Steel Column on Elevated Concrete Slab

[spaseur]

I have a steel column that will be supported on a concrete slab supported on a 24"x24" concrete column. The dropped panel at the top of the concrete column is 16" deep. There has been much discussion in our office of what the bearing capacity of the concrete. The discussion basically comes down to the what is A2? Is A2 determined by the plan size of the concrete slab? In which case the bearing capacity will increase by a factor of 2. If A2 is determined by the size of the column below - 24x24, the capacity will increase by factor of 1.2 (20x20 base plate). What are your thoughts/opinions? See link below for picture.

 

[radiocontrolhead]

I would consider the size of the column below as my maximum bearing area.

 

[JAE]

Sort of a gray question but for me I wouldn't go further than A2 = 24 x 24.
However, the commentary in ACI 318 Chapter 10 suggests that the A1:A2 increase in bearing is due to additional concrete beyond the direct bearing area A1 that helps confine the concrete and improve the bearing resistance.
Taking this to its extreme conclusion I just doubt, though, that you could take an A2 area based on extending out 32" (2:1 on 16") each side of the base plate.

 

[azcats]

JAE - A2 based on your theoretical 32" extension would be 7056 sq-in. This would make A2 irrelevant as the limiting equation from Table 22.8.3.2 (318-14) would be (b) which doesn't include A2.

So it's not really like A2 is unlimited but capped at 4 x A1. Which in this case would put that 2:1 angle at only 5" below the bearing surface.

In that case it seems conservative to use the 24x24 for A2.

I could be swayed - interesting question.

 

[JAE]

azcats - good point

 

[spaseur]

JAE - does this mean you are persuaded to an A2 plane located 5" below the slab?

 

[r13]

I would conservatively use 24x24, but the maximum you can go is d/2 on each side of column to avoid overlap the failure cone.

 

[azcats]

The way I'm reading the code is that A2 is really not related bearing stress at that A2 level - but more of an edge distance modifier to evaluate for surrounding concrete which confines the bearing area increasing allowable stresses up to the maximum in the second (b) equation.

The commentary specifically says that this frustum should not be confused with load path. Also says that the depth/thickness of the member is not a consideration for bearing.

IMO, for pure bearing I believe the equation (b) is correct in this scenario. (Not saying there's not a host of other checks to consider here though)

I didn't look into the Hawkins paper that referenced in the code, but I wonder if this is similar to the side face blowout research where it was likened to a fluid pressure inside the concrete and is controlled by bearing area and edge distance.

 

[JAE]

spaseur - I think what azcats has stated is pretty accurate.

 

[spaseur]

Gentlemen,

I would like to thank you for taking the time to express your thoughts and opinions. It has been very helpful.

 

[JLNJ]

In old ASD terms, the full .7f’c on a 20x20 base plate would be 0.486f’c on the 24x24 column. That’s a 1200 kip service load. Yikes.

 

[KootK]

My strict interpretation of the ACI provisions as written would be the lesser of these four checks.

 

[KootK]

But the story gets a little murkier the further one digs.

1) First expression below is what Hawkins originally proposed. It's a function of the concrete tensile strength as one would expect.

2) The ACI simplification of Hawkins' equation winds up being less conservative than Hawkins' equation above about 5ksi.

3) The Hawkins testing was done an a single punch model as shown below. On a long, double punch model the results are about the same. However, on a short, double punch model, the results worsen some because the lateral bursting effect from both the top and bottom loads kind of aggregates in one spot. This short, double punch model would actually seem to be the closest analog to OP's condition.

4) Rationally, if you're a short, double punch model but you can push on a bunch of neighboring concrete for confinement in addition to just relying on concrete tensile stress, I'd have to think that you'd be in fairly excellent shape. That, particularly given that there's probably an concentration of slab rebar in this area too. This might be a justifiable reason to throw some of it in the bottom of the drop panel.

 

[spaseur]

KootK,

Thanks for the input. It is very enlightening. Much appreciated.