Alma record. (Alma, Mich.) 1878-1928, 1888-04-13

THE FIRTH OF FORTH BRIDGE.
a Gigantic Engineering work-Two
spans of over seventeen Hundred
Feet Ench.

work W II be completed. and the time
given 10y the resident eng neer is toward
the end of next year. Describing the
progress of the work. The Scotsman
says that the huge "siraddle-l legged"
structures which are reared upon three
large piers were pract ically completed
before the end of the year. These rise
TO the great height Of three hundred
feet above high water mark. and torn
the points d'Gpp'H structural basis from
winch the "cintalcvers" are being built
out ON either side. The term cantlev-
ers is applied to the wing-shaped
structures which shoot out from the
large pier-towers and reach toward one
another across the great spaces that
have TO be spanned. The building of
these cantaleverS has at present reach
ed most interesting stage. From the
summit Of the towering pier structure
arms are being stretched out on each
side into IN d-ar. These are each com
posed of two pairs of huge booms. con
structed on the glider or lattice work
principle, and project sheer above the
waters, which are full three hundred
feet below. There is not the slightest
under support: the whole fabric relies
upon the strength OF the ties which
bind ii to the top OF the cantaleyer
towers. Incredible rs II may appear,
these top members have built out the
TO the distance of l25 feet. and seem
just now to hang in n precarous posi-
ton like the ash upon half smoked
cigar. II Is n this part of their work
that the designers and contractors clam
that they have vindicated The sOunduess
OF their calculations. Il was in the
sprnging of these top members or
arms. and carrying them out n,upport-
ed the necessary distance, that cavillcrs
and detractors of the scheme have
chiefly prophesied failure Yet from
both the Quecnsberry and Fife pier
structures these serial platforms have
been built out, two from each. w thout
a hitch, 20 the distance of l2j feet.
They now await the rearing of
temporary column which is running up
from below. and which viii form sup
port. enabling them 10 be carried still
further ont till they meet the first per-
manen. supports-the cross struts or
tubes which sprng from the bases OF
thc p.er-towcr~. the lower arms,
meantime have been proceeding even
more RAPIDLY. These are immense tubes
which prelude outward and upward.
and along with the pressure eNcrlcd bt
the weight of the spans is carried back
90 the stone piers. They have been
carried out 10 the distance OF fully 100
feet. it may be explained here that
the top arms. which are constructed on
the rider system are the tension MEM
Jers-that is 90 say, are subjected 10
a pull bv the weight Of the structure
which they support, while the lower
arms. which are of tubular construc-
ton, are the compression members, and
bear the down push. rs were These
lower tubes have been built out in
exactly the same way rs the upper ones
that is. W thout any support from
underneath The workers to-day are
practically standing upon their labors
of Yesterday. As soon rs a fresh round
Of steel plates is added to the tubes or
an additional or der sect on riycted to
the top arms, the platforms, with their
freight of men, cranes. and other
mechan cal appliances are slid ont cor
respondently and a new piece Of work
is begun, which again when completed,
VIII give the necessary standing sup
port for further extension Indeed,
This is characteristic Of the whole of the
WORK at the bridge. Every piece Of
work done becomes the basis of another
advance
The point however Of this great
work of Of engineering which is Of pre-
cmiueuti0tere-t and about which spec.
u.at on and prophecy are most bus ed,
is the problem OF bridging over the two
great spans of ~, lU feet each which eX-
tended north au south OF Inch Garyie
Ihe largest Il. bridge spans are iu,t
30 feet So ,la. ,he proposed spans at
Quocnsbarrv will bee seven Times as long
The largest span of any bridge in this
country is that of the Britannia br dge,
over the Menia strait which is 40 feet
i'm length. on somewhat over a quarter
of the ze of the Forth bridge spans.
20 throw q single span q cross such
space as LILO feet would, even under
ordinary conditions, be tremendous
undertaking. But in the present case
the difficulties are incalailablv cnhanc-
ell, because the whole distance has to
be bridged over without any support
from below The water is two humid
red feet deep on each side of Inch Gar
vie. so that il is impossible TO found
temporary scailolding or piers upon the
sea bottom. nor can any device Of pom.
toons be of practicable service The
problem then. comes simply this-to
join the Inch Garvie pier " In the north
and south peers leach l.710 feet apart
by simply building straight ont across
the waters at height sufficient TO nj
low the largest ships to pass freely up
and down the Firth. 1t is this that ex-
plains WHY the immense "siraddleleg
ged" towers which are the prominent
features of the structure Just now. have
been reared as high as $50 sect. These
great spans will never bee entirc-that
is. the wo haly will never be joined.
Each half Of each great span will hang
entirely 10y Is own supports ON the
main p ers Owing to the large cxpan.
on and contraction of so immense a
8 ia. q