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 foundation 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 foundation 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 84.P1an and section of a typical spread footing.