'Substructure Location, Elevation (Global Z)' is the bearing seat elevation, or the elevation of the top of the bent cap or abutment cap. 'Bearing Assignment, Elevation at Layout Line (Global Z)' is the elevation of 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:
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Figure 1 - Abutment bearing action point (Z = -49ft) and seat (Z = -51ft)
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Figure 2 - Bent bearing action point (Z = -49ft) and seat (Z = -51ft)