Evening star. [volume] (Washington, D.C.) 1854-1972, 1897-09-03
The Old Structure Officially De-
clared to Be Unsafe.
COMPLICATED PROBLEM PRESENTED
Capital Railway Line Authorized by
Congress to Cross It.
THE ENGINEERS REPORT


the Engineer Commissioner this morning
by the computing engineer, and brings the
Commissioners face to face with a compl-
Gated problem.
In the act of Congress chartering the
given the road to cross the bridge: The
Commissioners have decided it must cross
by an underground trolley. After consider
able pickering. the road agrees to put in
the underground system, and makes appli-
cation for a permit.
This necessltates, as the next STEP an
examination of the structure, to determine
Its strength. AS stated above, it has been
found that the bridge is in an unsafe con-
d1tion, and in the opinion of the engineers
not capable of carrying with safety the
loads that would be imposed upon it by
the cars.
No Alternative.
While the Commissioners have no alterna-
tive but to grant the permit, since Con-
gress has authorized the occupancy of the
bridge, they propose to point out strongly
the defects in the structure and throw
the entire responsibility upon Congress.
The report made by the computing engi-
neer. which was addressed to Capt. Lans-
ing II. Beach. assistant engineer commits
sioner of the District. is follows:
About n month ago l was orally in
structed by the Engineer Commissioner tc
examine into the structural sufficiency OF
the Navy Yard bridge over the Eastern
branch, with view to determining its
safety under the loads that would come
upon it. due to the anticipated operations
OF the Capital Railway Company.
have n ade such an examination, using
the best data obtainable, and while the
results are not so conclusive as a fuller
how Hye.f
sufhciently definite to permit me to re-
port certain circumstances and conditions
for your consideration. The structure was
built in 1876 by the Phoenix Bridge Com
pany. and consists of twelve spans of Pratt
trusses, each 100 feet long and a draw
span of plate girders thirty-two feet long.
The lOO-foot spans are through trusses di-
vided into seven panels, and are detailed
in the usual Phoenix style, with closed
section posts and cast-iron boxes at the
panel points. The iron is specifically in
fair condition.
I'm approaching such an investigation as
the present, the natural procedure would
be to examine first the probably deficient
me mberS of the structure, since the failure
to justify these would he a primary result
that would subordinate and make unneces-
Sary continuance of the examination.
Il have accordingly considered the first
truss vertical, the floor beam hangers and
stlrrups. the floor beams themselves both
as to shear and fiange strain, the cross
L
the main girder: OF the draw span, both as
to shear and as to iange strain.
1t Is proper to state here that the per-
missible unit strains for such purposes
as this examination are among the dis
puted data of engineering Should the
same strains used in designing a new
structure be here applied further con-
slderation of this case could be stopped
by the statement that the bridge is not safe
under the loads l have assumed would
come upon it. 1t is good practice, how
ever. in cases like the present 10 use a
hlgher unit strain tnan the one described
not that these are originally too low, but
That experience shows that when necessary
they may be increased to limited extent
without immediate danger What this
limit of strain should be is the cause of
much anxious thought to one charged with
the responsibility of its determinat ton
The literature on the subject is limited;
an epitome of it, and the best discussion i
nave seen, Is in Trans. Am Soc. c. E., VOL
81. NO. T. This paper. by Mr.. Greiner en-
g1neer bridges. b. and o., and discussion
by number of this couRtry's best bridge
engineers, seems to practically reduce to
the proposition that ordinarily the uni;
strains used In designing new struc-
ture may be increased 80 per cent when
reviewing an old one; before the bridge
should be condemned. 1t is of course. un-
derstood that the purpose of this increase
Is to possibly justify structure within
supposedly safe limits, in the interests of
economy
How far this motive should control in
this case has been another consideration
that has caused me some hesitation The
proposed use of heavy motors on this
structure mas never been encouraged by
this office, and was orally discouraged to
those who suggested it, and the greatest
advantage derived from its use accrues to
the Capital Railway Company, while the
risk. If any, is mainly on the District.
without appearing an alarmist, I believe
these last stated considerations justify me
in deciding to use not the full 80 per cent
increase over designing unit strains, but
p per cent
The assumed weight of the loaded motor
car is 10000 pounds on wheel base of
seven feet six inches. The unloaded motor
weighs 10000 pounds, and allowance is
made for packing 100 persons on the car,
each weighing $50 pounds. The statement
of results will be under the condition o.
two such motors passing on the bridge and
emitting all calculations