Substructure

SUBSTRUCTURE

The substructure of a bridge supports the superstructure. The substructure consists of:

ABUTMENTS

There are two types of end supports or abutments: footing and pile.

Footing

The footing abutment consists of—

· Footings. Footings transmit the load to the ground. They receive the load from the sill and distribute it over a sufficient area to keep the support from sinking into the ground.

· Sill. The abutment sill (Figure 10-2) receives the load from the stringers and transmits it to the footings.

· End dam. The end dam (or bulkhead) is a wall of planks at the end of the bridge to keep the approach road backfill from caving in between the stringers.

Pile

The pile abutment (Figure 10-3)
has three main parts:

· Piles driven into the ground,
transmitting the load to the
soil.
· A cap on top of the piles to
receive the load from the
stringers.
· Sheeting fastened to the
piles to hold the backfill in
place.

INTERMEDIATE SUPPORTS

The following are some of the
different types of intermediate
supports.

Pile Bent

The pile bent (Figure 10-4)
consists of the bent cap,
which provides a bearing
surface for the stringers and
transmits the load to the
piles; and the piles, which
transmit the load to the soil.

The support for the loads may come either from column action, when the tip of the pile bears on a 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 give some lateral
support; transverse bracing is also often used for this purpose.

The pile bent is used for highway bridges only. It is designed to carry both vertical and lateral loads and can be used for spans of up to 50 feet. Its ground-to-ground height is a function of its unbraced length.

Trestle Bent

The trestle bent (Figure 10-4) is like the pile bent except that posts take the place of piles. The posts transmit 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. The length of the posts varies according to the height of the bridge above the gap to be spanned. Transverse bracing like that used with the pile bent is provided.

The trestle bent is used for highway bridges only; however, unlike the pile bent, it is designed to carry vertical loads only. It can be used for spans of up to 30 feet and for ground-to-grade heights of up to 12 feet.

Pile-Bent Pier

The pile-bent pier (Figure 10-5, page 10-4) is composed of two or more pile bents with a common cap. The cap transmits the load to the corbels (short, stringer-like members) that, in turn, transmit the load to the individual bent caps and then to the piles and to the soil. Piers usually have cross bracing which ties the bents together, giving them longitudinal rigidity.

The use of multiple bents gives the pile-bent pier great strength. As a result, the pile-bent pier can be used for both highway and railroad bridges. It will carry both vertical and lateral loads, can be used for spans of up to 200 feet, and its ground-to-grade height is governed by its unbraced length.

Trestle-Bent Pier

The trestle-bent pier (timber-trestle pier) (Figure 10-6, page 10-4) is the same as the pile-bent pier, except that it has sills and footings which transmit the load to the soil.

The trestle-bent pier is used for highway bridges only. It is designed to carry vertical loads only and can be used for spans of up to 60 feet and for ground-to-grade heights of up to 18 feet.

Crib Pier

The crib pier (Figure 10-7, page 10-5) is quite different from pile and trestle piers. It is composed of logs or dimensioned timber fitted together in log-cabin style and is usually filled with rock or other stable fill material. The crib pier should be made so that it needs no exterior bracing. As an expedient, crib piers may be built to the height of the stringers, eliminating the trestle bents. The crib pier is used for highway bridges only. It is for vertical loads only and can be used for a span of up to 50 feet and a ground-to-grade height of up to 12 feet.

BRACING

Bracing consists of longitudinal bracing, transverse bracing, and diaphragms.

Longitudinal bracing (Figure 10-8) is used to stabilize the bridge centerline.

Transverse bracing (Figure 10-8) provides stability at right angles to the centerline. It is sometimes called sway or lateral bracing.

Diaphragms are braces
between stringers to prevent
them from deflecting laterally
(buckling) under load. In
spacing these diaphragms,
the ratio of distance between
diaphragms to the width of
the top of stringer (L/b ratio)
should not exceed 30 for
timber.

L = distance between
diaphragms

b = width of top of stringer
Example. If the stringer is 6
inches wide—

L = 30 L = 180 inches (15 feet)

In this example, diaphragms should be used every 15 feet
between stringers 6 inches wide.

CONSTRUCTION PROCEDURES

The following paragraphs contain construction procedures for a trestle-bent bridge. This includes laying out the centerline and constructing abutments, retaining walls, and trestle bents.

Layout of Centerline

The first task in constructing a trestle-bent bridge is laying out the centerline (Figure 10-9, page 10-6):

Step 1. Stretch a line or tape representing the centerline
across the stream or ravine.Step 2. Attach the line to stakes driven into the ground at least 15 feet behind the proposed location of the abutment sills. For defiles wider than 100 feet, use intermediate stakes as needed to
prevent sag.

Step 3. Place the line at the level of the intended top of the flooring or at some known distance above or below it.

Construction of Abutments

Saving time in abutment construction is especially important on short bridges. Abutment and approach preparation often requires as much time as the rest of the bridge. Use the simplest abutment possible; often a
timber sill with timber footings
is adequate (page 10-2).

The end dam is installed after
the stringers and planks.

After the centerline is fixed—

Step 1. Place the abutment sill at
approximately its correct
location under the tape. See that
it is at right angles to the
centerline by using a line from
the centerline stake 15 feet
behind the sill to each end of the
sill. Both distances must be the
same.

Step 2. Once the sill is properly located, mark its position and remove it to construct the foundation.

· Remove the earth as needed to provide a level surface for footings. The sill must be level and supported equally by each footing when installed. Make sure that the surface supporting the footings is about 2 inches higher than its final position to allow for settling. Do not dig too deeply. If this is done by mistake, do not backfill with earth.
Instead, raise the level with planking.

· Place the two outside footings so that their outer edges are under the ends of the sill. Place the long dimension of the footings parallel to the bridge centerline.

· Place the remaining footings, equally spaced, between and in line with the outside footings.

· Place the sill on the footing centerline so that the load is in the middle of each footing. Place the sill with the largest dimension vertical.

· Provide for drainage of the abutment area.

Construction of Retaining
Walls

Retaining walls and
revetments, when needed, are
part of the abutment
construction. The simplest type
of retaining wall is built of
planks or logs supported by
piles or posts. (Figure 10-10
shows an abutment and
retaining wall; Figure 10-11
shows retaining-wall details.)

· Use wing walls to prevent the earth from washing out behind the retaining wall.

· Drive piles or posts 4 feet into the ground.

· Fasten anchor cables from the top of the piles to a deadman behind the retaining wall or to the wing-wall end. These deadmen and anchors can be eliminated if two or three rows of piles, driven as far as they will go, are used.

· For long spans and heavy loads, the abutment and retaining wall are often constructed as a unit. This may also be necessary where steep banks and poor soil conditions exist.

Construction of Trestle Bents

After the position of the near-shore abutment sill is established, locate the position of the first trestle bent:

Step 1. Measure the length of the first span from the abutment sill along the centerline (Figure 10- 12).

Step 2. Drive a small stake under the centerline where the center of the trestle bent is to be. Use a plumb bob if necessary.

Step 3. Continue this procedure until all trestle bents and the far-shore abutment sill are located.

Step 4. Excavate and place footings under the trestle bent the same as for the abutment (page 10- 5). Outside footings under the trestle sill are centered under the outside posts of the bent.

· Measure the vertical distance from the centerline down to the top of the footings.

· If the centerline was placed at the intended top of the flooring, this distance minus the thickness of the tread, deck, and stringers gives the height of the trestle bent.

· If steel stringers are to be used, allow also for the thickness of the nailing strips.

Step 5. To obtain the correct height of the trestle-bent posts, subtract the thickness of the cap and sill from the height of the trestle bent.

Additional Construction
Procedures

There are some additional
procedures that should be
followed when constructing
a substructure.

· Make the length of the
cap and sill equal to the
roadway width plus 2
feet.

· Center the outside posts
under the roadway
edges 1 foot from the ends of the cap and sill.

Space other posts evenly between the outside posts.

· Use driftpins or bolts to fasten the sill and cap to the posts. Use scabbing instead of driftpins for fast erection.

· Nail transverse bracing across both sides of the bent. Usually 3- x 12-inch planks are used. Fasten the bracing to each post that it passes over. Cut the bracing so that the ends extend beyond where they are nailed, to prevent splitting.

· Put the bent into position, using a plumb bob to ensure that it is straight. Hold it in place with temporary braces nailed to stakes driven into the ground. Use these temporary braces until the permanent longitudinal bracing can be nailed to the outside posts of adjacent trestle bents.