The times., 1901-03-17


and justly So as they would have been
men Of exceptional genius. When these
bridges were built in 1893, the path had
been made so clear by mathematical IN
yestigation and the command of a better
steel that the task seemed easy.
| That which marks more clearly thah
anything else the great advance IN
| American bridge building during the
! last forty years is the reconstruction OF
the famous Victoria Bridge over the 81.
| Lawrence. above Montreal This prldge
! was designed by Robert Stephenson, ana
| the stone piers are n monument to his
| engineering skill. For forty winters they
| have resisted the great fields of ice borne
by a rapid current. Their dimensions
were so liberal that the new bridge was
| put upon them, although four times as
| wide as the old one.
The superstructure was originally made
| of plate-iron tubes, re-enforced by tees
| and angles, similar to stephenson
| Menai Straits Bridge. There are twenty
| two spans of 240 feet each, and n central
| one of 800 feet Perhaps these tubes were
! the best that could be had at the time
| but they had outlived their usefulness.
! Their interiors had become greatly cor
roded by the confined gases from the
| engines and the drippings from the chem
| icals used in cold-storage cars. Their
height was insufficient for modern large
! cars. and the confined smoke made them
so dark that the number of trains was
| greatly limited.
If was decided to build q new bridge of
open-work construction and of open
hearth steel. This was done and the
! comparison is as follows: Old bridge, l6
feet wide, single track, live load of one
ton per foot; new bridge, 81 feet wide,
! two railway tracks and two carriaGe-ways,
! live load five tons per foot.
The old iron tubes weighed looe9 tons,
cost s2,71s00O, and took two seasons to
erect. The new truss bridge weighs 22'-
600 tons, has cost between S,3Ooeo and
s.. .4e0. 600. and the time of construction was
| one year.
During his experience the writer has
! seen the roiling-load of bridges increase
from 2,CG0 to 1900 pounds per iineal foot OF
| track, with an extra allowance for con-
| centrated loads.
The modern high office building is an
! interesting example of the evolution of a
high-viaduct pier Such a pier of the re-
| uuired dimensions, strengthened by more
| columns strong enough to carry many
floors. is the skeleton frame. Enclose the
sides with brick, stone, or terra-cotta. add
windows and doors and elevators, and it
| is complete
Fortunately for the stability of these
high buildings, the effect of wind pres
sures had been studied in this country in
! the designs Of the Kinzua, Pecos, and
other high viaducts.
All this had been thoroughly worked
| out and known 10 our engineers before
| the fall of the Tay Bridge in Scotland.
! That disastrous event led to very careful
experiments on wind pressures by Sir Ben
| jamin Baker, the very eminent engineer
of the Forth Bridge. His experiments
showed that a wind gauge of 300 square
feet area showed maximum pressure OF
| IS pounds per square foot while a small
one of one and one-half square feet area
registered gusts of Il pounds per square
foot.
The modern elevated railway of cities
is simply a very long railway viaduct.
Some idea may be gained of the life of
| a modern riveted-ron structure from the
! experience of the Manhattan Elevated
Railway Of New York. These roads were
! built in 1883 TO carry uniform loads Of
| 600 pounds per lineal foot. except Second
Avenue. which was made to carry 2o09.
The stresses were below 1000 pounds per
| square inch.
These viaducts have carried in twenty
two years over ., 10000 trains. weighing
| over S.p9o ,o00 tons, at a maximum speed
| Of twenty-five miles an hour, and are still
in good order.
Bridge engineers Of the present day are
free from the difficulties which confronted
| the early designers of iron bridges. The
mathematics of bridge design was under
stood in 1870 but the proportioning of de-
tails had 10 be worked out individually.
| Every new span was a new problem. Now
the engineer tells his craftsman TO de
sign a span of given length, height, and
width, and to carry such load. BY the
light Of experience he does this at once.
Connections have become standardized
so that the duplication of parts can be car
rled to its fullest extent,
Machine tools are used 10 make every
part of a bridge and power riveters to
fasten them together. Great accuracy can
now be had and the sizes of parts have
increased in remarkable degree.
We have now great bridge companies
which are so completely equipped with
appliances for both shop drawings and
construction that the old joke becomes al
most true that they can make bridges and
sell them by the mile.
All improvements of design are now pub
lie property. All that the bridge COM
panies do is done in the fierce light OF
competition. Mistakes mean ruin, and the
fttest only survives.
Having such powerful aids, the Ameri-
| can bridge engineer of today has advan-
tages over his predecessors and over his
European brethren where the American
system has not yet been adopted.
The American system gives the greatest
possible rapidity Of erection of the bridge
on its piers a span of SIS feet weighing
1000 tons, was erected at Cairo on tho
Mississippi in six days. The parts were
not assembled until they were put upon
the false works European engineers have
sometimes ordered a bridge to be riveted
together complete in the makers yard
and then taken apart.
The adoption of American work in such
bridges as the Atbara in South Africa
the Gokteik viaduct in Burmah. 800 feet
high. and others, was due to low cost
quick delivery and erection as well as
excellence Of material and construction.
FOUNDATIONS, ETC.
Bridges must have foundations for their
piers. Up 10 the middle of the nineteenth
century engineers knew no better way OF