Highway bridge foundations must be designed to sustain (a) lateral spreading loads due to liquefaction or cyclic softening of native soils adjacent to piles (kinematics) and (b) inertial loads during shaking due to structure mass. However, there is no consensus in design codes on how to combine inertial and kinematic loads in design, as evidenced by varying recommendations by surface and marine transportation agencies. The transportation agencies in California and Oregon require combining 100% lateral spreading with 50% inertia (Caltrans 2012; ODOT 2014); Caltrans later updated this recommendation with higher performance criteria (Caltrans 2016). Washington State DOT recommends 100% lateral spreading + 25% inertia for long-duration motions (WSDOT 2021), while AASHTO (2014) recommends designing piles for the simultaneous effects of inertia and lateral spreading only for large-magnitude earthquakes (M>8). The combination of inertial and liquefaction-induced kinematic loads appears to be a site- and project-specific phenomenon affected by the type of foundations, the dynamic response of the structures, and ground motion characteristics. There is a need to characterize the inertial and kinematic load combination factors for highway bridges and update AASHTO Load and Resistance Factor Design guidelines with practice-oriented design recommendations.

A consensus has not yet been established in design guidelines on how to combine inertial and lateral-spreading loads on bridge foundations. The uncertainties related to the interaction of inertial and kinematic loads have more than an incremental impact on the foundation size. They often result in a change of foundation concept and raise the question of whether costly ground improvement methods should be incorporated to mitigate liquefaction risk. Research is needed to address this knowledge gap by developing inertial and kinematic load combination factors to be incorporated in design.

The objective of this research is to develop a design guide for practitioners that includes: (1) load factors that combine inertial and liquefaction-induced lateral spreading loads for bridge foundations that cover a wide range of soil and pile properties; (2) ground motion characteristics using validated numerical models to understand when the two loads need to be combined and when corresponding load combination factors should be incorporated; (3) performance metrics for determining failure; and (4) language for standards revisions suitable for consideration by the Seismic and Soils Structures technical committees of the AASHTO Committee on Bridges and Structures.