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

[ChadV]

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.

 

[KootK]

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.

 

[rapt]

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

 

[DETstru]

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

 

[ChadV]

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

 

[BAretired]

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

BA

 

[KootK]

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.

 

[Toby43]

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.

 

[ChadV]

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.