Beam Shear in Walls of Below Grade Structure
Beam Shear in Walls of Below Grade Structure
(OP)
I'm designing a pumping station that will be approx. 35' below grade with high groundwater. The resulting shears at the wall corners and base are high. To resist shear with Vc only requires very thick walls (thicker than otherwise required to resist moment). Are there good ways to provide shear reinforcement at the base and corners of the walls?






RE: Beam Shear in Walls of Below Grade Structure
RE: Beam Shear in Walls of Below Grade Structure
Incremental increases in concrete thickness for walls are relatively inexpensive, once you've paid for the formwork.
RE: Beam Shear in Walls of Below Grade Structure
RE: Beam Shear in Walls of Below Grade Structure
Buoyancy is an issue and mass is helpful, but the foundation is on piers so I can use the piers to resist uplift so I was hoping to go with thinner walls. I'm analyzing as 2-way plate now and have ult. shear of about 55 kips which gives 4' thick walls at the base.
Stirrups seem like a big constructibility problem. Are shear studs ever used in this type of application? The only thing I see them used for is punching shear at columns.
RE: Beam Shear in Walls of Below Grade Structure
RE: Beam Shear in Walls of Below Grade Structure
I've used that section of the code many times and never had a problem. Once again, it's not for the loads pushing out of the box (the liquid), only the soil load pushing in.
Are you using the factors from ACI 350? If you are, note that the extra factors for cracking resistance only apply to loads carried by reinforcing.
RE: Beam Shear in Walls of Below Grade Structure
Jed,
I have used d away from the support successfully in the past as well. When I was digging in to this one I read the fine print in R11.1.3.1 that says "two things must be emphasized: first stirrups are required...." Made me wonder if I applied that incorrectly (although successfully - so far
I'm not using the ACI 350 environmental durability factor for Vu when considering unreinforced shear capacity. The high loads are coming from 95pcf EFP soil pressures.
What do you think about pilasters in the walls to break up the horizontal spans?
RE: Beam Shear in Walls of Below Grade Structure
I never noticed that assumption in the commentary about stirrups. But in R11.1 it says that that the concrete shear capacity is the shear causing significant inclined cracking.
RE: Beam Shear in Walls of Below Grade Structure
RE: Beam Shear in Walls of Below Grade Structure
RE: Beam Shear in Walls of Below Grade Structure
no, i'll sharpen the pencil and try that along with 11-5
thanks
RE: Beam Shear in Walls of Below Grade Structure
Mike McCann
MMC Engineering
RE: Beam Shear in Walls of Below Grade Structure
Deep structure like this (with high ground water effect) should be handled conservatively for all the uncertainties. Remediation/retrofit is highly difficult, and costly.
RE: Beam Shear in Walls of Below Grade Structure
1. you mentioned the structure will be founded on piles. If designed properly, the base slab should not feel much of pressure from soil, if any, but hydrostatic uplift. Check with your geotech on this. Also, ask him to affirm that you can utilize pile friction at any circumstance (fully saturated soil for instance) to resist uplift.
2. Water proofing, water proofing. A slight leakage can leads to huge structural problems, and causing fears on workers.
RE: Beam Shear in Walls of Below Grade Structure
So, do you think 48-inch thick walls are reasonable? I have 35 feet deep with 95 pcf EFP. The horizontal span of the wall is 30 feet.
Is there any practical way to use shear reinforcement to help with the shear in walls rather than to handle the shear with thickness? Or any other ideas?
Thanks.
RE: Beam Shear in Walls of Below Grade Structure
The idea is like adding supports to a long cantilever beam, so the high base load/reaction would be passed out from bottom up. Take an unit strip, and give it a try, you would have better feeling on this method.
Or else, you may have to add physical beams and columns.
I wouldn't recommend special shear reinforcement for this case.
RE: Beam Shear in Walls of Below Grade Structure
i've used these when designing elevated swimming pools. they're meant to keep crack width to a minimum.
from my memory, the last edition of the envirionmental factors were still correlated to 1.4D / 1.7L. i don't think they've been recorrelated to 1.2D / 1.6L.
RE: Beam Shear in Walls of Below Grade Structure
ACI 350-06 adjusted to the newer load factors and mirrors 318-02/05/08...... (code revision is a dizzying industry in itself). The environmental factors are required for flexure and shear carried by reinforcement, but not for shear carried by the concrete. The main thing we were discussing above was shear in the walls carried by the concrete alone so the environmental factors don't apply there.