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