be provided for.

_Live Load on Railway Bridges._--The live load is the weight of the
heaviest train which can come on the bridge. In the earlier girder bridges
the live load was taken to be equivalent to a uniform load of 1 ton per
foot run for each line of way. At that time locomotives on railways of 4
ft. 81/2 in. gauge weighed at most 35 to 45 tons, and their length between
buffers was such that the average load did not exceed 1 ton per foot run.
Trains of wagons did not weigh more than three-quarters of a ton per foot
run when most heavily loaded. The weights of engines and wagons are now
greater, and in addition it is recognized that the concentration of the
loading at the axles gives rise to greater straining action, especially in
short bridges, than the same load uniformly distributed along the span.
Hence many of the earlier bridges have had to be strengthened to carry
modern traffic. The following examples of some of the heaviest locomotives
on English railways is given by W.B. Farr (_Proc. Inst. C.E._ cxli. p.
12):--

_Passenger Engines._

Total weights, tons ......... 84.35 | 98.90 | 91.90 | 85.48
Tons per ft. over all ....... 1.58 | 1.71 | 1.62 | 1.61
Tons per ft. of wheel base .. 1.92 | 2.04 | 1.97 | 1.95
Maximum axle load, tons ..... 19.00 | 16.00 | 18.70 | 18.50

_Goods Engines._

Total weight, tons .......... 77.90 | 78.80 | 76.46 | 75.65
Tons per ft. over all ....... 1.54 | 1.50 | 1.54 | 1.51
Tons per ft. of wheel base .. 2.02 | 2.02 | 2.03 | 2.00
Maximum axle load, tons ..... 15.90 | 16.00 | 13.65 | 15.50

_Tank Engines._

Total weight, tons .......... 53.80 | 58.61 | 60.80 | 47.00
Tons per ft. over all ....... 1.60 | 1.68 | 1.70 | 1.55
Tons per ft. of wheel base .. 2.45 | 2.52 | 2.23 | 3.03
Maximum axle load, tons ..... 17.54 | 15.29 | 17.10 | 15.77

Farr has drawn diagrams of bending moment for forty different very heavy
locomotives on different spans, and has determined for each case a uniform
load which at every point would produce as great a bending moment as the
actual wheel loads. The following short abstract gives the equivalent
uniform load which produces bending moments as great as those of any of the
engines calculated:--

Span in Ft. | Load per ft. run equivalent
| to actual Wheel Loads in Tons,
| for each Track.
|
5.0 | 7.6
10.0 |