exerted on the bridge is wv^2/gr tons. (7) In some
cases, especially in arch and suspension bridges, changes of temperature
set up stresses equivalent to those produced by an external load. In Europe
a variation of temperature of 70 deg. C. or 126 deg. F. is commonly assumed. For
this the expansion is about 1 in. in 100 ft. Generally a structure should
be anchored at one point and free to move if possible in other directions.
Roughly, if expansion is prevented, a stress of one ton per sq. in. is set
up in steel structures for each 12 deg. change of temperature.

i. _Live Load on Road Bridges._--A dense crowd of people may be taken as a
uniform load of 80 to 120 lb per sq. ft. But in recent times the weight of
traction engines and wagons which pass over bridges has increased, and this
kind of load generally produces greater straining action than a crowd of
people. In manufacturing districts and near large towns loads of 30 tons
may come on road bridges, and county and borough authorities insist on
provision being made for such loads. In Switzerland roads are divided into
three classes according to their importance, and the following loads are
prescribed, the designer having to provide sufficient strength either for a
uniformly distributed crowd, or for a heavy wagon anywhere on the
roadway:-- [v.04 p.0547]

| Crowd, | Wagon,
| lb per sq. ft. | tons per axle.
| |
Main Roads ....... | 92 | 10 with 13 ft. wheel base
Secondary Roads .. | 72 | 6 " 10 " "
Other Roads ...... | 51 | 3 " 8 " "

In England still larger loads are now provided for. J.C. Inglis (_Proc.
Inst. C.E._ cxli. p. 35) has considered two cases--(a) a traction engine
and boiler trolley, and (b) a traction engine and trucks loaded with
granite. He has calculated the equivalent load per foot of span which would
produce the same maximum bending moments. The following are some of the
results:--

Span Ft. |10. |20. |30. |40. |50. |
| | | | | |
Equivalent load in tons per ft. run, | | | | | |
Case a ............................. |1.75|0.95|0.70|0.73|0.72|
Do. Case b ......................... |3.25|1.7 |1.3 |1.2 |1.15|

Large as these loads are on short spans, they are not more than must often