economical to use spraying machines on bridges, if the labor cost saved will more than offset the decreased life of the paint. Painting by spraying machines would be satisfactory on large vertical surfaces, such as the sides of girders and beams and the faces of large posts and columns; but it is not recommended for lattice work and other small parts, on account of the wasting of paint. One advantage derived from the use of these machines is the application of paint in places that are difficult to reach with a brush. On flat surfaces like walls, recent experiments indicate that the air spray will decrease the labor cost by more than one-half, with a slight increase in amount of paint used, about 5%; but similar data for bridgework are not available.

Deterioration of steel structures due to wear will be apparent in the loosening of rivets and in the increased vibration caused by cutting in pins and other parts. When loose rivets appear in the structure, they should be cut out and replaced. They are generally most frequent in lateral bracing of stringers and girders and where single-angle bottom-laterals are attached to the stringers. Also stringer and floor-beam connections are likely to develop loose rivets when the structure is overloaded.

In old and heavy bridges the pins will often show considerable wear. This is an expensive matter to repair, as it involves putting the structure on falsework and the use of considerable machinery to re-drill the holes. If pins are replaced, the new pins should be slightly larger than the original ones. This class of repair work is not often done, because a bridge in this condition is usually very much overloaded and should be replaced with a heavier structure. Another location for wear is at the intersection of diagonal bars that are in contact. In order to prevent these bars from cutting away, a buffer should be placed between them and clamped thereto.

On old bridges it is frequently found that the eye-bars making up one member are not pulling evenly. If the member consists of two bars, a satisfactory repair can be made by cutting out a piece of the loose bar, inserting a turn-buckle, and drawing it up to the same tension as that of the tight bar. This can be done without falsework, but traffic must be kept off the bridge while work is in progress. The same procedure can also be adopted for the outside bar of members composed of more than two bars, but on the inside bars it is impracticable, because rivets cannot be driven.

When truss bridges are more heavily loaded than was originally contemplated, the overstress is likely to be greatest on the counters. Also on a long span the overstress on the floor system is liable to be considerably greater than it is on the main truss members. For this reason it is often possible to make the structure safe for considerably heavier loading by reinforcing these parts and keeping it in service instead of replacing with a heavier bridge. Also in some cases it is possible to improve the structure without strengthening it; that is, on a very light bridge, subject to large  vibrations  under  trainloads,  it  is  practicable  to  improve  the  action  by