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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 the 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 enclosed 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. Foundations

$114,069 00 From the foundations to low water, 31,578 cubic yards of masoary, at $7 50 per yard .

236,834 00 Piers and small arches under the inclined part of the road-way

40,108 00

to be granite

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38 large piers of rubble masonry, in common mortar, under

the horizontal part of the road-way, and above the low water 48,488 00 Abutments and filling

7,876 00 Coping, railing: dead walls of the inclined part of the bridge draws

31,815 00 Coffer dams

63,000 00

544,190 00

27,109 50

Contingencies, at 5 per cent.

569,299 50

Total substructure, masonry, draws, and railing
Superstructure of wood, 4,303 running feet, at

$3100 per foot, Mr. McCord's estimate $130,293 00 Contingencies, at 5 per cent.

6,514 65

136,807 65

Total, 706,107 15

JAMES KEARNEY,

Lt. Col. and T. E.

B. Estimate of the cost of a Bridge, substructure, &c., as before, the horizon

tal part of the superstructure to be cast iron arches. Substructure, &c., brought forward.

• $569,299 50 Superstructure37 cast iron arches of 100 feet span, 175 tons of

iron each, 6,475 tons, at $67 per ton $433,825 00 Workmanship, at $33 per ton

213,675 00

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CITY OF WASHINGTON, September 1st, 1838. Srr: Respecting the repairs which I have recommended to be made upte the old Potomac bridge, I have the honor now to state more fully the rear which I have taken of the subject.

It cannot be supposed that, over such a river as the Potomac, a work of the extent of that now contemplated, can be erected without frequent and serious embarrassment and interruption, unless we have more certain means of preserving the communication, and of transporting materials, than that afforded by the river; for, at the position of the bridge, it is nearly a mile wide-frequently frozen across—and, during the spring floods, carries with it great quantities of floating ice. Nor is it during the winter alone that it is impassable; for, at very low tides, there is not more than one or two feet

depth of water over the shoals, and, where the wind has blown strongly from . the northwest, it becomes in some places nearly bare.

Now, the only question is, whether we should at once put the remains of the old bridge in repair for this purpose, or construct, as the work progresses, the necessary scaffolding and gangways for the accommodation of the work?

The strength and cost of bridges of accommodation depend evidently not only upon the purposes for which they are designed, but upor, the character of the stream. In a quiet current, or in still water, the supports need not exceed the strength necessary to sustain the floorings and weights to be transported upon them; but evidently we cannot hazard as slight a structure in a more rapid stream, which, at certain seasons, carries heavy masses of ice, and is subject to floods. The strength of piling and bracing must therefore conform to that force, rather than to the weight of transportation; and hence there is a limit of breadth of piling below which we cannot, in establishing the proposed work, with prudence, venture; and it is by this view of the subject that I purpose to be giided, limiting, on the one side, the expenditure by the strictest economy, and, on the other, securing the safety of the work, by extending the breadth and strength of its supports no farther than necessity may require. Whether it be not more economical to use so much of the old bridge for our purpose as yet remains, rather than to construct anew the necessary gangways, appears to me therefore scarcely susceprible of being doubted.

It is proper also to acquaint you that there can exist no motive of economy forbidding this course, as connected with the existence of the old abutments, for the old foundations are utterly incapable of sustaining a structure of any magnitude; and, as to that part of them which is above ground, to fulfil the provisions of the law we must overwhelm them, were they even upon the same ground with the proposed abutments—the surface of the road being now but about six feet, whereas that of the new one must necessarily be from eighteen to twenty two feet above low water. But it may be objected that the old bridge and the one it is now proposed to erect are upon the same ground, and therefore that they must necessarily interfere with each other. Now, although the centre of the existing gateway is, in fact, nearly upon the centre of Maryland avenue, it stands also within Fourteenth street, and the abutment walls, extending thence to the line of wharves which constitute the water boundary of the city, completely block up the avenue, Fourteenth street, and Water street, with the exception of about forty feet of the breadth of Fourteenth street.

Besides which, the direction of the centre line of the bridge being oblique to the direction of the avenue, the termination, on the

water side of the face of the abutment, is placed at the lower side, instead of : being in the middle of the avenue. Now, it is clear that we should not per

petuate this obstruction to the public highways, and I have also respectfully to say that it seems to me equally clear that the face of the new ebutment ought to be nearly in line with the centre of the avenue, and that position would place it so far clear of the old work as to leave abundant and convenient room between the two for all purposes of construction. The old abutments are mere causeways pushed from time to time in advance of the an. cient shores of the river on the Washington side, to conform them to the progress of the improvements of the city, and on the Virginia side, for what purpose I know not. They are merely rivetted by dry rubble walling, to keep the earth in its place. They have also slight parapets of brick-work resting on the rivetments. That on the Washington side of the river may be usefully brought into the line of whartage by which it is proposed to control the direction of the current passing under the archways; and to no other useful purpose can it, as I conceive, be applied in relation to the bridge. The abutment on the Virginia side of the river is an incumbrance to the stream, and its position is equally pernicious to the navigation, and hazardous to its own existence, part of it being already ruined by the ice, which it had intercepied. The whole of this abutment will require to be removed, that the face of that to be erected for the contemplated work may occupy its proper position near the shore.

If, therefore, these works are misplaced and useless, as portions of a permanent bridge of the dimensions contemplated by the act of Congress, there is no reason why they may not provisionally be employed to some valuable purpose. I need scarcely occupy your time in showing also that the remains of the old wood-work, as well as the heaps of stone which have been thrown into the river upon the shoals, not only cannot constitute any pari of the proposed structure, but, as incumbrances and impediments to its prosecution, it would become indispensably necessary to remove them, as the work progressed, did the axis of the two routes coincide. Situated as the old and new works would be, in reference to their proper relative positions, the remains of the former become usefully available for construction, and enable us to economize in scaffolding and gangways, while we avoid the necessity of any expenditures for removing old snag piles from below the water line, or of stone which has buried itself under the mud—these operations being indispensable, wherever they fall within the area of operations, in founding stone piers.

It is for these reasons that I recommend that the remains of the old work may be converted into a bridge of accommodation for the service of the workmen and for the transportation of materials; and, in suggesting to you the propriety and economy of such an appendage, I am warranted not only by my own view of the subject, but by the practice in all similar works of any any considerable extent.

presume this work may be more advantageously undertaken now than hereafter; because the security and existence of that portion of the ancient bridge which remains, require that much of it should be repaired before the ice and floods of the ensuing winter shall attack it.

I have also respectfully to suggest, that this work, being in hand during the period for maturing the plans for the proposed structure, will not be so liable to interfere with the main operations as if it were postponed to a luture day.

The old Bridge Company had done something last season towards tempo rary repairs. Their intention was, over nearly the greater part of the shua's to make every alternate section (viz. every other iwenty-five fee:) a soud dry wall, and to bridge between them. Part of that design has been es ecuted or commenced, and however its propriety may have been questioned, we cannot now do otherwise than to retain the stone sections which are above the water mark; unless, indeed, we should resolve to incur the ex. pense of removing them. It is true, that ultimately this stone must be taken up, but it can be of little or no other use towards the proposed structure than to fill around the piers, with the view of defending the foundations against the action of the current, which will always be found at the shoulders of the piers.

For this purpose, the stone which the company has deposited in the line of the old bridge is very suitable, and it is therefore only that I would propose suffering it to continue in its present situation; and because, also, we will thus save the expense of transporting it to the shores, and thence back to the work.

The evil of obstructing so great an extent of the water line, would influence me to recommend, also, that no more stone shculd be brought on the ground for these sections, unless so far indeed as it may become absolutely and indispensably necessary, by reason of the difficulties that may present themselves, in'fixing firmly amongst them the necessary supports for a wooden

Granting the correctness of the views which I have the honor of presenting to your consideration in relation to this matter, there can remain, I should suppose, no question of the economy of using the remains of the old bridge, in preference to the employment, altogether, of new materials.

If we except the sections which have been wholly, or nearly carried away, we find that the material which will be mostly required will be the flooring joist, string-pieces, and iron; and that the greatest amount of labor will be in taking up the old caps and purlins, and readjusting them, and in splicing on short piles to the remains of the old ones.

The greatest extent of damage is over the channels: over the shoals but little new timber will be required for the under work.

I have the honor to be,
Sir, your obedient servant,

JAMES KEARNEY, Lt. Col. and T. E. To the Hon. Louis McLANE,

Secretary of the Treasury.

road-way.

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