tional maintenance required by the old bridge on account of the greater attention it receives.

For illustration, let us consider a few lengths of through spans designed for E-55 loading, replacing similar spans designed in the early 90's. New steel work taken at 5¢ per pound erected; falsework at $10.00 per lineal foot; removing old structure at $10.00 per ton; salvage on old spans at 21¢ per pound; additional cost of maintenance of the old span on account of additional inspection, classification, and supervision required, $1.00 per foot of span per year. The last column of the following table shows the amount which we could afford to spend per year in strengthening old spans rather than to renew them. The costs shown in this table are for illustration only. As they fluctuate from time to time, the resulting economics will vary accordingly:

Table 41a

The writer has in mind a bridge having three 400 ft. spans which, if renewed about ten years ago, as some Railroad Managements might have done, would have cost about $370,000 after deducting the salvage value of old spans recovered. The interest on this investment for the ten years would have amounted to about $185,000. Instead, however, of replacing these spans, they have been carefully maintained and inspected and the details strengthened wherever the classification showed that it was necessary to carry the heavier traffic. The actual cost of strengthening, together with the additional maintenance expense, has amounted to not over $20,000 during this period, showing a saving for this bridge of about $165,000; because of the policy of getting the longest practicable life out of structures.

This illustration is intended to show only one way in which the problem may be considered. With old and light bridges a limit is reached beyond which it is not economical to strengthen them; and replacement then becomes necessary. It must be recognized, of course, that a newly designed and heavy structure is preferable to a lighter one. It is possibly true that, in case of a serious derailment on a bridge, a light structure might be destroyed while a heavy new structure might withstand the same treatment without being seriously disabled. Such considerations must be taken into account in shaping the general policy concerning the keeping of light bridges in service.

From the data furnished by Mr. Heritage, the following has been excerpted:

Strictly speaking, the economics of maintenance and repairs starts with the design of the bridge. The most economical structure is that on which the total fixed annual charges are a minimum — these charges to include