This paper investigates the effect of bridge skew and soil-foundation interaction models, used by the Missouri Department of Transportation, on the response of highway bridges to earthquake loading. Current AASHTO Specifications in Division I-A give specific methods for the structural analysis and design of bridges during earthquake loading. It is less specific with respect to foundation modeling for the analysis. This is in part due to the complexities which are associated with the different foundation systems encountered in bridge structures in combination with the wide variety of soil types encountered in practice. In assessing the overall dynamic response of highway bridges, it is necessary to account for soil-foundation interaction effects.

Several soil-foundation interaction models are used by MoDOT in concert with soil boring data to generate the equivalent linear spring constants for spread footings, pile footings, abutment backwalls, beams and wingwalls. For abutments or pile footings, these equivalent linear spring constants are transformed to a master joint through the Rigid Body Transformation method (RBT) in order to reduce the total number of degrees-of-freedom in the analysis and take into account the geometric relationships among these spring constants. The master joint is usually chosen at the center of gravity of the superstructure for abutments and the center of gravity of the cap for pile footings. The response spectrum method is used for the dynamic analysis to determine the demand forces of these equivalent linear springs. The demand forces are compared with the corresponding structural components' capacities. The capacities include: abutment backfill passive pressure capacity, pile capacity, and footing bearing capacity. If the demand passive pressure of the backwall or wingwalls is greater than the corresponding backfill capacity, then the effective stiffness of the backwall or wingwall spring is revised and the next iteration is performed in the analysis. Two numerical examples are provided to show i) the structure's natural periods and responses are strongly influenced by the abutment soil-foundation interaction; and ii) the sensitivity of structural responses due to skew effects.

¹Structural Development & Support Engineer, Missouri Dept. of Transportation, P.O. Box 270, Jefferson City, MO 65102. Tel: 573-751-2920, Fax: 573-526-5488
E-mail: gerj@mail.modot.state.mo.us

²Structural Development Designer, MoDOT, Tel: 573-526-4855, Fax: 573-526-5488

³Structural Development Designer, MoDOT, Tel: 573-526-4855, Fax: 573-526-5488

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