Concrete Bearing Stress

[strucguy]

What is the immediate consequence of concrete bearing stress being exceeded in the case of steel column with base plate supported on a pad footing. What is the worst that can happen?

 

[Lion06]

How much are you over?

 

[strucguy]

About 40%.

 

[Lion06]

40% over? That's a lot. Are you taking advantage of A2/A1? If so, and you're still 40% over, that would make me uncomfortable. That seems like service loads could be approaching f'c.

Can you increase footing depth or bump f'c? What strength concrete are you using?

 

[ToadJones]

The concrete will crush.
But, you'd be surprised how far over strength many concretes will reach.
We regularly test all off our pours. Our pours usually require 4 ksi and we routinely test them at 6+ ksi.
This is on a DOT "MS" mix.

 

[frv]

ToadJones brings up a good point.. I've rarely seen 28 day breaks come in less than 2 ksi over their specified strength.. Since bearing is directly related to the square root of f'c, it'd be worth investigating what the breaks are.

 

[Ron]

The worst that could happen?...It could fail and the building could collapse locally.

You're on thin ice with this one.

 

[Lion06]

I've seen plenty of 28-day breaks come in within 10% of the specified strength. I've even seen some come in just uder f'c (admittedly, not many).

40% over is no joke. When you take into account the bump for A2/A1, the bearing capacity is right around the specified concrete strength. If you're 40% over, that means you're approaching f'c for service loads. If you get into a true overload situation, well, I wouldn't be comfortable with being that much over.

The only real remedies (that I can see) are increasing f'c, increasing the footing plan dimensions and/or depth (depending what is controlling the ratio of A2/A1), or a combination of the two.

 

[strucguy]

Thank you so much for all your responses. I am investigating an existing building and providing enhancements for resistance against progressive collapse. So, was wondering if foundation settlement, and concrete bearing over-stress and other foundation related issues are of major concern in the said scenario. Since life-safety is the only issue here, I was of the opinion that I could over look these components. I am having hard time imagining a situation where the columns penetrate through the footing and the soil to the point where they not supporting any loads from the structure. Your thoughts on this are greatly appreciated.

 

[hokie66]

I agree that you can't ignore overstress in bearing. But if bearing is that high, I would suspect other strength issues with the footing, i.e. bending and shear.

 

[Ron]

strucguy...

was wondering if foundation settlement, and concrete bearing over-stress and other foundation related issues are of major concern in the said scenario

Yes, they are. An example: Differential settlement of a pad footing induced a moment in the structure that it probably wasn't designed for. Bearing overstress, assuming you really have it, can have a variety of implications, most notably as hokie66 stated...you have other problems you perhaps haven't considered with shear and bending.

If the original concrete design strength (f'c) was 3000 psi and it is 40 percent overstressed in bearing, you now need 4200 psi. Not much of a stretch for most ready mix suppliers, unless the concrete was low strength to begin.

Find out what the actual strength is before you go further....and don't use a Swiss hammer or Windsor probe to check it...take a few cores so that you can visually check the concrete and test its compressive strength.

 

[strucguy]

The structure and the foundation system are just fine based on conventional design. I am only having issues when I am performing progressive collapse analysis (loss of first story column condition). So, isn't this kind of overstress considered instantaneous and short lived until the structure stabilizes and finds alternate load path to transfer loads to the foundation? I am talking about D+0.25L load combination here with dynamic effects.

 

[ToadJones]

Maybe slightly off topic, but does anyone have a paper discussing foundation settlements/ differential settlements?

 

[ishvaaag]

Respect the initial question about if there will be strength enough whilst exceeding a A2/A1 stress, one finds strange that this may become a controlling factor in progressive collapse except for very overdesigned columns.

It is also pertinent to investigate the kind of analysis being performed. Structures redistribute solicitations and as soon as some downwards movement (from initial crushing or sudden increase of settlement) starts, axial load will be transferred elsewhere, to other columns; for fragile concrete not the more envisageable prospective to arrest collapse yet something can do.

On the other hand the on A2/A1 factor resumes what codemakers feel safe to use of 3D constraint at the footing and so you may have found a condition requiring revamping the column at its base.

Respect papers on settlement and differential settlement I would say one should be able to find about the subject in regulatory codes; at least the spanish code is quite somewhat explicit on what must be checked, see attachment at section 2.4.3

 

[PEinc]

Jack the column up and install a larger and thicker or stronger base plate.

http://www.PeirceEngineering.com

 

[BAretired]

What is the worst that can happen?

The worst that can happen is that the footing fails in either bending or punching shear. It cannot fail in bearing.

BA

 

[hokie66]

I would agree with BA for any practical situation. But if you had a big column with a chiseled end, bearing would be an issue. Perhaps the OP can give the footing, load, and column details so we can comment more intelligently.