The timber-bent abutment shown in figure 8-2, view (C), can be used with timber or steel stringers on bridges with spans up to 30 feet. The DEADMAN is used to provide horizontal stability. These abutments do not exceed 6 feet in height.
Other types of fixed-bridge abutments are PILE abutments and CONCRETE abutments. Timber or steel pile abutments can support spans of any length, can be used with steel or timber stringers, and can reach a maximum height of 10 feet. A timber-pile abutment is shown in figure 8-2, view (B). Concrete abutments are the most permanent type. They may be mass or reinforced concrete, may be used with spans of any length, and may be as high as 20 feet. Use these abutments with either steel or timber stringers.
That part of a building or structure located below the surface of the ground is called the FOUNDATION. Its purpose is to distribute the weight of the building or structure and all live loads over an area of subgrade large enough to prevent settlement and collapse.
In general, all foundations consist of the following three essential parts: the
bed, which consists of the soil or rock upon which the building or structure rests; the
footing, which is normally widened and rests on the foundation bed; and the
wall, which rises from the foundation to a location somewhere above
Figure 8-3. - Common wall and column foundations.
the ground. The foundation wall, contrary to its name, may be a column or a pedestal instead of a wall. However, when it is a wall, it forms what is known as a continuous foundation. Figure 8-3 shows common types of wall and column foundations.
The continuous foundation is the type most commonly used for small buildings. The size of the footing and the thickness of the foundation wall are specified on the basis of the type of soil at the site. Most building codes also require that the bottom of the footing be horizontal and that any slopes be compensated for by stepping the bottom of the footing.
Another type of foundation is the grade-beam foundation. A GRADE BEAM is a reinforced concrete beam located at grade level around the entire perimeter of a building, and it is supported by a series of concrete piers extending into undisturbed soil. The building loads are supported by the grade beam, which distributes the load to the piers. The piers then distribute the load to the foundation bed.
A spread foundation, as shown in figure 8-4, is often required where heavily concentrated loads from columns, girders, or roof trusses are located. This type of foundation maybe located under isolated columns or at intervals along a wall where the concentrated loads occur. Spread footings are generally reinforced with steel. They may be flat, stepped, or sloped, as shown in figure 8-3.
Figure 84. - P1an and section of a typical spread footing.Continue Reading