Connecting a New Foundation to an Existing 4

[azogr]

I have been asked by a client to design a foundation for a building which will cover an existing structure (a concrete aggregate bin). The bin sits on a large mat foundation and the dimensions of this new building are very tight to the existing structure.

Before I had known that the existing foundation was so large I had sized footings which apparently will now interfere with the corners of the existing matt. The new building only has four columns. The contractor thinks this is a simple issue and wants me to simply put a large notch in my footing and tie into the existing matt.

I was going to resort to building a model in RISA to see how my new irregular footing and the existing mat footing would interact if knitted together. Is this overcomplicating the issue? I am interested to hear how others would approach this problem.

I also have a question about how they plan to tie the two foundations together. The contractor says that they just drill holes in the existing foundation and hammer in rebar that fits tight in the hole and then pour the new foundations around the bar. They claim that have done this for years and they do not use epoxy, grout or any other kind of bonding agent. Is this common? I plan to specify Hilti expoxy and whatever size rebar is required to meet the shear load.

Any thoughts or comments are greatly appreciated.

Thanks!

 

[shin25]

At least in the beginning, the new foundation being on virgin soil will experience more settlement compared to the existing foundation. This will only create unwanted eccentric loading on the new and existing foundations.

I have a feeling that the large mat foundation for the aggregate bin is large enough and a chip out should not create any problem? If this is the case, why to tie the new foundation with the old one? Chip out existing foundation as needed, place some bond breaker at the interface and pour the new foundation. This bond breaker should make the new foundation work as per design.

 

[jike]

Look at the relative loading of the bin versus the 4 building columns. It may be that there the building load is not large compared to the bin. If you need to take a closer look, then do a mat analysis by Risa or PCA Mats with all loads applied to see the bearing pressures and mat stresses.

I would guess the contractor has little appreciation for your work.

 

[msquared48]

Not knowing the degree of overlap or the relative sizes of the footings involved, I would consider underpinning the new footings. Thast way the problem goes away. You just have others.

Mike McCann
McCann Engineering

 

[azogr]

Thank you all very much for the suggestions. I just got a set of drawings which seem to show that I have no conflict. I guess we will not know the real story until they start excavating. Needless to say I am hoping that there is no interference. If there is it shouldn't be as much as we had originally thought and I could do something like shin25 has recommended.

You are right about the contractor jike. He is used to dealing with a fly by the seat of your pants old timer who had 50 years of experience and would eyeball alot of the stuff right on the spot for him. Unfortunately I do not have all that experience to draw upon so when I need to take a few days to look at something he doesn't understand the delay.

Regardless of whether or not there is interference with the mat foundation I was still planning to tie my new footing into the 12" thick slab which is poured over the whole site. This is to resist some of the large horizontal forces from the PEMB. So this brings me back to my other question regarding the rebar dry driven into holes in the existing concrete. Has anyone else used this method or heard of it being used?

Thanks again!

 

[CTW]

How about just extending the area of the mat and setting the columns at each corner? To me, trying to chip out existing foundation is messy and labor intensive, and usually does not turn out the way the engineer expects.

Typically the design of the mat for an aggregate bin is controlled by wind or seismic. So the size of the mat is more than likely related to the mass needed to resist the overturning of the bin. This could be checked rather easily from some rough estimations. If the bin foundation is controlled by wind, then once the bin is enclosed, the wind is only acting on the building. Just increase the area of the bin foundation to what ever is required by your design and set the columns. If the bin foundation is controlled by seismic, then the bin foundation would need to be checked a little more closely with the building loads added to it.

In either case, by extending each side of the bin foundation, your essentially creating a larger mat to support both the bin and the building.

The contractor's method of drilling a hole and hammering the rebar tight is probably not something I would want to take responsibility for on a design. If the rebar did end up fitting tight, then friction is the only thing resisting the forces. You're on the right track by specifying Hilti and the bar size required.

 

[Structures33]

using a hilti system or epoxy grouting the rebar in place is the way to go.

 

[azogr]

So when pinning into the existing mat how do you maintain the continuity? Or do you just assume that it is a hinge?

 

[Structures33]

you can't really count on the existing footing to provide much vertical support when you dowel rebar into it, although it's definitely more than a hinge -- the rebar attachment is more to resist settlement and have the entire foundation/mat act as one in lateral movement. if you wanted to try and draw more strength from the existing as far as vertical support, i would put some sort of epoxy bond material on the existing face prior to pouring the new (in addition to the dowels).

 

[jike]

Be careful with bonding agent! Some types are water-sensitive. Others are hard to work (short time to install and pour). With cold weather coming on be aware of ambient temperatures.