Seismic Evaluation of Regular MR Steel Frames Designed by Direct Displacement Based Method

During decades, engineers have tried to build safer and more economic structures. Since establishing primary structural design codes, it has been presumed that “strength” of a structure is synonymous to its “performance”. However, during many years by studying various structural damages occurring during strong ground motion earthquakes, it is obvious today that increasing strength of a building may not result in safer or stronger structure with better performance [1]. Park and Paulay [2] stated that the distribution of strength through a building is more important than the design base shear itself. As they stated, in order to achieve a better structural performance, two important issues should be devised. First, structures should be designed such that plastic hinge formation during nonlinear behavior of the building would occur in all beams before columns. In other words, weak beam/strong column mechanism should be assured. Second, high shear capacity of elements should be supplied to ensure the prevention of brittle failure of main structural elements. This is the case that our experiences during past strong earthquakes have shown that these issues are seldom achieved through applying force based seismic design code regulations. This was the true start to performance based seismic design [1]. During years, researchers found that although strength is an important issue to control floor displacement, the damage potential of structures should be directly related to deformations rather than strength. In the last twenty years, this concept has led to development of a large number of alternative seismic design philosophies based on deformation capacity of structures [3], among which a few methods are suitable as a standard method for implementation in new modern design codes. One of the most important methods is direct displacement-based design method (DDBD) that was introduced by Priestley in 1993. This method is rather more complete and simpler to apply, which has been also used for a wider category of structures [4].The main difference between DDBD and the traditional force-based design method is that DDBD characterizes the structure to be designed by a singledegree- of-freedom (SDOF) representation of performance at peak displacement response, rather than by its initial elastic characteristics. The characterization of the structure by secant stiffness avoids the many problems inherent in force-based design where initial stiffness is used to determine an elastic period, and forces are distributed between members in proportion to elastic stiffness [4]. In this article, the authors aim to consider performance of various level steel structures, which are designed by two completely different design approach; DDBD and a traditional forced-based design method. During this research, two goals are evaluated: the comparison between two displacement- and force based design, as well as the accuracy of formulations that are determined target displacement and damping capacity of the steel frames. This is because direct displacement-based approach is configured based on concrete frames and some correction are implemented in formulations to predict steel frame performance. Hence, three different regular moment resisting steel frames (MRF) with various heights: low, medium, and high-rise buildings, say 4, 10, and 16 stories, are designed by DDBD and the Iranian code of practice for seismic resistant design of buildings, standard No-2800 (the 3rd edition). Seismic behavior of all structures is examined by dynamic nonlinear time history analysis under a 7- acceleration record group. All members of this group have a response spectra matched to the design spectrum of the buildings. Extensive analysis is carried out and the results are compared together. Primary considerations show that the DDBD-buildings are so heavier than the similar 2800-buildings. These differences are much greater with increasing level of the steel frames, which is related to larger design base shear in DDBD approach. However, by considering drift ratios of the floors in the three frame structures, it is seen that DDBD-buildings is very stronger than they need to be. Differences between drift limit and drift ratios of the floors are obviously so large. This may be referred to the target displacement, design displacement profile and needed damping capacity of the frame. On the other hand, 2800-buildings are not meet drift limits specially in below levels of the structures, but by a rather simple modification, they may behave better. It should be noted that the weight of the 2800-buildings are still much lighter than the DDBD-buildings. Finally, it is expected that DDBD with some modifications for steel moment-resisting frames is a rigorous method that is a viable alternative to the current forced based design methods, which can consistently predict seismic behavior of all various level steel structures during strong ground motions. On the other hand, although steel frames designed by forced-based code have previously satisfied their design criteria, they may not be reliable to predict their performance during real earthquakes.