must be accompanied by stress diagrams and preliminary design of the whole structure in duplicate.

The successful bidder, when notified, must submit a complete set of shop detail drawings, which must be approved by the Commissioner of Indian Affairs before commencing work.

The work included in this project is the building complete and ready for use of one 72-foot steel span with 16-foot roadway and two concrete abutments, as shown on the accompanying plan.

The filling and grading of the approaches are not to be included in this project, and will be done by the United States Government after completion of the work.

LOADING AND UNIT STRAINS.

The loading and unit strains assumed shall be in strict accordance with class D bridges, as per Theodore Cooper's Specification for Steel Highway and Electric Steel Railway Bridges and Viaducts, 1909 edition, except that the concentrated load assumed shall be 10 tons instead of 6 tons.

SUBSTRUCTURE.

The substructure will consist of two concrete abutments as heretofore stated.

The abutment at the north end will be excavated into the ground to the depth shown unless the bedrock shall be previously encountered. The depth of the bedrock at this end of the bridge is not accurately known, and the excavation must be continued until the bedrock is reached.

Bidders in making their proposal will submit a lump bid including the abutment of dimensions shown and will also state the price per yard to be added for extra concrete if more is required to reach the rock, and shall state a price per yard to be deducted if bedrock is encountered at a lesser depth than that shown and the amount of concrete used is diminished.

At the south end of the bridge the bedrock comes to the surface of the ground and will be excavated to a depth of about 2 feet 9 inches for the reception of the abutment.

All concrete used in these abutments will be in accordance with the following specifications:

The concrete will consist of cement, sand, and broken stone mixed in the proportion of 1 part cement to 3 parts of sand and 5 parts of broken stone.

Sand may be obtained in the immediate vicinity of the bridge; it is finer than desirable but is otherwise of good quality.

The stone to be used consists of broken fragments of lava stone scattered about upon the surface of the ground and which will have to be gathered up and hauled to the site. Probably enough of this stone can be obtained within one-half mile of the bridge.

The approximate cost of gathering up and hauling this stone to the site may be about $3 per cubic yard; this is not guaranteed, however, and the contractor will verify this estimate before using it.

All concrete will be thoroughly mixed by hand or in a batch mixer and shall be what is known as a wet mixture. It is to be deposited

in a strong wooden form of exact shape of structure required. The exposed face of masonry will consist of clear mortar without stone. This facing will be placed at same time as main body of work. During the placing of concrete, one man will be kept inside of form working the fine material to the front. No stone should show on the exposed surface after removal of forms. No plastering will be allowed, but minor cavities may be pointed up to give workmanlike finish. After forms are removed the surface is to be painted with a thin grout of cement and water applied with a brush.

SUPERSTRUCTURE.

The superstructure will consist of one steel span 72 feet in length center to center of bearings and with a clear roadway of 16 feet.

The span must be a low truss-riveted span with steel handrail. Alternate bids will be considered on wood and steel joists; in either case the joists must be set on shelf angles on the beam, and the depth of the floor system reduced to a minimum in order that the maximum waterway may be obtained with the height of the floor shown. Lumber may be of first quality Arizona native pine or Douglas fir. Flooring to be 3 by 12 inch plank of even thickness.

All members, including lateral braces and handrail, shall be riveted in place, except that the steel joists, if supported on shelf angles, may be bolted to same. The shelf angles, however, must be riveted to the floor beams. Bidders are to submit their designs for the span in accordance with these requirements and with the loading and unit strains heretofore specified.

The span shall have a net camber of about 1 inch.

STRUCTURAL STEEL.

All plates and shapes used shall be of medium steel made by openhearth process.

No steel shall contain more than 0.08 of 1 per cent of phosphorus if made by the acid method, and no more than 0.04 of 1 per cent if made by the basic method.

All tests shall be made at the place of manufacture prior to ship

ment.

All test pieces shall be as per manufacturer's standard specifications. Such test pieces shall show an ultimate strength of 60,000 to 70,000 pounds per square inch, elastic limit not less than one-half of the ultimate strength, elongation 23 per cent, and bending test 180° to a diameter equal to the thickness of the piece tested without fracture on ouside of bent portion.

Rivets shall be made from special grade of rivet steel, having an ultimate strength of 48,000 to 50,000 pounds per square inch, elongation 26 per cent. Other requirements as per test conditions stated above.

The pitch of rivets in all parts of the work shall not exceed 6 inches, nor be less than three diameters of the rivet where possible. No rivet hole shall exceed diameter of rivet by more than onesixteenth of an inch. Use of drift pins will not be allowed, except for bringing pieces into alignment, and reaming must be resorted to where necessary.

Rivets must completely fill the holes, have full heads concentric with the rivets, and of a height not less than 0.6 the diameter of the rivet, and in full contact with the surface, and machine driven where practicable.

SPECIAL NOTES.

There is now a wooden bridge at this crossing, consisting of two spans about 29 feet long on stone and concrete substructure; this bridge, including center pier and north abutment, must be removed by the contractor. The material in pier and abutment must be removed from the bed of the wash at this point, and can be dumped over the falls a short distance below. All other material in present bridge to be delivered on the bank of the wash and to be the property of the Government.

The new bridge will be located at the site now occupied by the old bridge, and the floor elevation of the new bridge will be about 5 feet above the elevation of the old bridge.

The stream is usually flooded during the months of July, August, and September; the most desirable time to execute the work will be between October 1 and July 1.

Estimate of cost of steel bridge across Moencopie Wash, near Tuba, Western Navajo Reservation, Ariz., as per plans and specifications.

PROFILE OF MOENCOPI WASH, SHOWING OLD BRIDGE NOW IN PLACE.