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ECONOMICS OF BRIDGEWORK |
Chapter XVII |
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interpolation, 8,280 lbs. would give 3,184 lbs., showing that the assumed truss weight of 3,000 lbs. per foot was too light. Let us try 3,260 lbs. making the total load 8,540 lbs. Interpolating as before gives 3,262 lbs. As
this agrees with the assumed 3,260 lbs., it is correct; and, therefore, the
total weight of steel in the two trusses is
2 X 3,262 X 540 = 3,523,000 lbs.
Example No. 6
What is the weight of metal in the four shoes of a 380' span, double-
track, steam-railway, riveted-truss bridge for Class 60 live load?
Solution
From Fig. 6e, on p. 106 of B.E., we find the equivalent uniform live
load to be, |
| 2 X 6,830 lbs | = 13,660 lbs. |
From Fig. 7c, on p. 129 thereof, we find the impact to
be 18% or | 2,460 lbs. |
From Fig. 55y, on p. 1243 of same, we find the weight
of metal per lineal foot of span to be | 8,000 lbs. |
| Flooring for two tracks, say | 1,000 lbs. |
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| Total load per lineal foot of span | = 25,120 lbs. |
| Weight of span = 25,120 X 380 | = 9,546,000 lbs. |
| Load on one shoe= 1/4 X 9,546,000 lbs | = 2,386,000 lbs. |
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From Fig. 55mm, on p. 1257 of B.E., we find the average weight of one
shoe to be 18,400 lbs., hence the weight of the four shoes is,
4 X 18,400 = 73,600 lbs.
Example No. 7
What is the weight of metal per lineal foot of structure in a single-track,
steam-railway, steel trestle, 170' high, to carry Class 55 live load?
Solution
From Fig. 55rr, on p. 1262 of B.E., we find the required weight to be
3,140 lbs.
Example No. 8
What is the average weight of carbon steel per lineal foot of structure in
a double-track, steam-railway, riveted-truss, Type A, cantilever bridge to
carry Class 70 live loading, the length of main span, measured from center
to center of piers, being 1140 feet?
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