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of $61,000 be asked, and that section 1 of the act of February 23, 1881, authorizing the construction of the bridge, be amended so as to allow the expenditure of amounts required for contingencies of engineering surveys, advertising, &c., to be made from the sum appropriated, previous to the award of the contract. Very respectfully, your obedient servant,

S. T. ABERT,

United States Civil Engineer. The CHIEF OF ENGINEERS, U. S. A.

A.

BRIDGE OVER THE POTOMAC AT

66
THREE SISTERS."

Specifications for Foundation and Masonry.

FOUNDATIONS.

The foundation of the piers and abutments is to be on the solid rock of the river bed. A sufficient coffer-dam is to be built at the location determined upon for each pier or abutment.

When the coffer-dam is completed and the water excluded, all mud, loose rock, and other debris is to be removed from the area to be occupied by the footing courses of the masonry. The surface of the rock will then be carefully leveled to receive the foundation conrses by cutting away the projecting rock so as to secure a perfectly level bed of solid rock, with no pr ections, for the depth from the face of all headers and stretchers, greater than those allowed for the masonry.

Small holes or fissures in the rock will be filled with concrete. Where blasting is necessary to level the foundation it must be done with great care, under the immediate supervision of the inspector, so as not to crack or shatter the solid rock.

Where the declivity of the rock is so great that it would be impracticable to bring the surface to one level bed, the rock may be cut into level steps, in such manner as may be approved by the engineer, and in all cases where, in the opinion of the engiDeer, it may be necessary, the courses of masonry below the general level of the rock surface as prepared are to be doweled together and to the solid rock with 1-inch iron (0) clamps, or dowels, let into adjacent blocks and carefully run with lead or typemetal.

Fooling courses.—The footing courses of the masonry are to be commenced as shown on the plans, 3 feet ontside the general batter line of the piers at the base, being decreased in uniform steps, as shown on plan, until the batter line is reached, two courses, or from 5 to 6 feet above the base.

MASONRY.

Kind of stone.—The stone for the piers and abutments is to be of the best quality of the gneiss rock quarried along the Potomac River, between the Aqueduct Bridge and the Little Falls Bridge.

It shall be carefully selected, free from seams, or any other defects, of hard compact texture, and approved by the engineer.

Courxes.—The courses shall be not less than 18 inches nor more than 30 inches high, decreasing uniformly from the bottom to the top of the piers and abutments. Courses of stone shall be continuous around and through the piers and abutments, or may be occasionally broken with the consent of the engineer.

Finish of stone.—The face of the stone will be “quarry faced,” with a well-defined arris line pitched around the edges of the face, the rock projecting beyond the pitched lines not more than 3 inches.

The stone forming the face of the cut-water and ice-breaker on the up-stream end of the piers shall have their faces fine-pointed, to a uniform and regular surface from low-water to the freshet line, with a 2-inch draught line on the cutting edge of the ice-breaker.

Headers and stretchers.—The masonry is to be carried up in regular courses, and is to

consist of stretchers, headers, and backing. The breadth of the stretchers is to be at least one and a half times their thickness, and their length from three to four times their depth, but not greater than 6 feet.

The headers are to be at least 24 feet wide, and from 31 to 5 feet long, the latter being laid at the base of the pier.

The stone shall be laid at the rate of one header to two stretchers, and in each course the headers must constitute not less than one-fourth of the whole face of the course, evenly distributed so as to make an efficient bond. The stones will break joint by at least 15 inches in the lower courses and 12 inches in the apper courses.

Beds and joints.-Every stone, without exception, must be laid on its natural bed, and have both beds well dressed, parallel, and true to the proper line, and to the proper angle with the plane of the face.

The beds of the face stone must be dressed to their full width, and the vertical joints must be full avd square to the face for at least 12 inches back, and must not open more than 3 inches at a distance of 18 inches from the face, por more than 6 inches at a distance of 24 inches from the face. No face joint shall be more than one-half inch in thickness.

Backing.–The backing shall be composed of large and well-shaped stones with an area of not less than 3 square feet, except where smaller stones are needed to fill the openings between the face stones, to be carefully placed so as to break joint by at least 6 inches, and thoroughly bind the work; not more than two courses of backing shall be used for each course of face stone; the lower beds shall be dressed level and even, and all bigh projecting points shall be dressed from the top so as to give the succeeding stone a tirm bearing; the bed joints not to exceed 1 inch, and the vertical joints not to exceed an average of 24 inches, to be least along the face stone.

The backing must be made level with each course of face stone.

Any large vertical joints in the backing are to be carefully filled with spalls and cement, but no spalls or levellers are to be put under a stone for the purpose of raising it from its bed.

All masonry shall be well and carefully laid in full flush beds of hydraulic cementmortar as hereinafter specified, and every joint shall be completely filled with mortar.

Each stone before being laid shall be carefully cleaned and moistened, and masonry built in hot weather shall be protected from the sun as fast as laid by covering with boards No masonry shall be laid in freezing weather. No hammering will be allowed on a course after it is set.

The foll, wing modifications of the specifications apply to the up-stream pier-heads or ice-breakers.

Pier-heads.—The backing for a distance of 8 feet from the pier-heads shall be cat with vertical joints, not exceeding five-eighth inch in thickness, and the face-stone of the pier-heads must be made of stretchers, having not less than 3-feet bed with alternate headers, the rear end of both to be ont square to tit the backing.

The stones of thu pierhead and ice-breaker are to be clamped together, and to the backing with iron clamps of 1-inch round iron, 15 inches long, and 4 inches deep, from top of stone.

The dimensions of stone for pier-heads are to be in accordance with the drawings therefor.

The coping of all the piers and abutments is to be bush-hammered, to ten-cut work not less than 12 inches thick, and laid with alternate stretchers and headers, the headers to reach entirely across the pier, and the dimensions to be as shown on plans.

Pointing.-All the face-joints are to be raked out to the depth of 1 inch, and filled, and carefully pointed with finely tempered cement-mortar, 24 parts sand to one of cement.

The mortar is to be well driven into the joint and the exposed edge neatly finished or rubbed with pointing tools in a workmanlike manner.

Cement and mortar.—The stone throughout shall be laid in the best quality of freshmixed hydraulic cement-mortar.

Cement.—The cement is to be the best qnality of Rosendale cement, of such brand as may be found after tests and inspection to be best adapted to the work. The cement must be fresh ground, put up in well-made casks, so as to be reasonably secure from the air, and must be kept perfectly dry in a closed shed.

All cement furnished for the work will be subject to inspection and rigorous tests, and if found of improper quality must be immediately removed from the work.

The character and severity of the tests are to be determined by the engineer.

Sand.—The sand is to be of the best quality of clean, sharp, quartzose sand (such as is found in the Potomac River), to be free from loam or other impurities, and to be screened throngh a one-quarter inch sand screen.

Mortar.- The mortar is to be prepared from the cement and sand above specified in the proportion of one part of the cement to two parts of the sand.

The materials are in all cases to be measured in the proportions above required, and are to be thoroughly mixed dry, and a sufficient quantity of water is to be afterwards

added to produce a paste of the proper consistoncy, and the whole thoroughly worked with hoes or other tools.

The mortar is to be mixed in tight and proper boxes prepared for the purpose. It shall be mixed fresh for the work in hand, and any mortar that may have been left standing longer than one hour shall not be used, nor shall any mortar which may have set be remixed or used.

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The width of the piers at the top under the coping shall be 7 feet.

The length of the piers on top will be 23 feet 6 inches, and the batter on all sides shall be 1 in 20.

The height of the piers will be 40 feet above low-water under the coping, or soch height as may be necessary to bring the bridge-seat at the center of the truss.

The ice-breaker or up-stream pier-head of each pier shall have a batter of 6 inches to the foot, commencing at the food-line, 20 feet above low-water, and extending to low-water mark.

The top of the pier-head shall be finished with a cap or coping, as shown in plans.

The details of each pier will be shown on special drawings of the same, to be made by the engineer after the surveys are completed and the exact location is determined. They will, however, conform in general to the accompanying plan of a pier in 10 feet of water.

ABUTMENTS. The abutments are to be built of the same class of masonry as herein specified for the piers and foundations. They are to be built with wing walls and of such thickness as may be required for the purpose of a retaining-wall, and in accordance with detailed plans to be prepared by the engineer after the surveys have been made.

The foundation will start at solid rock.

Specification for the superstructure of a wrought-iron parabolic truss bridge at the site of the

Three Sisters," in Georgetown, D. C. General.—The structure shall be what is known as a deck-bridge throughout, and shall consist of five spans 187 feet each, center to center of end pins; one span 65 feet

center to center of end pins, and one span 55 feet center to center of end pins; carry: ing above the trusses a roadway 20 feet wide and one walk 5 feet wide, making a total clear width of 25 feet.

Capacity: -The capacity or live load of the bridge shall be 2,250 pounds per linear foot of bridge for the 107-foot spans, and 3,000 pounds per linear foot of bridge for the 85-foot and 55-foot spaus.

Unit strains.-Under any application of the above specified loads, no part shall be strained more than 10,000 pounds per square inch in tension, 7,500 per square inch in shearing and in compression, as given in the following table, taken from Volume IV, page 123, Transaction of American Society of Civil Engineers:

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When one end is square and the other end is rounded, a mean is to be taken between the two.

Wind loads.-Wind loads shall be taken at 56 pounds per square foot of exposed surface with unit strains taken at 15,000 pounds per square inch.

The bending strain on pins shall not exceed 15,000 pounds per square inch when the center of bearings are taken as the points of application of the strains.

The crushing strain on pin and rivet holes shall not exceed 12,000 pounds per square inch of the projected semi-in-trados. Compression strain in floor beanis 6,000 pounds per square inch.

Detailed specifications for spans 187 feet, center to center of end pins. Top chord.—The top chord shall be of rectangular section made up of two bars 16 incbes by 4 inch, one top plate 18 inches by } inch, and four angle-iron brackets 3 by 3 inches by 8 inch, making a combined section of 35.83 square inches.

The chord to be well stayed on the under side by 6 inches by t-inch plates, placed fire in each panel, and the whole united by three-quarter-inch rivets 4-inches pitch on centers. The chord shall be accurately drilled with pin holes at ends and at each panel point, and re-enforced so that in no case shall the bearing surface exceed 12,000 pounds per square inch (diameter of pin by thickness of bar), all as shown on detail drawings.

Botton chord.—The bottom chord shall be composed of four bars 5 inches by 16 inch, having a combined section of 31.24 square inches made with die-forged heads, without welds, enlarged so that the section through the pin shall be 50 per cent. greater than in the body of the bar, and back of the pin the section to be the same as in the body of the bar. The bars must be free from flaws, perfectly straight before drilling; the pin-holes to be in the center of the head on center line of bar, and to be drilled exactly perpendicular to the plane of the bar. The holes shall be drilled not more than onethirty-second inch larger than the pins, with no error in length to exceed one-sixtyfourth of one inch.

End post.–The end posts shall be of rectangular section formed of four bars, 3 by 3 inches by Pi inch angles, united by 14 inch by 1 inch lattice-bars, with one five-eighthsinchrivet on each end, as shown on detailed drawings. The end post shall be re-enforced and accurately drilled for pin holes as before specified for top chord, and in accordance with detailed drawings. Under end posts at one end of each 187-foot span, there shall be placed a nest of eight 2-inch rollers, each 30 inches long, carefully turned, and running between two three-quarter-inch planed wrought-iron plates. The rollers shall be beld in a frame, and the base plate shall be riveted to end post by 3-inch wroughtiron angle (L) brackets all as shown in detail drawings.

Web posts.-The web posts shall be formed of plates and angles arranged as shown on drawings, and united throughout by 14 inch by inch lattice bars, with one five-eighthyinch rivet in each end. Their size and section shall be as follows: 1. Two bars 6 inches by 1 inch, and four bars 2 by 2 inches by 7-inch angles combined, section 7.00 square inches. 2. Two bars 8 inches by 1 inch, and four bars 2 by 2 inches by t-inch angle combined, section 8.00 square inches.

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