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Substructure Location, Elevation (Global Z) is the bearing seat elevation, or the elevation at the top of the bent cap or abutment cap. Bearing Assignment, Elevation at Layout Line (Global Z) is the elevation at the bearing action point. Abutment and bent figures, taken from pages 1-13 and 1-15 of the Bridge Seismic Design manual, are shown in Figures 1 and 2:
Figure 1 - Abutment bearing action point (Z = -49ft) and seat (Z = -51ft)
Figure 2 - Bent bearing action point (Z = -49ft) and seat (Z = -51ft)