Selection of Comprehensive Design Criteria for Highway Bridges in the Vicinity of and Crossing Active Faults

The earthquake induced forces in bridge structures consist of the simultaneous action of dynamic (inertia) forces caused by ground shaking, quazi-static forces caused by oscillatory differential motions of the supports due to the wave passage, and, for bridges crossing faults, which is common in California - static forces caused by permanent offset on the fault during an earthquake. The dynamic forces are those that are considered normally in seismic design. In our previous METRANS project, we addressed the estimation of the static forces, by developing a probabilistic prediction model for the displacement across a fault, which is due to a seismic slip on that fault. In our previous work, we have considered the combination of dynamic and oscillatory quazi-static forces, but only for a particular (scenario) earthquake. The simultaneous action of all three types of forces has not been considered so far neither for a scenario earthquake, nor in a probabilistic hazard prediction model, and is important for achieving balanced design, so that a bridge is not overdesigned for one effect and under-designed for another effect. The objective of the proposed one-year effort is to: (1) develop a methodology for specification of comprehensive (universal) design criteria for highway bridges in the vicinity of and crossing active faults, and (2) demonstrate the methodology on example sites in the Los Angeles area. The comprehensive design criteria will include the simultaneous action of all the three types of forces: dynamic (inertia), quazi-static (due transient differential motion), and where appropriate, static (due to permanent fault offset). The design criteria will be expressed in the form of relative displacement spectra for direct design of bridge columns. We have developed previously physical models for construction of such spectra for the simultaneous action of dynamic (inertia) forces and oscillatory quazi-static forces, separately for in-plane (longitudinal) and for out-of-plane (transverse) response. These spectra are for shaking from a particular earthquake, and do not include the contribution from permanent differential displacement. The proposed work will include: (a) generalization of these Relative Displacement Response Spectra for a particular earthquake to include permanent displacement from slip on the fault, which is a novel concept in earthquake engineering, and (b) probabilistic description of these spectra within the framework of probabilistic seismic hazard analysis, which also has not been done previously. Different combination rules will be considered for the simultaneous contribution from oscillatory and static differential motions. The significance of the probabilistic approach is that it accounts for the different earthquake effects from all conceivable earthquakes that may affect the structure of interest, considering the likelihood of their occurrence during the life or service time of the structure. The outcome will be a new design criteria for bridges crossing active faults, which will offer balanced contribution from all earthquake effects on bridge structures. The probabilistic approach also would make it possible to compare the seismic risk with risks associated with other natural or man made hazards, e.g. congestion, pollution, etc. The proposed project will result in original and work that is publishable in scholarly journals. The approach such that the comprehensive design criteria are expressed in the form of a design spectrum, will guarantee its acceptance among practicing engineers. The proposed methodology is relevant for (a) deign and retrofit of bridges near and crossing earthquake faults, (b) seismic risk assessment for the ground transportation system, i.e. the risk for physical damage, loss of function, and overall economic consequences on the regional economy, and (3) emergency planning.