Bearing Pressure Question
Bearing Pressure Question
(OP)
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?
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?






RE: Bearing Pressure Question
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Bearing Pressure Question
I checked overturning based on service loads.
RE: Bearing Pressure Question
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
RE: Bearing Pressure Question
RE: Bearing Pressure Question
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Bearing Pressure 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?
RE: Bearing Pressure Question
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.
RE: Bearing Pressure Question
RE: Bearing Pressure Question
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
RE: Bearing Pressure Question
So, I was simply going multiply the max bearing pressure I obtained using service loads by 1.6.
RE: Bearing Pressure Question
RE: Bearing Pressure Question
RE: Bearing Pressure Question
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.
RE: Bearing Pressure Question
A lot of words that are of no help, sorry.
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Bearing Pressure Question
RE: Bearing Pressure Question
This question is totally academic since, as CANEIT may have said, bending and shear in the footing are of no consequence.
RE: Bearing Pressure Question
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.
RE: Bearing Pressure Question
"...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.
RE: Bearing Pressure Question
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.
RE: Bearing Pressure Question
I am not sure I follow you.
Is the 0.6 in your post from a 0.6D load combination?
RE: Bearing Pressure Question
Here what I see as your question.
Run calc for overturning using service loads - find bearing pressure.
Then do you take this pressure and multiply by 1.6 or do you rerun the calc with the ultimate loads and find new bearing pressures?
Right?
What happens if for your service loads the resultant falls out side of the middle third but then you apply ultimate loads and the resultant falls outside of the footing??
conservatively you could just apply your max allowable bearing pressure as a uniform load only the entire length of the footing and multiply it by say 1.5 or I guess just use 1.6. Depending on size I still don't know if this would control over minimum amounts of reinforcement.
EIT
RE: Bearing Pressure Question
Run calc for overturning using service loads - find bearing pressure.- YES
Then do you take this pressure and multiply by 1.6 or do you rerun the calc with the ultimate loads and find new bearing pressures? - I HAVE DONE IT BOTH WAYS, QUESTION IS, TO INCLUDE FOOTING WEIGHT AS DEAD LOAD IN THIS CALC IF YOU CHOOSE TO RE-RUN NUMBERS TO GET ULTIMATE LOADS
What happens if for your service loads the resultant falls out side of the middle third but then you apply ultimate loads and the resultant falls outside of the footing??
WHAT HAPPENS IN MY ACADEMIC CASE IS, I SIZED FOOTING USING SERVICE LOADS AND MADE SURE THE LOAD WAS IN THE KERN. IF I FACTOR LOADS AND RE-RUN TO GET ULTIMATE BEARING PRESSURE, LOAD IS OUTSIDE KERN BUT NOT OUTSIDE FOOTING
conservatively you could just apply your max allowable bearing pressure as a uniform load only the entire length of the footing and multiply it by say 1.5 or I guess just use 1.6. Depending on size I still don't know if this would control over minimum amounts of reinforcement. - THIS IS TRUE, QUESTION IS PURELY ACADEMIC AT THIS POINT. ANY FOOTING I HAVE EVER SIZED FOR SOMETHING LIKE THIS HAD MIN STEEL
RE: Bearing Pressure Question
EIT
RE: Bearing Pressure Question
I calculate the bearing pressures at service loads. Then for simplicity I multiply the bearing pressures by 1.6 to get something akin to ultimate pressures to design the footing. I do include the weight of the footing but for ease put the 1.6 factor on it as well to make my life easier.
RE: Bearing Pressure Question
RE: Bearing Pressure Question
RE: Bearing Pressure Question
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Bearing Pressure Question