# Influence of Multiple-Support Excitation on Seismic Response of Reinforced Concrete Arch Bridges
## 6 Conclusions  
This study investigated the influence of multiple-support excitation on seismic behavior of deck-type long-span reinforced concrete arch bridges. Results of the comprehensive analysis showed that applying a uniform seismic input at all bridge supports, as is commonly done in engineering practice, was not acceptable for these bridges. In all studied cases, multiple-support excitation had a detrimental effect on some aspect of the arch bridge seismic response, regardless of span length (the minimal span was 100 m). Hence, it may be recommended that this phenomenon is included in seismic analysis of similar structures, even if it is not required by the engineering design codes. Both ground-motion effects acting separately or simultaneously proved to be very important, with their relative significance depending on response values, analyzed arch locations, and utilized earthquake time histories. Therefore, it is suggested that both MSE effects are considered in seismic analysis of similar bridges. It should be pointed out that deck arch bridges of long spans are usually constructed on rock sites, because of the large arch thrust that needs to be transferred to the ground. Therefore, the effect of local-site conditions was not considered in addition to coherency loss and wave-passage. However, if the bridge location is at uneven site with non-uniform conditions at different supports, this influence on ground motion amplification and spatial variation should be considered. A simplified approach would be to apply the procedure presented in this paper (Figure 6), but to define different response spectra taking into account the soil category of different supports. However, one should bear in mind that the local site conditions have a much more complex effect than the spectral modification prescribed by the code design spectra and thus, a more comprehensive approach to this effect may be suggested, such as that applied in e.g., [18, 63]. Moreover, if the foundation interacts with the soil and the superstructure, thus significantly affecting the dynamic response of the bridge, this further complicates the analysis, and the state-of-the-art knowledge in geotechnical earthquake engineering and earthquake structural engineering should be applied accordingly. The soil-structure interaction may reduce the effect of multiple-support excitation by increasing the fundamental period of the structure and by introducing the dissipation of energy at foundations level. However, foundation rotations may induce large pseudo-static forces [64]. It was concluded in [18] that significant coupling exists between spatial variability, effects of local soil conditions and SSI effects, but their relative importance cannot be easily assessed in advance. In this research, no rule in response of bridges was apparent regarding the effect of dynamic structural properties and arch span, with few exceptions. Comparison of the various methods showed that the most suitable analysis procedure was time-history method, because other procedures proved to be less accurate and because of its general applicability. In particular, it was shown that simplified method proposed by valid European seismic codes systematically underestimates the MSE effect,so it is not recommended for use in engineering practice. EC8 rules, which specify that in the case of roughly uniform soil properties along the bridge, spatial variability must only be considered if the bridge length is larger than 400 m for rocky soils, cannot be safely used to analyze deck-reinforced concrete arch bridges. The other code deficiencies were also observed, such as the lack of guidelines on selecting the coherency loss functions and apparent wave velocities.