Differential settlement in new connector structure between two existing buildings

[dukeguy]

Hi,

I'm helping an architect friend on a home-remodel job. The existing situation I'm looking at has a two-story house and a detached one-story garage. The design calls for adding a second story to the garage and building a new enclosed two-story connector that links the house and the garage (the length of the connector would appx be the width of a two car driveway give or take).

Basically, my question is would you feel comfortable about there being negligible differential settlement between the new foundations for the connector and the existing garage/house foundations (i.e. wall footings) as long as you satisfied the soil bearing capacity requirements?

The thing that makes me a little unsure is I feel like I have three different bearing conditions: 1.) added second story onto existing first (new settlement?), 2.) brand new two stories, 3.) existing house w/ no new floors (no more settlement?). I didn't know if it was smart to just rigidly attach the connector structure to the buildings on either end, or should I just treat it as a completely independent structure and build in a vertical movement joint on either end so it doesn't interfere with the adjacent buildings...

Thanks!

 

[oldestguy]

If this is in northern climates, you also have frost heave considerations.

 

[oldestguy]

Whoops: If this is south of the equator, change "northern" to "cold". Sorry.

 

[jeffandmike]

Depending on the specifics of what's being done, I would probably avoid connecting the two buildings altogether (let alone "rigidly"), for a number of reasons, and have the bridge structure be independently supported, both vertically and laterally. I am guessing this won't be too popular with the architect friend, but if they want it built that way (i.e., connected), they should sign/seal it themselves, and assume responsibility for it (assuming this is allowed by the local jurisdiction). If it is their (the architect's) home, and/or if you're not being paid for the work, they should be assuming responsibility for it anyway.

I have a mechanical engineer friend, whom I have known for over 25 years, who was doing a remodel/addition on his home awhile back, and he asked for my assistance. I agreed to it... with the condition that he would do all of the drafting, he would assume responsibility for the final plans, and he would do all of the necessary interfacing with the building department and the contractor. I would merely offer him some advice on the design/plans. We sat down over dinner at his house, on a couple of different occasions, for maybe an hour total each time, and I pointed out issues that I saw with his design. He sometimes took my advice, and sometimes didn't. I thought it went really well overall. I got a couple of free meals out of it, we're still friends, and the remodel still looks good ten years later (they still live in the house, too).

 

[JAE]

You might consider oversizing the link portion footings to minimize settlements. That is really all you can do. Having an expansion joint in a wood framed residence is not very typical...i.e. never happens.

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[Ron]

Depending on your local soil conditions, the bearing capacity and settlement might be mutually exclusive. You can design the footing to exert less than the allowable bearing pressure on the soil, but still get settlement; however, you are dealing with lightly loaded structures and the performance history of the existing structures, so unless the garage is on significantly different soils than the house, the performance of the house with respect to settlement will give you your answer.

 

[XR250]

You could hire a geotech to evaluate the soils and estimate the potential settlement. If there are no foundation cracks currently, you are probably fine. You are not adding a whole lot of "realistic" weight.

 

[jeffandmike]

It may be a good idea to think about this in terms of issues other than differential settlement as well. This is kind of what I was getting at in my previous post when I said "for a number of reasons".

So... now that we've connected the buildings with a bridge, how good of a job did the designer do detailing the penetrations/connections through the building envelope of each of the two buildings, so that moisture doesn't get in? How good of a job did the contractor do with flashing/sealant at the penetrations/connections? How good of job is the owner going to do at maintaining those joints, and recaulking/addressing them as frequently as is actually required (if ever)?

In my experience, moisture intrusion/damage is one of the most frequent failures (on buildings) that occurs. Everyone assumes someone else has addressed it, and/or that the contractor installed things properly, and people (by and large) don't do a very good job of maintaining their buildings. This is even the case on very expensive buildings/construction... not just the cheap ones.