UBC 1997 Soft Story Stiffness Irregularity

[clarke1973]

I have a 3 story concrete framed building with a two level basement to design. The building design meets all of the requirements of 1629.8 to enable a static analysis to be carried out, however in the basement B1 structure I have a stiffness irregularity as the floor structure is partly open i.e does not extend the full basement foot print.
My question is this. Does the definition of the soft story apply to all levels of the building above and below ground, or is it only applicable for the levels above ground?

 

[JAE]

UBC 97 - Definitions Section 1627 -
"Base is the level at which the earthquake forces are considered to be imparted to the structure..."

For a basement, the whole of the "bottom" and the side walls have earth interface that can impart load on the building - so I would imagine the concept of your ground level being considered as the base has some validity, as long as you include the lower portions in the overall structural framing.

But the concern with the irregularity, below the ground surface, is a legitimate concern in that ground motions whipping through the structure could be amplified by the irregularity, even below the surface, and collapse/life-safety might be an issue.

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[JAE]

Here's an article from Structure Magazine on the topic:

http://www.structuremag.org/wp-content/uploads/2014/08/C-StructPerformance-Kelly-Dec091.pdf

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[JAE]

Also check this thread out: thread507-379456


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[clarke1973]

Thanks JAE