The Douglas independent. [volume] (Roseburg, Or.) 187?-1885, 1883-06-23

date. The design selected is what is
known as the canti-leyer bridge, the
principle of which is that of q trussed |
beam, supported at or near its center,
with the arms extending on each side,
and one end anchored or counter
weighted to provide for an equal loading.
It is In practice an entirely new design,
no bridge as yet having been completed
upon this principle. The Firth OF Forth
bridge in Scotland. with clear span of
1900 feet. is being built upon this plan,
and in this country the Fraser river
bridge, BIB feet clean span, on the
Canadian Pacific, is now being con-
structed after the same plan.
The bridge is to be adouble track steel
bridge, being built to carry upon each
track at the same time freight train of
the heaviest kind, extending the entire
length OF the bridge, headed by two
consolidation" engines and a side
pressure of thirty pounds per square
foot, which pressure is produced by
wind having velocity of seventy-6ve
miles per hour. Under the above loads
the structure is strained to only one-fifth
| of its ultimate strength. The total
! length of the bridge proper is 800 feet,
| divided into two canti-levers OF 375 feet
on the Canadian and 395 on the Ameri
can side, supported on steel towers
| rising from the waters edge. A fixed
span of 125 feet is suspended from and
| connects the river arms of anti.levers.
| The cl ar span across the river is BOO
| feet, being the longest double track
/ hrt rha fnyao
| are carried down until solid rock is
| reached, v hen blocks of ''Biton Col.
| guet' twenty feet vide, forty-fve feet
| long, and ten feet thick, will be put in.
| These form one single mass that will
Withstand pressure almost equal to the
best Quincy granite, and will so dis.
tribute the load OF 1900 tons that comes
| upon each pair of steel columns that it
will produce pressure of but twenty
Ave pounds per square inch on the
natural formation. Upon these Biton
blocks, four in number, will be built
masonry of the most substantial charac-
ter, carried up fifty feet above the sur
face of the water. On these the steel
towers rest, rising 13O feet above the
masonry. and upon these will set the
steel superstructures. The material for
this will be subjected to the most rigid
inspection and tests from the ore until it
enters the completed structure. For
erection at the bridge site temporary
scaffolding, using some 900, 900 feet of
timber, will be built on the bluff from
either side out to the edge of the water,
on level with the top of the bluff.
Upon these the shore arms of the canti-
levers will be erected, one end resting
on the steel towers and the other upon
I masonry on the bluff. The shore ends
will be firmly anchored to this masonary,
, so that it will take an uplifting force of
} 400 tons at each end to displace it. This
/ constitutes the counter weight to bal
| ance the unequal loading on the river
arm referred to above; as this, under
3 the most unfavorable conditions can
never equal 200 tons, the provision is
ample. After the structure is completed
} from the shore to the towers comes the
Z difficult portion of the work-to span the
400 feet across and 245 feet above roar
t ing river, whose forces no earthly power
r can stay ,and no temporary structure can
4 survive asmoment. Here the skill oi
3 the engineer comes in to battle nature
and laugh at her powers.
r The design of the canti-lever is such
that, after the shore arm is completed
! and anchored, as described above, the
/. river arm may then be built out, one
{ panel or section at time, by means o.
g great travelling derricks, and be self-sus
t tanning as it progresses. After one pane
of % feel is built and has its bracing ad
: iusted, ten travelling derricks are movet
r forward and another panel erected. Thu.
Q the work progresses, section by the sec
ton, until the ends of the canti.lever are
e reached, when there still remains gar
!. of 125 feet to close. Into this will be
swung and suspended from the can't
: lever arms an ordinary truss bridge
f forming the connecting link and com
Q pleting the structure. Compensation
+ for expansion and contraction is provided
for an ingenious arrangement between
( the ends of the canti-lever and fxeG
J. span, allowing the ends to move freely
6 as the temperature changes, but, at the
[ same time preserving perfect rigiditJ
against the side pressure from the wind
Y There will be no guys for this purpose
I as in the suspension bridge, but th,
e structure will be completed within itself
0 Neither will there be any of that wave
t motion noticed on suspension bridg,
as a train moves over it. When it i
{ considered that it took over three year!
e to build the present suspension bridge
which is for q single track, and that thi
4 bridge, for double track, must be fin
r ished within a months from the execu
/? ton of the contract, some idea may b.
| formed of the magnitude OF the opera
B tion.-Albany Journal.
1