The sun. [volume] (New York [N.Y.]) 1833-1916, 1902-01-19
FEATS DONE BY The ENGINEER
JUGGLING WITH GREAT STEEL
RAILROAD BRIDGES.
0ne structure slipped Into the Place OF
Another without Delay 70 Tramo
-~Excavatfng for the Chicago Canal
"A Shaft Through Quicksand.

ing structures Is, TO most people, the COM
monest, the modern railroad bridge. and
IL IL hard to realize that nothing at all like
il, even for highway purposes, existed
fifty years ago. Now almost every one
of the bridges earliest built has been found
inadequate for the enormous increase OF
tranlc and the weight OF locomotives. and
has been replaced by steel.
zach span In made according to draw
ings, with hundreds of beams, girders,
bars. rods and columns, riveted and forged
and bored In distant shops, but never fitted
together until they arrive by dozens OF
carloads at the site. Inc are quickly as
sembled TO make a slender. s~elf-supporting
framework with the rigid pieces which
weigh thousands Of pounds fitted accur-
ately and swiftly in place like the mechan-
ism OF a watch.
This u hard enough to do across n storm
swept bay, over an almost bottomless river,
where there Ia a fierce current or high tides,
or where. as in the Niagara gorge, tho
height Is 30 great that no support can be
built up from the bottom. and the span
has TO support itself during construction;
but all these difficulties are mall to tho
bridge builder, compared TO the require
ment. often met, that ar old bridge OF
perhaps many spans must be replaced by
a new one. In exactly the same place, OF
the same dimensions and supported on
the very same piers, without ~interrupting
appreciably the almost constant traffic
which demands passage nearly every hour
In the twenty-four for long freight trains
or rushing expresses.
Such was the case with the Florence
L-slz~ 1900. A.. Tann...aa Pix.or In Ala
bama. There were twelve spans Of about
100 feet each. which carried a single track
railroad on top and a highway between
the trusses.
The old wooden spans could not even
support the new steel ones during erection.
although the masonry piers had ample
strength. Wooden towers were built on
the top edges Of the old bridge. outside
the track. above two adjacent piers.
Above these towers. nearly twenty-five
feet over the track, a complete new tenn
porary span of good and iron as long az
The longest spaH Of the old bridge was
built. A new. permanent span was as
sembled together, suspended from the
under side OF il, clear OF the trains which
passed uninterruptedly through 1t and
under the temporary span.
When II was finished and track laid on
top OF il the under side was lashed 10 the
upper side Of h old span, and it was adj
dltlOnhlN tlSTfnuSl ITOm lut overhead
temporary span by fou vertical 81 inch
steel screw rods, about 10 feet long. one at
each corner. which had nuts bearing on
top Of the temporary span.
A suitable interval between trains was
selected, the ends of the old span chopped
away. the lashings between the new and
temporary spans removed, and then the
nuts on the great screws were Stacked OF
by gears operated by a steam engine at
each end, and both spans were lowered
- - ANAlaW ~-.. AlaUUs %
feet, In less than an hour: the old one drop
ping down almost 20 the waters edge and
the new one following 20 occupy its former
place and rest on masonry seats which were
meantime.
A few minutes later trains were running
across the new span. the old one was re-
a. leisure. and the overhead span
was rolled along until It's tower- rested
above the next piers. another span was
replaced. and 10 on. until the whole super
had been replaced. IL took
thirty-four men only eighty daysto rebuild
tho bridge.
The difficulty and expense ~ building
solid foundations for n bridge where the
water u very deep or swift or the bottom
unstable are very great. In soft soils
the utmost enor h made 90 prevent any
danger of settlement or Of undermining
by Hoods and swift current, and when"
pilas are not used IL IL generally necessary
10 carry the HnAEonrv down 90 solid rUsk I
W - y NnAN ~ ~ ~ 80 aAA 49,
This IL always very difficult on account 81
the tremendous pressure Of the earth ard
water az q great depth, and the danger -
working there even when u Il poss4ble
"a exclude the water. In THE country
cribs OF oaissons are often sunk by mo
chanical processes or by men working Ia
pneumatic pne--un. and then ar. hlled with
H. India where tho soft %AlluyiAl soil Ell
20 a great depth and 10 saturated
With xaisf and sill now hke mud. th.y
~ rupfoH haayy foundations partly
by itf s~tlolnIeas foT many yeaTr tlny
bllt hollow Cylindrical brick pisfs
from ~ "a io f.t dep Which hold hsayv
l0adf chiefly by ths furuou uu their si,sr
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