footing subjected to moments 2

[honthan]

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.

 

[UcfSE]

Does the moment act without lateral load? What happens to the footing under gravity only? Can the moment reverse?

 

[MarcbSE]

The formula I have always seend for a footing with net uplift on one side like you have is qmax = 4P/3L(B-2e) but your formula may work out the same depending on the definition of "a".

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.

 

[honthan]

UcfSE
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.

 

[msquared48]

honthan:

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

 

[civilperson]

Remember, the wind blows in all directions so solve again at 180 degrees and each of the 45 degree increments. A drilled shaft is vey effective in providing support for vertical beams,(small axial loads and large horizontal loads which produce moment).

 

[gwynn]

If you keep with seperate unlinked footings, your thinking is correct. The only reason I see to keep things symmetric if the moments are not symmetric is if you have settlement concerns.

 

[DaveAtkins]

I agree that your thinking is correct, except "b" does not equal 1'. It equals the width of the footing.

Are you using the weight of the footing, and any soil overburden, to your advantage?

DaveAtkins

 

[shin25]

I agree with msquared48, you should tie the footing together with grade beams.

 

[honthan]

unfortunatly, I am unable to interconnect the two individual footings with a grade beam due to another pair of columns with footings between the two carbis columns. thanks for the input.

 

[honthan]

the owner has also wanted to steer away from piles or drilled shafts due to cost. thanks for the input

 

[shin25]

honthan,

Then tie all four footings with a grade beam.

 

[JLNJ]

A good portion of the eccentricity is probably due to the overhanging nature of the Carbis platform.

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.

 

[UcfSE]

I asked because I didn't know if the moment was temporary or not. It sounds like it's permanent and in one direction only. If so, you may be able to design it asymmetric without worry, but be careful if you do so. Load reversal might be bad. There are plenty of footings designed on a property line with a column or wall right at the edge, so it's not a new thing but you do have to be careful.

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.