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underpinning adjacent buildings

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I was walking around San Francisco yesterday and I saw the basement
excavation for a new high rise. There are tied back wide flange piles and
lagging on three sides of the excavation. One side is underpinning a 6-7
story concrete/masonry building. It has been completely underpinned and the
underpinning piers have tie backs as well. The concrete underpinning piers
are about a story and a half tall, below what used to be a full height
basement. I know section 832 of the CCC addresses the underpinning
responsibilities, which BTW are rarely exercised, because it results in
delays. I was wondering if underpinning a building, and lowering the grade
would effectively change the base of the building and therefore change the
seismic response of the building? If so, this would be a sticky subject
because upgrading the adjacent building for lateral loads is usually not in
the scope of work.

Jeff Smith

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