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Wetland Trail Design and Construction

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Structures Requiring Foundations - (continued)

Bent Construction

Whether wood end-bearing or friction piles are used, once a pile is in place, the construction is similar. A second pile is placed on the opposite side of the trail centerline so that each is the same distance from the centerline. When both piles are in place, they are connected by one or two ledgers. The combination of ledgers and piles is called a bent.

On a one-ledger bent, the top of each wood pile is cut flat and level with the opposite pile. A 3 by 6 or 3 by 8 timber is placed flat on the top of both piles so that it extends a few inches beyond each of them. This timber, or ledger, is spiked to the top of each pile (figure 31).

Image of a one-ledger bent
Figure 31-A bent with one ledger. Spike the ledger
to the top of each pile. The pile and ledger
are collectively called a bent.

When two ledgers are used, one is bolted to the front and one to the back of each pile, spanning the space between the piles. Drill a hole through each pile parallel with the trail centerline. These holes (and ledgers) should be level with each other. A 3- by 6-inch ledger is held in place on one side of the pile, and the hole in the pile is extended through the ledger. This is repeated until each ledger can be bolted to each pile. The ledgers should be level and level with each other.

Another method for the same type of installation is to determine the proper height of the ledgers and clamp the pair of ledgers to each pile of the bent. Drill a hole through the ledger, the pile, and the opposite ledger, all at once. This is faster, but requires two large clamps that can open at least 1 foot (figure 32).

Image of a two-ledger bent
Figure 32-A bent with two ledgers. Trim the tops of
the piles at an angle so they will shed water.The
bolt goes through the pile and both ledgers.

After installing a pair of bents, pressure-treated 3- by 12-inch tread planks are nailed to the ledger or ledgers as described for the bog bridge on sleepers. If the planks are more than 2 feet above the ground or water, the tread should be at least two planks wide.

Where the deck will be more than 3 feet above the ground, diagonal bracing is needed to connect the piles of a bent. A single diagonal brace is adequate if the deck is just 3 to 4½ feet above the ground (figure 33). If the deck is higher than 4½ feet, two diagonal braces are necessary. These braces should be installed as a cross brace, forming an X between the piles. Diagonal braces are normally wood (figure 34). The angle of the braces should be between 30 to 60 degrees to the horizontal to provide enough support. Angles of 30, 45, or 60 degrees, or a 3-4-5 triangle, make the mathematics of carpentry easier in the field.

Image of a diagonal brace
Figure 33-A single diagonal brace is adequate
if the deck is no more than 4½ feet above the ground.
Alternate braces on successive bents.

Image of a double brace
Figure 34-Use a double brace if the deck
is higher than 4½ feet above the ground.

Occasionally, the ground is well below the surface of the tread. If the tread is 4 feet or more above the ground and the space between the bents is 6 feet or more, diagonal bracing may be needed to connect consecutive bents. Bracing between bents is done with wood members from the right pile of one bent to the right pile of the next, and the left pile of the bent to the left pile of the next (figure 35).

Image of braces between bents
Figure 35-Bracing between bents is sometimes necessary.

On national forests, all bridges require design approval from engineering before being constructed. Bridges are generally defined as structures more than 20 feet long and higher than 5 feet off the ground. Some of the more elaborate structures described in this report meet these criteria and require engineering review and approval.


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Updated: 04/14/2014
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