1" gap for earthquake on zero lot line in New York City

[ronimal]

Designing a two story building with a basement on a zero lot line in NYC. There is an existing adjacent building on the lot line. Code requires a 1" gap for movement in a earthquake but does not clarify if is required on the foundation wall below grade. Anyone know if they allow you to pour a foundation a wall against the existing building and then provide a 1" gap above that or does the gap have go all the way to the footing?

 

[JasonMcKee71]

Since basement is considered much stiffer compared to the upper structure because of the walls on its perimeter, I would suggest to keep 1" movement joint above grade only.

Jason McKee
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[MotorCity]

I am not familiar w/ the NYC code, but practically speaking, both building foundations will move in unison with the ground. The 1" gap is most likely the minimum allowance for different periods (sway) of both buildings to avoid pounding of the two adjacent structures.

 

[MrHershey]

Similar to MotorCity, not familiar with NYC code. But we typically will provide the gap only above grade.

One exception is if one of the structures has its perimeter basement walls designed as cantilevers and isolated from the rest of the building structure (as concrete, sheetpiling, or other). In that case, the building with isolated walls will have its structural base for lateral movement actually at basement level instead of grade and we'll carry the expansion joint down through basement level. This is more often done for flexible structures (high rise, moment frames), so would be pretty surprised if it applied to this case assuming the building next door is also 2-3 stories. I've done this in a 2-3 story structure before, but it was special moment frames and I wanted to keep my R value.

 

[KootK]

The shear demand time history of the the two basement structures will, in general, not be synchronized. This means that:

1) Sometimes you will get some small displacement pounding at the basement when the systems move counter to one another.

2) When the systems move in the same direction, shear load will redistribute between the two buildings intractably. Your neighbor's building might be adding to the sliding load on your basement walls. Or your parallel basement walls might exert significant bearing stresses on your neighbors subterranean slabs.

So, from a purely theoretical perspective, it would be nice to have the gap all the way to the footings. I agree with others that this is almost certainly not what the NYC code has in mind however. A few reasons for that:

1) Seismic problems below grade have proven pretty rare. And structural engineering is a pretty reactive science. No problems = no attention in most cases.

2) As a practical matter, it's not so easy to create and maintain a gap below grade. Doing it right would cost some money that owners are reluctant to spend on phantom problems.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.