How to widen existing footing 3

[tclat]

Hi,

I have been hired by a contractor to explore options to widen and existing footing.

The contractor cast the the column eccentric to the footing. 12x12 r/c column on 5x5 footing but 1 foot from the edge. Bearing pressures now exceed allowable.

One option is obviously to demolish and another to underpin although I'm not to keen on this option.

I was considering another option of casting a jacket around the existing column (say 5" around) and attach the two with epoxy dowels. I would then consider pouring a new footing over the existing footing and with the the reinforcment in the "jacket" embedded in the footing. This new footing would now be centered on the column. I would also have to expoxy dowel the new footing reinforcing bars into the existing column.

Has anyone seen this type of detail before? Any other options?

Thanks

 

[asixth]

Is it possible to splice bars which have been cut with chemical epoxy? I had a supplier in our office this week who said it is possible. But they also said that there product has sufficient resistance to fire but does not have test certification to confirm this.

You could demo the footing back and provide a welded splice. This could bring the lap length back to 6-8".

 

[3doorsdwn]

I remember I had a similar problem once and ran into the splice problem that others mentioned. (My situation was that the bearing pressure could not exceed a certain amount.) I remembered my way of handling it was to widen the footing as others have talked about....but to also build beams (2 per direction) on top of the footing to where [in each direction] it looked like an inverted double T beam. I not only anchored the new beams to the existing footing (including the addition), but also to the concrete pedestal on top of the footing (to help distribute the new load). Doing all this not only guaranteed the new load would be distributed evenly (due to the rigidity of the built-up footing), but it also made a splice unnecessary at the interface [between the old footing and new footing] because the load of the build out could be transferred by the beams.

The tricky thing I nearly forgot is: you have to size the new footing build-out (in plan view) for the NEW load. In other words: treat the existing footing like it already sees the DL+LL on a day to day basis. Then size the new (combined) footing so that the increase is not driving the bearing pressure of the old footprint too high [i.e. you will superimpose the pressure from the load addition (with the new plan dimensions) on the old pressure]. It will result in low pressure for the new [exterior] footprint....but I still designed the new concrete so that it could see the almost all the load in case the LL was not in place at the time the modification was made.

You need room to do it....but it can work if your existing concrete is good enough to anchor to on top (and on the pedestal).

My thoughts.

 

[Lion06]

3doors-
If the footing is a gravity footing what load does it see other than LL and DL? I would suggest that if the building is still under construction (as is usually the case when this kind of contractor error is discovered) that there is no LL on the footing and likely a small % of DL.

It is definitely something to think about though/

 

[3doorsdwn]

Structural EIT, the vast majority of the load it was under was DL+LL. (But questionable as to how much LL it was seeing on a day to day basis.) This particular footing never saw anything from lateral loads as it was not part of the lateral force resisting system.

The reason for my modification was not contractor error: it was an existing footing that was going to pick up about 30 extra kips from some new equipment. The icing on the cake (that help prompt my solution) was the fact that the concrete at the bottom of the footing made attachment questionable.

It was a tough problem but things turned out ok.......never got a phone call.