In fixing upon limiting unit stresses for loading old bridges, it is necessary to take into account the following:

Character of design; that the details are well proportioned and direct in action, and that there is no ambiguity or uncertainty as to how the members act.

Character of the workmanship entering into the structure as indicated by the reputation of the makers and by any material-test data that may be available.

Deterioration.

Action under load, such as rigidity and freedom from excessive vibration.

Speeds likely to obtain over the structure, and confidence as to the observance of any speed restrictions that may be imposed.

Element of certainty as to the assumed loading being the maximum to which the bridge will be subjected.

Importance of traffic, and the hardship which might result thereto from temporary disablement of the structure.

Probability of early renewal on account of change of line, etc. A higher limit might be allowed for a short time to meet an emergency than would be proper for a structure to be kept in service indefinitely.

General reliability of the data upon which the investigation of the structure is based.

Generally, judgment founded upon all of the factors surrounding the bridge, its location, service, and condition.

It must be recognized that there is danger in setting down a hard-and-fast rule for the limits to which structures might be stressed. In all cases it is necessary to exercise care, knowledge, and good judgment, in order to be always on the safe side and at the same time conserve the maximum life of the structure.

Standard Loadings

In the systematic investigation of a large number of bridges, it is necessary to have a unit loading as a basis of comparison. The familiar Cooper's Series of Standard Train Loadings furnishes a convenient and well-known basis. This series consists of two consolidation-type engines, followed by a train load having a fixed spacing of wheels and a fixed relation between the weights on the various wheels. These weights, however, are directly proportionable to the classes; i.e., the drivers for Class E-40 Loading have 40,000 lbs. on each axle; for Class E-50 Loading, 50,000 lbs. on each axle; etc. The unit loading in this Series is taken as Class E-1 Loading.

On account of the fixed wheel rearrangement for all classes and the proportionality of wheel loads, it follows that the stresses in all parts of bridges due to these loadings are directly proportionable to the classes; that is, the stresses in every part of the structure from Class E-50 Loading will be just fifty times the stresses from Class E-1 Loading.