Compressive Shear Failure
Compressive Shear Failure
(OP)
Situation: A pad/mat foundation is supporting 4 point loads (6" square bearing plates). The foundation thickness has been designed for one way and two way (tension) shear per ACI.
What I was wondering was how to calculate the resistive strength of a compressive type shear failure (see attached)? I would like to be able to calculate how close to the foundation edge is to close? Is this even a factor? This topic was stimulated by a contractor increasing rebar in the foundation and requiring end hooks to meet design strength of ACI 11.7 Shear Friction for the failure mode of breaking off a corner. I would think, if you stay far enough away from the foundation edge the concrete would have enough shear strength to not require the additional rebar or tension development, but how far away is far enough?
What I was wondering was how to calculate the resistive strength of a compressive type shear failure (see attached)? I would like to be able to calculate how close to the foundation edge is to close? Is this even a factor? This topic was stimulated by a contractor increasing rebar in the foundation and requiring end hooks to meet design strength of ACI 11.7 Shear Friction for the failure mode of breaking off a corner. I would think, if you stay far enough away from the foundation edge the concrete would have enough shear strength to not require the additional rebar or tension development, but how far away is far enough?





RE: Compressive Shear Failure
1. Determine bearing pressure under pad.
2. Design as a simple beam:
a. flip the lower sketch up-side-down, let point load as reaction on beam support.
b. place the bearing pressure as load applied on the beam.
c. determine critical location and check shear using ACI provision for oneway beams.
3. Design as two way slab:
Steps a & b are identical to above, but use ACI provisions on two way shear (punching shear) to check shear strength of the slab to support/resist the corner loads.
Wish this helps.
RE: Compressive Shear Failure
RE: Compressive Shear Failure
Given:
18" thick foundation, d=14",
top and bottom rebar with 3" clear cover and no hooks,
6" square bearing plate;
Question: Rather than hooking the rebar for the tension development requirements of shear friction design, how far from the foundation edge must the center of the bearing plate be so that the design is not controlled by shear friction?
Do just need to be far enough from the edge so that the compressive shear failure plane passes thru the entirety of the foundation? What would be the angle from the horizontal for this shear failure, 45 deg, 40 deg, less?
RE: Compressive Shear Failure
So to this effect you may make 3 D models of the different setups to find to where it works for you.
You may also use the same idea with simplified strut and tie schemes, where a compressive strut goes from the bottom of a corner column 45 deg to the bottom of the footing and then for a "tributary" diagonal beam in the footing analyze the tension present, and grossly, the tensile stress. A coarse method yet may give you soon an idea of to where you can go without causing too much tensile stress.
RE: Compressive Shear Failure
For multicolumn supports this could be another thing but, then, we are rarely designing the thickness of the footing to be mechanically critical, some minimum depths related to frost or just get to sound soil use to be respected, so if for a design of a multisupport for a bridge or main set of columns in buildings it is unlikely we would decide to be less conservative.
RE: Compressive Shear Failure
If this is a corner column in two way slab, and there is adequate concrete shear strength through two way action, then you need not to worry about edge distance. The mode - chipping through the corner/edge is not likely to occur. However, you may always assume a failure plan (from highest compressive stress drawn a line towards free edge of the pad), and check using shear friction concepts (similar to corbel for this case).
If design is controlled by shear friction, you need to develop the shear friction bars to achieve yield strength at the critical location, therefore, it controls your edge distance - either for a full development length, or use hooked bars. However, I really do think this (shear friction) is controlling factor.
RE: Compressive Shear Failure
Based on a shear friction design (ACI 11.7) would the minimum distance between the foundation edge and the failure plane be as shown in the attached drawing? 2 to 4 feet seems like a significant oversizing of the foundation? Also, the longitudinal rebar size and spacing will need to be increased to account for shear friction in addition to bending?
Shear friction assumes that the concrete has cracked along this shear plane. What is the plain concrete strength of this plane prior to the failure?
Would it be correct to assume the following: Using Mohr's circle tau max = sigma/2, therefore would the concrete compressive shear strength of 4000 psi concrete be 2000 psi? Can I use this value across my failure plane to determine whether or not the failure would occur in the concrete without accounting for the rebar/shear friction?