THE COLLAPSE ASSESSMENT OF THE LOW-RISE ASYMMETRIC DUCTILE RC FRAME BUILDINGS WITH A LESS-SENSITIVE INTENSITY MEASURE TO THE SPECTRAL SHAPE EFFECTS

Nonlinear dynamic analysis is frequently applied for the earthquake-induced collapse estimation. Thus, it is important to understand the ground motion properties that are more effective in a building collapse. The ground motion intensity is a well-known property that is used in a dynamic analysis. It should reveal the impact of earthquake record characteristics on a structure. A proper intensity measure is sufficient and efficient enough when it results in small variability of demand and reduces the number of dynamic analysis assessing structural performance. Spectral acceleration at the fundamental period of a structure, Sa (T1), is a very common intensity measure used in the collapse assessment of structural systems. However, it is not the most efficient and sufficient IM for most applications (Luco & Cornell, 2007). Therefore, researchers have introduced some new measures (Mousavi et al., 2011). For instance, Baker and Cornell (2005) indicated that the acceleration spectrum of rare earthquakes was much different from the code design spectrum. They introduced a vector-valued IM based on the spectral shape parameter named Epsilon (ε). Later, several studies implied its importance on the structural collapse capacity (Haselton & Baker, 2006). A study by Haselton and Baker (2006) also showed that the spectral shape effect on the variance of collapse capacity of the single-degree-of-freedom systems was changing while the fundamental period of the system was varying. When a ductile building subjects to strong ground motions, it behaves nonlinearly and the structural period elongates. This study investigates if the spectral acceleration at the extended period is a more proper IM than the common intensity measure, i.e., Sa (T1) to utilize for assessing the collapse of the asymmetric reinforced concrete specialmoment- frame buildings.The reinforced concrete special-moment-frame (RC-SMF) buildings are the example buildings. The buildings are designed using SAP2000 based on ACI 318-05 and Iranian seismic codes. The code fundamental period (Tcode) and the associated design base shear coefficient (Cy) of the 5-story buildings are 0.6 (sec.) and 0.078, respectively. To examine the example buildings’ collapse, OpenSees is adopted for modeling and analysis. In the building models, structural elements (i.e., beams and columns) are idealized using an elastic beam element with nonlinear rotational springs at each end. The spring model developed by Ibarra et al., (2005) is capable of capturing the monotonic and cyclic deterioration modes. The models’ irregularity is simply 10 and 20% mass eccentricities induced in the one-way of the plan. However, the effects of all other important parameters keep constant in each asymmetric building.

Keywords:

Collapse , Asymmetric , Spectral shape , Intensity measure