Bearing Pressure Question 1

[ToadJones]

Rather dumb question...

Designing a footing with large OT moments but very little axial load.
The engineer supplied me with an OT moment for the footing that was nearly all from live load.

Can I simply multiply the bearing pressure I determined with service loads x 1.6 (ACI) in order to obtain the ultimate bearing pressure for the footing design?

 

[msquared48]

You will still need to include the dead load of the footing in your analysis for overturning Toad. If I understand your question correctly, I don't think just a 1.6 factor will work here.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[ToadJones]

that's kind of what I was getting at...

I checked overturning based on service loads.

 

[msquared48]

I always check overturning (stability) based on unfactored loads, but determine the factored soil pressures so the footing rfeinforcing can be designed.

The size (footprint) of the footing is determined based unfactored loads.

Is that what you mean then?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[Ron]

With a large OT moment and low axial load, loss of contact on the footing is a real issue. Check footing bending and loss of contact to get the bearing pressure. Can sometimes increase the bearing pressure by factor of 2 or more.

 

[msquared48]

Yep. Gets into an entirely different set of equations. That's where the use of a grade beam might be considered.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[ToadJones]

I was worried that I wasn't being clear enough but I got in a hurry to post my question....

The issue that has me a little befuddled (embarrassingly enough) is that I am using concrete weight in the form of a spread footing to resist overturning at the owners request (can't believe how many times I have been asked to do this over the yrs instead of a pole footing). So the overwhelming majority of the dead load is from the concrete itself and soil over burden. I checked OT using service loads which was mostly concrete wt and have a F.O.S of nearly 3.

Normally I would not include footing weight in calculating my ultimate bearing pressure for use in designing the footing itself but in this case it is the only real dead load.
I was considering taking my soil pressures that I obtained from service loads and simply multiplying the max soil pressure by 1.6 even though it would be conservative (bending not likely an issue at all anyway).

Clear as mud?

 

[DRMABZ]

There are well used methods for calculating bearing capacity for a footing with horizontal load, including horizontal loads that produce a moment. This is a slightly more complex bearing capacity equation to a foundation under pure vertical loading

Essentially the moment reduces the effective foundation area and the horizontal load reduces the bearing capacity. The vertical load including the foundation self weight is an important factor on how detrimental this H and M loading is.

You will find the methods for foundation design under HVM loading in most advanced geotechnical textbooks and many foundation design codes. However, if your not familiar with this aspect of foundation design it may also be worth seeking some help with this.

 

[CANPRO]

I understand...it just doesn't feel right not having any dead load when you're factoring something. Obviously you can't go wrong counting the dead load...but physically the weight of the footing will always counter the bearing pressure, so you can't go wrong leaving it out either

 

[ToadJones]

I guess I'm not making my point.

I am aware of the methods for determining bearing pressures and the bearing area with biaxial moments and shears that cause overturning. I have made some crude spreadsheets that calc the bearing area where the line of zero pressure is on any angle across the footing as I have had to deal with this in the past.

I intentionally sized this particular footing so that the full footing is in contact under service loads.

However, my question really has nothing at all to do with this

 

[ToadJones]

ACI used to have some provisions for transforming the actual bearing pressure into an ultimate bearing pressure by using a ratio of dead and live load....I guess my question is, I really have no superimposed dead load, only the dead load of the footing and a moment due to live load.
So, I was simply going multiply the max bearing pressure I obtained using service loads by 1.6.

 

[CANPRO]

I think I get what you're saying now...if you ignored the dead load to get the factored bearing pressure, it wouldn't make any sense because the dead load of the footing is the only thing that keeps your resulatant force anywhere near the footing. Is that your problem?

 

[ToadJones]

Yes, basically that is what I am trying to say.

 

[CANPRO]

If your footing is sized just to get the dead load, bending and shear probably won't be an issue, even with dead load counted in the factored pressure, as you had said.

But I think you would still be justified in taking your specified bearing pressure, subtracting the pressure from the concrete and overburden soil, and then multiplying that by 1.6 to get the factored bearing pressure.

 

[paddingtongreen]

I always used the footing mass in the calculation, it is part of the structure for me. The factored case has always bothered me though. As I interpreted the requirements, the bearing stress had to work under service loads for the allowable stress and under factored loads for ultimate bearing stress, and sometimes you can't get there from here. Most of my colleagues disagreed and believed that the phony bearing stress is only for concrete design, not the soil.

A lot of words that are of no help, sorry.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[Ron]

Like Mike McCann, I don't use factored loads for overturning or bearing pressure. I do use the weight of the concrete, because in some of the aluminum structures I've designed, the overturning moment is huge, and the only appreciable resisting moment you have comes from the footing.

 

[ToadJones]

So I should probably determine my factored bearing pressure by re-running the calcs and factoring the dead load of the footing itself + the live load OT moment?

This question is totally academic since, as CANEIT may have said, bending and shear in the footing are of no consequence.

 

[ToadJones]

Ron-
Absolutely....I use the weight of the footing and the overburden if I can reasonably count on the soil to be there in calculating the foundation stability.

 

[ToadJones]

Pad-
"...Most of my colleagues disagreed and believed that the phony bearing stress is only for concrete design, not the soil...."

In my interpretation, the factored soil pressure acting on the bottom of the footing is no different than a factored load acting on a beam in concrete design. So, I guess I too think it is just there for the footing design.

 

[Lion06]

Toad-
I understand what you're getting at and I think you should probably use the factored loading for the footing design and here is why - if you multiply the service load stresses by 1.6, you're taking advantage of 1.6*0.6 = 0.96 on the DL axial load, when in the strength equations you only use 0.9. Admittedly, it's a small difference, but if the OTM to axial load is so large, it might make a big difference.