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Re: Residential URM

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I believe that jurisdictions that have adopted URM ordinances have specifically targeted commercial buildings and apartment buildings.  Single family and duplex residences are generally excluded.  Perhaps Fred Turner could be asked to weigh in on this.
The 1997 UCBC is the ASD code that currently has been adopted for seismic strengthening of URM buildings.  It does not exclude residential buildings; the city's adopting ordinance is usually the document that states the scope of the ordinance with working that excludes single-family residences.  The Standard that I have used most recently is "Guidelines for Seismic Retrofit of Existing Buildings", published by ICBO.  It is a strength-based document that is otherwise essentially the same as the UCBC that I have found to be acceptable to the jurisdictions I have designed for.  It also does not exclude residential buildings.  However, I have not encountered a jurisdiction that applies either as mandatory for single-family or duplex residential buildings.
But, an addition to a URM residence is a different matter; it triggers other requirements: see Section 101.3 and Chapter 34 of the California Building Code.  Additions and alterations are allowed without requiring the existing to comply with the modern code providing the new construction complies with the modern code AND providing that the addition shall not cause the existing building to become unsafe.  This generally is interpreted as meaning that seismic interaction between the addition and the existing needs to be evaluated, and the seismic demand on the existing shall not be increased by the new work unless the existing is made to comply with the modern code.  My approach to deal with this interpretation has been to design the addition to brace itself plus the tributary portion of the existing with rigidity to assure that the seismic loads to the existing shear walls are reduced or at least are unchanged.  This generally means that new shear walls must be as rigid as [or more rigid] and stronger than the existing shear walls in the same line and thus they must be of masonry or concrete.  In this way, I can show that the shear loads to the existing shear walls are not increased.  Also, a system of drag-struts that interconnect aligned new and existing shear walls is needed.  The result is that the new work is quite disruptive to the existing.
Nels Roselund
Structural Engineer
South San Gabriel, CA