We are now aiming to restrict fiber stresses in all structures to the lower limit indicated in the paper, with the understanding that some features of the design or other conditions may make even this lower limit inadmissible and vice-versa.

The more important parts of the paper mentioned are as follows:

General Considerations

In the maintenance of bridges there are two general considerations to be observed:

1. Safety in carrying the necessary traffic.

2. Economy—i.e., obtaining the maximum life from the structure at reasonable maintenance cost.

On all railroads which are twenty-five or more years old, there are usually a number of light-capacity bridges which impose more or less restrictions on the train loadings that can be handled over the lines. This is a very serious problem on railroads which have many bridges that were built during the eighties and early nineties.

New bridges are generally designed for the heaviest engine and car loadings in existence at the time. In proportioning them there is, however, a certain margin between the unit stresses which are used and the maximum unit stresses which the material can safely carry. This margin provides an allowance for some future increased engine and train loadings, in addition to the contingencies which are usually embraced by the term "factor of safety."

Classification of Bridges

The term "Classification of Bridges" is used to describe the systematic investigation of light-capacity bridges, with the view to determining the maximum loads which can safely be carried.

Formerly, the common practice, when a new engine loading was up for consideration, was to investigate all of the light bridges on the lines where the use of the heavy loading was contemplated. Stresses throughout the structures for this loading were figured, and decision then made by the one responsible for the said structures as to whether the load could be safely handled. Each time a new loading came up for consideration the process was repeated; and little or no use was made of the previous computations.

The present practice on the C. M. & St. P. Ry. is to make an investigation or "classification" of each structure. Its carrying capacity is determined in terms of a standard series of train loadings. New engine and car loadings that come up for consideration are classified in the same series of standard loadings, and it is then a matter of direct comparison to tell whether such proposed loadings can be safely handled over the various bridges. Every bridge whose date of construction indicates that it is of light design, or which is known to be, or suspected of being, overloaded, is thus classified. Every part of the structure is figured or taken into consideration.

In making these classifications it is necessary first of all to establish the maximum unit stresses to which the various materials can safely be subjected. For the different materials these maximum safe stresses are taken as near the limit of strength of the material as is considered safe. The maximum safe stresses must be assumed low enough so that there is no danger of the material yielding, altering its character, or reducing its strength for carrying loads after being subjected to this limiting stress for any number of times.

As an illustration of what may be considered as safe limiting unit stresses, the following are given, and may be taken to apply where the design and physical condition of the structure are known to be first-class: