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To assess seismic vulnerability of existing reinforced concrete buildings in Iran, cyclic and monotonic load test on six beams was carried out. Three of the specimens represented semi-engineered buildings and three of them represented well designed buildings that have been designed according to the seismic provisions of ACI3 18-99. Seismic provisions and standard detailing requirements are not met adequately in the semi-engineered buildings. The semi-engineered and standard specimens had similar span length and identical seismic demand in the buildings, but they differ in dimensions of the section, amount and arrangement of both longitudinal and lateral reinforcement, and concrete strength. All the specimens were half-scale and had 1250 mm length and 150 to 200 mm width. The aim was to determine seismic characteristics in terms of energy absorption capacity, ductility, and vulnerability of the beams under strong cyclic and monotonic motion. Based on the test results, parametric idealized curves for cyclic response and backbone curve of both standard and semi- engineered beams were established. The idealized curves could be used for numerical simulation of the existing buildings under seismic load. During the tests, the semi- engineered specimens showed much less strength, ductility, and energy dissipation capacity in comparison to the standard ones. The reduction in strength, ductility, and energy capacity was in average about 50, 40, and 70%, respectively. The standard specimens, that were detailed according to the intermediate ductility provisions and that is commonly applied in Iran, demonstrated relatively good cyclic capacity, but the provision of using at least 113rd of the main longitudinal reinforcement in the opposite side of the section seems insufficient and it is recommended to be increased to 1/2nd