1 Way Shear Check in Two Way Slab System

[polskadan]

Good Morning All!

I've looked over the forums and have found some topics regarding one-way shear in a two way slab system however none cover the issue that I am currently facing. At my office we are currently having a debate among the structural engineers and was hoping I could get some more insight from anyone on the boards.

I am analyzing an existing concrete building that has a two way concrete slab system with capitals and drop boxes. My company is currently designing support steel that will be support new tank loads. The new support steel columns are located very close to the existing concrete columns (base plate is 1" off of the existing concrete column). I am doing calculations to check the transfer of this new load from the steel to the concrete columns and an issue has arisen regarding shear.

I know I need to check: (A) Punching shear of the new base plate with its respective critical perimeter, (B) 1-way shear in the slab system, & (C) 2-way shear in the slab system. I am using MacGregor's Reinforced Concrete Text and of course ACI. I understand that 1-way shear rarely controls in two way slab systems however in my new scenario it is. Thus the debate. I am dividing my loads from my tributary area over the entire width of the drop box for one way shear as discussed in McGregor's text (pg 586). For one way shear the critical distance is located at 'd' from the face of the column. If my new load from the steel column is within this 'd' then does this additional load contribute to the 1-way shear or does this action more or less act in 2-way shear (?even if it was outside 'd'?). I believe that I should consider this new load for both 1-way shear & 2 way shear action regardless if the load is located within our out of the 'd' critical distance (I am practically breaking it down to a concrete beam with a shear diagram at this point). What I am doing is getting a shear load per unit width and comparing it to the effective unit width of the new base plate. No where in the text nor code do I understand that if my load is within the critical distance, that I need not consider 1-way shear. Another structural engineer at my office is stating since I am so close to the existing column (within 'd') that 1-way shear need not be considered and the 2-way shear will be the controlling failure. I am having a difficult time validating this.

If I could get any help on this issue it would be greatly appreciated! Also, I have attached an image for further clarification.

Thank you all in advance!

 

[cliff234]

That's an interesting one. I tend to be conservative with shear checks on two-way slabs because punching shear failures are sudden and unforgiving. I would check the 1-way shear limit state when investigating the post load because the post load is on one side of the column. It is trying to "beam shear" it's way through the edge of the drop box. (The 60 kip load will not find it's way onto the other three sides of the drop box.) I know you didn't ask this, but I would do whatever it takes to get that new load on top of the existing column. If the new post can't be moved, can you put a bracket or corbel of some sort at the base of the post to get the load onto the column? 60kips is a substantial load. I would not put a load of that magnitude directly onto the slab. It just doesn't feel right to me.

 

[polskadan]

Cliff,

That was my original point of thinking, the point load will create more of a "beam shear" rather than distribute its way around the column and thus I wanted to send the load directly to the existing columns through some kind of mechanism. I guess the concern was if I am within the "d" critical distance from the face of the column, then the new loads contribution to the system need not be considered for 1-way shear (Or if i'm within d/2 then need not be considered for 2-way shear). If I were to do a shear diagram for the beam action for 1-way shear it was hard to ignore this huge jump of shear near the support due to the new load even if it were within distance 'd.' I however just got off the phone with another structural engineer and he did put it in perspective that does validate the need to not consider this load for this shear. If my load is within the critical distance 'd' for one way shear or 'd/2' for two way shear then the shear load will break down at a 45 degree angle and will already enter the column before shear across the slab would ever take into effect, thus is why the codes have created the critical distance that we need not consider. This seems more clear now that he put it in such a simple way, I guess it was hard to justify this since it was such a large load (and I was over complicating the issue)...still I do have to check punching shear but in terms of 1-way shear distribution the load will distribute its way into the column well before it would be a consideration for 1-way shear. This makes more sense now but I am still very open to further input!

 

[BAretired]

If the columns were square in cross section, I would agree with the structural engineer you phoned. Because they are circular in section, it may be prudent to consider an equivalent square column with side dimension 0.8*30 = 24" when checking the critical section for one way shear.

BA

 

[hokie66]

The point load is in a non-flexural zone, like a corbel, and I would check it using strut and tie principles. The strut should not be a problem because the drop panel provides good confinement, but the tie might be an issue if there is not much top steel through the column region.

 

[slickdeals]

Agree with Hokie. One of my colleagues had a similar situation, see this thread. thread507-256671

 

[RFreund]

Slick - do you recall the final solution for that problem?

EIT

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