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Concrete Wall Shear at Slab Intersection

Concrete Wall Shear at Slab Intersection

Concrete Wall Shear at Slab Intersection

(OP)
Hello,

I am fairly new to concrete design and am trying to determine a way to increase allowable shear resistance in a concrete wall at the intersection of a slab. I am currently dealing with a cantilevered, laterally loaded, wall that has a slab that provides lateral restraint to the wall. My main question is there any way to increase the shear capacity of the wall at the slab intersection without thickening the wall? Any help would be much appreciated.

TWG84

RE: Concrete Wall Shear at Slab Intersection

Could use truss type reinforcement within the wall.

BA

RE: Concrete Wall Shear at Slab Intersection

Stirrups would work too.

BA

RE: Concrete Wall Shear at Slab Intersection

Increase the compressive strength of the concrete.

RE: Concrete Wall Shear at Slab Intersection

(OP)
BA. How would the stirrups have to be located in the wall? Would additional longitudinal reinforcement work as well?

JAE.. Increasing f'c does not give me enough additional shear capacity.

RE: Concrete Wall Shear at Slab Intersection

Switch from a cantilevered wall design (yielding) to a non-yielding, basement wall design if you can.

Some of the shear will be transferred to the diaphragm at the top, if there is a diaphragm present. May not be the case though...

Mike McCann
MMC Engineering

RE: Concrete Wall Shear at Slab Intersection

(OP)
In using the non-yielding situation, since I cannot tie the top of the wall to an actual diaphragm, could I possible cast in a really stiff element at the top of the wall (wide flange beam for example) to act as a diaphragm to transfer some of the shear? My wall is spanning between 2 concrete columns, which would allow me to transfer the shear from the top stiffened element to each column. If this theory is possible are there any references on how stiff the element would have to be so act as a top support of the wall?

RE: Concrete Wall Shear at Slab Intersection

Provide a sketch.

BA

RE: Concrete Wall Shear at Slab Intersection

Then this is already a basement wall.

Well, statically speaking, if you lowered the steel beam level, the bottom of the wall would see less shear.

Mike McCann
MMC Engineering

RE: Concrete Wall Shear at Slab Intersection

(OP)
The sketch that I provided was a proposed theory. If it is an applicable theory, do you know of any good references that could point me in the right direction as to know how stiff the beam would have to be to act as a diaphragm (pin) at the top of the wall.

RE: Concrete Wall Shear at Slab Intersection

Bracing the top of the wall would help, as you have suggested. But what is causing the shear issue? Inadequate shear capacity in retaining walls is uncommon, and is best dealt with by increasing the wall thickness.

RE: Concrete Wall Shear at Slab Intersection

You have a wall supported on three sides and free on the fourth. This can be designed by yield line analysis or you can find tables for moment and shear values at various coordinates on the wall.

The steel beam is a bad idea as you cannot pour concrete around it. Much better to use a monolithic pour with the beam having the same depth as the wall.

I would be surprised if shear stress is critical, but you have given no dimensions or loads, so who knows?

BA

RE: Concrete Wall Shear at Slab Intersection

(OP)
hokie66,

The load induced on the wall based on the material that I am designing the wall to resist is causing the issue. I am designing the wall to hold a bulk storage material. The issue is where the slab braces the wall. At that intersection I am getting a very large shear force that the wall needs to resist. The material that I am designing the wall to withstand is approximately 70pcf with a 27° angle of repose. Based on that density, and pressure distribution, that I calculated, it is putting a lot of shear into my wall when trying to design the wall as a cantilevered wall. I am trying to keep the wall thickness no greater than 24". From the pressure distribution analysis that I performed, I am loading the wall with 1300 psf. I feel that I may be overlooking something minor.

Thanks,

RE: Concrete Wall Shear at Slab Intersection

Your sketch indicates uniform lateral loading on the wall. The load from the stored material would vary with height. How high would the stored material be at the wall face?

RE: Concrete Wall Shear at Slab Intersection

(OP)
Based on constraints of highest point of bulk material and the confined bin width that holds the material, the uniform loading provided in my sketch is based on the triangular distribution reaching the adjacent wall a distance above the top of my concrete wall. In my understanding of other research that I did, from that point down the loading does not increase, therefore remaining uniform. Above the concrete wall I have an additional build up wood wall to resist the material above the top of the concrete wall.

RE: Concrete Wall Shear at Slab Intersection

If the columns are capable of resisting the beam reactions, the wall is not going to act as a cantilever but as a wall supported on three sides. This will reduce your shear values considerably, but you have not provided enough information for anyone to make a reasonable assessment of the situation.

BA

RE: Concrete Wall Shear at Slab Intersection

I think you are doing something wrong with your calculations.
I don't get any shear issue if I take ALL of the load sideways into the columns or ALL of the load down to the slab or footing.

The total lateral force I get is 1300 psf x 8 ft x 12 ft = 125 kips.

If you use a live load factor of 1.6 you get a factored total lateral force of 200 kips.

If you look at the shear capacity of the wall, using b = 12" (a one foot section) and d = 20", f'c = 4000 psi
I get a phiVc capacity of the wall of about 23 kips/ft. Over the 8 ft. base of the wall you get 184 kips capacity.
Using f'c = 5000 psi you get 206 kips capacity.

If you do what BAretired correctly states - use yield line analysis to take advantage of the three-way behavior of the wall the total shear will come down significantly.

RE: Concrete Wall Shear at Slab Intersection

  • From your earlier sketch, I thought the columns were deeper than the wall. The new sketch indicates they are both 24" deep. How they interact depends on the height of top of column.
  • You also mentioned that you have a wood wall above the concrete wall to resist lateral forces. Does it span horizontally between columns? Or does it span vertically from wall to roof? Either way, the reactions from the wood wall will affect the behavior of both the wall and columns.
  • You must consider the wall and columns acting together, i.e. strain compatibility, but based on the loads you have provided, shear in the wall does not appear to be a problem.
  • BA

    RE: Concrete Wall Shear at Slab Intersection

    (OP)
    JAE, Do i not have to account for an increase in shear at the slab intersection (assuming the slab is bracing my wall @ 2ft from the top of footing). I was using that assumption and looking at the wall as a single span w/ cantilever. I was assuming a fix condition at the foundation and a support @ 2ft from the foundation (slab). From that analysis i agree with your 23 k/ft (that you mention above) as the shear approaches the slab, however at the slab would the shear not increase due to the slab acting as some type of brace/support?

    BA. i apologize for an earlier confusion on my previous sketch. The wood wall above I have spanning between the columns so I wasn't accounting for any direct interaction with the concrete wall. I understand that in carrying the loads from the wood wall to the columns would interfere with the behavior of my columns. There is some shear that my wall is having to resist, but I am not having any problems with them resisting the shear that I am transferring to the columns. For my wood wall I am using LVL beams with plywood sheathing on ea. side. Because I am distributing the load to the columns, I have my columns to be @ the 10ft center to center, to keep from developing too much shear in my wood members. As the load gets to approximately 2ft above the concrete wall the load distribution begins to decrease as it carries further up the wall.

    I have attached another sketch to hopefully provide more clarification of the section that I am looking at. Please note that the Columns are not shown in this section. This section is only showing the wood and concrete wall with how I laid out my pressure distribution on the wall. Thanks for all of your help so far.

    RE: Concrete Wall Shear at Slab Intersection

    (OP)
    I also plan to look into the yield line analysis. I have not every looked into this theory. Does anyone have a any good references?

    RE: Concrete Wall Shear at Slab Intersection

    TWG84,

    Your information comes in bits and pieces, each contradicting the previous one. You must learn to start thinking like a structural engineer. I don't mean that as a criticism as we all had to start somewhere. It takes time and until you are able to do that, you are going to need supervision from a qualified engineer, which you don't appear to be getting now.

    The first thing you need to do is to develop a conceptual framing arrangement for the entire structure. Before doing a lot of calculations, you may wish to discuss it with your boss and get his input. Or you may find that your first idea didn't work after doing a few calculations. It is far better to spend time developing a workable concept than to get bogged down in detail.

    Forget about yield line theory for now. It may have been applicable to your first sketch, but no longer makes much sense. Focus instead on how you are going to frame these storage bins from foundation to roof.

    Regarding your latest sketch:

    1. What is the elevation of exterior grade?

    2. What happens at top of columns 33' above the wall? Are the columns tied by beams across the 40' bay or are they just flapping in the breeze?

    3. Do you have a tentative Roof Framing Plan?

    4. Why is top of footing 2'-0" below top of slab? Would it be better to cast the slab directly over the footings?

    BA

    RE: Concrete Wall Shear at Slab Intersection

    (OP)
    BA,

    Thanks for all of your input. I do not take any of this as criticism as I am merely trying to fully understand and know that what I am designing is structurally adequate plus the qualified engineer that I am under is not of much help. To answer your questions:

    1. Elevation of existing grade will match finish floor elevation

    2. The columns will be tied together each direction by glulam members.

    3. I have a Roof Framing Plan laid out based on my column locations and spacing. The plan consist of what is described in 2 above. Also I have a steep slop on my roof. Roof slope on one side if 4/12 and a 12/12 on the other side.

    4. the wall on the right side of the sketch is an exterior wall, therefore I have it 2ft below finish floor in order to achieve the minimal frost depth. being that the other footing is interior it could be raised up to cast the slab directly over the footing.

    Thanks,

    RE: Concrete Wall Shear at Slab Intersection

    It is a very unusual structure to say the least. You appear to be relying on 24"x24" cast-in-place concrete columns spaced at 10'-0" centers to resist lateral pressure from the wood wall above Elev. 114'-0" but it is not clear what provisions will be made to handle the column reactions at roof level. The cost of casting those columns to a height of nearly fifty feet is not trivial and I suspect they are not deep enough to do the job without excessive deflection.

    What governed the wall cutoff level of 114'-0"? Why not use a consistent framing system with wood wall framing between columns all the way from bottom to top and avoid those messy cantilevered walls. From an erection standpoint, it would be simpler and cheaper to use glulam or structural steel columns.

    That is my first reaction, but others may have different opinions.

    BA

    RE: Concrete Wall Shear at Slab Intersection

    Correction: Elev. 114' should read Elev. 112' in the previous post.

    BA

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