Assessment of the Bill Emerson Memorial Bridge - FTP Directory ...

4.4.4. Connection bearings between towers and decksThe connection of the deck to the towers presents a special challenge to the developmentof the FE model. Generally, two approaches exist to model bridge bearings as discussed inSAP2000 Manual. One approach is to attach elements to separate joints at the samelocation and constrain their degrees of freedom using an “Equal” or “Local Constraint.”The other approach is to attach several elements to a common joint and use frame elementend releases to free the unconnected degrees of freedom. The first approach was adopted inthis study. The pot bearings used between steel girders and pier cap beams at Piers 2 and 3were modeled to allow for the longitudinal translation and free rotation about any axis. Theearthquake lateral restrainers at the center of the floor beam at Piers 1 to 4 were modeled toprovide lateral restraints between the floor beam and the cap beam. Two earthquake shocktransmission devices were installed next to each pot bearing, which will limit thelongitudinal movement in the event of a strong earthquake but leave it nearly free to moveunder slowly varying conditions such as thermal effects. As such, the devices weremodeled in this study as a hinge in the longitudinal direction for seismic analysis. Effectivemodeling of support conditions at bearings and expansion joints requires carefulconsiderations on the continuity of displacement components in horizontal, longitudinal,and vertical directions.4.4.5. Foundations in main and approach spansIn the Bill Emerson Memorial Cable-stayed Bridge, Pier 2 is based on rock. Piers 3 and 4are supported on caissons, which is sufficiently rigid to be modeled as a fixed support.Soil-pile interaction effects can be neglected in the cable-stay span. On the other hand,Piers from 5 to 14 in the Illinois approach are supported on pedestal pile-groupfoundations. Each pier has two columns and each column is supported by 5 round piles of1.83 m (6 ft) in diameter. The length of piles varies from 24.1 m to 30.5 m (79 ft to 100 ft).In the FE model, the soil-pile interaction was simulated by linear dampers and springs invertical, longitudinal, and traffic directions, respectively, as illustrated in Figure 4.3 forsprings. The linear dampers, not shown in the figure, were modeled similarly withLink/Damper elements. The spring and damping coefficients of soils were listed in Table4.4. Their calculations are detailed in Appendix A.TrafficLongitudinalVerticalFigure 4.3Modeling of pile-group foundation for the base of the piers46