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Seismic responses of ordinary highway bridges that feature stiff superstructures have been shown to be strongly affected by abutment-backfill interactions. Seismic risk assessment of these bridges at the regional scale faces the added challenge of having to deal with the large uncertainty in backfill properties. In this regard, the study develops probabilistic backfill models to better quantify the uncertainties in modeling the abutment. First, efforts to establish the validity of the numerical method in predicting the pushover response of backfills are described. Advanced plasticity materials are used in finite element models (FEMs) to simulate sandy and clayey soils that yield consistent backfill force-displacement relationships against full-scale test results. Second, a probabilistic analysis framework is constructed to incorporate soil uncertainties that are identified from field investigations. Statistical moments are extracted from the resultant pushover curves to fully define the probabilistic backfill models, which are verified to bear appropriate uncertainty treatment and reasonable height adjustment factors. Further, statistical analysis tools are used to investigate the influences of different backfill models on the bridge demand estimates of two common bridge classes. The study reveals that backfill models will affect the response estimates of different bridge components in both diaphragm and seat-type abutment bridges. However, probabilistic models shall be especially considered on backfills for the bridge components that are expected to have dominant responses in the longitudinal direction. The proposed backfill models appear to outperform previous deterministic models in predicting realistic bridge responses. The models can be employed in the task of regional seismic assessment of various bridge classes.

Skewed bridges are often encountered in the highway bridge system when the geometry cannot accommodate straight (unskewed) bridges. The objective of this study is to investigate the influence of skew angle on the seismic response of bridges using nonlinear time-history analysis and probabilistic seismic assessments. Six types of skewed and straight bridges (including multi-span simply supported, multi-span continuous, and single-span skewed bridges with steel or concrete girders together with non-integral abutments) commonly used in the central and southeastern United States (CSEUS) are considered for establishing three-dimensional numerical bridge models. The six bridge types are further categorized as: (1) non-seismically designed (NSD) bridges, (2) bridges with seismically designed (SD) columns, (3) bridges retrofitted by (i) column jackets, (ii) isolator bearings (IBs) and keeper plates (KPs), (iii) restrainer cables (RCs) and shear keys (SKs), or (iv) seat extenders (SEs) and shear keys (SKs). Probabilistic seismic demand models incorporating geometric and material uncertainty parameters for the bridges under a suite of ground motions are established to develop corresponding sets of fragility curves in terms of vulnerable bridge components. System fragility curves are further developed through a combination of the component fragility curves in the bridges. Comparisons of the fragility curves between the straight and skewed bridges indicate that the larger the skew angle, the more vulnerable the bridges, regardless of NSD bridges, bridges with SD columns, and retrofitted bridges. Formulas that consider effect of skew on the values of fragility parameters in the fragility curves are derived for each bridge class, component type, and limit state. Finally, the retrofit of columns and seismically designed columns can reduce column damage probabilities without significantly increasing demands to the other bridge component types, leading to a lower bridge system risk than that for the NSD bridges. However, although the other three retrofits (IB&KP, RC&SK, and SE&SK) can reduce transverse and/or longitudinal demands on the bearings, the column demands remain a similar or worse damage level than that for the NSD bridges, resulting in a similar or higher risk for the three retrofitted bridge systems.

A hybrid seismic device that provides both energy-absorbing and re-centering capabilities to overcome external forces is developed and evaluated. The hybrid device is composed of three main components: (1) a set of re-centering wires fabricated from shape memory alloy (SMA) material, (2) two energy-absorbing struts, and (3) two high-strength steel tubes to guide the movement of the hybrid device. The SMA wires are located within the guiding high-strength steel tubes and designed to be sufficiently long such that their deformation strain is within the 6% target strain limit. A conservative value of 6% strain, instead of 8%, or 10%, is adopted to (1) avoid the SMA stiffening phase that increases strength up to 5 times that of its forward transformation yield forces, resulting in a serious damage to the adjacent structural members, and (2) to retain the full re-centering capability of the SMA wires even when the hybrid device is under large displacement. The energy-absorbing struts are pin-connected outside of the guiding steel tubes and may be fabricated of mild steel or low strength aluminum. To reduce the possibility of buckling in the energy-absorbing struts when subjected to compression, they are designed to be stocky and seismically compact, or buckling restrained. An optimal proportion of the SMA wires and energy-absorbing struts is formulated such that the hybrid device retains re-centering capability, while maximizing energy dissipation. Results obtained through the seismic analysis reveal that the hybrid braced frame system exhibits a similar energy dissipation capacity to the buckling restrained braced system, while also having excellent re-centering capabilities.