There are various damage models for use during seismic performance of RC bridges. One of the most widely used damage model is column drift ratio ((Tavares et al. 2012), (Akbari 2012), (Billah and Alam 2015) etc.). However, due to the reasons further explained below column drift ratio is not suitable for this research involving mainshock aftershock sequences.
In the Fig. 1, the maximum column drift during mainshock of Tohokhu earthquake is 1.15%. Similarly, if we consider aftershocks too the maximum drift during the mainshock aftershock sequence doesn’t increase beyond 1.15%, due to which it seems the application of aftershocks doesn’t have any significance. However, this is not true as aftershocks do add some level of damage to the structure. Similarly, residual displacement at the end of the sequences also may not give the exact state of damage as it is dependent on the spectral characteristics of the aftershocks itself (Qian 2012). Therefore, it is clear that lateral displacement alone cannot properly justify the true damage state of the structure.
Therefore, it is necessary to include the energy dissipated during each earthquake in conjunction with the displacement ductility terms in order to represent true damage state of the structure and for that Park Ang Damage Index is a good choice (Park and Ang 1985). Park Ang damage scale is frequently used damage index to quantify the damage in concrete where the damage is expressed as the function of maximum deformation and repeated cyclic loading. The expression of Park Ang damage index is given by the following Eq. (1).
DI= \(\frac{{\delta }_{m}}{{\delta }_{u}}+\frac{\beta \int dE}{{Q}_{y}{\delta }_{u}}\)(1)
Where,
DI = Damage Index
\({\delta }_{m}\) = Maximum deformation under given earthquake
\({\delta }_{u}\) =Ultimate deformation under monotonic loading. Given by; \({\delta }_{u}={\delta }_{y }\text{x} {\mu }\)
\({\delta }_{y }\) =Deformation at start of yielding
\({\mu }\) =Deformation ductility
β = cyclic loading factor taken as 0.05–0.2.
\({Q}_{y}\) =calculated yield strength (if the maximum strength\({Q}_{u}\), is smaller than\({ Q}_{y}\), \({Q}_{y}\)
is replaced by \({Q}_{u}\))
\(\int dE=\) Total absorbed hysteretic energy
Here, the expression for the total absorbed hysteretic energy added to the deformation parameter adds the much needed accuracy in the damage state of the structure and it is given by Eq. (2).
$$\int dE={\sum }_{i=1}^{n}\frac{{P}_{i+1}+{P}_{i}}{2}{(u}_{i+1}-{u}_{i})$$
2
Where;
P = Force applied/Reaction and u = deformation.