Evening star. [volume] (Washington, D.C.) 1854-1972, 1904-07-20

more ornamental than was contemplated
in those plans. The plans were then sub
mitted to Col. Theodore Bingham, then SUI
perintendent of public buildings and
grounds, for report This officer return
ed them, with suggestions for improve
ments in the elevated approach to the
north end of the bridge permitting the pas
sage beneath of several additional drive
ways of Potomac Park.
A large number of borings were made
through the river bed at the proposed site
to ascertain the character Of the strata
upon which the foundations would rest. 1t
was found that bed rock was more than
100 feet below low-water level. Above the
rock was stratum of coarse gravel, and
1t was deemed sufficient to drive piles I'M
til they rested in this to form stable
foundation for the piers, with the excep-
ton of the pivot pier, which supports the
draw span and machinery 1t was neces-
sary here to build an enormous caisson
which was sunk to the river bed and air
pumped in while men went down and ex-
cavated until the gravel stratum was
reached. This caisson was hexagonal, for
ty-four feet in diameter, the walls and
deck or roof being constructed of a double
layer of 12x12 timbers. The circular gran
lite pier, which was to support the greatest
weight of all those in the bridge. was built
upon the deck Of the caisson as it sank by
reason of the removal of material from be-
meath. large steel pipe ran through the
center of the pier to the air chamber.
through which the compressed air was fur
When the layer of gravel was encountered
the work of excavating was stopped and
the caisson itself packed full of concrete.
The other piers were built in cofferdams.
which, like the caiSSOn were built on shore
and launched in the same manner as ship
might be, then floated to the point where
they were to be used.
MARYLAND Granite Used.
The new bridge is of the type known as
the Pratt truss bridge, developed by Mr..
Pratt, the engineer of bridges of the Penn
sylvania railroad It's total length is 2,52S
feet six inches There are twelve piers
built Of granite from the quarries at Port
Deposit, Md. There are eleven spans be-
sides the draw span, which is virtually Two
spans, the ends resting on the pivot pier
being connected by tie bars at the top so
that when their outer ends are swung away
from the two piers on either SIDE or the
draw, they counter balance each other.
Their construction is varied from that of
the other spans in order to meet this strain,
the connecting ends being higher than the
outer ends All engines and machinery for
the operation of the draw have been in
stalled and are in working order. They are
situated in a small house built on a plat
form at the center of the draw about thir-
ty-six feet above the railroad tracks.
Without the steel rails of the tracks which
are to cross the bridge, each of the other
spans is separate from adjoining ones and
not connected in any way, merely resting
end to end upon the supporting piers. If
force sufficiently potent We're ever exerted
one might be entirely swept away without
disturbing the others.
The number of men engaged in the actual
steel construction at each end is about
twenty-five, and this force is ample to ac-
comp,sh the work with good speed The
many others engaged in handling material
and preparing the temporary support Tor
the spans are equally capable, however, of
attending to any of the details OF bridge
construction.
In the process of erecting the steel work
it Is necessary to provide platform upon
which the truss may rest until 1t is Sc
curely riveted and prepared to bear its
own weight This temporary support is
built upon piles which have been driven at
close intervals between each of the piers
except the pivot and adjoining piers. The
piles have been sawed off at an equal
height, and across these heavy, beams are
laid upon which the floor beams of the
bridge rest while the remainder of the
structure is built upon them. Outside the
lines upon which these beams are to be
laid steel rail is laid upon stringers,
forming a track the width of which Is
greater than that of the bridge itself. Upon
this track runs the traveller framework
of heavy beams, which is utilized in NOV
ing the heavy pieces of steel to their places
in the truss work. This structure is about
fifty feet high and has a space under 1t
large enough to permit it to move back and
forth over the completed portion or the
bridge. Numerous puleys are fixed upon
1t at various points, and as it is moved for
ward the bridge work comes out from under
1t with every beam bolted in place.
The riveters then come along, taking out
one bolt at time and with powerful pneu-
matte hammers forcing tremendous bolts
into the many joints of the framework. The
power and rapidty of the blows of these
hammers operating against the ringing
steel makes sound suggesting the action
of an elephantine locust, the singing Of
h nl, he I'd a
How the Work is Done.
AS the work progresses ties are laid, and,
upon these, railroad tracks, so that cars
loaded with the steel required may be moved
directly to the point of use. An engine of
the regular freight type is constantly en-
ployed in shifting the train loads of steel
used up in the work. This serves to test
the strength of the bridge to some extent,
for as the work of the riveters is finished
the timbers are removed from beneath the
spans and the steel truss takes up the
strain. The painting of the bridge Is now
under way, and this In itself is no small
undertaking. Two coats of red will be put
on, and more than 1000 gallons OF paint
will be required.
Tne building of elevated approaches to
either end of the bridge is well under way,
that on the south being nearly ready for
the laying of tracks. trestle is first
built upon which the track is laid, and
subsequently earth is unloaded from cars
run upon the trestle, making embank-
ment of sufficient height, the stability of
which is insured by the umbers below. The
approach from the District side Is about
1900 feet long and begins at the bridge
over the head of the Washington channel.
The southern approach is about 2,6OO feet
long.
The new bridge begins about sixty feet
to the west of the old one on the District
side, but Its southern end is about 260 feet
west of the old structure. This change OF
direction was decided upon in order to
bring the line of the bridge more nearly at
right angles to the course of the river cur
rent in the channel, the piers thus Offering
less obstruction to the flow of water, which
in time of food Is an important considera-
ton. The wooden bridge has a far greater
number of piers than the steel structure,
and its length being less by about 200 feet