The building codes have changed over many years, but many buildings built under previous codes have not been upgraded to the present code level. New materials and methods have been developed for construction that were not available in the past. Decisions on how and whether to upgrade existing structures have generally been a matter of public policy and local law. Various methods are used to evaluate and upgrade existing structures are explained breifly below. APT staff are experienced in all the listed methods:
National evaluation methods:
FEMA 154, Rapid Visual Screening of Buildings for Potential Seismic Hazards. Intended to be a quick method of classifying a number of buildings for seismic risk. Additional, in-depth analysis of high risk structures would typically follow.
FEMA 178 - NEHRP Handbook for the Seismic Evaluation of Existing Buildings. Provides quick engineering analysis of an existing building to classify areas or portions of high seismic risk.
FEMA 273 - NEHRP Guidelines for Seismic Rehabilitation of Buildings. Provides methods of designing upgrades to existing buildings. Replaced by FEMA 356.
ASCE 31/FEMA 310, Handbook for the Seismic Evaluation of Buildings--A Prestandard. Revised FEMA 178 to incorporate experience into a potential standard document.
ASCE/FEMA 356, Prestandard and Commentary for the Seismic Rehabilitation of Buildings. Incorporates experience gained since FEMA 273 into a potential standard document.
Others exist but have not shown up in ordinances at present.
Each government entity can have their own local ordinance. You should consult a qualified local engineer for your area.
City of Portland, Seismic Design Requirements for Existing Buildings - Indicates the required modification standards based on changes of occupancy type, percentages of floor area changed, and various specific design types. Required analyses may be to upgrade to existing code requirements, to evaluate according to ASCE 31, or to strengthen roof structure and brace parapets on unreinforced masonry structures. Check with www.portlandonline.com under Chapter 24.85.