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Re: criteria for retrofit of existing structures

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You are correct, that the UBC is not appropriate for design of retrofit of 
existing buildings, although, the California State Historical Building Code 
allows for historic buildings, use of the UBC and design seismic loads of 75% 
of the loads required by the 1995 California Building Code (1994 UBC).

The 1997 edition of the UCBC (published by ICBO) has retrofit standards for 
unreinforced masonry buildings and conventional light wood-framed residential 
buildings.  The City of Los Angeles Building Code includes the following 
retrofit standards:
Division 88 Unreinforced Masonry;
Division 91 Tilt-up Concrete Wall Buildings;
Division 92 Residential Buildings with Weak Cripple-walls and Unbolted Sill 
Division 93 Wood-framed Residential Buildings with Soft, Weak, or Open-front 
Division 94 Hillside Dwellings;
Division 95 Concrete Buildings and Concrete Frame Buildings with Masonry 
Division 96 Concrete and Reinforced Masonry Wall Buildings with Flexible 

All except Division 88 were developed by the City of Los Angeles and seaosc 
to deal with seismic weaknesses in existing buildings that became apparent 
due to the Northridge Earthquake of 1994.  Division 88 has been in use 
(modified with updates from time to time) since the early '80's.  All are 
available from ICBO by ordering the 1999 Los Angeles City supplement to the 
1997 UBC.

Nels Roselund
Structural Engineer