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In an ordinary quadrangular truss, these strains are taken up by the chord at each panel point and transmitted directly to the points of sup port, but in the case of the deck parabolic truss the tendency of longi tudinal strains is to cause the truss to revolve around the point A (see Fig. 1), the point B having a pin connection. The force of this objec tion was admitted by the Corrugated Metal Company when their plans were first under consideration, and the inclined brace A C (Fig. 2), was introduced to obviate this difficulty. In order to prevent the introduction of new strains in the truss itself from this strut, the company propose to rivet it in place after the truss has been swung clear of the false works. The introduction of another strut, A D (Fig. 2), running to the second panel point, would tend to stiffen the bridge system still further. The Corrugated Metal Company claim that this construction is analogous to the ordinary deck quadrangular truss; but that this is not the case will be very clear from the examination of the plan of such a truss, when it will be seen that the floor system is a part of the truss itself, and that at every panel point the longitudinal strains are taken up by the members of the web system. In the through bridge of the Corrugated Metal Company these horizontal strains are transmitted by the floor system directly to the points of support. In the deck bridge of the parabolic arch truss, however, the floor system is not a part of the truss, but is supported upon vertical posts which are analogous to the spandrils of an arch, these posts having no diagonal bracings. The shocks upon the floor system can therefore only be taken up at the cen ter of the truss, where the truss and the floor system coincide. At all intermediate points the tendency is to overturn the vertical posts which carry the floor loads to the upper chord.

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In view of these considerations, it seemed to me that, unless the maSoury were carried up to the center of the truss proper, the bridge would, in the course of a few years, be found deficient in stiffness and longitudinal strength. A provision for building the piers to the full height was therefore inserted in the new specifications. As the reduction of the width of the bridge would reduce the cost somewhat, it seemed probable that the Corrugated Metal Company might be able to build up the piers to the full height. This, however, they state that they are unable to do on account of the cost. If the piers are carried to the height shown in the last specifications, of 40 feet above low water, under the coping, the batter may be reduced to 1 in 20 or even one-half inch per foot. I estimate that the additional cost of building these piers over that of building the piers proposed by the Corrugated Metal Company (with a batter of 1 in 12) will be about $7,500. Deducting from this the amount saved by omitting the iron end posts and braces ($1,500) and by the reduction in the width of the bridge about ($1,000) will give a net increase of cost of $5,000. The plans of the Corrugated Metal Company as submitted show the extreme length of the iron cross floor beam with overhanging sidewalks as 30 feet, so that the available bridge width would not greatly exceed 28 feet, if built according to their plans.

While the Corrugated Metal Company are willing to make the reduction in width, they offer no increase in the masonry as an equivalent therefor.

CONTINGENT EXPENSES.

Before work or any contract could be commenced, there would be required an additional appropriation for the payment of contingent expenses of engineering and superintendence.

In addition to the ordinary engineering and office expenses, there will be required an inspector of masonry on each pier in progress, and an expert inspector of bridge and iron work to be present at the works while the iron is being rolled and the members of the bridge assembled for the truss. It is important that the work should be thoroughly inspected and kept under constant supervision, and I would recommend an appropriation for contingencies of not less than $14,000, as the work may extend over a year and a half.

CONCLUSIONS.

I have thus far given the history of the transactions between this office and the Corrugated Metal Company, and now submit the following conclusions and recommendations:

In the last letter of the company (dated February 16), addressed to the Chief of Engineers, they propose to build the additional masonry for the piers for $50,000 in addition to their contract price. As their original bid for the entire masonry was only $36,000 (or about half the actual cost), and as the net cost of raising the piers to the center or point of support of the trusses is only $5,000, I presume that the amount named by Mr. Douglas is a mistake. In any event, his proposition, being a violation of act of appropriation and also of section 3733 of the Revised Statutes, could not be entertained.

The company at first proposed to build solid masonry piers to the point of support of the trusses, and subsequently modified their original proposals by the substitution of the cheaper and inferior construction of iron posts. On similar grounds of economy they might have removed

the stone work to a much lower level and replaced it by iron posts. I think there can be no question that if these posts shall be accepted in lieu of masonry, the bridge would be found after the wear of a few years deficient in longitudinal stiffness and stability.

The introduction of a second series of struts running (shown on Fig. 2) from the pier to the second panel point and the introduction of diag onal bracing above the arch would, as already stated, strengthen the construction, but this would involve further correspondence and delay, which might be prolonged until it was too late for the action of Congress and thus delay the work another year.

The King Bridge Company have already written several letters complaining that the Corrugated Metal Company have been allowed to modify bids and plans, and have formally requested that they be allowed to modify their plans, and bids also and to bid for substructure. The King Bridge Company made no bid, originally, for substructure, and for that reason their proposal was not considered. As the question has been raised, however, I think there are doubts as to the propriety of allowing bids to be modified. I inclose copies of the letters of the King Bridge Company on this subject (inclosures D and E).

Upon reviewing the entire subject, I can see no reason why the United States should erect a bridge which shall be inferior in any respect, or one in regard to which there can be any question in the future as to durability or strength.

As already stated, an additional appropriation for contingencies will be required, in any event, before the work can be commenced, and I believe it would be wise economy at the same time to ask for a sum sufficient to build a substantial and durable bridge. I submit the following estimates for such a bridge.

As it has not been practicable, for want of funds, to obtain any precise information as to the depth of rock, any estimate of the cost of the bridge must necessarily be approximate.

I estimate the cost of a bridge with the iron structure below grade, with masonry of somewhat better class than proposed by the Corrugated Metal Company, as follows:

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The cost of a bridge with the same class of masonry as now proposed would be as follows:

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The estimate for a bridge with the iron structure above grade with piers 60 feet high would be $220,000.

For the above reasons, I would recommend that the proposals of the Corrugated Metal Company be declined, that an additional appropriation

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,

The CHIEF OF ENGINEERS, U. S. A.

S. T. ABERT,

United States Civil Engineer.

A.

66
BRIDGE OVER THE POTOMAC AT 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 débris 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 courses by cutting away the projecting rock so as to secure a perfectly level bed of solid rock, with no projections, 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 engineer, 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.

Footing courses.-The footing courses of the masonry are to be commenced as shown on the plans, 3 feet outside 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.

Courses.-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 3 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 upper 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 and square to the face for at least 12 inches back, and must not open more than inches at a distance of 18 inches from the face, nor 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 high projecting points shall be dressed from the top so as to give the succeeding stone a firm 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 following 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 cut 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 cut square to fit the backing.

The stones of the pier-head 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 quality 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 through 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

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