Analysis of Closely Located Point Loads Over a CMU Wall 5

[dursundrsn]

Hi Folks,

I am working on a boxed shape wood framed house.Its foundation/retaining walls consist of 8" solid grouted CMU walls at all 4 sides. The house owner is adding a floor for that reason I am verifying the additional compressive stress over the CMU walls, and the existing soil bearing capacity.
I have new point loads (from new beams and columns), which are closely located over the existing CMU walls. I am distributing the point loads along the height of the CMU wall to the footing with 45 degree at each in-plane direction which is giving me the breadth, and multiplying it with the depth of the footing and the allowable soil bearing pressure to justify the sufficiency of the soil bearing pressure.
How do you guys distribute the point loads over a CMU wall when your point loads are closely located ? I also attached my thought process. I would love to get some input on that ?
What do you guys think ?

 

[Agent666]

I think if the wall was high/tall enough (read stiff enough) then the regularly spaced loads can pretty much be resolved to a single load acting at a certain point location and then look at the wall/strip foundation under this equivalent point load and moment to determine global bearing pressures. In effect the loads at regular centers (where the ratio of load centers to overall length of wall is appropriate) basically become a uniformly distributed load acting at foundation level. If tall enough even a single central point load will pretty much result in a constant bearing pressure over the foundation.

Similar in a way to a series of evenly spaced point loads on a simply supported beam, you get to a point where estimating the loading as a UDL is deemed acceptable (when compared to the true answer of distinct point loads).

 

[BAretired]

I don't agree with that assessment. The sum of point loads would be 90,000# on a 28' length of wall (area of 42 square feet). If the eccentricity of applied loads plus the weight of wall and foundation is considered, the bearing pressure would exceed the allowable by a factor of two or more.

BA

 

[XR250]

How much load are you dealing with? Does not seem like it could be too significant.

 

[KootK]

It's also worth remembering that the load distribution rules were developed considering reinforced masonry. If you've got horizontal rebar in there near the top and bottom, you may be able to call the thing a giant beam and have an effectively linearly varying bearing pressure at the footing. There's a pretty good chance that will be a close approximation of true behavior anyhow.

 

[masonrygeek]

If you look at TMS 402 (formerly ACI 530) they give a load dispersion of 2 vertical to 1 horizontal, not 45 deg. It's in Section 1.9.7 (2011 edition) or 5.1.3 (2013 and 2016 editions). I like the idea of a bond beam at the top, but this is an existing wall and it may not be there.

 

[JLNJ]

I can't tell from the sketch, but it looks like you have an opening in there. Such an opening may serve to concentrate your forces at the footing level. Especially if it's a door.

The settlement of the soil may also help distribute out the loads, but this might also cause cracking in the wall.

 

[Ron247]

I agree with BA. Your footing is 28'x1.5'x1500psf = 63 kips total allowable but you point loads add to 90 kips. There will be places where your footing loads overlap. Do your distribution independently for each point load on whatever angle you think appropriate. Then superimpose the results. Superposition shows you have areas that are well beyond 1500 psf.

That is before you add the existing loads and possible eccentricity of any of the loads as already mentioned.

 

[dursundrsn]

Thank you all for the opinions.
I like the idea of treating the cmu wall as a beam. I think I will utilize that.
-There is no opening on the wall.
In the real case there is a beam sitting on the existing cmu wall applying 12kips of total load , there is a column at 4 ft left and right of the beam applying 11 kips of total load, there is a uniform load applying 984 plf, and the weight of the 8" cmu wall ( 8 ft tall ).
Bearing pressure is 1500 psf, existing footing width 2.5 ft.