Superstructure

SUPERSTRUCTURE

The superstructure is the spanning structure consisting of stringers, flooring (decking and tread), and other features such as curbs, handrails, sidewalks, and end dams.

STRINGERS

When wood stringers are used, they are usually long enough to extend across the abutment sills and trestle caps on which they rest. Stringers of one span are lapped with those of the next span.

Placing Stringers

After the abutment and trestle bents are in place, the stringers are installed (Figure 10-13). When stringers are lapped, place one outside stringer so that its inside face is under the inside face of one curb. Place the other outside stringer so that its outside face is under the inside face of the other curb. Stringers can then be lapped with a similar spacing on the next span. The remaining stringers are usually spaced evenly between the outside
stringers. On some narrow
one-lane bridges, stringers
may be grouped closer
together under the vehicle
tracks.

When stringers are butted, or
continuous across the span,
place the outside faces of both
outside stringers under the
inside faces of the curbs.

Fastening Stringers

Fasten stringers (Figure 10-
14) as follows:

Wood Stringers. Fasten
wood stringers by driving
nails diagonally through the
side of the stringer into the
cap or by driftbolts. When
using driftbolts, bore a hole,
smaller in diameter and 3 inches shorter than the driftbolt, through the stringer and into the cap.

Steel Stringers. Fasten steel stringers by—

· Driving railroad spikes into the cap beside the flange.
· Driving 60d nails partially into the cap and bending them over the bottom flange.
· Driving nails or driftbolts through prebored holes in the
bottom flange.

NOTE: When steel stringers are not fastened through their flanges, frequent inspection is necessary to be sure the stringers have not shifted. Fasten wood nail strips to the top flange
of steel stringers to provide a means of fastening
the flooring.

When a laminated deck (planks placed on edge) is to be
installed, the planks may be fastened to steel stringers
either by using metal clips provided for the purpose or by
driving nails partially into the deck and bending them
around the stringer flange (Figure 10-15).

FLOORING

The flooring system of a typical timber-stringer trestle
bridge consists of two main parts: the decking and the
tread.

Decking

The decking is the part of the
structure that is laid on the
stringers to form the roadway
across the trestle bridge.

Decking may be laminated or
solid plank.

Laminated. Laminated decks
may be solid or open with
uniform spacing between
members.

· For an open laminated deck
(Figure 10-16) where the
planks are long enough to
reach completely across the
width, use two spacing blocks between laminations. Place spacers on the stringer nearest onethird the length of the lamination. Where the laminations are not long enough (usually true for two-lane bridges), lap the laminations on a central stringer. Put a spacer block at each outside stringer.

· For a solid laminated deck, place laminations solidly against one another.

Solid Plank. For a solid-plank deck (Figure 10-17), lay planks horizontally, at right angles to the stringers. Leave a 1/4-inch space between planks to allow for swelling when wet. Extend the decking about 2 feet at approximately 5-foot intervals to support the handrail posts (Figure 10-18, page 10-12).

Tread

The tread consists of planks
placed over the decking and
between (but not under) the
curbs. The planks are usually 2 or
3 inches thick, of varying lengths,
and are laid parallel to the
direction of traffic. On one-lane
bridges, the tread is limited to the
path of the wheels or track. Twolane
bridges are fully covered
with tread. (Figure 10-19, page
10-12 shows tread placement.)

CURBS

A curb system on a timber-trestle
bridge is used to guide traffic on
the bridge. When assorted sizes
of lumber are available, make
curbs of 6- x 6-inch timber
supported on 6- x 12- x 30-inch
curb risers, spaced on about 5-
foot centers. The curb is usually
bolted to the decking with 1/2-
inch bolts, two per curb riser.

HANDRAILS

Handrails (Figure 10-20, page 10-
12) mark the bridge route and
provide safety for pedestrians
crossing the bridge. Make
handrails of 2- x 4-inch or larger
material, if available. Over a
laminated deck, make handrail
posts and knee braces of the
same material as the deck so that
they can be fastened snugly between the laminations, which are extended to receive them.

For solid-plank decks, toenail 4 x 4 posts, or two 2 x 4s nailed together, to the extended planks. Make the posts 42 inches high and space them on 5-foot centers. Place the posts so that the distance from the inside face of the curb to the inside face of the handrail is at least 10 inches.

SIDEWALKS

If sidewalks are necessary, form them by extending the decking an additional 36 inches. Place stiffening members underneath the outside edge. Support them with braces attached to the stringers, where necessary.

END DAM

The end dam is the wall that withstands the earth pressure of the abutment of a bridge (see Figure 10-1, page 10-1). After the stringers and flooring are in place, construct an end dam of flooring planks across the end of the stringers. The end dam should extend across the roadway from the top of the footing to the top of the tread. After placement of the end dam, complete the approach up to the top of the bridge deck. Post the traffic-control and classification signs. The bridge is now ready for traffic.