Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
CHAPTER 9 FINDINGS In many applications, GRS bridge abutments offer a relatively low-cost and easily constructed design alternative for single-span, simply-supported bridges. Previous research produced NCHRP Report 556 that provided design and construction guidelines for GRS bridge abutments with a flexible facing under static loading conditions. The research described in this report was undertaken to extend the earlier guidance to GRS abutments in seismically active areas. This research entailed design and shake-table testing of a model GRS abutment with modular block facing. The model was designed using an ASD method to withstand a peak ground acceleration of 0.2 g. The resulting abutment design had a configuration identical to a GRS abutment designed only for static loading according to the static design method presented in NCHRP Report 556, with the exception of bearing pads and expansion joints added at the end of the bridge. This report describes both ASD and LRFD design methods that may be used for GRS bridge abutments in seismically active areas. In the shake-table test, the model withstood the vertical and horizontal loads placed on it during ground accelerations of 0.15 g at 1.5 Hz, without experiencing any structural failure or significant movement. The model also safely withstood the bridge loads while being subject to ground accelerations up to 1.0 g at 3 Hz. Data related to the internal behavior of the abutment during testing, such as accelerations transferred within the model, the envelope of maximum stresses in the geosynthetic reinforcement, and the pressure distribution showed favorable performance of the GRS abutment-bridge system even when subjected to horizontal accelerations exceeding 1.0 g. Parametric studies using finite element analysis subsequently assessed likely behavior of ASD GRS abutments subject to actual earthquake acceleration histories. These studies then indicate that the proposed seismic design methods presented here can be used for preliminary design of GRS abutments when seismic loads are a concern, i.e., when the peak
248 ground acceleration exceeds 0.2 g. The ASD design method is similar to that presented in NCHRP Report 566, except for the addition of bearing pads and expansion joints at the ends of the bridge. Construction guidelines developed in this study for GRS abutments in seismic areas also are similar to those presented in NCHRP Report 566. While this research indicates that GRS abutments, even when designed for static loads only, are capable of withstanding significant ground accelerations, their load-carry capacity at high ground accelerations may be compromised by sliding of the sill if the bearing pads are not properly designed. As the only link between the superstructure and the substructure, the bearing pads play a key role in the abutmentâs performance. If elastomeric bearing pads are chosen for a GRS bridge abutment, they should have a lower natural frequency than the expected high energy frequency range of ground motion anticipated on the construction site. As seen in testing, if the natural frequency of the bearing pads is below the ground motionâs frequency, the horizontal motion of the superstructure can be isolated from the substructure, hence significantly reduces the horizontal forces exerted on the abutment. The single shake-table test conducted in this study should be supplemented by additional tests to confirm and extend this studyâs findings. These additional tests should be made using actual earthquake acceleration histories.
