The Earthquake Engineering Online Archive

New design and analysis procedures for intermediate hinges in multiple-frame bridges

DesRoches, Reginald; Fenves, Gregory L.

UCB/EERC-97/12, Earthquake Engineering Research Center, University of California, Berkeley, 1997-12, 202 pages (630/D47/1997)

During an earthquake, adjacent bridge frames can vibrate out-of-phase, exceeding the range of support provided by the hinge seat, leading to collapse. The collapse of bridges in recent earthquakes due to hinge unseating emphasized the importance of providing an adequate number of hinge restrainers to limit the relative displacement between hinges. Current restrainer design procedures are not adequate because they do not account for controlling factors in the response of multiple-frame bridges. A nonlinear numerical model representing the longitudinal earthquake response of two frames connected at a hinge is developed. The model has nonlinear force-displacement relationships for the frames and nonlinear elements accounting for tension-only restrainers and friction. Pounding is accounted for directly in the equations of motion. A parametric study shows that the response of the hinge is governed by the frame period ratio, the target ductility demand of the frames, and the stiffness of the hinge restrainers. A multiple-step design procedure based on a linearized numerical model is developed. The procedure accounts for the phasing between frames with a modal analysis. Yielding of frames is linearized by using a substitute structure method, and optimization theory is used to obtain the restrainer stiffness. Parametric studies show that the procedure works well in limiting the relative hinge displacement for a wide range of parameters. The required number of restrainers decreases as the frame period ratio and the frame target ductility increase. A simplified single-step restrainer design procedure for hinge restrainers is also developed. The single-step design procedure is based on a nondimensional value of the restrainer stiffness that is determined by performing a large parameter study. The results from the single-step procedure are generally more conservative compared with the multiple-step procedure. Comparisons with current restrainer design procedures show that the new multiple-step and single-step procedures are more accurate than current procedures for designing hinge restrainers. Pounding of frames and engaging of restrainers produce forces and displacements significantly different than what is typically assumed in design. Pounding typically increases the demand on stiffer frames and decreases the demand on the most flexible frames. Although estimates from bounding models generally report the maximum elastic forces, they do not bound frame ductilities.

Available online: http://nisee.berkeley.edu/documents/EERC/EERC-97-12.pdf (16 MB)