GIRDERS AND SUSPENSION CHAINS COMBINED. 243 
___ To compare this weight with that of a girder alone of the same length and 
depth as that used, which would be equally rigid with the suspension girder, 
_ we have to multiply 150 x 25=3750 tons, or more than eight times the 
amount of metal; but it may be correctly argued that a simple girder would 
be made deeper, and it is therefore fairer to make the comparison with an 
» actual girder, of which we have an example nearly the same span in the 
Britannia Tube. : 
The weight of the pair of the Britannia Tubes is 3100 tons, or more than 
seven times the amount, a difference which will be received with surprise; 
but it is perfectly consistent with the fact that the Derry Bridge has nearly 
three times the depth, and has 2660 tons less of its own weight to support. 
The weight of metal in the Londonderry Bridge does not in fact exceed 
that of the sides of one of the Britannia Tubes without the top and bottom 
webs. 
ti It should be observed that the proportion of the cost of anchorage will 
vary under different circumstances, but in the case of the Londonderry 
Bridge it will be under 15 percent. 
_ It should also be noticed, on the other hand, as a set-off to the cost of 
anchorage, that the foundations will be increased in a girder bridge, from 
_ their having to support 3110 tons as compared with 432 tons in the suspension 
bridge, which will produce an amount in saving in average cases equal to 
the anchorage. 
We will now compare the rigidity of the suspension bridge with that of 
the tube. 
____ The deflection from 1 ton per foot all over the suspension bridge (see 
_ Appendix A) will be 14 inch. 
____ The deflection of one of the Britannia Tubes from 1 ton per foot all over 
is 34 inches. 
_ The greatest wave that will be produced by a train of 200 tons covering 
one-half of the Londonderry Bridge, the other portion being unloaded, will 
be readily found from the experiments. 
_ The calculated extreme deflection of the girder with 200 tons all over, 
separate from the chain, is 41°25 inches (see Appendix A): — =1°65, the 
greatest deflection of the wave if simply suspended from the chain; but, as 
the chain in the actual bridge is attached to the girder for nearly one-half 
the length, the rigidity will be much greater than here indicated. 
It thus appears that the deflection of the Londonderry Bridge, with a sus- 
pended girder and loaded all over, equals the wave when the bridge is half 
ded, and they are each about half the deflection of one of the Britannia 
bes when loaded all over with the same weight per foot. 
[t is necessary to explain, that the estimate given of the deflection of the 
nia Tubes assumes that they act separately; when united at the top 
become suspension girders, and the deflection is reduced; on the other 
nd, it has to be noticed that I have not taken into account the increased 
idity from uniting the girder to the chain, instead of simply suspending it, 
h will have a most material influence. 
I will also call attention to the fact, that in estimating the deflection of the 
Londonderry Bridge, I have treated the point of support as a fixed point, 
which is the case if all the spans are equally loaded ; but in the event of one 
Span being loaded and the adjoining span unloaded, the point of suspension 
will not be a fixed point, and the deflection will be greater than I have 
mated. : 
Thus with one span loaded and the second span unloaded, the girder bridge 
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