Figure 10-5.-Locating seats for trestle bents.
less the thickness of a footing, will be the height of the
first bent.
Next, stretch the tape from the location of the first
bent, level it as before, and again drop a plumb bob from
the 12-foot mark. The position of the plumb bob will be
the location of the section bent. The vertical distance
from the location of the bob to the horizontal tape, plus
the height of the first bent, less the thickness of the
footing, will be the height of the second bent.
Finally, stretch the tape from the location of the
second bent and proceed as before. The vertical
distance from the location of the bob to the horizontal
tape, plus the height of the second bent, less the
thickness of a footing, will be the height of the third
bent, and so on.
CONSTRUCTING A TRESTLE BENT
When a trestle bent is laid out and constructed, the
length of the posts is equal to the height of the bent, less
the combined depths of the cap and sill. In a four-post
bent, the centers- of the two outside posts are located
from 1 to 2 1/2 feet inboard of the ends of the sill, and
the centers of the two inner posts are spaced equally
distant between the other two.
Sills, caps, and posts are commonly made of stock
that ranges in size from 12 by 12s to 14 by 16s. If a
sill or cap is not square in a cross section, the larger
dimension should be placed against the ends of the
posts. The usual length for a sill or cap is 2 feet more
than the width of the roadway on the trestle. The
minimum width for a single-lane trestle is 14 feet; for
a two-lane trestle, 18 feet.
Layout
Part of the terrain at an assembly site may be graded
flat and used as a framing yard, or a low platform may
be constructed for use as a framing platform. To assem-
ble a bent, lay the posts out parallel and properly spaced,
and set the cap and sill in position against the ends. Bore
the holes for the driftpins through the cap and the sill
into the ends of the posts, and drive in the driftpins. Cut
a pair of 2- by 8- by 18-inch scabs for each joint and
then spike, lag-screw, or bolt the scabs to the joints.
Finally, measure the diagonals to determine the
lengths of the transverse diagonal braces. Cut the
braces to length and spike, lag-screw, or bolt them to
the sills, caps, and posts. Transverse diagonal bracing
is usually made of 2 by 8 stock.
Trestle Bent Erection
After assembly, the trestle bent is moved to the
abutment, and set in place on the footings at the seat.
Carefully plumb the bent and temporarily brace it with
timbers running from the top of the bent to stakes driven
at the abutment. Lay the superstructure (girders and
decking) from the abutment out to the top of the first
bent. The second bent is then brought out to the end of
the superstructure and set in place. Plumb the second
bent and measure the diagonals to determine the lengths
of the longitudinal diagonal braces between the first and
second bents. Then, cut the braces and spike, lag-screw,
or bolt them in place.
The superstructure is then earned out to the
second bent, after which the third bent is brought to
the end of the superstructure. This procedure is
repeated, usually by parties working out from both
abutments, until the entire span is completed.
SUPERSTRUCTURE
Timber girders are usually 10 by 16s, 14 feet long,
spaced 3 feet 3 1/2 inches on center (OC). Various
methods of fastening timber stringers to timber caps
are shown in figure 10-6, view A. Various methods of
fastening steel girders to timber caps are shown in view
B. This view also shows three ways of fastening a
Figure 10-6.-Methods of fastening timber stringers and steel
girders to timber caps.
10-3
