Beam Shear in Walls of Below Grade Structure 2

[dmessayons]

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?

 

[kslee1000]

Try to avoid shear reinforcement to the best you can, especially in wall. The first thinking popped up is that weight is the preferred/cheaper means to resist hydrostatic uplift. The 2nd, how about increase strength of concrete mix? Finally, have you analyzed the slab and wall as 2 way plate, or conservatively treated as one way element? It makes big difference.

 

[JedClampett]

You can reduce the shear to a point "d" from the support per code (for exterior loads only). Beyond that, I'd make the wall as thick as shear dictates. You'll find if the wall is skimpy for shear, you end up with congested moment reinforcing (something I could never logically explain).
Incremental increases in concrete thickness for walls are relatively inexpensive, once you've paid for the formwork.

 

[kslee1000]

I like Jed's idea to provide chamfer (bull leg) at the corners, it would help, if you can.

 

[dmessayons]

thanks, kslee1000.


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.

 

[dmessayons]

Thanks, Jed. By ACI 318/350 R11.1.3.1 don't you need stirrups to be able to move "d" away from the corner and base? Or do you think this requirement is lifted for walls and slabs like minimum shear reinforcement reqts?

 

[JedClampett]

I defer to the Figure R11.1.3.1(a) that implies that a shear failure would occur at a 45 degree angle from the support, stirrups or not.
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.

 

[dmessayons]

I appreciate the responses.

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 . What do you think?

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?

 

[JedClampett]

I'd stay away from anything fancy like pilasters. I know it doesn't seem economical, but just pour the entire wall to the thickness required by the shear loads. Pilasters end up being very heavily reinforced and then might have their own shear issues.
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.

 

[kslee1000]

Have you tried eq. 11-4 to cal Vc?

 

[dmessayons]

Thanks, Jed. I'm reluctant to go with 48" thick walls. Any other ideas would be appreciated.

 

[dmessayons]

kslee,

no, i'll sharpen the pencil and try that along with 11-5

thanks

 

[msquared48]

Why not extend the footings beyond the walls to pick up more soil load to help resist the uplift?

Mike McCann
MMC Engineering

 

[kslee1000]

Double check your model and analysis results. For a wall with 3 sides fixed and top pinned, or fixed, when subjected to a trapezoidal load, the largest reaction shall be at the middle of the bottom edge, and at approx. 2/5 wall hight on the side walls. The lower wall corners shouldn't see much of reaction, because deflection simply couldn't occur (the corners are stiffened by the fixed edges). Turn on the deflection, shear stress and bending stress diagrams, you shall see the wall has deflected into bowl/dish shape, with high stresses concentrated at locations mentioned above.

Deep structure like this (with high ground water effect) should be handled conservatively for all the uncertainties. Remediation/retrofit is highly difficult, and costly.

 

[kslee1000]

2 add'l notes:

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.

 

[dmessayons]

Thanks all.

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.

 

[kslee1000]

You may place an embedded beam a few feet above the base, then design, and reinforce it accordingly (as a beam). You can repeat it upward until local staffening is no longer required. By doing so, you should be able to reduce the wall thickness down to 30"-36" range.

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.

 

[vandede427]

interesting that someone brought up the environmental factors for water tightness. I would definitely use them in this case

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.

 

[dmessayons]

Thanks vandede,

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.