GArage Addition, how to tie foundations together...

[Apexpredetor]

Good Morning,

I'm a civil P.E. in NY. I am planning a garage addition and I was running through some numbers and I am a bit stumped on how to tie the garage addition foundation to the existing addition?

Existing structure is a two story colonial, full basement (8ft deep).

I am looking to add a two story garage addition to the side of my existing house.

I am looking for the proper method to tie them together?

I plan to use CMU's, 4ft below grade, 2ft wide, 8" deep footer (a bit overkill, but it is my own home). Since the two footers will be offset by about 4ft, should I pin to existing CMU by grouting void spaces?

 

[bimr]

It is typical to install something like #5 horizontal bars @ 16-Inch on center vertically. Drill into existing foundation wall and install bar. Fill block void spaces with concrete. 18-Inch long bars are adequate.

 

[oldestguy]

A common approach for footings at different levels is to step up from lower to higher elevation. If not done, at least make sure the old backfill is compacted.

 

[racookpe1978]

Draw the problem, plan, elevation and section view. Let us think about it.

Can you drill or bore into the side of the existing slab?

 

[SimpleJohn]

I agree with seeing an elevation view first. I would also be concerned with immediately tying the new foundation into the old one, especially if the addition is founded upon fill. The initial settlement of the addition could stress the existing foundation and cause cracking. You might consider grouting the horizontal bars into the home first, but oversize the penetrations into the CMU's to allow for initial settlement. Only after the new structure is built come back and grout the block voids through a preparred opening.

 

[racookpe1978]

oldestguy (Geotechnical)
10 Feb 14 16:47

A common approach for footings at different levels is to step up from lower to higher elevation. If not done, at least make sure the old backfill is compacted.

How does he prevent a trap for water accumulation in that lowest-to-highest? Even if it were not flowing down the yard, snow, rain, ice would build up if not drained away very carefully.

 

[CELinOttawa]

There are two fundamental solutions:

1) Bind the new and old foundations together.
2) Isolate the new foundation from the old, and allow for differential movement in your detailing.

Both are valid, and have several ways of going about doing them.

Most commonly I will ask the client which they want, explaining th risks and potential benefits to each, and pursue a solution under their direction.

 

[pat75h]

Is the existing foundation block? If it is you can line up the new wall with the cells and tooth them together by knocking out some of the old foundation block. If it is concrete drill and epoxy rebar is the best.

May I ask why you are using block and not a poured wall especially a wall formed with ICF's.

 

[Apexpredetor]

Sorry for the delayed response.

IT is a CMU wall. I will put op some renderings of the set up later today.

Thanks for all of the responses thus far.

 

[msquared48]

If the CMU wall is solid grouted, then drill and epoxy at the center of the wall the same as a concrete wall.

If it is not grouted, drill horizontally in two cells minimum (16"), grout both solid, and insert horizontal rebar into the newly grouted cells, stubbing the bars out at least 24" to receive the new horizontal steel.

Mike McCann
MMC Engineering

 

[racookpe1978]

Mike: You would not drill first, stick the rebar lengths in the holes, then pour the concrete/grout in over the rebar?

 

[msquared48]

racookpe1978:

I think that's what I said... Or at least what I intended to say.

Mike McCann
MMC Engineering

 

[RHTPE]

Mike,

You say:

If it is not grouted, drill horizontally in two cells minimum (16"), grout both solid, and insert horizontal rebar into the newly grouted cells, stubbing the bars out at least 24" to receive the new horizontal steel.

Makes perfect sense if you have access to the top of the wall. But in this case there's a house sitting on top of the wall, which will then require breaking into the appropriate cell "columns" somewhere near the top of the wall in order to grout the cells. This also creates additional difficulty when constructing the new CMU walls to accommodate the projecting bars.

I would opt for CELinOttawa's suggestion

Isolate the new foundation from the old, and allow for differential movement in your detailing.

Which then leads to additional considerations when framing the 2-story garage against the existing house.


Ralph
Structures Consulting
Northeast USA