for the full Pbr.

Appreciate any and all responses. Thank you for your time!

 

[wannabeSE]

Off the top of my head, I would say the flange forces will act in opposite directions, so the brace needs to resist the resulting moment. I believe AISC 341-10 provides guidance for torsional bracing.

 

[jdgengineer]

My gut says no. I believe the intent of the provisions is to ensure that the full plastic moment capacity of the beam section is met (most stable and predictable hinging mechanism). Therefore, they want you to brace the beam close enough to ensure this happens. As EQ loading reverses the flange that is in compression will switch between the top and bottom flange. Therefore, both flanges would need to be braced, but the bracing force from the flanges would not be applied simultaneously.

I might be wrong but this sounds right to me. I didn't look it up though.

In a wood diaphragm I think your biggest problem will be the stiffness requirements for the bracing.

 

[kgengr]

Thank you for your responses. AISC 341-05 references A-6-7 and A-6-8 equations from section 6.3.1 in Steel Manual under "Lateral Bracing" instead of 6.3.2 "Torsional Bracing", leading me to think the design forces could be required in the same direction simultaneously? My gut initially said no to the 2*Pbr - but AISC design examples use the same bracing force for top and bottom bracing separately - of course, no specific example where the brace for both occurs at the same location. Also states that stability bracing for MF beams is required for inhibiting both lateral, and lateral-torsional, however only one design brace force is specified for "bracing". Where it falls apart (for me) to use only Pbr to brace both top and bottom - if I were to stagger said braces, appointing every other as bracing top or bottom flange - I would use Pbr there individually for each brace - so would I not use 2*Pbr if I have them together instead? For my detailing I have the MF beam in the joist space, wood plate on top flange is directly under the sheathing - for compression I am solid blocking at brace locations with PSL out the distance to have capacity in the diaphragm and strapping across the top of the blocking - the tension is addressed with holdowns to either side of the farthest block with threaded rod back to a WT tab welded to MF beam web and flanges top and bottom. I'd prefer 1*Pbr as I will double my blocking distance with 2*Pbr, but am trying to hedge my bets against a plan check.