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Strut and tie for "Deep Beam" above opening in foundation wall

Strut and tie for "Deep Beam" above opening in foundation wall

(OP)
Hi folks!

I'm assessing an existing foundation wall with a large opening which effectively forms a 10 ft span x 10 ft deep x 4 ft wide "beam" above the opening. There is longitudinal reinforcement in the bottom of the "beam" but not a sufficient amount to act as a tied arch in a strut-and-tie assessment (see attachment, top image). If I was designing it new, I'd provide enough steel there, but alas it's been around longer than I have, and now there's a question of whether the load on it can be increased.

My thinking is that since it's a very thick wall, it would be reasonable to take the thrust from the compression struts as a horizontal compression load (over an area based on the S&T node size) and check that the walls can carry the load, rather than relying on the rebar at all (see attachment, bottom image) - essentially an arch with abutments rather than the tied arch you'd typically consider in a strut and tie model of a simply supported deep beam.

I'd appreciate any thoughts on this approach. I recognize that the failure mode would be brittle because it's not based on rebar yielding, but I'm thinking if the capacity is high enough this would be justifiable.

RE: Strut and tie for "Deep Beam" above opening in foundation wall

I like the abutment approach so long as you do your due diligence in evaluating the capacity of the abutment. Clever.

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: Strut and tie for "Deep Beam" above opening in foundation wall

As long as you can accept the probable cracks in the wall above the opening and the compression stresses are acceptable.

RE: Strut and tie for "Deep Beam" above opening in foundation wall

How far does the wall extend either side beyond the opening? Is there something rigid enough there to provide the "T" reaction?

RE: Strut and tie for "Deep Beam" above opening in foundation wall

(OP)
About 8' on one side, framing into a perpendicular wall, and about 25' on the other side.

RE: Strut and tie for "Deep Beam" above opening in foundation wall

Can you add external reinforcement to make up the required area of steel?

BA

RE: Strut and tie for "Deep Beam" above opening in foundation wall

8' might not actually be all that much for the application. Is there much weight holding it from tipping over? Or a slab tied in at the top?

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: Strut and tie for "Deep Beam" above opening in foundation wall

I would think that the buttress approach would likely be the stiffer path. Without knowing all the particulars of your project (total wall height and opening dimensions etc...), a quick plane frame truss analysis with compression only diagonals, decreases the "Tie Force" at the lintel to approx. 1/3 of that from idealized S&T. The tie force also spreads up higher over lintel (assuming R/F exists there) and "bursting" tie forces exists near load application of similar magnitude as found at the lintel. Lateral forces through wall/buttress obviously make up the difference and would need full load path assessment.

RE: Strut and tie for "Deep Beam" above opening in foundation wall

(OP)
Thanks for the input folks! In response to a common thread - for sure there is a fair bit more assessment needed to make this actually work. I appreciate all the high-level comments to get a feel for whether the approach seems reasonable before diving in.

BAretired - external reinforcement is an option being considered as well. Just trying to cover all the bases before recommending too much remedial work.

KootK - yes, tied into a thick slab at the top. Will definitely have to check that the whole thing is tied down sufficiently.

Toby43 - thanks for the comment, much appreciated. Definitely needs assessment of the full load path.

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