footing subjected to moments
footing subjected to moments
(OP)
I have a footing that is subjected to a rather small vertical load, a rather large moment, and with site constraints keeping the footing from being symetrical about the center of the column. The current design example I am using to design this footing bases the design off of a symetrical footing.
As of now i have tried to design resolving the moment into a eccentric vertical load. The eccentricity is so large that it is outside of the footings kern causing the 'heel' of the footing to be in tension. In this case qmin = 0, 3a away from the edge of the footing, and qmax = 2P/3ab. This is due to the fact that the Resultant of the triangular shaped soil pressure is placed directly under the vertical eccentric load to keep the footing in equilibrium.
I know qmax and P, and have assumed b=1'-0". In this case i can solve for 'a'. Now that i know 'a' i should be able to solve for the distance from the center line of the column to the edge of the footing in the direction of the 'toe' of the footing. Due to the fact that the heel is in tension does the distance from the center of the column to the edge of the heel need to be the same distance, symmetrical, as the center of the column to the edge of the toe. My thought is no since it really isn't doing any work. This will help with my site constraints keeping the heel side from being symmetrical with the toe side.
Please help with any advice. Thanks in advance for the responses.
As of now i have tried to design resolving the moment into a eccentric vertical load. The eccentricity is so large that it is outside of the footings kern causing the 'heel' of the footing to be in tension. In this case qmin = 0, 3a away from the edge of the footing, and qmax = 2P/3ab. This is due to the fact that the Resultant of the triangular shaped soil pressure is placed directly under the vertical eccentric load to keep the footing in equilibrium.
I know qmax and P, and have assumed b=1'-0". In this case i can solve for 'a'. Now that i know 'a' i should be able to solve for the distance from the center line of the column to the edge of the footing in the direction of the 'toe' of the footing. Due to the fact that the heel is in tension does the distance from the center of the column to the edge of the heel need to be the same distance, symmetrical, as the center of the column to the edge of the toe. My thought is no since it really isn't doing any work. This will help with my site constraints keeping the heel side from being symmetrical with the toe side.
Please help with any advice. Thanks in advance for the responses.






RE: footing subjected to moments
RE: footing subjected to moments
To answer your question directly, there is no reason why the heel distance would have to be symmetrical with respect to the column. In fact it often is not. However, this comes with a trade off in that your eccentricity can be moderated somewhat by the footing weight itself. If the footing becomes narrower by reducing the heel length then your moment arm for the footing weight is also reduced.
RE: footing subjected to moments
The moment is due to the column being one of two supports of a cantilevered carbis truck access system where most of the load is vertical on the platform of the carbis system therefore causing the moment at the column base. There are wind loads that occur laterally, the worst case causing a moment in the same direction as the vertical live load and dead load on the platform of the system. The other directions of wind load occur in the direction of the center line of the two carbis columns, therefore the loads are supported by the system.
RE: footing subjected to moments
Any possibility of using an interconnecting concrete grade beam between the two footings to simplify the design - grade beam giving further leverage to resist the overturning.
Mike McCann
McCann Engineering
RE: footing subjected to moments
RE: footing subjected to moments
RE: footing subjected to moments
Are you using the weight of the footing, and any soil overburden, to your advantage?
DaveAtkins
RE: footing subjected to moments
RE: footing subjected to moments
RE: footing subjected to moments
RE: footing subjected to moments
Then tie all four footings with a grade beam.
RE: footing subjected to moments
Just design yourself a big fat footing to resist the overturning and be done with it. You can make the footing asymmetrical about the posts but this may confuse the contractor.
RE: footing subjected to moments
I wouldn't try to cut the footing down because the "a" distance says you aren't using the whole thing. Those equations are based on a lot of assumptions and give us something to design with, not something that necessarily represents the actual stress condition or location.