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CCEER-16-08: Effect of Skew on Seismic Performance of Bridges with Seat-Type Abutments
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It is well known that skewed bridges with seat-type abutments are more vulnerable to unseating during strong earthquakes than straight bridges of the same length, due to excessive in-plane rotation. This rotation is believed to be due to the eccentricity between centers of mass and stiffness, and abutment pounding. Despite the common occurrence of this type of damage, little experimental research on the interaction between a bridge deck and abutment has been conducted to confirm this behavior, quantify its effect, and validate numerical models. As a consequence, many design codes specify the minimum support length for skewed bridges based on engineering judgment and not on rigorous analysis. In this study, an unseating mechanism is proposed after examining the behavior of skew bridges in recent earthquakes. It is hypothesized that the obtuse corner of superstructure engages the adjacent back wall during lateral loading and the superstructure then rotates about this corner, causing large displacement at the acute corner at the other end of the span. These displacements can be large enough to unseat the deck, especially in bridges with small seats. In addition, shake table experiments on skew bridge models are described. These experiments were conducted in the Earthquake Engineering laboratory of University of Nevada, Reno in order to: 1) test the proposed unseating mechanism and investigate the seismic behavior of skewed bridges; 2) confirm the applicability of a simplified method for estimating the minimum support length requirement for skewed bridges; and 3) validate a detailed 3D analytical model developed in OpenSEES, that considers pounding and friction effects at the abutment-deck interface. During these experiments, excessive in-plane rotation of the deck was observed after the expansion gap closed and the deck impacted the abutment. This rotation increased the support length demand on the opposite abutment seat, which confirmed the proposed unseating mechanism. However, the experimental results also indicate that, after contact, the deck rebounds from the abutment and, for a short interval of time, rotates in free vibration about the center of stiffness of the substructure, until contact is made at the opposite abutment. This additional rotation further increases the support length demand and it is concluded that it is the combination of forced vibration in translational modes and free vibration in a rotational mode that leads to the unseating of symmetrical skewed bridges at their acute corners. Moreover, abutment pounding plays a dominate role in the in-plane rotation, and should not be neglected in any rigorous numerical model. Furthermore, friction should also be included in the numerical model to capture the sliding of bridge along the abutment at the acute corner and the rotation of bridge about the obtuse corner. Furthermore, a simplified method for estimating minimum support length requirements for skewed bridges is described, which is based on the proposed unseating mechanism, and results are compared with the experimental data. The comparison suggests this method can be used for the preliminary design of bridges with typical expansion gap sizes, but should be modified to include impact effects when the gap is large. Finally, a rigorous OpenSEES model, which accounts for impact and sliding effects between the bridge deck and abutment, is described. Good correlation is shown between the experiment and the OpenSEES model. Dynamic analysis was then performed on a family of prototype bridges using the validated OpenSEES model, to investigate the skew effect on the minimum support length requirements to avoid unseating. The numerical results indicate that the required additional support length, when expressed as a fraction of the length required for a straight bridge, varies linearly with skew angle from 0o to 60o for both near-field and far-field ground motions. This is in contrast to the AASHTO requirement which varies as a quadratic function of the skew angle. It is recommended that the skew factor for the minimum support length be 0.0074θ for far-fault bridges and 0.0082θ for near-fault bridges, where θ is skew angle in degrees. The empirical formulas of the skew factors specified in various codes and documents were then compared with the dynamic results. All of these factors show significant differences with the dynamic results. For example, AASHTO expression gives similar results at 0o and 60o skew with the dynamic results, but it falls well below the OpenSEES results between 15o and 45o skew. Even though the AASHTO value for additional support length is believed adequate because the support length for a straight bridge is very conservatively prescribed, these recommendations are necessary since they reflects the true effect that skew angle plays on the minimum support length requirements. The dynamic analysis results were also used to investigate the connection forces for shear keys and assess the adequacy of AASHTO’s provisions for the minimum connection forces for skewed bridges in seismic zone 1 and SDC A. It is shown that if shear keys are provided at the abutment seats to prevent lateral movement of the deck, the forces in the keys at the acute corners can be very high. This is most likely to occur when the longitudinal component of the ground motion closes the gap, and the transverse component along with the impact and friction forces at the abutment-deck interface must be resisted by the shear keys. Minimum design connection forces, such as those specified by AASHTO for seismic zone 1 and SDC A, are shown to be inadequate for skew bridges in these zones/categories. To reduce these forces in low seismic zones, it is recommended the expansion gap be increased in size to avoid abutment pounding, which is an affordable solution since the increase is expected to be small. In high seismic zones, it is recommended that the keys be designed as sacrificial elements to protect the abutment piles from damage. In addition, a simplified method was developed to estimate the maximum forces of shear keys. Good agreement was achieved between the simplified method and the dynamic results. Therefore, it can be used as the preliminary design for shear keys.
Report No. CCEER-16-08