Mat Foundation Weight

[slickdeals]

All:

Do you typically include the weight of your mat/raft foundation in calculating the ultimate bearing pressure values for designing your flexural/shear reinforcing?

For rafts that are 5-6' thick, it will cause significant additional pressures which doesn't really exist. The same would apply even for simple spread footings.

Appreciate your thoughts.

 

[bootlegend]

If you do include the weight to calculate the bearing pressure you should also be using the weight to calculate the shear and bending in the mat. This means there should be no net effect on the shear and bending due to self weight.

 

[CANPRO]

The additional pressure from the foundation does exist. Gravity pulls the foundation down, and the soil pushes back up. If you include the self weight of the foundation in the bearing pressure when checking bending/shear you just need to remember that the self weight of the foundation counters the pressure from the soil...or you could just get the bearing pressure ignoring the foundation weight and go from there.

 

[ToadJones]

The wt of existing soil removed down to the bottom of footing level comes into play here no?

 

[ToadJones]

oops...

you said:

"Do you typically include the weight of your mat/raft foundation in calculating the ultimate bearing pressure values for designing your flexural/shear reinforcing?"


Include the weight of the footing for gross soil bearing pressure, which should not exceed the allowable.
Design the footing itself for the net bearing pressure, which does not include the effect of the weight of the footing or soil above.

 

[jdgengineer]

Most soils reports I have seen (at least in California) typically allow you to neglect the weight of the footing when calculating bearing pressures on the soil.

 

[ToadJones]

do you think part of the reason is that at the soil bearing level, all the weight of the soil above has been removed and replaced with concrete & more soil?
Concrete to soil wt. ratio is somewhere around 1.3-1.5

 

[jdgengineer]

That's what I always assumed the reason was. The soil is over consolidated from the removal of the soil weight that adding in the concrete has a marginal impact on the consolidation of the soil. I'm not a soils expert though...

 

[hokie66]

A 6' thick ground supported concrete mat, without any loading from above, would not be subject to bending or shear...the load goes straight down. So the bending and shear are strictly due to superimposed loads, including hydrostatic uplift. If the mat is on piles, the answer is totally different.

 

[DST148]

For a structure with a symmetrical geometry and loading, inclusion or exclusion of the weight of the foundation would not affect the flexural and shear design of the foundation. However, for unsymmetrical geometry and loading, coupled with lateral loads etc.; inclusion of the weight of the foundation does affect the soil pressure distribution, effective area of the foundation (portion of the foundation in contact with the soil) and there by influencing the flexural and shear design of the foundation proper.

As an aside.... Ref- Suggested Analysis and design procedures for combined footings and mats, reported by ACI Committee 336: The pressure causing settlement in a mat analysis may be computed as
Net pressure = (Wt. of structure + Wt.of mat - Wt. of excavated soil(overburden) ) / Mat Area. The report clearly states that the overburden should be determined by the geotechnical engineer.