Enlarging a wall footing 1

[Ben29]

We are adding a 66-foot wide addition onto the side of an existing building. The arch is showing the new roof (approx 66-foot span) to bear on an existing 12" block wall. The existing wall footing is only 12" deep by 24" wide with (3) #5 X cont rebar in the footing. There are no tie bars. See image below.

The architect suggests enlarging the wall footing - "whatever it needs to be" - in order to get the footing to work for the additional load. I checked it and it doesn't work due to eccentricity. Also, I don't have a dowel bar into the footing so there is no fixity between the wall and the footing.

Even if I could get the footing to work, How do I develop that rebar if there are no existing tie bars in the existing footing? Do you have to worry about developing the bars if you are using Hilti HY 200 + rebar?

They are hell bent on getting this to work.

 

[XR250]

Maybe add some helical piers instead?
Also, have you had a subsurface investigation done to confirm the 2000 PSF?

 

[Ben29]

I suggested helical piers. They didn't bite.

There was a geotech report done in 2009. I called the geotech to see if we could get more capacity out of the soil. They are being very slow to get back to me (understandably) - but it has been almost 2 weeks since I first contacted them and they aren't communicating much at all.

 

[KootK]

The architect suggests enlarging the wall footing - "whatever it needs to be" - in order to get the footing to work for the additional load. I checked it and it doesn't work due to eccentricity. Also, I don't have a dowel bar into the footing so there is no fixity between the wall and the footing.

Your concerns are valid. Turning a "do-nothing" plain concrete strip footing into a reinforced, significantly eccentric strip footing is going to land somewhere between difficult and impossible depending on what you've got in there.

Even if I could get the footing to work, How do I develop that rebar if there are no existing tie bars in the existing footing? Do you have to worry about developing the bars if you are using Hilti HY 200 + rebar?

Without existing rebar to lap to, this becomes an anchorage / plain concrete design problem for the most part. And it's unlikely to work out for the reasons that you've identified.

The "whatever it needs to be" approach needs to pay some homage to what you need it to be here. Since the eccentric scab isn't like to work, that means that it will need to be something else. Some additional options to add to those that have already been proposed:

1) Bite the bullet, demo some of the existing SOG, and underpin symmetrically as shown in the sketch below.

2) Turn the framing if possible.

3) Support the framing from beams and columns against the wall that would be supported by strap beams to deal with the eccentricity.

 

[Ben29]

Koot - thank you for the response. And thank you for validating my thoughts on the matter.

To add to the confusion... the as-built drawings show the following detail (image below). If lean concrete was used, would you be able to attach the helical piers to the lean concrete? Or would you need to chip away at the lean concrete in order to attach the helical pier directly to the footing?

 

[KootK]

If lean concrete was used, would you be able to attach the helical piers to the lean concrete?

I would think that you'd want to chip away to more robust concrete as you said.

That said, if the lean concrete is there, I'd be tempted to try to use that as the underpinning that I recommended earlier.

 

[EngineeringEric]

Can you underpin with a wider foundation?

I agree, you can't rely on what is shown in your first detail. I see it built, but I don't agree it works on paper for any substantial load transfer.

 

[hokie66]

You need a new soils guy, or at least get the old one to cooperate. 2600 PSF, unfactored, on competent subgrade is usually achievable. Maybe the original 2000 PSF was just an eyeball number? As you have access to the new construction side, should be easy to determine.

Doing as the architect suggests will not help.