thoughtofthis
Structural
- Jan 16, 2012
- 31
1)
Historically, ACI has explicitly exempted footings from the minimum shear requirements. In ACI 318-14 the chapters were rearranged and minimum steel requirements were put into chapter 7 (one-way slabs) and 9 (beams). Foundations are in chapter 13.
Foundations says to design one-way shallow foundations according to applicable provisions of Ch 7 and Ch 9....
7.6.3.1: A minimum area of shear reinforcement, Av,min, shall be provided in all regions where Vu > ϕVc.
9.6.3.1: A minimum area of shear reinforcement, Av,min, shall be provided in all regions where Vu > 0.5ϕVc except for the cases in Table 9.6.3.1. For these cases, at least Av,min shall be provided where Vu > ϕVc.
The table goes on to talk about things that do not appear directly applicable to footings.
CRSI guides and SP-17 have examples that indicate the historical interpretation is accurate, but there is very little discussion in the commentary. Obviously, you're not going to put shear reinforcement in the heel, but keeping Vu > 0.5ϕVc could increase the thickness quite a bit. I'll stick with the examples for now, just curious if anyone had a different interpretation.
2)
ACI 7.4.3.2 says critical section for shear is at d from the face, as long as: (a) Support reaction, in direction of applied shear, introduces compression into the end region of the slab (b) Loads are applied at or near the top surface of the slab (c) No concentrated load occurs between the face of support and critical section
(b) and (c) seem easy for retaining walls, but not sure how to evaluate (a).
AASHTO C5.13.3.6: In the general case of a cantilever retaining wall, where the downward load on the heel is larger than the upward reaction of the soil under the heel, the critical section for shear is taken at the back face of the stem in which dv is the effective depth for shear.
What's up with that?
Historically, ACI has explicitly exempted footings from the minimum shear requirements. In ACI 318-14 the chapters were rearranged and minimum steel requirements were put into chapter 7 (one-way slabs) and 9 (beams). Foundations are in chapter 13.
Foundations says to design one-way shallow foundations according to applicable provisions of Ch 7 and Ch 9....
7.6.3.1: A minimum area of shear reinforcement, Av,min, shall be provided in all regions where Vu > ϕVc.
9.6.3.1: A minimum area of shear reinforcement, Av,min, shall be provided in all regions where Vu > 0.5ϕVc except for the cases in Table 9.6.3.1. For these cases, at least Av,min shall be provided where Vu > ϕVc.
The table goes on to talk about things that do not appear directly applicable to footings.
CRSI guides and SP-17 have examples that indicate the historical interpretation is accurate, but there is very little discussion in the commentary. Obviously, you're not going to put shear reinforcement in the heel, but keeping Vu > 0.5ϕVc could increase the thickness quite a bit. I'll stick with the examples for now, just curious if anyone had a different interpretation.
2)
ACI 7.4.3.2 says critical section for shear is at d from the face, as long as: (a) Support reaction, in direction of applied shear, introduces compression into the end region of the slab (b) Loads are applied at or near the top surface of the slab (c) No concentrated load occurs between the face of support and critical section
(b) and (c) seem easy for retaining walls, but not sure how to evaluate (a).
AASHTO C5.13.3.6: In the general case of a cantilever retaining wall, where the downward load on the heel is larger than the upward reaction of the soil under the heel, the critical section for shear is taken at the back face of the stem in which dv is the effective depth for shear.
What's up with that?
