Evening star. [volume] (Washington, D.C.) 1854-1972, 1886-06-12

board of public works and by the vote of a ma
jority of the stock of the said canal COM
piny, the president and directors of the said
canal company were empowered and directed
To lease the said canal for the period of ninety
nine years; and that on the 18th day of May
1866, the said president and directors did
grant, lease and convey the said canal, its aqile-
duct locks, banks, lands. gates and property of
all description to- Henry III. Wells, Philip
Quigley and VIM. W. DUNCAN. Section 1 de-
ciareS the aqueduct to be a legal structure
Section tour provides THAT as soon as the chief
engineer of the army shall certify to the Secre-
tary of War that the said bridge is so far COM
pieted as to be ready, tit and convenient tor TIE
passage OF persons, animals and vehicles the
SAID lessees and their legal representatives may
demand, have and receive, in advance, the fol-
lowing tolls, to wit: For any foot passenger
crossing on said bridge, two cents; tOrany horse
mule or jack, any OK or other horned cattle,
HVe CENTS for any vehicle drawn by one ami
NAL fifteen cents; drawn by two animals,
twenty-five cents; drawn by four animals,
thirty-five cents, but no extra charge shall be
made for the driver of such vehicle; tor any hog,
sheep or other live creature, one cent; which
certificate shall be published for three weeks in
TWO daily papers IN the city of Washington at
the expense of the company: Provjded, howecer,
Ihaf IL shall be lawful Tor said lessees to conn
mute those rates to persons requiring yearly
passes. Prorided, hOwever. That said
bridge Is open and free for the passage OF troops
and munitions OF war by the United States
without charge or compensation of any kind
DESCRIPTION OF THE BRIDGE.
The piers of the aQueduct, constructed to
carry the Chesapeake and Ohio canal across the
Potomac river, were built under the direction
of Capt William Turnbull, corps OF Topographic
cal engineers, and are composed of heavy blocks
OF gnelss, quarried about one mile below the
Little Falls bridge, on the Potomac; each pier
has an lce-breaker of cut granite. quarried in
Sandy Bay, Massachusetts. Between the abut-
ments are eight piers, about 40 feet between
ALI is Is feet wide at top: the remaining 6 piers
are feet wide. The top of each pier is about
30 feet above ordinary high-water mark. Ac-
cording to the first design the superstructure
was to be OF stone arches. but, tor reasons OF
economy, a trough OF kyanized wood was
substituted of the he pattern designed by
neers. The aqueduct was used by the
government during the war, the water
laid upon the floor. This bridge becoming
much decayed, its place was supplied by the
lessees who built the present structure, which
Is Howe truss, strengthed by arch-ribs. The
trough OF the aqueduct and the path rest
on the lower cord, and highway bridge and
foot-path rest on the upper chord.
WITH RATCHET AND AUGER.
The report then gives the result of an exam
ination concluded March 10th last. In making
the examination a Ratchet was used to sound
the timber, and pick and auger were en-
plOyed to penetrate all such places exhibited
signs of decay. For convenience OF description,
MIR. Albert has divided the bridge into three
general parts, beginning at the north end:
First, the Georgetown approach; second, the
aqueduct proper; and, third, the Virginia APP
proach. OF the first part, the Georgetown
approach, Mr.. Albert says the abutment is in
fair condition; the trusses strong and in
good conditions; but the floor beams, q by IL
inches in cross section, resting on the lower
chords and supporting the carriageways and
footpaths, are In very bad condition.
THE AQUEDUCT PROPER.
Concerning the Aqueduct proper, the report
gives detailed statement Of the decayed tim
Der and other parts said to be defective Not
withstanding the support of the substruts, the
bridge exhibits a tendency to turn about it's
horizontal axis; which tendency, first noted in
W q nj i'm md
March, 1856. This twisting or overhanging
is due to the fact that the center of gravity OF
the bridge, when loaded with water, Is 1.3 feet
TO the west OF the center of gravity OF the
bridge without the Aqueduct and water. This
dangerous defect, noted in 1876, and first meas
ured in 1854, was much reduced in March last.
This Improvement has been made by the SUPER
1ntendent OF repairs.
THE VIRGINIA APPROACH
The Virginia approach the report says, is
composed of bents, or trestles, extending to a
crib abutment; each bent is formed of a ground
sill, three posts, and a cap, supporting a road
way. The distance from the end OF the aoue-
duct to the abutment on the road is AT feet
These trestles have been repaired since the
examination made in 1884 and may be con-
sidered to be in a fair condition. Some few
rotten timbers, posts, and floor beams should
be replaced. The piers. completed about the
year 1889, are in good condition. A certain
amount OF deterioration was observed and
noted but 1t was not measured. 1t consists in
the removal of the cement from between
the joints of the ice-breakers, and also
from the joints on each side of the piers This
defect is progressive, and although it may
apprehended from this cause, the repairs should
be thoroughly made and at an early date
THE STRENGTH OF THE BRIDGE
The report then gives elaborate calculations
of stresses on the truss and arch of the aque-
duct, remarking that it is evident that the re-
pairs which have been made annually tor the
past ten or twelve years have not been cock
tensive with the decay, and that progressive
deterioration is noticeable throug bout the struc-
ture. The weakest part of the bridge appears
to be the top chord. the factor of safety, in hich
is 2.3o.
GENERAL CONCLUSIONS.
The following conclusions are drawn from the
report of inspection and from the calculation:
The two approaches can be put in Il satisfac-
tory condition by replacing with sound timber
the decdyed parts mentioned.
The aqueduct, when tine water is withdrawn
can be made safe for the passage of persons an
finals and vehicles by the completion of the
following repairs: All decayed and defective
timber should be replaced by sound timber of
the proper dimensions. The floor fioor-beams
and much of the old trunk can be removed
The straining beams and sub-struts will not be
needed, and should be replaced by the lateral
braces. The braces now wanting in the central
panels of each span should be supplied. The
OF the a4ueduct are in good condition,
but should be properly fitted into new
floor beams placed beneath the aqueduct.
The old flooring of the roadway should be re-
placed by new flooring. The trusses and
tine arches should be restored to their original
vertical position and should be maintained by
diagonal ties. Braces can be added If neces-
sary. Many OF the carriage blocks in each
span defective useless. The efficiency of
| the arch depends on the condition of these
blocks, and forty-five have been counted which
should be at once renewed. The chords and
parts most liable to decay should be kyaniZed
or covered with zinc. After the completion OF
The repairs the bridge should be property ad-
justed by screwing the nuts of The tie-rods, and
unremitting attention should be given to
| the maintance OF the proper adjustment OF the
load upon the trusses and arches. When the
bridge is repaired further settlement of the
| arch may be prevented by replacing the
Old iimbers with new OF exactly the same di-
| mensions and by employing heart pine for the
skew-back and the part OF the arch adjacent
| thereto. Iron plates should be employed at the
/ abuting ends of the arch-ribs
THE WEAKEST POINT
| In concluding the report Mr.. Albert saws
| 1.-The strength of a bridge is determined by its
weakest part. The factor OF safety of the weak
| est part of the Aqueduct bridge is found in the
/ Top chord, In which the coeftcient is 2 rs
| when the bridge is loaded with water and with
| the moving load. II is therefore evident that
| when thus loaded the bridge cannot be pro
| nounced To be safe for q moving load of 80
pounds per square foot, and that Tor the security
| Of ordinary travel, thorough repairs and in.
| creasing diligence will be necessary to prevent