Seismic Retrofitting of Steel Ordinary Moment Frame Buildings

The seismic rehabilitation of exsisting buildings is a key issue in seismic zones. In fact, moderate to strong earthquakes are threatening the lives of many people and inflict severe damages on structures and infrastructures, so that they put a stop to daily life and cause the economy suffer from downturn in regional scale [1, 2]. Hence, extensive researches on the evaluation of seismic performance of the steel moment frames have been carried out in recent years [3]. Ghodrati et al. studied ordinary steel moment frame buildings designed by Code 2800 (second edition) [4] based on the Seismic Retrofitting Provisions [5] and came to the conclusion that buildings with moderate-to-high hieghts need retrofitting in the base upgrading level when using nonlinear static method to control [6, 7]. They also concluded that all buildings do not satisfy the base upgrading level when the spectral dynamic method is employed to control the accuracy of the design. In this study, it is intended to investigate and employ the most appropriate method in order to rehabilitate steel buildings in a way that not only it shows the real performance of the structure, but also it has the lowest cost.2.1. Studied models The plan for designated sample buildings is a 15*15m square with frames that are 5 meters away from each other. The studied buildings were selected in four types concerning the height: two, four, six and eight-story in which the height of each story was 3m. The buildings were assumed residential types with intermediate importance in the south of Tehran. The ground type was selected IV. In all buildings, the resistant system against the lateral loads in both directions has been steel ordinary moment frame. In order to bear the gravity load of stories, the oneway slab system (rib and block) has been used in ceiling. The connections are assumed to be rigid. Due to the type of selected plan and the direction of ribs, all of the frames in buildings have almost similar situations, and the required conclusions for the whole building can be reached through studying one frame. In all of these buildings, the frame being studied is the frame of axis-2 (a middle frame) that has more critical state due to the loading, especially in the columns. Fig. 1 shows the plan of buildings under the consideration and the frame being studied. Previous researches showed that ordinary steel moment frame buildings which have been designed according to the Code 2800 (2nd edition), in controlling through the some methods of Seismic Retrofitting Provisions were not accepted and they need to be improved [6, 7]. In the present study, it is intended to investigate and identify the most appropriate method in order to rehabilitate such buildings in a way that it shows the real performance of the structure, meanwhile, it has the lowest cost. To this end, after rehabilitating of the members using different analyses in Seismic Retrofitting Provisions, the weights of structures and new members are calculated and compared with their previous weights, and overweight percentage, due to rehabilitating, is obtained. Fig. 2 shows the overweight percentage diagram of rehabilitated buildings in different methods.