# Influence of Multiple-Support Excitation on Seismic Response of Reinforced Concrete Arch Bridges
## 5 Discussion
### Influence of Analysis Methods and Applied Ground-Motion Time-Histories  
As previously mentioned, the largest average peak time-history results obtained using generated records were assumed as the reference values for validation of all methods. The simplified method proposed in EC8 systematically underestimates response values and, therefore, it is not suggested for the analysis of similar bridges. This conclusion is in accordance with the findings of some other studies in which EC8 simplified method was applied. For example, in the recent study of the integral bridge [22], it was concluded that EC8 approach provides results in the order of \(\pm\)40-50% of the reference values, owing to the inability of the method to predict the contribution of higher modes as well as the location of structural elements which are favorably or unfavorably affected by spatially variable seismic actions. The research presented in [18] dealt with the response of bridges to MSE and evaluation of EC8 provisions very comprehensively. The authors concluded that in some cases the provisions of the EC8 on the simplified method may be safely applied, but in other cases ignoring the effect of MSE or performing simplified calculations can significantly underestimate the actual seismic demand. In the study of an arch bridge response to multiple-support excitation presented in [33] the authors also suggested reexamining the limit for consideration of spatial variability effect in concrete arch bridges, proposed by Eurocode 8. Moreover, the modification of the method in which the combination of the most adverse dynamic response with the most adverse effects obtained by the quasi-static analyses is calculated by applying the sum of absolute values, applied also in [18], cannot be recommended for the use since the response values are still underestimated, with a few exceptions that cannot influence the method selection. It can be observed that the response spectrum method MSRS does not accurately estimate the response of arch bridges to MSE, with some values being significantly lower and some higher than the reference values, which is in line with the research presented in [62], proposing its modification. Therefore, bearing in mind also the general inapplicability of the method to any non-linear analysis, this method is not recommended for the calculation of similar bridges to MSE. The use of earthquake records generated by unconditional simulation produces very conservative results, and it should be kept in mind that such \"artificial\" records are used because of the lack of recorded ones and they can never realistically define an earthquake excitation. The advantage of using such a simulation is that it simply and quickly generates a large number of records once a calculation algorithm is made, which is useful for parametric analyses. The procedure of conditional simulation using recorded motions is more demanding, because it is firstly necessary to find a series of appropriate records from a strong-motion database with respect to the bridge location (which is not always possible), taking into account the code rules and, if necessary, to process them (baseline correction and filtering). Then, for each selected record, the time windows and the spectral power density function for each of these should be determined, taking care that the duration of each stationary time window is \(2n\) since inverse fast Fourier transforms are used in the simulation. It is evident that it is very challenging to fully automatize such a process. Only after this initial selection and adjustment procedure has been carried out can the process of generating records for the other supports of the bridge be initiated. The records generated using this method retain the properties of the original recorded time-histories (for example, the frequency content of the signal) so that they can more realistically define the earthquake excitation than the \"artificial\" records generated on the basis of a probabilistic model.