$2.00 per ton, not including rent for the apparatus. The paint should be applied immediately after cleaning, and before rusting has started.

For additional data on paints see the author's "Structural Engineer's Handbook."

The most common shop coat is red lead paint, although some engineers prefer linseed oil alone, or mixed with sufficient lampblack to give a black coating.

The specifications for paint and painting as required by various commissions follow. The author's specifications for field painting are given in Appendix I.

Examples of Paint Specifications.-The practice in painting highway bridges will be shown by the following abstracts from specifications.

Iowa Highway Commission.-Painting Metal Structures.-One shop coat and one field coat are required. All metal must be cleaned of rust, scale, dirt or grease and must be dry before applying shop coat. Shop coat to be applied after assembling and riveting. Parts not accessible after erection to be painted two coats. Machined surfaces to be coated with lead and tallow. Shop paint shall be one of the following pigments mixed with pure linseed oil or China wood oil, with not more than 10 per cent Japan drier. Red lead paint, not less than 65 per cent pigment; sublimed blue lead paint, not less than 60 per cent pigment; sublimed lead sulfate, not less than 60 per cent pigment; basic lead or zinc chromate paint, not less than 60 per cent pigment; iron oxide paint, not less than 55 per cent pigment; graphite paint, not less than 35 per cent pigment. Iron oxide is to be mixed with not less than 10 per cent basic lead or zinc chromate. Natural graphite is to be mixed with not less than 20 per cent basic lead or zinc chromate. All percentages are in terms of weight of finished paint.

After structure is erected complete it is to be cleaned of dirt, grease or oil and is to be given one coat of paint. Field paint shall be one of the following pigments mixed with pure boiled linseed oil or China wood oil, with not more than 10 per cent Japan drier. Red lead, not less than 55 per cent pigment; sublimed blue lead, not less than 50 per cent pigment; sublimed sulfate of lead, not less than 55 per cent pigment; iron oxide, not less than 50 per cent pigment; pure graphite (natural), or pure carbon, not less than 25 per cent pigment. Red lead is to have some tinting pigment in sufficient quantities to eliminate fading of straight red lead paint.

No painting shall be done in wet weather, or when the temperature is not above 45 degrees F for at least 10 hours per day.

Before repainting old bridges all loose paint, scale, rust and dirt shall be removed by a sand blast, with metal scrapers, wire brushes, or the painters torch. The quality of cleaning shall be equal to that produced by the sand blast. First coat of paint shall be applied as soon as practicable after cleaning. If rusting results before painting, surface must be recleaned. One coat of prime paint and one coat of field paint shall be applied. The first coat shall have time to dry before applying the second coat.

Wood structures shall be given two coats of white paint made by mixing pure white lead 65 per cent, pure zinc white 20 per cent, and not more than 15 per cent inert material. The paint shall contain from 60 to 65 per cent pigment and pure boiled linseed oil, Japan drier shall not exceed 10 per cent.

Illinois Highway Commission.-Three coats of paint shall be used as follows: A shop coat of pure sublimed blue lead and pure boiled linseed oil; a second coat, applied in the field of a mixture of 80 per cent pure sublimed white lead and 20 per cent pure blue lead and pure boiled linseed oil; the third coat to be of pure sublimed lead and pure linseed oil. The mixed paint shall contain not less than 50 nor more than 54 per cent of pigment by weight.

The specifications for details of applying shop and field coats, and repainting old bridges are practically the same as given in the specifications of the Iowa Highway Commission.

Minnesota Highway Commission.-Shop coat of red lead made by mixing 25 lb. red lead, 94 per cent pure, with one gallon pure boiled linseed oil. Field coat to be an approved graphite paint. If specified, trusses and railing and rail posts of beam spans shall be painted with pure sublimed white lead and pure boiled linseed oil so mixed that the paint contains not less than 50 nor more than 54 per cent pigment by weight. The use of tar and asphaltum paints is prohibited. Michigan Highway Commission.-Shop coat to be red lead and linseed oil. Two field coats to be of very distinct colors. The final coat being black, either red lead and a linseed oil; or other paint as specified.

Virginia State Highway Commission.-Shop coat of red lead and oil. Two field coats, the first pure sublimed white lead and oil; the second coat of pure white lead, with four ounces of lamp black in oil and eight ounches of French ochre per 100 lb. of pigment. Mixed paint to have 55 per cent of pigment by weight, approximately 18 lb. of pigment per gallon of linseed oil.

Oregon Highway Commission.-Shop coat of best carbon primer or red lead and linseed oil in proportion of 30 lb. paste to one gallon of oil. Two field coats as specified. All three coats of paint to be of different color.

Pennsylvania State Highway Department-Shop coat to be red lead (85 per cent pure) mixed with pure raw linseed oil. Field coats to be one coat of red lead (85 per cent pure), and two coats of white lead and zinc white, mixed in proportion of 75 lb. white lead to 25 lb. zinc white, in pure linseed oil without turpentine or drier. The final coat of paint may be tinted as desired.

Engineering Institute of Canada.-Shop coat of pure red lead and lampblack mixed in the proportions of 25 lb. of red lead, 4 oz. of lampblack, and one gallon boiled linseed oil. Shall not be thinned with turpen ine, benzine or other liquids, and no drier will be allowed. Parts inaccessible after erection to be given two coats of shop paint. Planed and turned surfaces to be coated with white lead mixed with tallow before leaving the shop. Metal surfaces thoroughly cleaned before applying shop coat, using sand blast if necessary and dusted with stiff bristle brush. Two field coats of approved paint after erection. Field coats to be of different colors.

In "Bridge Engineering" Mr. J. A. L. Waddell after a thorough discussion recommends a shop coat of red lead ground in linseed oil, and two field coats of carbon or graphite paint. Painting Railroad Bridges.-An analysis of the practice of painting railway bridges (Proc. A. R. E. A., 1915) shows:

(1) For a shop coat, 29 out of 50 used red lead; 5 out of 50 used linseed oil alone; 6 out of 50 used linseed oil, painting parts in contact with paint; 8 out of 50 used graphite and carbon pigments, including lampblack; 2 out of 50 used miscellaneous paints.

(2) For field coats on new steel, 12 out of 48 used red lead; 24 out of 48 used carbon or graphite or both; 12 out of 48 used miscellaneous paints.

(3) For maintenance, 13 out of 46 used carbon; 7 out of 46 used graphite; 4 out of 46 used both carbon and graphite; 11 out of 46 used red lead; 11 out of 46 used miscellaneous paints.

Painting Timber Structures.-Ohio State Highway Department specifies three coats of white paint for timber structures. The white paint is made by mixing 75 lb. white lead in oil, 25 lb. zinc oxide in oil, 4 gallons raw linseed oil, and 1 quart drier.

For specifications for painting timber structures, see § 49, General Specifications for Timber Bridges and Trestles in Chap. XVI.

CHAPTER XVI.

DESIGN OF TIMBER BRIDGES AND TRESTLES.

Introduction.-Timber highway bridges were formerly quite generally used, and are still in use for temporary structures and in localities where transportation is difficult and suitable timber is available. Timber truss bridges are usually made with the Howe type of truss with timber top and bottom chords and diagonal braces, and with steel rods for vertical ties. Timber floorbeams and joists, and a timber flooring are used. Timber Howe through truss bridges of more than about 70 ft. span are commonly housed to protect the bridge timbers from decay.

Timber Howe through truss bridges are commonly made with cast iron packing blocks to increase the resistance to crushing, for the reason that the bearing on inclined surfaces of timber is much less than square end bearing. Low truss timber bridges are commonly made without cast iron details.

In the west, combination timber and steel highway bridges have been quite generally used, and when well designed and constructed give excellent service. Combination bridges are usually made with the Pratt type of truss, with timber top chords, posts, struts, floorbeams, joist, and floor,