rocking of the wall on the footing. Earth pressure against the wall is of little importance in this connection as it but tends to reduce the tension in the girder steel during expansion and to cause the abutment wall to follow the superstructure during contraction. It does not increase the stress in the compression area of the girder as the load is applied at the bottom of the girder tending by this eccentricity of application to reverse the dead and live load stresses in the girder.

"This method has been found to be entirely successful, but is somewhat objectionable as a slight movement of the wings due to earth pressure and unequal settlement sometimes causes the wing walls to move forward slightly at the top, making a somewhat unsightly off-set between the wing and abutment walls. This has never been more than 2 or 3 inches for the highest walls, but as it is not understood by the ordinary observer, an impression of weakness is sometimes caused.

"The present method of providing for expansion is to design the abutments and wings in the ordinary way, separating the superstructure completely from one of the abutments by a thick paper joint and supporting each girder at the free end on a single cast iron rocker of large diameter. The reaction is transmitted to the girder and abutment from the rocker through planed structural steel plates stiffened with I beams when necessary. The rocker surfaces in contact with the bearing plates are turned to insure perfect bearing on the plates. The diameter of the rocker is made proportional to the load imposed per lineal inch, in the same manner as is commonly used in proportioning roller nests for steel bridges. The upper and lower plates are bedded in the concrete of the superstructure and abutment. The rocker is located in a pocket built in the abutment. This pocket is filled with a soft asphalt to prevent the entrance of water or dirt and to protect the metal from corrosion.

"The rocker method of providing for expansion has proved very satisfactory, and is but little more expensive than the other method, especially when it is considered that the wings may be tied to the main wall when rockers are used, and advantage taken of the mutual support thus obtained." Fig. 12 shows how the rockers are arranged at the free end of the girders.

EXPANSION ROCKERS.-The Illinois Highway Commission requires that all reinforced concrete through or deck girder bridges designed as unconstrained structures shall be provided with cast-iron expansion rockers at one end of each span. The rockers to have a thickness not less than 2 in. for spans of 45 ft. and less, and not less than 3 in. for spans over 45 ft., but in no case shall the unit compressive stress exceed 9,000 40l/r lb. per sq. in. Rockers to have bearing surfaces turned to uniform radius and smooth surface, and have 2-in. holes to facilitate handling. Bearing plates to be steel plates not less than I in. thick with planed bearing surfaces. Rocker pockets are made 2 in. longer than rockers. The top of the rocker is placed in. above surface of concrete. The rockers are blocked up by means of sticks with cross-section of one inch, and the pocket is filled with asphalt. The top plate is then blocked up with sticks during the pouring of the girder. The space between girder and abutment is filled with bituminous felt. The Minnesota Highway Commission has the same specifications for expansion rockers.

Segmental Roller Bearings for Concrete Girders.-The segmental roller bearings for concrete girders shown in Fig. 12a were designed by Mr. C. V. Dewart for the Michigan State Highway Department. The bearing consists of a hollow casting containing in a medium of heavy oil the segmental rollers which fit into the girder shoe. The pier is first poured up to the under side of the coping, and in this run it is necessary to place the locating bars. In placing these bars the pedestal plate may be used as a template. The turnbuckles are then placed on top of the locating bars and the pedestal castings on top of the turnbuckles, and leveled up by means of the turnbuckles. The vertical castings are held in place by means of temporary wooden struts. The open joint between the ends of the girders is protected with an expansion plate.

OVERFLOW BRIDGES.-In many localities in the west and southwest, streams which at flood carry a large volume of water are entirely dry or have a very small flow except for the occasional flood flow. In many cases the building of a permanent bridge with a waterway sufficiently large to carry the flood flow is not possible on account of the cost. The overflow bridge must be so designed as to carry the normal flow, and at the same time withstand the action of the heavy current, and not lodge drift; and that when the flood has subsided will leave the roadway free from flood deposit. Two types of overflow bridges are used.

FIG. 13. STANDARD CONCRETE DIP. U. S. BUREAU OF PUBLIC ROADS.

1. Concrete Dip.-Where there is no flow or a small flow, except at flood, the roadway is depressed to make a channel for flood flow, the embankment is paved with concrete on each side to prevent erosion, and a small flow culvert is placed at the lowest point of the channel. The grade of the road descending into the channel and ascending should not exceed 5 per cent. Details of a standard "Concrete Dip" as designed by the U. S. Bureau of Public Roads are given in Fig. 13.

FIG. 14. STANDARD CONCRETE OVERFLOW HIGHWAY BRIDGE. U. S. BUREAU OF PUBLIC ROADS.

2. Overflow Bridge.-Where it is desirable to take care of normal storms the waterway is provided by means of several culverts, and the remainder of the bridge consists of fill between reinforced concrete walls. The railing should be of a type that will not hold the drift and will pass the flood flow freely. Details of a standard reinforced concrete overflow bridge as designed by the U. S. Bureau of Public Roads, are given in Fig. 14. The rail on top of the curb is made so that it will wash away in flood, and will not collect drift. The openings in the curb will produce an increased velocity of the water over the top of the bridge floor that will keep the surface clear of silt.

FIG. 15. CONCRETE OVERFLOW Bridge, Bexar COUNTY, TEXAS.

The low water bridge shown in Fig. 15 was built in Bexar County, Texas. The bridge consists of a reinforced concrete slab bridge resting on concrete piers, and a filled portion between concrete retaining walls. The total length of the bridge is 252 ft. In 1915 the cost was $21.70 per lineal foot of bridge. For rock footings the foundations are carried 12 in., into rock and are anchored with fish bolts. The foundations on clay and gravel are made of a continuous slab footing, and at the upstream and down stream edges the wall extends four feet below the footing slab. Fill is placed between the retaining walls, and the roadway is paved with concrete.

The curbing is 12" wide and 12" high, and has openings at intervals. These openings cause

a current that cleans the roadway of silt. There is no rail.

CONTINUOUS GIRDERS.—A continuous reinforced concrete girder bridge of three spans is shown in Fig. 16. The roadway for a single track electric railway is laid on ballast carried on

There is no

the reinforced concrete floor. The structure is reinforced as shown in the cut. expansion joint in the structure. The left hand abutment and the piers rest on solid rock, the right hand abutment rests on clay.

Continuous reinforced concrete bridges require heavier foundations than for bridges with simple spans, if settlement is to be prevented. The shear near the piers over which the girder is continuous is greater than at the ends of simple girders, and there is no economy in continuous reinforced concrete girders over simple span reinforced concrete girders. Continuous reinforced concrete girders usually require more material, and more careful workmanship, than simple span reinforced concrete girders and are not to be recommended for highway bridges.

CANTILEVER REINFORCED CONCRETE GIRDERS.-The Colfax-Larimer reinforced concrete viaduct shown in Fig. 17, was designed as a cantilever bridge. This design was worked out by the designing engineer, Mr. H. S. Crocker, M. Am. Soc. C. E., to obviate the difficulties incident to the construction of a continuous structure and at the same time obtain a more economical structure than could be obtained with simple spans. The spans vary from 40 ft. to 50 ft. The suspended span has a span length equal to one-half the total span length. All columns are supported on independent footings. The main viaduct carries a roadway and electric tracks, each of which is carried on a viaduct with four columns in each tower, making four columns in line in a transverse direction. The roadway floor slab and electric railway floor slab are connected by a slab free to move at the ends parallel to the roadway. A very unusual rain occurred during construction and several columns near the west end settled several inches, without causing any serious cracks or injuring the structure. These columns were easily jacked back into place. The same settlement with a continuous bridge would have wrecked the structure. Details of the suspension link are shown in Fig. 17. The cantilever reinforced concrete bridge is especially suitable for viaducts.