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.

Intermediate Supports

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 bent laterally.

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 feet.

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 longitudinal direction.

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.