The response of an existing building to earthquake motions reflects the performance level inherent in the codes, standards, and construction practices in existence at the time of the design and construction of the building. Deterioration and improper alteration during the service life of a building affect the actual margin of safety provided by the building. Thus there is a need to evaluate the potential seismic hazard of existing buildings. Following evaluation of the original structure, the effectiveness of appropriate strengthening or retrofit procedures may be determined. A methodology is presented in the user documentation for evaluating the need to upgrade existing buildings by identifying items requiring strengthening or repair, and then by verifying the adequacy of rehabilitation. This evaluation is given in terms of the behavior of the critical structural elements in a building. The determination of this behavior requires an analysis of the structural response of the building to prescribed forces. An analytical model of the building is defined with data collected from a field inspection of the building, original structural calculations and drawings, test records, and test of the existing building materials. The 1973 edition of the Uniform Building Code (UBC73) serves as the basis for calculations made with the analytical model. A building's description, plan dimensions, inertia and stiffness properties are input to the ACE program. UBC73 earthquake loads are calculated either by (1) the UBC73 building period formulas and basic shear distribution equation, or by (2) the building period and/or fundamental mode distribution of base shear obtained from the detailed analytical model. Member forces for user defined critical elements are calculated and used to find critcal stress ratios for these elements. Among the critical building elements the user can consider are steel, reinforced or unreinforced masonry, and reinforced or unreinforced concrete.