Lateral Bracing of a Steel Beam using CMU Wall

[PittEng88]

I am currently working on a project that requires a partial height removal of a CMU Wall for a building addition. A steel beam header will be installed to pick up the loads from the remaining portion of the CMU wall above. If the steel beam is only grouted to the underside of the CMU wall with non-shrink grout, would it acceptable to consider the top flange of the beam as fully braced? I know this sounds trivial, but I am just having a tough time seeing this work.

 

[WARose]

I'd run it by Appendix 6 (AISC) like anything else. But with a conservatively low coefficient of friction.....and not much out-of-plane wall stiffness.....I doubt you will make it either.

 

[KootK]

I'll do that with new masonry where there's studs or deformed bar anchors on top. A little scarier with nothing but self rectifying dead load and friction though. Are you making it happen with needle beams etc? If so, could you demo a little higher than you need to so that you can get some studs on the beam?

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.

 

[PittEng88]

Thanks for the replies!

Are you making it happen with needle beams etc? If so, could you demo a little higher than you need to so that you can get some studs on the beam?

Yes, I was planning on using needle beams for the demo. I was thinking about using angles every so often and thru bolting them through the masonry to help brace the top flange, but I think the studs will be a cleaner look at the end.

 

[PittEng88]

Or I could just use a beam that works at that unbraced length. As long as it doesn't encroach on the headroom too much.

 

[Wallache]

Stress rarely governs lintels for me using L/600 deflection.

 

[MotorCity]

You could also weld plates or angles to each side of the top flange after the beam is in place to "grab" the wall.

 

[jayrod12]

I've done MotorCity's detail many times with success. I do agree with Wallache however, it's likely that stress will not govern if you're designing for the correct deflection.

 

[PittEng88]

Thanks for the responses! I agree that stress will not be an issue designing to the L/600 deflection. Is it just me or does that limit seem awfully strict? Especially since it includes dead and live loads.

 

[KootK]

Especially since it includes dead and live loads.

That point gets debated here from time to time. Some feel that the limit applies only to deflection resulting from loads applied after the masonry is in place. That water gets a little muddier in a retrofit application, though, and I'd be inclined to err on the side of extra stiffness.

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.

 

[PittEng88]

Some feel that the limit applies only to deflection resulting from loads applied after the masonry is in place.

I never considered that, but it does make sense. I guess the argument can be made that the deflections due to the masonry will be set/locked-in before the grout and mortar hardens. In the Section 1.13.3.1 of the ACI 530 Commentary it states that, "The deflection limits apply to beams and lintels of all materials that support unreinforced masonry. The deflection requirements may also be applicable to supported reinforced masonry that has vertical reinforcement only."

So, if the existing wall contains horizontal reinforcement than the deflection limit of L/600 does not apply?