Existing Pad Footing Enlargement

[neaveyoung]

Recently I deal with one project that need to enlarge the existing footing due to the additional loading. I have some doubt to design it.

1.Is it possible to enlarge the footing because I’m not allowed to remove the existing footing?

2.The new footing area is base on the existing footing area plus the new footing area?

3.How about the new reinforcement? Can I embed the new reinforcement from the existing footing?

4.Do I need to check the punching shear? Can I deepen the existing pad footing?

I will very appreciate if somebody can give me some concrete idea and suggestion.

Thanks.

 

[Stego1]

Hi

It would probably be better to underpin the existing footings to carry the loads down to a more suitable stratum.
You would have all sorts of issues to consider if you widened the existing pads such as additional settlement, uneven settlement, tie in problems etc etc. Although I suppose it depends on the magnitude of the additional load and the tolerances of the structure......

Hope this helps

 

[worldofmud]

not an expert on this but it may, depending on the age of current footing and proposed loads, pay dividends to investigate the soil conditions underlying the existing foundation-it may be able to support the additional load without further work. Perhaps details of existing and proposed loads/soil type/age of existing footing would help a bit.

 

[csd72]

We used to allow an extra soil capacity due to the fact that the soil would have already consolidated. Getting a geotech to verify this is a good idea.

Otherwise underpin and cast a U shaped footing around the existing.

 

[kslee1000]

How many footings you need to strengthen, 1 or 2 out of many, or the entire building columns?

 

[jike]

There are many different ways to approach this problem.

If existing footing and pier have sufficient structural capacity (bending, punching shear and beam shear) then it might be advisable to test whether soil is actually stronger than assumed.

You can enlarge a footing to increase bearing area but as you alluded to, you would need to check shear and bending. If one of these fails then you may need to enlarge the pier, base plate or add concrete to the overall thickness. You may run into limitation on how deep the footing can be. Make sure that you dowel the new and existing footing together sufficiently so that it functions as one monolithic footing. I do not suggest relying on bonding agents! Make sure that you can develop the reinforcing.