STUDY OF ENERGY DISSIPATION CAPACITY OF RC BRIDGE COLUMNS UNDER SEISMIC DEMAND

Abstract

Field studies were carried out to investigate various parameters of bridges found in northern part of Pakistan. After the large Kashmir earthquake of M w7.6 in 2005, detailed field investigations to study the seismic performance of bridges was also undertaken. A mathematical function to define the functionality of bridges was developed which is helpful for quantifying the seismic resilience of bridges. Criterion for minimum required functionality for different bridges and limit states were defined for extremely large rare earthquake and for moderate occasional earthquakes. From the field data, typical parameters of reinforced concrete bridges were established. A series of experimental studies were undertaken in the laboratory on four scaled models of a typical bridge that consists of pier having single column. The pier column was of low strength concrete with solid circular cross section. The objective of the study was to experimentally determine the energy dissipation capacity of low strength concrete piers. Two types of tests were done on the four bridge piers: quasi-static cyclic tests and free vibration tests before, during and after the quasi-static tests. From the experimental results on four scaled low strength bridge piers damping was seen to decrease with increase in damage, natural period of piers doubled near failure, energy degradation was seen to be more in low strength piers. Energy based strength degradation and pinching is predominant in low strength concrete piers along with large permanent deformations. Response modification (R) factors based on natural period of bridge are found to better represent the energy dissipation and are accordingly proposed. The values of R-factor calculated for low strength concrete piers are lower than AASHTO LRFD 2007 thus more conservative. The fragility curves plotted for the bridge columns indicate that for peak ground accelerations (PGA) of seismic Zone 3 and above of the seismic hazard map of Pakistan (for 475-years return period) pushes the bridge in to damage state that is allowed for large earthquakes only (with return period of 2,500 years). Mathematical function for the quantification of seismic resilience of bridges is proposed for the first time. It is demonstrated that using the general guidelines of AASHTO LRFD 2007 quantification of seismic resilience is possible.