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?

 

[ToadJones]

Lion-
I am not sure I follow you.

Is the 0.6 in your post from a 0.6D load combination?

 

[RFreund]

Toad -

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

 

[ToadJones]

RF-

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

 

[RFreund]

TJ - Thanks I see now. Looking back at my class notes, we reran the calc using a factored dead load and factored live load. Then becomes do you need to check all factored load cases? I suppose technically yes.

EIT

 

[dcarr82775]

Toad,

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.

 

[ron9876]

Toad I simply multiply the service soil stresses by 1.6. I know it is conservative but usually doesn't have a large impact.

 

[ToadJones]

Glad to see my reasoning isn't far off from what others do.

 

[msquared48]

Are you absolutely sure of that Toad?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[ToadJones]

No