258 APPLIED MECHANICS 
the wind load on the windward girder to the leeward girder. The form: 
shown in Fig. 379, p. 247, would be suitable in this example. ‘ 
The Roller and Fixed Bearin, ys and any other details will cond pial Ap’ 
the design for the superstructure. 
The probable deflection, necessary camber, quantities and weights will 4 
complete the calculations. 7 
Should the finished weight come out much in excess of that estimated, — 
it will be necessary to re-design the structure to allow for this. 4 
Exercises XV. 
1. A road bridge is 80 feet long and 15 feet clear width between the main § 
girders. Each main girder is of the Warren type, and is divided into eight e . 
bays of 10 feet each. The weight per foot-run of each main girder may be n u 
as 4 cwt., and the total weight of cross girders, flooring, etc., per foot-run as — 
i ton. The girder has to be icons ie to support a crowd of people weighing 1 cwt. — 
per square foot of roadwa also to be strong enough to sustain a traction 
engine. The wheel base o the traction engine may be taken as 14 feet, and the ~ 
loads on the axles are 7 and 15 tons respectively. Estimate the greatest force to 
which each member is subjected. Also sketch a section of the booms and, — 
starting from a point of support, proceed to determine the scantling of the | 
members and to design the joints, Choose your own material, working ne 
and scales. Your calculations must be handed in with your drawings. (U.L.] — 
2. Design for a single track railway bridge. Span, 120 feet. Ratio of d 
to span, y'5. The girders to be of uniform depth, divided into ten equal oa 
Web bracing to be of N type. The bridge is to carry a uniform travelling load — 
of 2 tons per foot-run, the maximum axle load being 18 tons. 5 
3. Fig. 387 shows a hinged lifting bridge, The span is 40 feet, and it is a 
divided into five equal 
bays, each of 8 feet length. z Hine of pull of lifting chains. % ; = 
K- - - -7 in q 
The bridge -load is 
equivalent to a uniformly §& 
distributed dead load of § 
of a ton per foot-run, and to s 0 
auniformly distributedlive & 
load of } ton per foot-run. 
Determine, (a) the stresses 
in the various bars of the Fig. 387. 3 
bridge when it is closed and fully loaded ; (b) when it is being lifted and is just — 
clear of the free support, carrying then, of course, only the dead load. 
Choose your own working stresses, and design the top and bottom booms. 
All drawings to be neatly finished in pencil and fully dimensioned. All calcula- — 
tions must be handed in with the drawings. (U.L.] 
4. Fig. 388 shows a bowstring girder for a proposed road bridge, which has 
also to carry a tram line; the span of the bridge is 140 feet, the or at ae 
centre 26 feet 6 ; 
inches; width of _ 
bridge from centre “o, 
to centre of main gir- 
ders, 18 feet. The _ = 
dead load is to be +  - 
1300 Ibs. per lineal |} — — —— — ——- 4+ — +274. See 
foot, the live load Fic. 388. ‘ 
3000 lbs. per lineal 
foot. Determine in any way you please the stresses in each member of the — 
girder due to dead and live loads. Design the top and bottom booms. You — 
are not required to draw the section of the booms, but to determine the necessary 
cross sectional area, and to sketch the sections. (U-Ls 
5. Design for a single track railway bridge of the American type (see p. 245). 
