down with tight joints, and shall be fastened to the outside spiking strips with two 6-in. lag screws at each end of each plank, and to the intermediate stringers with two spikes in each stringer, the length of the spikes to be at least twice the thickness of the floor planks. The felloe guard shall be bolted to the stringers with -in. bolts spaced not more than 5 ft. centers.

WEARING SURFACES FOR HIGHWAY BRIDGE FLOORS.—The wearing surface of a highway bridge floor should satisfy the usual conditions for a pavement and in addition should not have an excessive weight; as an increase in dead load on the bridge increases the necessary amount of steel in the floor supports and the trusses and increases the total cost. The most common wearing surfaces will be briefly described.

Concrete. A concrete wearing surface is laid on top of the concrete slab by the Illinois Highway Commission as follows:-The wearing surface shall have a thickness of not less than 4 inches. The lower 2 in. of the wearing surface shall be made of concrete mixed in the proportions of one part Portland cement, 2 parts clean sand and 4 parts clean gravel or broken stone that will pass a 1-in. ring. The concrete shall be thoroughly mixed in a batch mixer to a jelly-like consistency and shall be placed immediately on the sub-floor slab. Upon this concrete layer shall be immediately laid a 2-in. layer of mortar made by mixing one part Portland cement and 2 parts of clean, coarse sand. The mortar shall be mixed to a jelly-like consistency in a batch mixer and shall be immediately placed upon the freshly laid concrete. Before the mortar has begun to set it shall be finished off with a wood float, and before it has hardened it shall be roughened by brushing with a stiff vegetable brush or broom.

The concrete slab and the concrete wearing surface are commonly laid in one operation, the wearing surface being finished as for a concrete pavement.

Creosoted Timber Blocks.-The blocks shall be made of prime sound long-leaf yellow pine or Oregon fir and shall contain no loose knots, worm holes or other defects, and shall be well manufactured. No wood averaging less than 6 rings to the inch, measured radially from the center of the heart shall be used. The blocks shall have a depth as specified, but the depth shall not be less than 3 in. The blocks shall be from 6 to 10 in. long. The width shall be from 3 to 4 in., but the blocks in any contract shall have the same width. A variation of in. in depth and in. in width will be permitted. The width shall be greater or less then the depth by not less than in. The blocks shall be impregnated with creosote oil by the full cell process. The creosote oil and the method of creosoting timber blocks shall be the same as specified for creosoted timber. All creosoted timber blocks shall contain not less than 16 lb. of creosote oil per cubic foot of timber. Laying Creosoted Timber Blocks.—When the creosoted timber blocks are laid on a creosoted timber base, a layer of tar paper shall be laid on the timber base. When creosoted timber blocks are laid on a concrete floor slab, a layer of dry cement mortar made by mixing dry one part of Portland cement and four parts of clean dry sand shall be spread on the dry floor slab. The cement cushion shall be rolled to a thickness of in. As the blocks are laid on the concrete slab the sand and cement shall be moistened by sprinkling and the blocks shall be laid before the cement has had time to set. The blocks shall be laid at right angles to the length of the bridge in parallel lines, with the grain vertical. The blocks shall break joints at least 3 in. Two lines of blocks shall be laid next to the curb with the long dimension of the block parallel to the bridge, and the remainder of the blocks shall be laid at right angles to those blocks. The blocks shall be laid with open joints, -in. open joints transversely, 4-in. open joints longitudinally. Expansion joints not less than I in. thick the full depth of the block shall be provided along each curb, and transverse joints not less than in. thick shall be provided every 50 ft. in length of the bridge. These joints shall be kept closed until the blocks are all laid, and the space is then to be filled with a bituminous filler. After the blocks have been laid they shall be tamped or rolled to firm bearing. All defective, broken, damaged or displaced blocks shall be removed and replaced with sound blocks. All joints and expansion joints shall then be filled to a depth of two-thirds the depth of the block with a satisfactory bituminous filler. The filler shall not be brittle at o° F. nor flow at 120° F. The filler shall be applied at a temperature of not less than 300° F. After the first application has

set the joints shall be filled to the proper height with a second coat. Joints shall be filled only in dry weather, when the temperature is not less than 50° F. Before the second coat has hardened a layer of sand in. thick shall be spread on the surface and shall be swept into the joints.

Bituminous Wearing Surface Floors.-Bituminous wearing surface floors may be laid on a creosoted timber sub-floor or on a concrete sub-floor.

Bituminous Wearing Surface on Timber Sub-Floor. The bituminous wearing surface may be put on hot by the standard method, or by a cold process. The specifications adopted in 1917 by the Illinois Highway Commission are as follows:

Bituminous Wearing Surface-Hot Penetration Method. Illinois Highway Commission. Asphalt. The asphalt used for bituminous wearing surface shall conform to the following requirements: Asphalt shall have a specific gravity at 25° C. of not less than 0.97 nor more than unity. It shall be soluble in cold carbon disulphide to the extent of at least 98 per cent. Of the total bitumen, not less than 22 per cent nor more than 30 per cent shall be insoluble in 86° B. naphtha. When 20 grams (in a tin dish 2 in. in diameter and in. deep with vertical sides) are maintained at a temperature of 163° C. for 5 hours in a N. Y. testing laboratory oven, the evaporation loss shall not exceed 2 per cent and the penetration shall not have been decreased more than 25 per cent. The fixed carbon shall not exceed 16 per cent by weight. The penetration as determined with the Dow machine using a No. 2 needle, 100 gm. weight, 5 seconds time, and a temperature of 25° C. shall be not less than 30 nor more than 50. The asphalt shall contain not to exceed 6 per cent by weight of paraffine scale.

Aggregate. The aggregate shall consist of screened gravel, which shall have been approved by the engineer, dry, free from dust, dirt and clay, and graded in size from in. to in.

Cleaning Sub-Planking. Before placing the wearing surface, the sub-planking shall be thoroughly cleaned from all foreign material and the cracks shall be filled and the plank covered to a depth of approximately in. with asphalt of the character herein specified, which shall be applied at a temperature of not less than 400° F. The sub-planking shall be dry when the asphalt is applied. Placing Wearing Surface. The gravel shall be spread on the asphalt covering while the same is hot and in a quantity which will just cover the asphalt. The thickness must not exceed that which will be formed by a single layer of the gravel pebbles.

Upon the material thus spread, there shall be poured hot asphalt until the interstices are all filled, the asphalt being at a temperature of not less than 400° F.

Upon the layer of asphalt thus poured there shall be spread a second layer of gravel which shall not exceed the thickness of a single layer of pebbles, but which must be spread in sufficient quantity to cover completely the layer of asphalt.

Upon the layer of gravel thus spread there shall be poured hot asphalt until all the interstices are filled, the asphalt having a temperature of not less than 400° F.

Finish. The surface shall then be covered with a layer of pebbles just sufficient to cover the asphalt, the pebbles to be well rolled or tamped into the asphalt and the surface finally covered with coarse sand sufficient to take up any free asphalt. After the surface has stood for one day, it may be opened to traffic.

Bituminous Wearing Surface-Cold Mixing Method, using an Asphalt Emulsion. Illinois Highway Commission.

Asphalt Emulsion.—The emulsion shall consist of asphalt, water and fatty or resin soap thoroughly emulsified. It shall conform to the following requirements:

Total bitumen..

Specific gravity of dehydrated material.

Penetration of dehydrated material, 25° C., 100 gm., 5 sec..

Not less than 60.0 per cent . Not less than 1.000 .150 to 200

Total Bitumen.-The total bitumen shall be considered as being 100 minus the sum of the percentages of water, of fatty or resin acids, of organic matter insoluble in carbon disulphide other than fatty or resin acids from the soap, or mineral matter (ash), and of ammonia.

For percentages of water, fatty or resin acids, organic matter insoluble in carbon disulphide, mineral matter (ash), and ammonia, see United States Department of Agriculture Bulletin 314, P. 41.

Specific Gravity.-Standardized pycnometers, United States Department of Agriculture Bulletin 314, p. 4. Penetration.-A. S. T. M. Stand. Test D 5-16.

Aggregate. The aggregate shall consist of crushed stone chips uniformly graded from in. down to dust with all dust removed, to which shall be added sufficient sand to fill all remaining voids, but not to exceed 20 per cent of the volume of the aggregate.

Cleaning Sub-Planking.-Before placing the wearing surface, the sub-planking shall be thoroughly cleaned from all foreign material and all cracks shall be filled with wood strips or oakum. Mixing Materials.-The aggregate and the asphalt emulsion shall be mixed cold in the proportions of 1 gal. of emulsion to I cu. ft. of aggregate. To facilitate mixing, water to the extent of 20 per cent may be added to the emulsion. The proportions given above for mixing the aggregate and the emulsion are based on the undiluted emulsion. The mixing shall be done on a tight mixing board or in a batch concrete mixer, and shall continue until all particles of the aggregate are thoroughly coated.

Placing Wearing Surface. After mixing, the material shall be spread upon the roadway in sufficient quantity to provide a thickness of in., after rolling or tamping.

Finish. After the material has been rolled or tamped smooth and to a uniform thickness of in., the surface shall be given a paint coat of the emulsion applied at the rate of gal. per sq. yd., and then shall be covered with coarse sand sufficient to take up any free asphalt and to fill all voids in the surface. After the surface has stood for one day, it may be opened to traffic.

Bituminous Pavement on Concrete.-A bituminous wearing surface may be laid as on the creosoted plank sub-floor, or the wearing surface may be laid according to the following standard method. The concrete shall be dry and thoroughly clean. A bituminous wearing surface two inches thick is applied as follows: The aggregate consists of broken stone or gravel passing a one-inch screen with the dust screened out to which is added sand equal to about one-quarter to one-half the volume of the stone. The aggregates shall be heated and mixed with the bituminous material in a mechanical mixer or by hand with hot shovels. The asphalt shall be mixed not less than 20 gallons to the cubic yard of aggregate at a temperature of 350° to 400° F. The mixture shall be applied hot to the concrete surface and shall be raked with hot hoes or rakes and rolled with a roller weighing not less than 5 tons. After the surface has been rolled a layer of hot asphalt shall be applied and a layer of coarse sand rolled into the hot asphalt.

Examples of Highway Bridge Floors.-The following examples of highway bridge floors specified by different highway commissions are of interest.

The Illinois Highway Commission uses the following standard floors: (1) A reinforced concrete sub-floor 4 in. thick, and a concrete wearing surface 4 in. thick, weight 100 lb. per sq. ft.; (2) a reinforced concrete sub-floor 4 in. thick and a creosoted timber block wearing surface 3 in. thick, weight 65 lb. per sq. ft.; (3) a creosoted plank sub-floor 3 in. thick and a wearing surface of creosoted timber blocks 3 in. thick, weight 32 lb. per sq. ft.; and (4) a creosoted timber ship lap floor 3 in. thick and a wearing surface of creosoted timber blocks 3 in. thick, weight 26 lb. per sq. ft. For details of Illinois Highway Commission's standard floors see Fig. 2.

The Michigan Highway Commission uses the following surface treatment on concrete floor slabs. The surface of the concrete is thoroughly cleaned and } of a gallon of coal tar per sq. yd., heated to a temperature of 250° to 350° F. is spread over the slab. While the tar is hot the surface is evenly covered with a layer in. thick of clean, sharp, coarse sand.

The Wisconsin Highway Commission does not specify a wearing coat on top of concrete floor slabs.

The Iowa Highway Commission uses either a 3 in. fill of gravel or a creosoted block floor 3 in. thick. Concrete slabs are covered with a bituminous coating made by applying to the clean dry slab of a gallon of hot tar per sq. yd. A layer of coarse dry sand is heated and sifted on top of the tar.

For additional examples of highway bridge floors and data on concentrated loads on bridges and details of the design of highway bridge floors, see paper entitled "Highway Bridge Floors," by Professor Charles M. Spofford, Proceedings Society Western Pennsylvania, Vol. 31, pp. 727–826.

Cost of Floors.-The costs of highway bridge floors were estimated by Mr. Clifford Older, bridge engineer, Illinois Highway Commission in 1915 as follows: Concrete in sub-floors including. reinforcing steel, $12.00 per cu. yd.; concrete wearing surface, 4 in. thick, $0.90 per sq. yd.; creosoted sub-plank (12-lb. treatment) in place, $70 per thousand feet B. M.; creosoted blocks 3 in. thick, in place, $1.80 per sq. yd.; bituminous gravel wearing surface, in. thick, $0.60 per sq. yd. The weights and costs of the Illinois Highway Commission standard floors were as follows: concrete sub-floor 4 in. thick and concrete wearing surface 4 in. thick, weighs 100 lb. per sq. ft.,

and cost $2.95 per sq. yd.; concrete sub-floor 4 in. thick, and creosoted blocks 3 in. thick, weighs 65 lb. per sq. ft., and cost $3.25 per sq. yd.; creosoted plank sub-floor 3 in. thick, and creosoted blocks 3 in. thick, weighs 32 lb. per sq. ft., and cost $4.10 per sq. yd.; creosoted plank sub-floor 3 in. thick, and bituminous wearing surface in. thick, weighs 26 lb. per sq. ft., and cost $3.00 per sq. yd.

DESIGN OF STRINGERS.-Stringers or joists support the floor and in turn are supported by the floorbeams. The joists may be supported on the tops of the floorbeams or may be framed into the floorbeam by the use of connection angles. Where concrete floors are used the steel joists should either be supported on the tops of the floorbeams or if framed into the floorbeams should have the upper flanges of the beams coped so that the tops of the joists will be on the same level as the floorbeams. The loads carried by the joists are (1) the dead load which is made up of the weight of the joists, the floor slab and the wearing surface; (2) a uniform live load, or a concentrated moving load. The uniform live load and the concentrated moving loads are the same as the loads used in designing the floor slabs, but the distribution of the concentrated load is not the same. The distribution of the moving concentrated load to the joists as specified by different highway commissions and others, and by the author have already been given.

Steel Stringers.-The sizes of steel I-beams of minimum weights required for stringers with different spacings to carry a dead load of 100 lb. per sq. ft. and a 20-ton auto truck with 30 per cent impact or a live load of 125 lb. per sq. ft. with 30 per cent impact are given in Fig. 8; and to carry a dead load of 100 lb. per sq. ft. and a 15-ton auto truck with 30 per cent impact or a live load of 100 lb. per sq. ft. with 30 per cent impact are given in Fig. 9. The sizes of steel I-beams of minimum weights required to carry a dead load of 100 lb. per sq. ft. and a 15-ton auto truck without impact or a live load of 100 lb. per sq. ft. without impact are given in Fig. 10. The steel stringers used by the Wisconsin Highway Commission to carry a 15-ton road roller without impact, and the steel stringers used by the Iowa Highway Commission to carry a 15-ton traction engine without impact are practically the same as those given in Fig. 10.

The proportion of the concentrated live load carried by one joist shall be taken equal to the spacing of the joists in feet divided by four feet.

Joists were designed for allowable stresses as follows: Cross-bending, 1,500 lb. per sq. in.; bearing across the grain 400 lb. per sq. in.; longitudinal shear 140 lb. per sq. in.

Spacing of joists for spans to left of heavy line are determined by longitudinal shear.

Timber Joists.-The sizes of timber stringers or joists for different spacings and spans to carry a 20-ton auto truck are given in Table V; to carry a 15-ton auto truck in Table VI, and to carry a 10-ton auto truck in Table VII. The timber joists were designed for the following unit stresses, to be used without impact: Allowable bending stress, 1,500 lb. per sq. in.; allowable bearing across the grain, 400 lb. per sq. in.; allowable longitudinal shear in beams, 140 lb. per sq. in. The maximum spacings of timber joists for short spans are determined by the longitudinal shear.

The proportion of the concentrated live load carried by one joist shall be taken equal to the spacing of the joists in feet divided by four feet.

Joists were designed for allowable stresses as follows: Cross-bending, 1,500 lb. per sq. in.; bearing across the grain, 400 lb. per sq. in.; longitudinal shear, 140 lb. per sq. in. Spacing of joists for spans to left of heavy line are determined by longitudinal shear.

The proportion of the concentrated live load carried by one joist shall be taken equal to the spacing of the joists in feet divided by four feet.

Joists were designed for allowable stresses as follows: Cross-bending, 1,500 lb. per sq. in.; bearing across the grain, 400 lb. per sq. in.; longitudinal shear, 140 lb. per sq. in.

Spacing of joists for spans to left of heavy line are determined by longitudinal shear.