Study on the additional support length requirements of single-span bridges due to skew using a simplified method

Skew bridges with seat-type abutments are frequently unseated in earthquakes due to large transverse displacements at their acute corners. It is believed these large displacements are due to in-plane rotation of the superstructure. Lack of detailed guidelines for modeling of skew bridges, many current design codes give empirical expressions rather than theoretical solutions for the additional support length required in skew bridges to prevent unseating. In this paper, a parametric study has been carried out to study the influence of skew angle, aspect ratio and fundamental periods of bridges on the additional support length requirements of single-span bridges due to skew using a shake table experiment validated Simplified Method, which is capable of simulating gap closure based on response spectrum analysis. This method is developed based on the premise that the obtuse corner of the superstructure engages the adjacent back wall during lateral loading and rotates about this corner until the loading reverses direction. A design response spectrum specified in AASHTO LRFD Specifications was employed to represent the design-level earthquakes. The results show the additional length required to prevent unseating due to skew increases with the skew angle in an approximately linear manner when the angle is less than a critical value and decreases for angles above this value. This critical skew angle increases with the aspect ratio approximately in a linear manner and shows negligible dependence on the fundamental periods of the bridges, and combination of span length and width. In addition, the critical skew angle varies between 58° and 66°, when the aspect ratio is varied from 3.0 to 5.0. The results also show that the empirical formulas for minimum support length requirements of skew bridges in current codes and specifications can not accurately reflect the influence of skew.

Seismic Study of Skew Bridge Supported on Laminated-Rubber Bearings

Skew bridges consisting of simply supported girders, continuous decks, and laminated-rubber bearings are widely used in western China; however, they are highly vulnerable to strong earthquakes. To investigate the seismic performance of skew bridges considering the sliding behavior of laminated-rubber bearings, the Duxiufeng Bridge located in Sichuan, China, was used as a prototype bridge. This bridge is a skew bridge that suffered seismic damage during the 2008 Wenchuan earthquake. The possible seismic response of this skew bridge under the Wenchuan earthquake was simulated, and the postearthquake repair methods were analyzed considering the effects of bearing types and cable restrainers. Parametric studies, using the finite element method, were also performed to investigate the effects of the skew angle and friction coefficient of the bearings on the seismic response of the skew bridge. The results indicate that pin-free bearings could effectively control the seismic displacement of the bridge, and the cable restrainers with an appropriate stiffness could significantly reduce the longitudinal residual displacements. The effect of skew angles is less significant on skew bridges with laminated-rubber bearings than on rigid-frame skew bridges because of the sliding between the girders and bearings. The residual displacements of the bearings were more sensitive to the variation in the friction coefficient between the laminated-rubber bearings and the girders compared to the maximum seismic displacements.

Role of Skew on Bridge Performance

Gravity load paths of high-skew bridges differ from the ones with no skew. High skew can also lead to stresses or displacements that adversely affect service performance. This paper demonstrates the effects of skew on bridges through finite element analyses, bridge inspections, and statistical analyses. Five deck-girder type bridges with and without skew were inspected. A database of more than 1,400 deck-girder type bridges was analyzed to seek relationships between skew and National Bridge Inventory (NBI) ratings. Practices of Departments of Transportation (DOT) were compared with each other and to provisions of AASHTO LRFD Bridge Design Specifications (BDS). Acute deck corner cracking and bridge movements were documented on some high-skew bridges. Field inspections and database analyses showed that not all high-skew bridges have performance issues, and NBI ratings are in general not sensitive to skew. This is likely because of many factors affecting performance and certain details mitigating skew effects.

Dynamic Analysis Of Horizontally Curved Bridges

—Horizontally curved bridges are the most feasible options at complicated interchanges or river crossings where geometric restrictions and constraint of limited site space, make difficult the adoption of standard straight superstructures. Usually these bridges are of cellular cross-section so that high torsional moment can be well resisted economically. In this paper a parametric comparison was made between straight bridge and different curved bridges and skew bridges. Then these bridges were analyzed for dead, modal and moving load cases. This was done in order to study difference in the results obtained between straight, curved and skewed bridges for dead and moving load cases. The modeling part of the both bridges was done by using SAP 2000 in which there is an option named bridge wizard by which modeling of the bridge can done in a sequential order. After analyzing for dead load case unlike straight bridge there is torsion in the curved and skew bridges along the length of the bridge as there is unsymmetrical mass distribution in curved bridge about horizontal axis. Modal analysis showed the curved and skewed bridges have more initial torsional modes but whereas for straight bridge the initial modes were transverse and longitudinal. The amplifications in torsion were large compared to other parameters for curved and skewed bridges compared to straight bridge.