Evaluating response modification factor (R) for some types of steel structure

Abstract

Response modification factor (R), tabulated in the Colombian Design Code as NSR-98, is used in this paper for evaluating internal member forces produced by design earthquake action on steel structures and the inconsistencies pre sent when designing structures when 1% drift limits must be complied with. The article presents the design of 45 frames corresponding to the seismic resistance system of 5 buildings: 15 special moment frames (SMF), 15 special concentrically-braced frames (CBF) and 15 eccentrically-braced frames (EBF). External loads and their combination were used in estimating internal loads and rigidity demands (1% drift) were evaluated in line with NSR-98 requirements. Member strength requirements were evaluated by using the AISC-2005 seismic provisions for steel structure buildings. Modal pushover analysis was used for evaluating the response modification factor for the 45 given frames at different structural performance levels. It was found that this factor was not constant for any of the three structural systems (SMF, CBF and EBF) suggested by NSR-98 and that the values of the response modification factor found in the present investigation were smaller than those tabulated in this design code governing everyday structural design. This would lead to significant errors being made in evaluating design forces, not only in the structures but in the support elements (base-plates, foundations, shear walls) and any structures attached to buildings constructed in line with the seismic resistance system.