structure. For this purpose the Washington abutment is excellently situated; but the Virginia abutment, standing as it most injuriously does, in the stream, can be of no use, farther than the materials of which it is composed are capable of being employed in the backing and embankments. In relation to the breadth and form of the piers, I deem it my duty to present some very general observations. That these should be useful to the present purpose, it were very idle to discuss the principles which connect themselves with the theory of arches, or to expose the analytical formula by which equilibrium is established between the force exerted by an arch and the resistance of the pier. The weight of an arch may be sustained by comparatively slender supports, and experience has given to us examples that would appear to be extremely hazardous. With these, I presume, we should by no means meddle. In the erection of an extensive series of arches, a very important item of expense is the wooden frame work, which sustains the arch stones until the upper tier or keys are inserted: these are denominated centres. Were the piers not in equilibrium with the thrust of the arches; or at least nearly so, it were essentially necessary that a centre for every arch should be erected, and that the work on the whole series should be carried on at the same time; or that abutting piers, whose increased thickness should compensate for the slightness of the piers of support, should be interposed at stated intervals. In either case, stability and economy are sacrificed to effect, and the ruin of any one arch of a series involves the probable destruction of others It is for these reasons that I would recommend that the piers should have the dimensions and strength necessary to resist all the force to which they may, by any possibility, be exposed. As to the form of the piers, it may be enough to remark, that those of square ends or of other fornis, whose tendency is to accumulate a head of water under the tympanum, and to throw the current broadly off from the sides of the piers, not only impedes the navigation, but, by concentrating the sums of the velocities upon the centres of the archways, also tends to the destruction of the piers themselves, in sweeping deeper the bottom of the river which lies between them. It is apparent, that the more accute the angle presented to the stream, the less is its tendency to turn it suddenly from its course. The liability, however, of accute points to be broken or injured should limit their use, and it is only in very rapid streams, whose beds are extremely liable to be washed away, that they ought to be recommended. Semi-circular, heads, although they create a swell above the shoulder of the pier, leaves the water-way under the crown of the archway nearly level: wherefore, and because of their power to resist crushing; the little ground which they occupy; their economy of materials, and their agreeable form; they are very generally employed. They should rise so far above the waer as to protect the body of the pier against the shock of ice or other floatng matter. The down stream end of the piers, at least in the channel, should also roject beyond the dead wall of the bridge, to prevent the eddies which night otherwise form, behind the piers, shoals, to prejudice the navigation. The solidity and durability of the structure depends essentially upon its Dundations: when these are firm and unyielding, the rest of the work is ealy secured, unless grandeur, rather than solidity, is aimed at an error which cannot be apprehended in the present undertaking. But few stone ridges, indeed, have failed in the construction of their piers and arches, com pared with the great number of them which owe their destruction to defective foundations, to ignorance, and to carelessness in securing them. Heavy masses of masonry have been risked upon wooden platforms reposing upon yielding soils: examples are to be found along our own seaboard at the fortresses which the engineers of fortifications erect successfully upon the unfathomable alluvion of the Mississippi. To these platforms, the resources. of necessity, and designed also to support the mere dead weight of the walls, a large area is given, that the points of support may be multiplied. However happily these may have fulfilled their object, we have evidence of the hazard at which they are employed, in the failure of structures similarly founded, but under the guidance of a less intelligent, or of a less vigilant supervision. The tendency of vertical walls would be to consolidate the soil on which they stand; but of a pier subjected to the thrust of an arch, the tendency is to turn around the exterior edge of its base, or to slide horizontally. In constructing bridges of a few arches over the bed of a river in which it is very difficult to attain the solid bottom, platforms have, nevertheless, been used, care being taken to carry up all the arches at the same time. By this process, the thrusts of the arches resist each other, and the force resolves itself into a vertical weight Necessity has also compelled, at enormous expenses, the construction of general platforms, extending for the breadth of the But of all the means bridge across the river, from abutment to abutment. pr posed for supporting massy structures upon yielding soils, the safest, and the most usual and appoved, is piling. To be effectual, however, the piles should be driven until they cease to yield very sensibly to the percussion of the ram-covering the surface pressed by the structure with such number of piles as may be found necessary; they are then cased with pile planks, and between their heads are rammed broken stone, cement, and sand, which unite them as one mass. The depth to which the broken stone is carried depends upon the consistency of the mud, the length of the pile, and the depth at which it may be determined to maintain the ed of the river. The indes tructability of wood when it is continually immersed, and defended from Piles thus properly secured, abrasion, is familiarly known to every one. would endure without limitation; but should they be exposed to the action of the current, we can no longer assure ourselves of their duration, nor of their stability The platforms of the piers, which may be built upon piles, should, therefore, be laid at such a depth as to be below the influence of the current. It is in relation to the work below the water line, that we can, with the least hope of success, predetermine the means which it may be necessary to employ, the precaution that should be used, the difficulties and obstacles to success that may be encountered, or the cost that may be incurred. We have so often seen the efforts of the most skilful and enterprising frustrated by unforeseen casualties in conducting hydraulic works, that we might well incur the imputation of arrogance were we now to assume, for our estimates, an assured character of certainty; and when we refer to experience, we may well question the intelligence, the knowledge, or the honesty of purpose, of him who might presume to predict, with certainty, the cost or difficulties of this part of the structure. Nevertheless, it is far from unreasonable to assume, that, considering the depth of the foundatious, and the extent of the work, we may require but ordinary care and exertion to carry it to low water mark for a sum, which may be estimated within the limits of probability. The ooze, so far as it has been found practicable to examine it, is capable of preventing filtrations. Should our anticipations, in this respect, be verified, we shall have little difficulty in avoiding the enormous expenses which are so often incurred in freeing the enclosures, in which the piers should be built, from water; and it may therefore be assumed as probable, that, in building all the piers between the two principal channels, coffer dams, or cribs, may be economically employed. Over the greater part of the shoals, and in the Washington and swash channels, piling will be necessarily used to support the piers, but wherever it may be found to be more economical to descend with the masonry to a firm bottom, that will be done. The bed of the Virginia channel, for a distance of 450 feet, is gravel and loose stones; those, being carefully levelled, will receive the platforms of the piers without the intervention of piling. Care will be taken, however, to secure the piers against the under-wash of the current. Upon the flat it is not proposed to carry the masonry more that three feet below low water, nor in the swash channel, more than 8 feet. The beton or the broken stone may descend a few feet more. Hence, for a distance of 1032, yards there may be anticipated but little or no difficulty. The Washington channel is 700 feet broad, recko ing from shore to four feet water. On the edge of the flats its greatest depth is 9 feet at low water, and the breadth, for 6 feet or more in depth, is about 300 feet. Whether situated, as this channel is, along the borders of the city, and in the vicinity of an enterprising community, provisions should be made under the anticipation of the channel being deepened hereafter, is a question belonging to the wisdom and discretion of the Executive. The estimate for the foundation supposes that, for the breadth and depth to which we may be allowed to limit it, and in which the piers would be ca pable of encountering the action of the current, we may, for the present, and until that question is settled, assume its breadth to be 750 feet, and its depth 12 feet, at low water. In relation to the moveable parts of the bridge, it may, at this period, be premature to decide upon the details. It should be enough, considering the progress of invention, to limit the decision concerning them to general principles. Of draw-bridges, some move perpendicularly upon an axis placed at right angles to the road way; others, turning upon a pivot, describe arcs parallel with the plane of the road; and others act upon rollers, as a carriage does upon a railway, and in the direction of the axis of the bridge. Of these modes of action, it is very clear, that, were the railway draw equally manageable, it would affect the stability of the piers less than either of the others. The leverage of the hoisting, and the extra space required for the revolving draw bridge being objections, the force of which cannot be obviated by any of the contrivances of art. To the rolling draw, it may be objected that it occupies, very inconveniently, the road-way; that it is more liable to injury from the shock of carriages; that it does not lie sufficiently low. These defects, to me, may be measurably or wholly avoided. The designs herewith submitted, marked will suffice to indicate the means by which it is proposed to adopt the rolling principle to a draw bridge of 66 feet span. Should these fail, as I am persuaded that they cannot, then it will be expedient to use a hoisting draw. The piers immediately at each side of the draws, for this reason, will have no more breadth than may be necessa appears ry to them as abutting piers, and they will present no greater impediment to the current. In relation to the designs and estimates for the work, as well as to those facts and observations which more immediately belong to the details, I beg leave to refer you to the accompanying specifications, estimates, and schedule of prices, and to the plans and profiles herewith. All of which is respectfully submitted to the consideration of the honorable the Secretary of the Treasury. JAMES KEARNEY. CITY OF WASHINGTON, December 24th, 1832. To the President of the United States: The President having directed us to report to him the considerations which should determine the choice of a superstructure, the same being limited to be of wood or of iron, for the bridge to be built across the Potomat river, we have the honor, in fulfilment of that direction, to state That the design of an iron arch which we have before us, and which is offered in the name of Mr. Charles F. Le Barron, appears, from the drawings, to be on the principle of the open voussoir, which, since the erection of the Sunderland bridge in the year 1796, has served for all the large structures of this kind. No estimate accompanies Mr. Le Barron's drawing, nor does he give the dimensions and details which would enable us to ascertain the quantity and kind of iron which he proposes to use in his arches; nor does he exhibit the methods by which he proposes to unite the voussoirs of each rib; nor those, equally essential, by which he would connect the ribs, and limit the amplitude of the horizontal oscillations. We assume, however, that, in accordance with the received practice, and because of its economy, that his vouissoirs are to be of cast iron; and we have taken for the quantity of metal in them, the proportion which belongs to the difference of breadth of the two designs, in comparing his arch with those erected at Bristol, in England, by the Messrs. Jessop, and which seem to us to be sufficiently like this of Mr. Le Barron to warrant the comparison, with the view, at least, of an approximate estimate of the cost. It appears, upon this hypothesis, that the cost of a superstructure of iron for the Potomac bridge would be $723,949 for the horizontal part of the road-way. Taking for a superstructure of wood the model of the lower Schuylki!! bridge, (Palmer's) which is proposed by Mr. Isaac McCord, together with Met ord's estimate of thirty-one dollars per running foot, we would have for Mc the cost of the horizontal part of the road way, on that hypothesis, the sum of $136,808-wherefore, the difference of expense would be $587,141. Hence, in an economical point of view, although an iron bridge were indestructible, the wooden superstructure would be decidedly preferable; for it might be shown that the tenth part of the interest upon this difference would for ever keep up a wooden superstructure on the plan of the lower Schuyl ki bridge, which, although erected in the year 1803, is yet sound, and requires little repair. But, although iron may be more easily protected against the chemical action of the atmosphere than wood, and will therefore decompose less rapidly, there are other causes which tend to limit the duration of cast iron bridges. The ribs are composed of a great many parts, united in such a way that every impulse creates an oscillatory motion throughout the whole system, and brings into action numerous rubbing surfaces; besides which, there is the shock of heavy bodies acting upon a very brittle substance, under the influence of rapid changes of temperature. Now, an experience of thirty-six years, dating from the completion of the Sunderland bridge, is not sufficient to test the durability of a cast iron arch, subject to the action of such powerful causes, especially in our climate; and we would hesitate to recommend the pursuit of an experiment, which we believe may be justified only on the gounds of an expediency which does not apply to the condition of the United States, in respect to the other materials employed in bridge building. We think that we are sustained in this view of the subject by the practice of other nations; for, although in some parts of England and France, the comparative market value of wood, iron, and stone, together with considerations of policy in relation to a great staple commodity, have tended to encourage the use of iron arches-we do not find, in countries where timber is abundant and cheap, that it has yet been supplanted by iron as a substitute for stone. But iron arches, as well as wooden ones, are liable to fail, and they frequently require to be repaired. Now, while in the one case the means of reparation may at all times be readily and cheaply procured by us, our forests being at hand for that purpose, in the other case we must be dependant upon the ability of a foundry, and the fluctuations of a market. In our opinion, the main reason in favor of iron in preference to wood, is its security against fire; but, recurring to experience, we do not find that the risk is, in the case of wood, very great. We therefore recommend (preserving the arrangement and dimensions of he abutments and piers proposed by the engineer of the work in his report upon that matter) that the superstructure, on the horizontal part of the line, shall be of wood. All which is respectfully submitted. C. GRATIOT, Brig. Gen. Chief Eng. JAMES KEARNEY, Lieut. Col. and Top. Eng. P. S. For the entire cost of the work, we beg leave to refer to the enlosed estimates of the engineer, marked A and B. A. Estimate of the cost of a Bridge to be erected across the Potomac River; the substructure and inclined part of the bridge to be of granite, and the horizontal part of the superstructure of wood. oundations $114,069 00 rom the foundations to low water, 31,578 cubic yards of masonry, at $7 50 per yard iers and small arches under the inclined part of the road-way to be granite 236,834 00 40,108 00 |