Concrete Wall Shear at Slab Intersection

[TWG84]

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

 

[BAretired]

Could use truss type reinforcement within the wall.

BA

 

[BAretired]

Stirrups would work too.

BA

 

[JAE]

Increase the compressive strength of the concrete.

 

[TWG84]

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.

 

[msquared48]

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

 

[TWG84]

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?

 

[BAretired]

Provide a sketch.

BA

 

[TWG84]

BA.. Attached is a sketch of what I described above.

 

[msquared48]

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

 

[TWG84]

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.

 

[hokie66]

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.

 

[BAretired]

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

 

[TWG84]

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,

 

[hokie66]

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?

 

[TWG84]

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.

 

[BAretired]

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

 

[TWG84]

BA,

The attached sketch should show the dimensions, wall layout, and lateral load for more clarity.

 

[JAE]

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.

 

[BAretired]

[li]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.[/li]

[li]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.[/li]

[li] 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.[/li]

BA