Substructure

as a multispan bridge. Although bridges may be either fixed or floating, only fixed bridges are covered in this training manual (TRAMAN). The following information covers the components of a fixed bridge. SUBSTRUCTURE The substructure supports the superstructure and consists of abutments, footings, sills, posts, bracing, and caps. Abutments There are different types of fixed-bridge abutments. First, let’s cover the footing type of abutment. In figure 8-2, views (A) and (C) show two types of footing abutments. View (A) shows a timber-sill abutment, and both of these views, you will see that three elements are common to a footing type of abutment. Specially each type has a footing, a sill, and an end dam. Notice that the timber-sill abutment shown in figure 8-2, view (A), is the same footing type of abutment shown for the bridge in figure 8-1. In this type of abutment, loads are transmitted from the bridge stringers to the sill which distributes the load to the footing. The footing then distributes the combined load over a sufficient area to keep the support from sinking into the ground. The end dam is a wall of planks that keeps the approach-road backfill from caving in between the stringers. The timber-sill abutment should not be more than 3 feet high. It can be used to support view (C) shows a timber-bent abutment. By studying spans up to 25 feet long, Figure 8-2.—Types of fixed-bridge abutments. 8-2