연성파괴메커니즘에 기초한 교량의 내진설계

Abstract

Bridges as part of main roads are very important infrastructures for life-saving and public order after earthquakes. In this regards 'Earthquake Resistant Design Part' in the 2010 'Roadway Bridge Design Code' is presently applied after inclusion in the 1992 Code and revisions of design provisions in the mean time. Of course it is also required to achieve intentional seismic performance of existing bridges as well as earthquake resistant design of new bridges. The fundamental concept of earthquake resistant design is 'No Collapse Requirement', which is obtained by preventing falling-down of superstructure and supplying emergency traffic after earthquakes.

Earthquake resistant design should provide a description of the structural failure mechanism under earthquakes as well as satisfy the requirement of other designs, e.g. design strengths of each structural member should be equal or greater than the required strengths. The reason of such a requirement is the randomness of seimic loads different from other definable loads. Therefore designers should satisfy the 'No Collapse Requirement' by providing that structural yielding process is principally designed with the 'Ductile Failure Mechanism'. In order to get the 'Ductile Failure Mechanism' for typical bridges, pier columns yielding should occur before that of connections. However domestic conventional bridge design with unnecessary stiff substructure leads to unnecessary seismic loads and makes it difficult to get the 'Ductile Failure Mechanism'. Such a problem arises from the situation that earthquake resistant design is not carried out in the preliminary design step.

'Roadway Bridge Design Code' provides the spectrum analysis method using response modification factors as a basic design method for earthquake resistant design of typical bridges. Because the spectrum analysis method is a linear analysis method, non-linear behavior of bridge structural members is controlled by given response modification factors. In other words, the 'Ductile Failure Mechanism' is obtained by applying response modification factors for connections and substructures. However this method in the 'Roadway Bridge Design Code' is a simplified method without considering overstrengths of structural members, which should be considered for the verification of actual failure mechanism. Therefore, considering that korean peninsula is classified as moderate seismic regions and domestic circumstances for bridge design and construction are different from other countries, the applicability of this Code is not yet proved and it should be verified that applying the spectrum analysis method fulfills the 'No Collapse Requirement'.

In this study typical bridges - two with steel bearings but different T-type / ∏ type piers and one with lead rubber bearings - are selected as analysis bridges and earthquake resistant designs are carried out by applying the spectrum analysis method and design conditions for moderate seismic regions. To provide the 'Ductile Failure Mechanism', design strengths for connections and pier columns are determined in the preliminary design step.

Based on the results obtained through analysis procedures, the role of response modification factors and fulfillment of the 'No Collapse Requirement' are discussed, from which supplementary provisions for the design code are identified. It is shown that the earthquake resistant capacity of typical bridges can be achieved by way of redesign of bridge system, e.g. determination of pier design section for substructure and change of bearing function for connections, with structural members determined or required by other designs.