REPOET OF THE To the DIRECTORS of the NEW YORK BRIDGE COMPANY. Avery Architectural and Fine Arts Library Gift of Seymour B. Durst Old York Library NEW YORK, May, 18G9. TO THE DIRECTORS OF THE Gentlemen : Having received through Mr. Roebling, Engineer of the New York Bridge Company, the request of the Directors that we should, as a Board of Engineers, make careful examination of the plans proposed by him, for the erection, over the East River, of a Wire Cable Suspension Bridge of 1,600 feet span and 135 feet elevation above low water, and make report as to the prac- ticability, adequacy, safety and durability of the structure as pro- posed, and having accepted the trust reposed in us, and having given the subject the full consideration and careful investigation which its great magnitude and the large interests involved de- manded, we have now to submit a report of our proceedings and of the conclusion at which we have arrived. On coming together as a Board, our first action was, to ob- tain a clear knowledge of the object for which the Board had been convened. It was then ascertained, that the object was not one of service to the Directors in their consideration of the plans proposed, or their action on the adoption of the plans ; for that 4 consideration had been given, and that action had been taken, and was before the public ; and also that it was not the object that the Engineer of the Company should have the aid of the counsel and suggestions of this Board, in maturing his plans, and deciding the many important questions involved in the proposed structure ; for the plans had been matured, and the decisions had been presented in a report, stating fully and clearly the general character of the structure, with such reference as to details of combination and proportions as was appropriate. Through the inquiries which led to this information, the Board learned, that after the adoption of the plans recommended by Mr. Roebling, and after the decision of the Directors to go forward with the undertaking, Mr. Roebltng requested of the Directors that a Board of Engineers should be convened, before whom the plans in all their details should be laid, in order that their professional judgment should be obtained as to the prac- ticability of the erection of a Suspension Bridge 1,600 feet span and 185 feet elevation above the water on the plans proposed by him ; as to the adequacy of the structure when erected to sustain the greatest load it would have to carry, alike under the extremes of summer and winter, and when subjected to the action of the greatest storms known to take place. This Board was informed by Mr. Roebling, that in view of the magnitude of the under- taking and the large interests connected therewith, in the cities of New York and Brooklyn, he had suggested to the Directors that it was right and proper, before going forward with the work, that his plans should be subjected to the careful scrutiny of a Board of Engineers convened for the purpose, in order that if his plans received their approval, the enterprise would stand before the public sustained by their professional judgment, and would not rest solely on the judgment of one single Engineer. 5 The Board have undertaken the responsible duty thus defined ; a duty requiring the greater care, because their action is at the request of the Engineer whose plans they were to scrutinize. The plans, including foundations, towers, and superstructure, have at numerous meetings, (between March 25th and April 20th,) held for the purpose, been laid before this Board by Mr. ROEBLING, from whom they have received at the same time the fullest ex- planations on every point. In the consideration of what has thus been brought before them, the Board have been impressed by the manner in which Mr. ROEBLING has sought to have an independ- ent investigation of his plans and proportions, of the severest character. Having completed the examination of the plans, and the in- vestigations of the combinations and proportions proposed for the service required, the Board deemed it an appropriate part of their duty to examine the structures of the same general character, erected by Mr. Roebling, across the Alleghany and Pittsburgh in 1860, and across the Niagara at Niagara Falls in 1854, and across the Ohio, at Cincinnati, in 1867. They have thus had under their eye and personal examination the successive steps, which, beginning with a span of 200 in 1844, was followed by a span of 825 feet in 1S54, and by a span of 1,057 feet in 1867, all standing this day a practical demonstration of the soundness of the principles and proportions on which these structures have been erected, and rendering unnecessary, at least for spans of 1,000 feet, any other demonstration, and affording the best source of information as to the practicability of taking another step in a span of 1,600 feet. The report which, after these investigations, we now concur in making, is presented in the following statements, having reference 6 therein to a Steel Wire Cable Suspension Bridge of 1,600 feet between the towers, 135 feet above the water, on the plan, ma- terials and proportion of parts set forth in the printed report of Mr. Roebling, made to the New York Bridge Company. First. That the structure, when erected according to the plans and specifications proposed by Mr. Roebling, will possess a strength of parts not less than six times the strain to which they will be subjected, when under the greatest load which will be on the Bridge, and that a strength of six times the greatest strain is a greater degree of provision for safety than is usually made in structures having the same object in view. Second. That in the combinations and relations of parts, as proposed for the 1,600 feet span, there are none of a novel char- acter, being a repetition of what has been done in the Niagara Bridge, and on enlarged proportions in the Cincinnati Bridge ; the difference for the span of 1,600 feet being a provision of a total strength to provide the same degree of excess of strength in die greater span. Third. That the combinations of parts provide in the same manner for the changes of temperature which attend the ex- tremes of summer and winter, which has proved perfectly ade- quate in the Cincinnati Bridge. Fourth. That the structure is amply prepared to sustain the action of the greatest storms of which we have any knowledge. Fifth. That adequate foundations for the towers can be had in the manner proposed by the Engineer, the particular plan to be used depending on the character of the material developed in the progress of the work/ ami that if Vood is used in the manner proposed under certain circumstances, its durability in such posi- tion is known from the experience of centuries. 7 Sixth. That, as to the durability of iron put together and placed out of the reach of all elements of decay, in the manner proposed and practiced by the Engineer, no limit can be assigned : the facts being well established, that iron not acted on by oxygen is as durable as any material of which we have knowledge, and that air and water, the means of furnishing oxygen, can be readily and with ordinary care permanently excluded. Seventh. That the pressure on the foundations of the towers will not be increased by the superstructure, and the greatest load carried by the Bridge more than six to seven per cent.; and that, therefore, the permanent stability of the towers depends on the principles well known and used in heavy structures of masonry. Eighth. That there are no peculiar difficulties in erecting the proposed structure ; that the same means and arangements by which the Suspension Bridge of 1,057 feet span was erected over the Ohio at Cincinnati, are the means and arrangements to be made use of in the erection of a span of 1,600 feet over the East River. Ninth. That in the use of six trusses of large dimensions in combination with the wire cable and stays, there is provided all the rigidity vertically, lengthwise the Bridge, that is required, and that in the roadway of 80 feet width, with its horizontal trusses, there is secured a stability horizontally far exceeding that pro- vided in any previous structure of the kind. To the preceding may be added, that in the inertia of the mass comprised in the superstructure of a Suspension Bridge 1,600 feet long and 80 feet wide, there exists a stability not to be sensibly disturbed by any action which we can anticipate. Such being the conclusions we have to present, it will be seen that they sustain the following summary expression ; 8 That it is beyond doubt entirely practicable to erect a Steel Wire Suspension Bridge of 1,G00 feet span, 135 feet elevation, across the East River, in accordance with the plans proposed by Mr. Roeblixg, and that such structure will have all the strength, stability, safety and durability that should attend the permanent connection by a Bridge of the cities of New York and Brooklyn. With this expression of our professional judgment we could, and perhaps should, close this report. But in the consideration of the circumstances that, in some degree, rendered advisable the action of the Directors in calling together this Board, we have been led to see that it would be in accordance with that object, and also be of some practical value in reference to the use of similar structures in other places in our country, which has yet so much to do in spanning its wide streams and valleys, to present, in a manner not requiring professional attainment to apprehend, the grounds on which confidence in the practicability, safety and durability of structures of the character proposed by Mr. Roebling can be attained to a degree that will be of practical value to those interested in structures requiring wide spans. The safety of any structure to sustain strain, depends on the relation of strength provided, to the strain to be sustained; thus, a sound steel wire, about one-sixth of an inch in diameter, will sus- tain, hanging on it vertically, 4,000 pounds — if that weight be in- creased to 4,100 pounds, the wire will probably be broken. This capacity of a sound steel wire one-sixth of an inch diameter to sustain 4,000 pounds hanging vertically, is a practical fact, only known by actual numerous trials ; it is in no degree a calculation. When six wires of one-sixth of an inch diameter are put together to sustain 4,000 pounds, known to be within the capacity of a single wire, it is said that a provision of six times the strength is made, and this is what is to be understood when strength " by 9 calculation" is referred to. But when six wires arc to be relied on to furnish six times the strength, it must be known that they arc so combined that in use the strain of the weight is sustained by the six instead of the one. This is matter of detail not easily presented on paper ; the manner of combination must be seen. On examination of the manner in which the wires used by Mr. Roebling are made, tested and combined, it will be found that they are all straight wires, put together in parallel position, and bound together by temporary wire bands, making what is called " a strand." Strands so formed are put together in parallel posi- tion to each other, and form what is called a wire cable. In the use of wires to sustain weight between towers wide apart, the weight is not sustained by wires hanging vertically, but is sustained by wires suspended between, and passing over the towers, and held in position by the anchorage on the land side. In such case, calculation and trial are made use of to ascertain what strain a wire thus supported will be subjected to, when a known weight distributed along the wire is sustained with a known deflection of the wire. What that strain is, under known circum- stances, is as fully and plainly a matter of fact, as the strain is known when the wire hangs vertically. It is thus known as plain matter of fact, that a wire hanging between two towers 1,600 feet apart, with a deflection at centre of 128 feet, will sustain 2,300 pounds beyond its own weight, and that if six wires so placed are combined to sustain the 2,300 pounds, there will be a provision of six times the strain created by the sus- pended weight. It is also well known, that the less the depression of the wire hanging between two points, the greater the strain on the wire ; and that by taking the proper depression, its own weight may be 10 used to test the strength and soundness of the wire, and determine its actual strength. Mr. Roebling makes use of this means in the making up of the strands of his cables. When the towers are ready to receive the cables, the wires are one at a time stretched over the towers, and drawn by ade- quate power on the land side to a depression in the centre of 57 feet, the wire will be subjected to a strain of 500 pounds. When wires have been so placed and so proved, they are closely bound together by wire wrappings, and thus is formed what is called a " strand," each wire of which is known to have sustained a strain of over 500 pounds. The strands so formed and so proved are then lowered to a depression of 128 feet. When the required number of strands are brought together, they are tightly bound together by wire wrappings, using powerful clamps to bring the strands closely in contact and to the proper form, before the wire wrapping is wound, round as close together as possible. This being done from one end to the other, the wire cable is formed. In each of the four wire cables proposed for the East River Bridge, 19 of these strands are used, which will swell their size to nearly 15 inches diameter. It is proposed to use four steel wire cables of this character in the East River Bridge, every wire of which will have been proved after it has been stretched between the towers. It is to be under- stood that in this combination of wires in a cable they are brought by powerful pressure into the closest contact ; it will then be per- ceived that it would be impossible to draw out a single wire by itself; that if adequate power was brought to act on drawing out a single wire it would be broken, but could not be drawn out. Experience has shown that wire cables, although of large di- mensions and great strength, are to some degree flexible, and that 11 some means that will give greater stiffness lengthwise the Bridge is necessary. Such means are introduced by Mr. Roebling, in the Niagara Bridge, and thereby the structure was made adequate to the passage of railroad trains. They were again used in the Cincinnati Bridge. As proposed to be used in the East River Bridge, they may be thus described : They consist in connecting with the four wire cables, six trusses of iron 12 feet deep, which arc to carry the roadway and extend from tower to tower ; in con- struction they are of the kind now used so extensively to form railroad Bridges, of up to 300 to 400 feet span ; in dimensions of parts they are such, that any 400 feet in length, supported at the ends and properly proportioned, would more than sustain their own weight without aid from the cable ; while the fact is, that the truss is supported by the cable every 7£ feet. The use of this combina- tion at Niagara and Cincinnati has demonstrated that the re- quired stiffness is thereby attained. In addition to the use of wire cables in combination with the trusses as described, the engineer proposes to support about 570 feet of each end of the Bridge next the towers by wire ropes, of 2$ inches diameter, which being connected with the towers near the top and with the road bed at distances from the tower of 50 feet up to 570 feet of the roadway, in a manner well known and extensively used. The strength of each of these wire ropes, called, from their posi- tion and supporting action on the roadway, Hays, is practically known, and also their sustaining power, brought to bear on the first five hundred feet from the tower. From the familiar description of the East River Bridge as pro- posed by Mr. Roebling, it will be seen that the structure will be a combination of four steel wire cables, of near 15 inches diameter, 12 with six iron trusses extending from tower to tower, and 128 stays attached to each tower ; a combination which in principle has been adequately proved at Niagara and Cincinnati, and which provides as matter of fact a strength equal to six times the strain that will result from the greatest load that will ever be on the structure. As to the adequacy of the towers to perform their part, it is to be understood that the pressure on the towers due to the weight of the wire cables, trusses, roadway etc., act vertically through the tower, because the wire cable, lying in the top of the towers, ex- tends on both sides at about the same angle of depression, and is on the land side held by a mass of masonry whose total weight exceeds largely the tension of the cables. To this may be added, that a force equal to about 4,000 tons, acting horizontally on the top of the tower in the direction of the Bridge, would be required to upset the tower when held in posi- tion only by its own weight, and constructed as a solid mass. As to durability. The durability of any material or structure depends on excluding all causes tending to change the condition of the material or of the combination of the structure. A structure of masonry in this climate, from which water was not excluded by proper means, would have less durability than one of iron under same exposure. The action of cold in the win- ter, if water was allowed to be present, would soon show that care must be taken to secure the desired durability. The known durability of wood, when used in foundations at great depths and always saturated with water, finds its explana- tion in the fact that there are no changes in the condition. There are no alternations of wet and dry, and no material changes 13 of temperature, and a long experience has proved that under such circumstances the condition of the material remains the same. In the use of iron and steel, the same kind of care must be taken. Change in the condition of iron or steel can only take place from rusting ; that is, from combination with oxygen. Oxygen can only have access to the iron or steel from water or air. If water and air are excluded entirely from contact with the iron or steel, there can be no oxidation, and therefore no change in the condition, consequently no change in strength. That it may be understood in what way and by what means water and air are to be permanently excluded, it is to be stated, that when the strands, containing about 270 parallel straight wires, are made up, all the interstices between the wires are thoroughly filled with pure boiled linseed oil, and that when the cable is made up from 19 of these strands, oil is introduced so thoroughly, that when the cable is wound, there is not an interstice between the wires that is not filled with oil. The winding of the cable thus formed and saturated with oil, is by means of wire one-tenth of an inch diameter, which is wound so close together on the exterior of the cable as to form a surface nearly impervious to water; that surface of wire is then painted with pure oil in several coats. It will be readily understood, that as long as the external painting is properly renewed from time to time, neither water nor air can ever have access to the wires, and as long as this state of things continues, the unchanged condition of the wire must con- tinue, and consequently the strength of the material must continue unimpaired. \