The daily morning journal and courier., 1902-01-25
fEATS DONE BY ENGINEERS
JUGGLING WITH CHEAT STEEL
 R1ILROAD BRIDGES.
One Structure Slipped Into the Place of Another Without Delay to Traffic --Excavatins For the Chicago Canal - shaft Through Quicksand.

About the newest of all great en-
 sineering structures is, to most people,
 the commonest, the modern railroad
 bridge, and it is hard to realize that
 nothing at all like it, 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 traffic and the
 weight of locomotives and has been re-
 placed by steel.
 Each span is made according to draw
 ings, with hundreds of beams, girders,
 bars. rods and columns, rveted and
 forged and bored in distant shops, but
 never fitted together until they arrive
 by dozens of carloads at the site, and
 are quickly assembled to make a slen-
 der, self-supporting framework with
 the rigid pieces which weigh thousands
 of pounds fitted accurately and swiftly ill place like the mechanism of watch
 This is hard enough to do across a storm swept bay, over an almost bot
 tomless river, where there is a fierce current or high tides, or where, as in the Niagara gorge, the height is so 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 small to the bridge builder, compared to the require
 ment, often met that an 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 inter- rupting 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 bridge across the Tennessee River in Alabama. There were twelve spans of about 120 feet each, which carried a sin gle track railroad on top and high way between the trusses.
 The old wooden spans could not even support the new steel ones during erec- ton, 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 complete new temporary span of wood and iron as long as the longest span of the old bridge was built. A new, permanent span was assembled together suspend ed from the under side of it, clear of the trains which passed uninterruptedly through it and under the temporary span.
 When it was finished and track laid ! on top of it the under side was lashed to the upper side of the old span, and | it was additionally suspended from the | overhead temporary span by four verti- cal 1-2 inch steel screw rods, about 30 | 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 slacked off by gears operated by a steam engine at each end, and both spans were lowered smoothly and safe ly together, about 22 feet, in less than an hour, the old one dropping down all most to the waters edge and the new one following to occupy it's former place and rest on masonry seats which were prepared meantime.
 A few minutes later trains were run ming across the new span, the old one was removed at leisure, and the over head span was rolled along until it's towers rested above the next piers, an- other span was replaced, and so on, un- til the whole superstructure had been replaced. It took thirty-four men only eighty days rebuild the bridge.
 The difficulty and expense of building solid foundations for a bridge where the water is very deep or swift or the bottom unstable are very great. Tn soft soils the utmost effort is made to pre- vent any danger of settlement or of un- dermining by floods and swift current and where piles are not used it is gen. srally necessary to carry the masonry down to solid rock or to a safe distance in earth and sand. This is always very difficult on ac- count of the tremendous pressure of the aarth and water at a great depth and i the danger of working there even when t Is possible to exclude the water In ( this country cribs or caissons are often { sunk by mechanical processes or by /, men working in pneumatic pressure { md then are filled with concrete. f In India where the soft alluvial soil I extends to a great depth and is saturat- ( .d with water and will flow like mud, f hey make it support heavy foundations partly by it's stickiness. For many 6 years they have built hollow cylindri- f ,al brick piers from 20 to 100 feet deep f which hold heavy loads chiefly by the | friction on their sides f Originally thtSe hollow piers, a few i eet in diameter, were built up on wood | ,n rings laid on the ground in the bot c om of pits dug down to water level A T, native diver went inside and dug up the bottom with an adze-like tool la I, undermining the walls, so h hat they gradually settled a few inch Q IS week as the tops were built on high fl bove the surface of the ground f Afterward the excavation was done 6 " dppers attached to long poles and ometimes by buckets with valves in q he bottom. Recently large pier foun- a ations for railroad bridges have been q made with two or three brick cylinders T welve to fifteen feet in diameter, shod o rith circular iron girders having sharp, uttine edges and horizontal shelf to z. eceive the brick work. fl Steam dredging machines were set on r, np of the piers, and undermining the V