Shallow Footing Increasing

[tbone73]

Here's one for you Geotech guys. I have an existing shear wall footing that supports the wall obviously as well as s column load and two other smaller concentrated loads. It has been in service for about 5 years. We need to support two new columns on this existng footing (See attachment). Can you simply add on to the footing to increase the area to satify bearing capacity? Will the original pressure distribution change and redistribute once more footing area is added and the new columns loaded. Will the new pressure distribution be fairly linear assuming small moments due to eccentricities?

 

[concretemasonry]

Only the structural engineer can give the new added loads and a connection details to the existing footing that will carry the load and what the resultant effect might be on the combination of the old and new footings/loads.

Information on the depth of the footing and any lateral loads on the wall or columns will also be needed.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[VAD]

tbone73

Adding on to the existing footing is possible. The addition is to be such that the dimensions of the combined footing would produce a uniform pressure under the combined footing. This would result if the resultant passes through the centroid of the footing contact area. The width of the footing should be determined such as not to cause an increase in the contact pressure above that which was existing at the base of the existing footing. Finally, at the depth where the stresses from the existing and new footings overlap, the stress should not exceed the allowable pressure at the the depth of the overlapping stresses.

 

[rscassar]

The bearing pressure distribution below the footing is going to depend on more factors then just the geotechnical conditions.

Factors which are going to influence the bearing distribution below the footing is shoring of the wall and columns during footing widening; and the relative stiffness of the new portion of footing compared to the existing.

There are some good threads which are similar to the question which you pose.

thread256-241177
thread507-273074

 

[six06]

I would better recommend that (if the loads are considerably high)you underpin it with micropiles. With that you dont have to concern about the new loads and just transfer them to the MP.

 

[tbone73]

six06, supporting it with micro piles would be the expensive alternative, and the consequently the last approach. All other foundations are spread footings, so that is the option we are pursuing.

kikflip, thanks for the post of other threads they were insightful. But my question is still not completely answered. I know about typical footing design and pressure distribution, but this is a case where the existing footing has probably already undergone settlement due to dead load. Now we are looking to add on to this footing then load it with more load. Will any of the existing footing pressure transfer to the new footing? Or will the new footing simply share support of the added loads with the existing footing, while the existing footing supports all the original load?


(See the attachment and FYI the added footing will have nearly the same stiffness as the existing footing)

 

[Mickney]

tbone73:

The transfer of load will be a function of how the additional footing is connected to the existing footing. Will it be "structurally connected" to act monolithically? If so, then treat the combined footing as one footing. If not, then you have a different animal.

Settlement for the combined footing will be a trickier problem. Your geotechnical engineer for the project will have to analyze for the different phases of construction. His first analysis should be for the existing footing under the current load. The second analysis should be for the combined footing under the new load. The geotechnical engineer must keep in mind that the existing footing has already achieved some level of settlement. The geotechnical engineer should also have a good understanding of the subsurface conditions to provide you, the structural engineer, an indication of the anticipated settlement.

 

[InDepth]

I've done this several times with good results. Just add what you need for footing width using dowels and epoxy. Once you start adding additional load, the footing pressure will redistributed (deformation compatibility). Ie if the soil yields beneath a certain portion of the footing it has to redistribute itself (assuming you detailed the addition considering shear and flexural continuity).

Now, most of what I said above was for minimal changes in load (don't forget geotech FS of 2-3). If there are huge loads that exceed 2-3x then you need to have the geotech perform a more rigourous settlement analysis based on preloaded soil conditions.

 

[tbone73]

Thanks everyone for all your input, I appreciate it.