Figure 8-5 shows the plan and section of a typical
mat foundation. In this type of foundation, a heavily
reinforced concrete slab extends under the entire
building and distributes the total building load over the
entire site. This minimizes problems created by unequal
settlement when the subsoil conditions are uneven.
BENTS and PIERS provide support for the bridge
superstructure at points other than the bank ends. A
BENT consists of a single row of posts, or piles, while
a PIER consists of two or more rows of posts, or piles.
The following text covers some of the different types of
bents and piers.
The PILE BENT shown in figure 8-6 consists of the
bent cap, which provides a bearing surface for the bridge
stringers, and the piles, which transmit the load to the
soil. The support for the loads may be derived either
from column action, when the tip of the pile bears on
firm stratum, such as rock or hard clay, or from friction
between the pile and the soil into which it is driven. In
both cases, earth pressure must provide some lateral
support, but traverse bracing is often used to brace the
A TIMBER PILE BENT consists of a single row of
piles with a pile cap. Brace it to the next bent or to an
abutment to reduce the unbraced length and to provide
Figure 8-5.Plan and section of a mat foundation.
Figure 8-6.Typical pile bent.
stability. This bent will support a combined span of 50
The TRESTLE BENT shown in figure 8-7 is similar
to the pile bent, except the post takes the place of the
piles, and transmits the load from the cap to the sill. The
sill transmits the load to the footings, and the footings
transmit the load to the soil. Timber trestle bents are
normally constructed in dry, shallow gaps where the soil
is firm. They are not suitable for use in soft soil or swift
or deep streams. The bent can support a combined span
length of up to 30 feet and can be 12 feet high.
The PILE BENT PIER (fig. 8-8) is composed of two
or more pile bents. In this figure, notice the common
cap. The cap transmits the bridge load to the CORBELS
which transmits the combined load to the individual bent
caps. Piers are usually provided with cross bracing that
ties the bents together and provides rigidity in the
A TRESTLE BENT PIER is the same as the pile
bent pier, except it has sills and footings to transmit the
load to the soil, as shown in figure 8-7. This bent is
designed to carry vertical loads only and is used to
support spans up to a combined 60 feet and for
ground-to-grade heights of up to 18 feet.