In through bridges the stringers can be reinforced by additional riveting, by the placing of additional stringers, either timber or steel, and by shifting existing stringers to secure better distribution of the load. Where stringers are spaced so that some of them do not carry their full proportion of load, it is possible to introduce cross bracing so that all the stringers in the panel shall act togethar to carry the total load and thus relieve the excessive burden on certain stringers.

Floor beams can be reinforced by cover plates or angles added to the flanges, by additional riveting, or by shifting the stringers toward the trusses so as to reduce the bending in the beams.

In very old bridges the floor beams are frequently of much lower classification than the remainder of the bridge; and they can sometimes be replaced with entirely new beams at a reasonable expense so as to get additional life out of the rest of the structure.

In trusses, diagonals and counters can usually be reinforced with additional bars or rods having loops over the truss pins and being connected by turn-buckles to provide adjustment. Similarly, bottom chords of eye-bars can be reinforced with additional bars having yokes bearing on the heads of the original eye-bars.

End posts of through bridges, whose low classification is due to eccentricity of members, can be strengthened by placing angles or plates on the sides of the said members, so as to make the cross-sections better balanced, thus reducing eccentricity.

The bottom chords of truss spans may be reinforced by adding an auxiliary bottom chord above the present chord and sloping the end panels to meet as nearly concentric with the end pins as conditions will permit. Auxiliary web members may be connected to the new auxiliary bottom chord and the top chord. This is the method employed on the North Halsted Street Viaduct over the C. M. & St. P. Ry. Co.'s tracks in Chicago. This method has also been employed by the City of Chicago on several bridges in that city.

Where pins have low classification, it is sometimes possible to move the members on the pin and reduce the bending. In some cases, diaphragms placed in built-up members will relieve the bending on the pins. The pins themselves can be strengthened by replacing them with high-carbon or special-alloy steel pins of the same size; or, if still more strength is required, by boring out the pin holes and putting in larger pins. This operation has been done a number of times; but it requires rather elaborate arrangements for holding the members in position while the pins are removed.

Timber truss-bridges can be strengthened by placing floor beams, diagonal braces, or truss rods where needed.

Where timber trusses are old and have commenced to open slightly in the joints or to show other signs of diminished strength, they can be temporarily strengthened and carried for a few more years by placing timber bents under the panels points at two or three panels from the end of the span. This has the effect of reducing the span-length and stiffening the structure.

Timber-trestle bridges can be readily strengthened by additional stringers.

The cost of strengthening bridges varies with the size of the job, the amount of staging required, the amount of moving the same about so as to reach different portions of the work, the size of the crew available, the distance traveled by the crew, the tools at hand, etc. In a general way, it has been found that the cutting out and replacing of rivets on ordinary strengthening jobs costs from 25¢ to 75¢ each. Drilling new holes and driving new rivets therein cost from 50¢ to $1.00 each; i.e., the cost of such work will be given by the total number of rivets driven at these unit prices, plus the cost of the additional material required.

With the maintenance of old and light-capacity bridges, the question continually arises whether it is more economical to strengthen the structure or to renew it. As a general proposition, it would be permissible to spend each year for strengthening an amount equal to the interest on the investment in a new bridge, less the cost of addi-