Flat Slab Shear
Flat Slab Shear
(OP)
I have an existing 10" flat slab with drop panels projecting 8" below the slab soffit (total thickness 18"), with 40" diameter concrete columns (no capitals). f'c = 4000 psi.
Bay size is 28 feet x 40 feet.
Shear is 400 kips factored as per CSA A23.3-2004.
I want to bring down two new steel column loads onto the flat slab, one column load of 80 kips at the north face of the existing round concrete column and another new column load of 80 kips at the south face of the existing round concrete column (total additional load =160 kips).
Total load = 400 kips existing + 160 kips new = 560 kips factored.
These new columns will apply their load within about d/2 of the round column face (d average = 18" -1.5" cover - 1" top bar size = 15.5"; d/2 = 7.75').
Question: How can I check whether the flat slab floor works for shear?
Bay size is 28 feet x 40 feet.
Shear is 400 kips factored as per CSA A23.3-2004.
I want to bring down two new steel column loads onto the flat slab, one column load of 80 kips at the north face of the existing round concrete column and another new column load of 80 kips at the south face of the existing round concrete column (total additional load =160 kips).
Total load = 400 kips existing + 160 kips new = 560 kips factored.
These new columns will apply their load within about d/2 of the round column face (d average = 18" -1.5" cover - 1" top bar size = 15.5"; d/2 = 7.75').
Question: How can I check whether the flat slab floor works for shear?






RE: Flat Slab Shear
RE: Flat Slab Shear
RE: Flat Slab Shear
Ref CRSI Handbook 1996, Chapter 13 - Piles: For a case of a pile cap with reactions from piles located less than d from face of the column, the handbook provides special investigation for two-way shear based on the report by Joint ASCE-ACI Committee 426 "The shear strength of reinforced concrete". For such a case, a < 45 deg. Failure when a < 45 deg can occur but only at increasingly higher values of shear stress. Special investigation for two way shear is made with a critical section located at the face of the column.
I am not sure if the theory in that report would be directly applicable in your case but may provide some pointers.
RE: Flat Slab Shear
RE: Flat Slab Shear
Thanks to hokie66 too.
Just one thing, when I wrote the description in my original post, I was not in the office and was trying to remember the loads but got them very wrong. They are much heavier, namely 275 kips factored times 2 loads per existibg column location = 550 kips. Nevertheless I think it may still work by the CRSI method. will let you know.
RE: Flat Slab Shear
Can anyone explain why the interface shear formulae in CSA A23.1 give such different answer when the load is very close to the column face (about 3" away)?
RE: Flat Slab Shear
RE: Flat Slab Shear
RE: Flat Slab Shear
I have had a similar situation in the past. A Strut and tie analysis as suggested would be the most appropriate one. For your loads, like Hokie suggests, you may not have enough top steel to cater for the tie force required to keep the two inclined struts from separating.
One option might be to groove cut the slab and post-install some rebar, if needed.
It's no trick to get the answers when you have all the data. The trick is to get the answers when you only have half the data and half that is wrong and you don't know which half - LORD KELVIN
RE: Flat Slab Shear
It's no trick to get the answers when you have all the data. The trick is to get the answers when you only have half the data and half that is wrong and you don't know which half - LORD KELVIN
RE: Flat Slab Shear
If I ty to transfer the load all with 24 mm Hilti HSL bolts or equivalent, I need 8 rows of bolts in 2 columns (16 bolts total) extending the full storey height and then this only takes 75% of the load (may be good enough, because it is unlikely that all post-tensioning strands in the girders that are being braced are going to fail, especially since they are currently being dried and grease injected). Would have to do scans of the columns to locate the column verical rebars and then custom fabricate the holes in each plate to suit. Will also require drilling thru the spiral ties in the existing concrete columns, which are at 2.75" centes.
I am thinking of investigating the strut and tie model further by using a curved vertical plate that extends about half way around the perimeter of the existing concrete column, with a curved based plate welded to the bottom of the curved vertical plate. This should allow a narrower base plate and a narrower compression strut, thereby reducing the eccentrity and required tie steel, as well as mobilizing more tie bars. The curved plate would have vertical bending and would have to be designed as a flangeless beam so that the compression edge of the plate does not buckle.
Any comments on any of the above? any other suggestions?
RE: Flat Slab Shear
RE: Flat Slab Shear