Retaining Wall - Shear at stem wall & footing interface
Retaining Wall - Shear at stem wall & footing interface
(OP)
thread507-122089: Shear-Friction Design Method for Reinforced Concrete
I have thoroughly confused myself regarding shear friction at the construction joint of a retaining wall & footing interface. (see attached diagram)
ACI 22.9.1.1 says that we should consider shear transfer at this interface.
Assuming I have the perfect amount of flexural reinforcement at this interface (no excess steel area); do I need ADDITIONAL shear friction reinforcement using ACI equations 22.9.2.4 or 22.9.2.3?
My reading of these provisions are that I DO need to provide bars to specifically resist the shear forces; or I could possibly increase the flexural steel area.
I have reviewed several design documents and have not found an answer. The 2008 CRSI manual contains specific “D’ Bars to resist shear at this interface, but I have a 2014 CRSI retaining wall design guide that does not show these bars.
Opinions?

I have thoroughly confused myself regarding shear friction at the construction joint of a retaining wall & footing interface. (see attached diagram)
ACI 22.9.1.1 says that we should consider shear transfer at this interface.
Assuming I have the perfect amount of flexural reinforcement at this interface (no excess steel area); do I need ADDITIONAL shear friction reinforcement using ACI equations 22.9.2.4 or 22.9.2.3?
My reading of these provisions are that I DO need to provide bars to specifically resist the shear forces; or I could possibly increase the flexural steel area.
I have reviewed several design documents and have not found an answer. The 2008 CRSI manual contains specific “D’ Bars to resist shear at this interface, but I have a 2014 CRSI retaining wall design guide that does not show these bars.
Opinions?

RE: Retaining Wall - Shear at stem wall & footing interface
RE: Retaining Wall - Shear at stem wall & footing interface
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: Retaining Wall - Shear at stem wall & footing interface
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
www.americanconcrete.com
RE: Retaining Wall - Shear at stem wall & footing interface
1) Canadian code separates cohesion as well. You'd get 0.5 MPA here. I'd wager about 2.5 MPA here from rebar. Is a 20% improvement of about the right order?
2) Since cohesion is not just rough contacted surfaces but molecular bond, is there a greater need for quality control and inspection of the mating surface?
3)Is the nature of cohesion such that it can be present when there is some degree of gap between surfaces? At ultimate flexural capacity, there may be a millimetre-ish physical gap over 80% of the cross section. Differential stem shrinkage might also open things up a bit.
4) Do you know if the ASHTO stuff is based on different testing from 318? I believe that the basic ACI testing was done in a setup producing nearly flexureless shear and no shrinkage restraint. Pretty cohesion friendly.
I agree though, if one gets cohesion and didn't account for it in design, there would be a pretty healthy buffer against my original concern. I like it.
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: Retaining Wall - Shear at stem wall & footing interface
- shear friction resistance = (cohesion factor, c) * (shear friction cross-sectional area) + (friction factor, μ) * (Avf*fy + permanent net compressive force)
There is also two limits placed on the shear friction strength, one based on shearing of the area and the other on compressive strength. A phi factor is also applied as per typical LRFD fashion and a minimum area of shear reinforcement is required. The "cohesion factor" is actually a combined cohesion and aggregate interlock factor. fy is limited to 60 ksi.The cohesion factor and friction in AASHTO varies depending on the surface cast against. For example, against a clean surface roughened to 0.25 in. amplitude you get a whopping c = 0.24 ksi (1.65 MPa) and a friction factor of μ = 1.0. Without roughening you get c = 0.075 ksi (0.52 MPa) and μ = 0.6, very similar to yours.
The few times I've used the AASHTO shear friction equation to it's full potential the EOR required inspections of the mating surfaces prior to the secondary pours. This seems reasonable and appropriate to be a special inspection item.
I believe that cohesion would be lost if a gap opened up. Though, of course it would be hard to tell if that was just a gap at the exterior of the concrete or whether it was through-thickness. However, AASTHO is quite clear that their "cohesion factor" also accounts for aggregate interlock so I believe they're considering more than just the straight cohesive bond between the two surfaces. Thus, I would think a crack/gap wouldn't be an outright cause for rejection but would be cause for further investigation.
AASHTO seems to think similar as "brackets, corbels, and ledges" have a specific cohesion factor of zero due to the potential for a vertically oriented crack.
AASHTO cites the following for their factors: Loov and Patnaik, 1994; Patnaik, 1999; Mattock, 2001; Slapkus and Kahn, 2004; Hofbeck, Ibrahim, and Mattock, 1969; Mattock, Li, and Wang, 1976; Mitchel and Kahn, 2001. These appear to be different from the references given in ACI 318-14 but I have not reviewed any of the AASHTO research listed. Definitely something on my "to-do" list, though.
And, yes, AASHTO also has the same requirement that shear reinforcement be "fully developed on both sides of the interface" and gives no credit for excess reinforcement quantities. However, the not that bars with fy > 60 ksi to have an assumed fy = 60 ksi is quite interesting.
Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries
www.americanconcrete.com
RE: Retaining Wall - Shear at stem wall & footing interface
That caught my attention. Vewy intewesting...
RE: Retaining Wall - Shear at stem wall & footing interface
"It is imperative Cunth doesn't get his hands on those codes."
RE: Retaining Wall - Shear at stem wall & footing interface
"It is imperative Cunth doesn't get his hands on those codes."