added to produce a paste of the proper consistency, 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. 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 such 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 flood-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 5 feet center to center of end pins, and one span 55 feet center to center of end pins; carrying 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 187-foot spans, and 3,000 pounds per linear foot of bridge for the 85-foot and 55-foot spans. 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: 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 beams 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 inches by inch, one top plate 18 inches by inch, and four angle-iron brackets 3 by 3 inches by inch, making a combined section of 35.83 square inches. The chord to be well stayed on the under side by 6 inches by 2-inch plates, placed five 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. inch, Bottom chord.-The bottom chord shall be composed of four bars 5 inches by 1 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 inch angles, united by 14 inch by 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-eighthsinch rivet in each end. Their size and section shall be as follows: 1. Two bars 6 inches by inch, and four bars 2 by 2 inches by 4-inch angles combined, section 7.00 square inches. 2. Two bars 8 inches by inch, and four bars 2 by 2 inches by 1-inch angle combined, section 8.00 square inches. 3. Two bars 10 inches by inch, and four bars 2 by 2 inches by 4-inch angle combined, section 9.00 square inches. 4. Two bars 12 inches by inch, and four bars 2 by 2 inches by 4-inch angle combined, section 10.00 square inches. 5. Two bars 12 inches by inch, and four bars 2 by 2 inches by 2-inch angle combined, section 10.00 square inches. The top plates upon which rests the floor girdors shall be three-eighths inch thick, and secured to post by angle (L) iron brackets. The posts shall be accurately drilled and re-enforced for pin holes with pin bearings the same as given before for top chord. In posts 1, 2, and 3, the 6-inch, 8-inch, and 10-inch plates shall be omitted in section of post above top chord, the angle being latticed by 14 by 4-inch lacings, with one five-eighths-inch rivet in each end as shown on detail drawings. Web ties. The web ties shall be formed of round bars of following sizes and sections: Second panel, main ties two bars 18-inch round, section 2.96 square inches. Third panel, main ties two bars 18-inch round, section 2.96 square inches. Fourth panel, main ties three bars 14-inch round, section 3.68 square inches. Fifth panel, main ties three bars 18-inch round, section 4.45 square inches. Second panel, counter ties two bars 14-inch round, section 2.00 square inches. Third panel, counter ties two bars 14-inch round, section 2.45 square inches. Fourth panel, counter ties two bars 14-inch round, section 2.45 square inches. Fifth panel, counter ties two bars 14-inch round, section 2.45 square inches. Sixth panel, counter ties two bars 14-inch round, section 3.53 square inches. They shall be made with loop eyes of square iron so as to give good bearings on the pin, and all screw ends shall be upset so that the diameter of the bar under the thread shall be one-sixteenth inch greater than in the body. They shall be adjusted by sleeve nuts with right and left hand threads. Wind braces.-There shall be a lattice brace placed between each panel of lower chord, and between each panel of upper chord except the two center panels, composed at ends of four bars 2 by 3 inches by -inch angles, latticed at first panel point four bars 2 by 2 inches by 4-inch angles latticed, and at other panel points of four bars 14 by 14 inches by 4-inch angles, latticed, connected direct to chord-pins by three-eighthsinch hitch-plates of a width to accommodate the connecting detail, but in no case narrower than the smallest diameter of the connected brace. Sway rods.-The vertical cross-ties between the trusses shall connect direct with braces as shown in drawings, and shall be 14 inch round bars at ends 1-inch round bars at first panel, and 4-inch round bars in all other points. The horizontal lateral rods shall be for upper or roadway truss First panel, 24-inch round, section. For lower chord sway system: Square inches. 3.97 3.14 2.40 1.76 1.00 .78 1.00 .78 .60 .44 41 .44 Those in floor truss shall be adjusted by means of thread and nut at each end jamming against a wrought-iron level plate securely riveted to floor beams as shown in drawings. The lower chord sway rods shall attach direct to lower chord pins and be provided with sleeve-nut adjustment with right and left hand threads. Floor line chord.-The floor line chord shall be composed of plates and angles of following sizes and sections for different panels: First panel, two bars 6 inches by inch, and four bars 2 inches by 2 inches by 4-inch ang's, aggregate, section 7.00 square inches. Second panel, two bars 6 inches by -inch angles, aggregate, section 7.00 square inches. Third panel, two bars 10 inches by -inch angles, aggregate, section 9.00 Fourth panel, two bars 12 inches by -inch angles, aggregate, section 10.00 Fifth panel, two bars 12 inches by -inch angles, aggregate, section 10.00 Sixth panel, two bars 12 inches by inch, and four bars 2 inches by 2 inches by inch angles, aggregate, section 10.00 square inches. inch, and four bars 2 inches by 2 inches by inch, and four bars 2 inches by 2 inches by square inches. inch, and four bars 2 inches by 2 inches by square inches. inch, and four bars 2 inches by 2 inches by square inches. They shall be latticed together by 14 inches by 1-inch lacing bars with one fiveeighth-inch rivet in each end. They shall be placed between the floor beams and attached to same by gusset hitch-plates and securely bolted, all as shown in detailed drawings. The nuts on connecting bolts shall be riveted down after being drawn up snug to prevent working loose. Floor beams.-There shall be a wrought-iron floor beam at each panel point having a capacity of 100 pounds per square foot of roadway and walks, or a wheel load of 6 tons on each pair of wheels for each wheelway. The floor beam shall be so proportioned that the chord strains are resisted by the upper and lower flanges and the shearing strains by the web plates. It will be constructed of 24 inches by 4-inch plate, with four bars 3 by 34 inches by -inch angles riveted to the flanges and 2 by 2 inches by inch, and 2 by 3 inches by 4-inch angle iron stiffeners riveted to the web, two of the latter on each side over the points of support and one of the former on each side, 6 feet from the points of support all by five-eighths-inch rivet. The floor beams shall be secured to web post top-plates by four five-eighths-inch rivets on each end, and, omitting the plate outside of the trusses, the lower flange shall level up to 7 inches, all as shown on detail drawings. Rirets. All rivets shall be made of best quality of soft fibrous iron which shall be capable of bending 180 degrees upon itself without fracture. They shall, as far as possible, be driven by machinery and completely fill the holes; the heads shall be hemispherical directly opposite and shall be formed in a neat and workmanlike manner. All rivet boles shall be carefully spaced and punched by a die not over one-sixteenth inch larger than diameter of rivet, and must match in different pieces to be riveted together, so that hot rivets can be readily inserted without using the driftpin to strain the iron. The pitch of rivets shall not exceed 5 inches (except for open lattice work), nor to be less than three times the diameter of the rivet (which will generally be five-eighths inch to seven-eighths inch diameter); where practicable rivet holes shall be two diameters from the edge of the piece. The use of drift-pins will in no case be allowed after the work has been once assembled ready for riveting, and only to hold parts in place ready for bolting up. Within two diameters of the end of riveted compression members, the pitch of rivets shall not exceed four times the diameter of the rivets. Pins.-All top chord pins shall be 3 inches in diameter, the bottom chord pins 5 inches in diameter, and the end connection pins 6 inches in diameter. They shall be turned straight and smooth and to exact size as given, and shall be provided, as far as possible, with cast-iron end caps, set screws, and chamfered ends, all as shown on detail drawings. No variation of more than one-sixty fourth of an inch will be allowed between the diameter of pins and pin-holes. Cast-iron packing-rings or washers are to be used whenever required to inake joints snug and tight. Pins are to be proportioned so as to resist bending as well as shearing strains. The diameter to be not less than twothirds the width of the largest tension member attached. Railing. The sidewalk railing shall be made of open lattice work 34 feet high with projecting pickets, and shall be secured to floor beams at each panel point by wrought angle-iron posts. General.-All abutting surfaces must be planed or dressed so as to insure even bearings, taking light cuts so as not to injure the end fibers of the piece, and must be protected by white lead and tallow. Pieces where abutting shall be brought into close contact by the use of clamps or other approved means before being riveted together. In all built struts or posts the segments must be all of one length without break and must be rigidly riveted together so as to act collectively as one solid post. All rivets in pin re-enforce plates shall have the same bearing on the plate that the plate has on the pin. They shall be so placed as to utilize the full section of the plate metal, but in no case shall their distance apart exceed 5 inches nor be less than the diameter of the rivet. All bed plates must be of such dimensions that the pressure on the masonry shall not exceed 250 pounds per square inch. Rollers shall be not less than 2 inches in diameter and shall be so proportioned that the pressure per linear inch of voller shall not exceed the product of the square root of the diameter of the roller in inches multiplied by 500 pounds. The roller shall be placed between planed surfaces. The ends of all square ended members shall be planed smooth and exactly square to the center line of strain. No lateral or diagonal rods shall have a less area than three-fourths square inch. Eye-bars which are to be placed side by side in the bridge shall be bored at the same temperature. All iron work shall be neatly and accurately fitted up in a thorough and workmanlike manner. It shall be painted with one good coat of metallic paint and boiled liu |