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Full text of "The bascule drawspan of the Arlington Memorial Bridge / by R.W. Carroll"

'THE BASCULE DRAW SPAN 

OF THE 

ARLINGTON M E M R I A L BRIDGE 



BY 



R, W. CARROLL. 
March 24, 1939. 



m 



SUUMAUY 



The object of this thesis is not to discuss the whole of the 
-Arlington Memorial Bridge, but rather to give some facts concerning 
the bridge in general ruid discuss in more detail the construction, 
erection, and o bion of the barscule draw span. 

The bridge consists of -aeae main spans, the middle one of 
which is a steel draw span, and the four flanking on each side are 
masonary. The Bmson4ry arches pre of reinforced concrete construction, 
i.lth all the visible surfaces faced with granite. In order to keep the 
symmetry of design, as the bridge vras designed with the specific object- 
ive to make it a irork of art, the bascule lift span was used. , i 1 he 
design was carried out in the lift span by making its sides of fascia 
of pressed ornamental molybdenum steel to simulate the construction of 
the mason/ry spans. In the actual construction of the bridge, several 
problems were met. It was necessary to establish a contractors' yard 
on the Arlington side of the river where materials were received and 
subsequently transported to the site of the bascule by means of barges. 
The various pieces i r ere lifted into position by means of a derrick on the 
bascule abutments and on the boats. 

In the construction of the counterweights it was necessary to 
use a large quantity of punchings to give the necessary weight. As the 
necessary amount of scrap steel which is commonly used could not be ob- 
tained a cargo of Swedish ore v/as imported and used. One leaf of the 
bascule was constructed at a time so that navigation could proceed. 
After the copper-molybdenum had been placed on the masking trusses of the 
Washington leaf which was completed first, the leaf was raised and found 
to balance excellently. 



The entire draw span was built at a cost of a little over 
)Q,000. It can easily be seen from the fact that the ornamentation 
for the draw cost -'.400,000, almost one-half of the total, that great 
pains were taken by the Special Coimnission in charge to improve the 
appearance of the bridge. 



-bibliography 

Architecture and Tfeildi ng Vol. 61, ,ov. IS 

"Rlectrical Equipment for Arlington ; emorial Bridge" 

Engin eer -I fev/s -R ecord Vols, for Jan. to June of 19f,7, 1938, 1929, 1932 

Oood Roads Vol. 71, July, 1928 

"Construction Features of Arlington i;i emorial Bridge" 



Interviews 



Mr. Beech, U. S, Bureau of Public Roads 

Mr. Gartside, If. S. Dept. of Interior (Ilember of Special Com- 
mission for Arlington Llemorial Bridge) 



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THE BA 'CULE DRAW SPAN OF SEE AHLniGTOU MEMORIAL BlilDGE 

The Arlington Memorial Bridge spans the Potomac River from a 
point on the north bank about 100 yards from the south west corner of the 
Lincoln Memorial to a point on the south bank approximately one fourth 
of a mile from the Arlington Cemetery. The bridge, v:ith the exception of 
the bascule draw span, is of reenforced concrete construction, with all 
the visible surfaces, except the soffits of the arches, faced with brush- 
hammered granite. The granite on the lower portions of the arches is 
from Stone Mountain Quarry in Georgia, The remainder of the granite vjas 
taken from the Mount Airy Quarry in I-forth Carolina, 

The bridge has an overall -legntfe- of 2,16^ feet and a clear 
width betv/een the balustrades of 90 foet. The width of the roadway 
between the curbs is 60 feet, and the width of each of the two sidewalks 
is 15 feet. 

The design v;as governed largely by architects 1 considerations, 
In appearance it is a nine-span arch bridge, but it renlly consists of 
tho eight main, concrete barrelk arches of 166 to 180 foot 3pans and the 
216 foot span bascule in the center which is faced with ornamental metal- 
vork to harmonize with the brush hammered granite surfaces of the concrete 
arches. The substructure of the bridge consists of four abutments, one 
at each shore line and one at each end of the bascule span, and six piers. 
The piers are founded on solid rock which was found at a depth of from 
35 to 45 feet below the surface of the river. 

The total cost of the bridge was .; 7, 250, 000. 00. An unusual 
feature of this cost is the fact the granite, including the granite 



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earving arid tiie ornamentation of the draw 3pan made up almost one-half 
of the total cost. The structure was built within the cost as originally 
estimated and approved. 

The bridge was built by the Arlington Memorial Bridge Con- 
mission which had charge of the entire project. This commission i 
headed by Colonel U. S. Grant 3rd who acted as the executive officer, 
and John L, Nagle, the designing engineer. The Commission had for its 
specific objectivo the designing and construction of a bridge that 
would be a work of art. Since the bridge crosses the Potomac between 
the Lincoln Memorial and the Arlington National Cemetary it was para- 
mount that it should be a structure of great beauty. Therefore, to 
carry out the symmetry of the chosen design the bascuJe lift span was 
used. This type was the most logical choice because the lift span could 
be covered with an ornamental iron frescia to present an appearance in 
heeping with that of the other eight granite faced concrete arches. 

DESIGN OF THE BASCULE 

The movable bascule draw span is one of the largest and heaviest 
bascules in the world. Each of the two 108 foot leaves and its concrete 
counterweight weigh 3,800 tons. Design, fabrication, and erection con- 
sequently involved difficult problems. 

Each leaf of the bascule Includes two bascule trusses mounted 
on a main trunnion resting in bearings on either side of the truss. The 
bearings are carried on vertical trunnion posts that transfer the load 
directly to the abutments. The counterweight under the floor hangs as a 
pendant from counterweight trunnions at the rear end of each bascule 



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truss, the arrangements being made in such a manner that the counter- 
weight moves p-- rallel to itself as the leaf rotates upon the main trunnions. 
At the center of the span the two leaves are locked together by a detail 
design so as to transmit she?r and to maintain equal deflection for both 
leaves under all conditions of loading. 

THE CO!I,::TRUGTION AND EEEGTION OF THE BASCUIi; 

The approaches to the bascule were not complete when its 
erection was started, so that it was necessary that all materials be 
delivered by water. A contractor's yard was established and materials 
were received in the government reservation on the Arlington side of 
the river. A derrick car and crone in the yard tsansfered the materials 
to the barges, on one of which an A-frame derrick was installed, From 
the yard to the site was one-third of a mile, and a tugboat was used to 
move the floating equipment between the two places. At the bascule 
site a thirty ton steel stiff- legged derrick was installed on each abut- 
ment, and materials were placed by these and by the derricks on the 
boats. 

As the first step in the construction, the grillages under 
the trunnion posts and the trunnion bearings resting on the posts were 
set with extreme care. Before grouting, the grillages were leveled by 
set screws. Dowels of one and one-quarter inches were used for locating 
and adjusting, and dummy trunnions with a taut wire through their axes 
were utilised in aligning the trunnion bearings before planing and in- 
serting the necessary shims. The distance between trunnion axes on 



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ppposite abutments was carefully measured to insure proper matching of 
the leaves at the center of the channel, Machinery supports were also 
placed with great care. The trunnion posts were among the heaviest 
gle pieces that had to be lifted, each section weighing 32 tons. 
After these and the bearings nere pi- ced, the rear portions of the main 
trusses and the heavy counterweight truss suspended between them were 
built. The heaviest single lift in the bridge weighed 51?? tons and 
was composed of an assembley of the two main truss members and the gusset 
plates connecting them at the trunnion, the trunnion pin and the collars. 

The first step in the construction of the count en ■/eight was 
the placing of steel falsework, resting on the pit fl~>or, to carry the 
mass of the counterweight, dimensions of which were 16 x 25 x 62 feet. 
Three inch planking in the falsework beams served as the bottom forms. 

The timber side forms were built in sections at the Arlington yard ready 

( 

for assembleyfet the site. The -re±«ma- of each counterweight is .,89 cubic 

yards. The weight per cubic foot required for the Washington leaf was 
SG5 pounds and for the Virginia leaf was 255 pounds. The extra './eight, 
of the ... Bhlngton loaf counterweight was caused by the center locki: 
device. To secure the required unit weights punching were specified to 
be used in the concrete mixture for the counterweight. The most satis- 
factory forms of stee 7 scrap to be used for punchings as concrete in- 
gredients are structural shop punchings. However, the counterweights on 
e would have required nbout 3,000 gross tons of punchings. 

a 

This .Amount was much to large to be accumulated within the specified 
time from the available soureoo. To solve this problem an effor 



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raade to find a material of a high specific gravity as a substitute for 
the structural steel scraps. The first material considered t.tis "heating 
furnace cinder" (lumps of 'SeJ}* obtained from heating furnaces). Then 
copper slag and various ores were tried. Borne of these while having the 
high specific gravity proved to be to easily crumbled and thus unfit. 
Chilean ore was found to be suitable but unfortunately it we s not avail- 
able in sufficient quantities. The solution was to us a Swedish ore 
which ha^ the required physical characteristics an a specific gravity of 
4.94. The entire ccrgo of a ship from Swedes which carried 2,400 gross 
tons xias secured and brought by rail, making up a train of 59 carloads, 
to the Arlington yard. There it was run through a crusher to break up 
the large lumps, and then transported to the concreting plant on the 
bascule abutments. 

Compression tests of concrete specimens r:hich contained the 
Swedish ore showed that tho strength was entirely satisfactory and so 
the materials were proportioned so as to get the desired unit weights 
and the concrete for the counterweights was mii:ed. The amounts of tho 
irediente in the concrete as mixed were :200 barrels of cement, 125 
tons of sand, 91 gross tons of fine Swedish ore, 1490 tons of coarse 
Swedish ore, and 1136 gross tons of punch ings. About 146 tons of w 
were retained in the mixture. Exclusive of the concrete reinforcement 
rods, the total mixture in the counter eights weighs 4787 tons. All 
materials were necessarily proportioned by weight. When the concrete 
wag placed it was attempted at first to use a compacting machine, but 
this was found to be impractical because of the large amount of rein- 
forcing steel. 



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The construction of the bascule was carried out by erecting 
on leaf at a time, in order that the concrete floor slab could be placed 
with the leaf closed. To protect navigation a fem'er 8 feet wide and 
143 feet long was built around the abutments, when these fenders and the 
Washington counterwei ;ht were finished, navigation ras restricted to the 
south half of the channel. Then the main trusses, masking trusses and 
all the bracing for the Washington leaf xi&ve -rooted by the derrick- 
boats. In this part of the work the heaviest sin ;le load lifted was 
the 25 ton end floor beam over the center of the channel. When all 
but the bottom chord splices of the trusses h d been riveted, forms 
for the ro;^dway and sideiralks floor slabs wore put in place and the 
concrete was poured. 

Before opening the leaf it was necessary to put on the cop- 
per-r!olybdenum on the masking trusses and to place and fid just the cast 
aluminum balustrades on the deck. The installation of the machinery 
and power line went along with the other work. When the finished leaf 
was raised it was found to h:l; nee perfectly. 

The construction of the Tir^inia leaf was carried on in the 
same manner as the '.'.ashington one. Navigation now was confined to tho 
north half of the channel. 

TION OF THE BASO 

Each leaf of the bascule is operated by two motors directly 
connected to a herringbone- gear speed reducer. This gear has an etual^ 



_7- 



^zing device which enables it to deliver one-half of the total driving 
torque to each of the two main operating pinions, meshing with r ck seg- 
ments which are attached to the bascule trusses, jiiach oper; ting motor 
is provided with a motor-mounted solenoid service brake. Then on each 
side of the shaft of the speed-reducer thare is another brake, making 
a total of four in all. These -a uxilar y brakes can be used one at a time 
and they have sufficient braking power to hold the leaf fully open vtainst 
a vind pressure of 15 pounds per square foot. 

The power service at the bridge is in the form of alternating 
current. This is 3 ph^oe, 60 cycle, 4-000 volts. A 4000 volt, 300 H. P. 
notor is connected to two 100 Kw», 600 volt direct current generators, 
which supply current to the operating motors. They are set up so op- 
erating motors of either leaf can be supplied v:ith power from either 
generator* The speed of the leaf varies directly with the voltage, so 
it is possible to get an extremely slow speed for seating the bascule 
leaves. In case that the regular power line supply should fail, a -gas- 
oline engine-driven generator has been provided. 

CONTRACTS 

The general contract for all the work on the bascule span was 
taken by the Phoenix Bridge Company of Phoenixville , Fa. They were able 
to erect the bascule at a total cost of ^900,000 exclusive of the piers. 
.1400,000 of this represents the cost of ornamentation necessary to keep the 
same appearance throughout the entire length of the bridge. 



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The Hunkin-Gonkey Construction Company had the subcontract 
for the stone nasonary work and concrete backing for abutments as well 
as for the concrete for houses and floor Slabs. 

The machinery for the job wos contracted for by the Francis 
Liachine Company of Cleveland, Ohio, and the W # V. Panghorae Company of 
Philadelphia, Pa. had the subcontract for the electrical equipment. 

The design of the bascule span was handled by the Strauss 
Engineering Corporation which is noted for many large bridge designs, 
among them being the Golden Gate Bridge in San Francisco. J. F. Winter 
in general charge of the construction of the bascule and the field 
operations were handled by H, IT, Archinal and Ifrank Pierce. 

The bridge was opened for traffic, which is limited to non- 
commercial vehicles, on May 6, 1932. A traffic count was taken on ilay 
30 and it showed that in a period of 24 hours 26,460 cars crossed over the 
bridge.