combined footing design: self-weight included? 2

[scoteboo]

This is my first post, so please bear with me.
I’ll keep this brief, I have two questions related to the design of combined footings:

1) I am wondering if a designer should/can account for the self-weight of the actual footing when designing a footing, specifically a combined footing?
The standard examples (available in text books and online)that I’ve seen show a footing with two columns. The footing is to be designed for the axial loads in the column, P1 and P2 by combining them and determining the resultant, and determining the eccentricity (if any). If the self-weight of the footing is considered, a large eccentricity can be reduced, but this never seems to be shown in the examples. Is it acceptable?

2) One of my axial loads is actually a tension load, causing a large eccentricity. Hence why question 1 is extra important for this situation, as it is the only resisting moment. Any red flags related to tension loads on a combined footing? All my reference material only addresses compression loads, and I find it strange.

Thanks in advance, I hope I have made this clear.

 

[msquared48]

If you are talking about including the self weight to resist overturning or uplift, absolutely include it.

As for the dead plus live soil pressures, I rarely include it.

Mike McCann
MMC Engineering

 

[Once20036]

If you're looking at a gravity only situation, I believe that whether or not you include the self weight of the concrete is dictated by your geotech report.
Sometimes they provide a net allowable bearing (no concrete weight) and sometimes they provide a gross allowable bearing (yes concrete weight).

If you have a tensile load, I'd absolutely include the weight of the concrete. Hopefully the examples you're looking at indicate that the bearing pressure under the footing will not be uniform. I typically consider it to be a linear variation across the footing.

 

[steellion]

Yes, footing weight can and should be included when trying to resist uplift or overturning. Confirm with your geotechnical engineer if the soil bearing pressure provided is gross or net to determine if footing weight should be considered for soil bearing pressure.

 

[OUe]

scoteboo: yes, you may include it. Be careful to include the reduced weight in the load combination. Also, if you are resisting for seismic design, you may be able to use the 25% reduction in loading (or 1/3 increase) for overturning based on ASCE 7. This can be found in section 12.13.4 if my memory serves. Some folks will design for overturning with no ASD coefficients in the combination and use a 1.5 safety factor much like a retaining wall. I think this is incorrect, but will get you a very similar solution to if you just check for stability with the load combinations and make it a 1.0 safety factor with the geotech parameters.

The only time you can use a 1/3 increase is if you use the alternate load combinations in the governing building code or if it is for seismic overturning and follows the ASCE foundation design definition.