Paper Information

Journal:   MODARES CIVIL ENGINEERING JOURNAL   APRIL 2017 , Volume 17 , Number 1 #F0062; Page(s) 115 To 126.
 
Paper: 

EFFECT OF NEAR AND FAR FIELD EARTHQUAKES ON STRENGTH REDUCTION FACTOR AND INELASTIC TO ELASTIC DISPLACEMENT RATIO: DEMAND DUCTILITY CONCEPT

 
 
Author(s):  SIAHPOLO NAVID*, GERAMI MOHSEN, VAHDANI REZA
 
* CIVIL ENGINEERING DEPARTMENT, ACECR INSTITUTE FOR HIGHER EDUCATION, KHUZESTAN, IRAN
 
Abstract: 

The experience of previous earthquakes shows that the inelastic response of structure is related to the intensity and content of ground motion. In this case, the evaluation of nonlinear response of structure demonstrates the reduction in the base shear force. This reduction which leads to inelastic base shear is defined by Behavior Factor (strength reduction factor) in seismic codes. One of the important parts in R factor is ductility reduction factor Rm. While Rm is related to the type of earthquake, it seems that for near fault motions there would be a different value in comparison to ordinary earthquakes. For the near fault earthquakes, due to the direction of fault rupture from the site, the directivity effect becomes an important parameter. Previous researches show that for forward directivity effect, there would be two components for earthquakes. One is normal strike and the other is parallel strike. In this paper, these components are regarded as SN and SP. Also, in the concept of performance-based design, the ratio between inelastic and elastic response of structure is an important index in calculating the target displacement. This ratio is called CR, hereafter. It is good to mention that CR factor is defined as C1 coefficient in FEMA440. In previous researches, the evaluation of CR for near and far fault motions has less been considered.
To evaluate R
m and CR, the extended number of SDOF systems (from 0.2 to 4 Sec.) are considered for four levels of target ductility (2, 3, 4 and 5). Accordingly, Rm and CR are calculated for near field (normal and parallel component) and far fault earthquakes. The normal strike component is traced by a sensitivity analysis, changing the strain hardening ratio and inherent damping. To perform the analysis, the nonlinear time history analysis was selected in Opensees. The steel material was also defined to be bilinear. To set the required ductility with the prescribed target ductility -during trial and error procedure- the yield strength of SDOF was changed, since the target ductility was achieved. To solve the inelastic equation of motion, the Newmark-Beta method was selected. The inelasticity in Opensees was modeled with distributed plasticity using the fiber element. Finally, to calculate Rm and CR for near and far field motions, approximately 84000 nonlinear time history analyses were carried out. In addition, to study the sensitivity of Rm and CR to damping and strain hardening ratio for the normal strike earthquake, approximately 22400 nonlinear time history analyses were carried out.
The results show that for all three sets of earthquake, the R
m increases up to a specific value and after that, becomes constant while the fundamental period (T) increases. For small values of ductility (m), increase in T may lead to convergence of Rm to target ductility. In the near field, when the values of T and ? are increased, Rm becomes almost greater than ?. However, for small values of T, Rm is not dependent on demand m. The study shows that: using far field value of Rm for near field motions may lead to a non-conservative value. Furthermore, while T increases, the CR value converges to the unit. In the short period, CR depends on m and T, severely. Using CR of far field against SN component leads to Non-conservative result. For a constant value of m and T, increase in damping may increase CR. Using C1 for near field motions is non-conservative for near field motions. Also, for short periods and high ductility demand, CR, corresponding to SN component is about 40% greater than C1. Evaluation of the ratio of displacement modification factor to behavior factor shows that the Cd/R ratio for T -greater than 1 Sec.- converged to the unit. For small period values, this ratio is significantly dependent on the duration. Also, using Cd/R of far field for near field motions may lead to inaccurate results.

 
Keyword(s): NEAR FIELD MOTIONS, DUCTILITY, NONLINEAR TIME HISTORY ANALYSIS, BEHAVIOR FACTOR, FEMA440
 
References: 
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