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Abstract of proposals for free bridge across Potomas River about 500 feet above Aqueduct Bridge, opened Saturday, October 29, 1881, at 12 m.

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Charles H. Bliss & Co* Washington, D. C. Similar and equal to Ana- Not stated.

costia Bridge.

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Abstract of proposals for free bridge across the Potomac River at the "Three Sisters," opened Saturday, October 29, 1881, at 12 m.

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$183, 600

73 pounds average.

12,500 to 10,000 to 8,000 20 16,000 13,400

5 $36,000 $100,000 $4,000

$140,000

† Not stated.

* Proposals not accompanied by bond or detailed specifications, and plans informal. Proposals not accompanied by bond or plans, and specifications informal.

for approaches.

Shearing.

Roadway.

2 King Iron Bridge and Manufacturing Company.§

iron parallel chord, Pratt truss, spans 100 to 300 feet. Deck bridge.

60 to 80 pounds.

12,500

10, 000

9,000 25

5

102, 000

3

Chas. H. Bliss & Coll

Washington, D. C

Similar and equal to Anacostia Bridge.

Not stated

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Same as Aqueduct Bridge. § No bid for piers and abutments nor

145, 400

[PUBLIC-NO. 38.]

AN ACT to authorize the construction of a bridge across the Potomac River at or near Georgetown in the District of Columbia, and for other purposes.

Be it enacted by the Senate and House of Representatives of the United States of America in Congress assembled, That the Secretary of War be, and he is hereby, anthorized and directed to cause to be constructed across the Potomac River, at or near Georgetown in the District of Columbia, at such point as he may select, a substantial iron and masonry bridge, with approaches; and the sum of one hundred and forty thousand dol lars be, and the same is hereby, appropriated, out of any money in the Treasury not otherwise appropriated, for the construction of said bridge and approaches, the same to be maintained as a free bridge for travel.

[1] Provided, That the said Secretary of War shall construct a bridge upon such plan as shall cost no more than the amount herein appropriated, and which cost shall include the construction of a substantial bridge over the canal, and any and all approaches to the said iron bridge; and no part of this appropriation shall be paid out of the Treasury until contracts shall have been entered into with responsible parties, and with good and sufficient sureties to be approved by the Secretary of War, for the construction and completion of said bridge, including the masonry, iron-work, and approaches, at a cost not to exceed one hundred and forty thousand dollars.

[2] And provided also, That a draw of sufficient width to permit the free passage of vessels navigating that part of the Potomac River shall be constructed in said bridge unless said bridge shall be constructed upon or by the side of, or up the river from, the present aqueduct and at the same or greater elevation above the water.

[3] And provided also, That the sum which may be expended under this act shall be treated and regarded as a part of the general expenses of the District of Columbia, and the United States shall be credited with the amount which it may pay under this act for the erection of said bridge upon its fifty per centum of the expenses of the District of Columbia, as provided in the act of June eleventh, eighteen hundred and seventy-eight, entitled "An act providing a permanent form of government for the District of Columbia."

[4] Provided further, That the Secretary of War shall, as soon as may be, fix and determine the location of said bridge, and cause a survey of the river to be made at such place or location, determine the length, width, and height of said bridge, and the length of draw, if one is required, and thereupon advertise for plans and price for the construction of such bridge; such advertisement to be inserted in one or more daily newspapers published in Washington, District of Columbia, New York, Cleve land, Ohio, Detroit, Michigan, Philadelphia, Pennsylvania, and Chicago, Illinois, for the space of one week.

SEC. 2. That for the purpose of establishing a free bridge, and in lieu of erecting the bridge provided for in the preceding section, the Secretary of War may, in his discretion, purchase the aqueduct bridge now crossing the Potomac River at Georgetown [5] Provided, said bridge with all the appurtenances, rights and franchises connected therewith including piers and real estate for abutments and approaches can be purchased for a sum not exceeding eighty-five thousand dollars; which sum or so much thereof as may be necessary may be paid out of the money appropriated by this

act.

[6] Provided further, That a good and sufficient title thereto can be secured to the United States, to be approved by the Attorney-General of the United States.

[7] It is further provided, That the Alexandria Canal Company or its present lessees shall have the right to maintain at their own cost and expense, a canal aqueduct of the same width and depth as the one now in use, and to attach it to or suspend it from said bridge; and whenever a permanent bridge shall be erected upon said site, the same shall be of sufficient strength to sustain the weight of such canal aqueduct; but the construction attachment and maintenance of such aqueduct shall be such as the Secretary of War may determine and shall be without cost or liability to the United States or the District of Columbia.

[8] And it is further provided, That if upon the erection of such permanent bridge the said canal company or their present lessees shall neglect or refuse to reconstruct secure and attach the said aqueduct at their own expense, or if at any time for the space of six months, they shall fail to use such aqueduct for the purposes of a canal, or fail to keep the same in good condition and repair, or if at any time, they shall use the same for other than canal purposes, then all rights of said canal company, its lessees or assigns in said bridge and property, shall cease and determine, and the said aqueduct shall be detached and removed by the Secretary of War.

SEC. 3. And the Secretary of War is further authorized, in his discretion, in the event of said purchase, to repair the wooden bridge nowon said piers, and for that purpose is authorized to expend, of the moneys hereinbefore appropriated, a sum not exceed ing ten thousand dollars.

Approved February 23, 1881.

REPORT OF MR. S. T. ABERT, UNITED STATES CIVIL ENGINEER, OF FEBRUARY 27, 1882.

UNITED STATES ENGINEER OFFICE, Washington, D. C., February 27, 1882. GENERAL: I have the honor to submit herewith the following report upon the modified plans of the Corrugated Metal Company for the construction of the free bridge across the Potomac River at the Three Sisters, near Georgetown, D. C., referred to me by your indorsement of December 20, 1881:

The preparation of this report was unavoidably delayed by reason of the fact that at the time of the receipt of the papers relating to this subject I was engaged in the completion of my report upon the proposed improvement of the harbors of Washington and Georgetown, D. C., which was needed for immediate consideration. As soon as this report was completed the subject of the bridge was taken up. As several modifications of the original plans and specifications had been made, it seemed to be essential, in order to avoid any misunderstanding in the case of the award of a contract, to prepare a complete specification for substructure and superstructure, embodying these various modifications and other provisions which had been omitted, but which were required. to make the specifications complete.

These specifications (a copy of which is herewith inclosed), inclosure A were sent to the Corrugated Metal Company on January 31, with the request that they would state whether they would accept them as a basis for a contract, but owing to the illness of Mr. Jarvis, the chief engineer of the company, no reply was received until February 13. The reply made by Mr. W. O. Douglas, agent, is herewith inclosed-Inclosure B. Referring in detail to the various propositions of the Corrugated Metal Company, I would report as follows:

1. Foundations. In the letter addressed to the Chief of Engineers by the Corrugated Metal Company on December 19 they call attention to the crib foundation shown in Trautwine's Handbook, and ask that they be allowed to construct the piers of the bridge on such foundations. On the 27th of December I addressed a letter to the company inquiring whether this was intended as a substitute for the foundations built by coffer-dam on solid rock. The agent of the company, Mr. Douglas, replied that in case the crib foundations were not accepted the company would still adhere to the original plan of foundations of masonry built in a coffer-dam and starting at the solid rock.

The crib foundations would manifestly be inadequate to give the proper strength and security to the structure, and I would respectfully recommend that this proposition be declined and that the foundations. be built by means of a coffer-dam, the masonry starting from the sol d rock as originally proposed, and as provided in the new specifications inclosed.

2. Strains on the iron.-The unit strains on the iron proposed by the Corrugated Metal Company are as follows:

For tensile strains, 10,000 pounds per square inch.
For shearing strains, 7,500 pounds per square inch.

of

For compressive strains as per table on page 124, volume IV, Transactions of American Society of Civil Engineers. An addition should, however, be made to the table of compressive strains, in order to make it complete, as in the report, as follows:

When one end is square and the other end rounded, a mean is to be taken between the two.

The vertical posts which carry the load from the bridge floor to the truss will have one end square and one end round (pin), and a provis ion of this kind will therefore be necessary. This table, however, includes compressives strains of 10,000 pounds per square inch, exceeding the limit of 8,000 pounds fixed by your letter to the company of December 2, 1881. As the Corrugated Metal Company had your letter before them while preparing their revised specifications of December 17, it is fair to assume that the compressive strains of the committee of the Society of Civil Engineers represent the limit to which they are willing to go in the matter of compressive strains, and they therefore fail to conform to your requirements in this particular.

3. Capacity of the bridge. The capacity of the bridge proposed by the Corrugated Metal Company is 75 pounds per square foot over the roadway and footwalks. This load of 75 pounds per square foot conforms to the requirements named by a majority of the committee of the American Society of Civil Engineers in the report already referred to, although three of the members of that committee recommended a load of 80 pounds per square foot for spans of 187 feet. While a capacity of 75 pounds per square foot would perhaps be sufficient to support a crowd of people on the bridge, it is equally important to provide for the effect of shocks and dynamic loads as well as for the element of uncertainty due to the defects of the metal. The bridge when fully loaded may be subjected to a sudden and severe strain by the rapid movement of a crowd of people upon it, or by a high gale of wind; or the movement of cattle, or troops, or heavy loads may induce shocks and dynamic strains. It is true that when the bridge is new and all the members in proper bearing these strains will be but little felt, but in the course of a few years under the influence of wear and successive loading, it will be found that these shocks have a most injurious effect upon the life and stability of the structure.

The allowance which should be made for the working load of highway bridges has been variously estimated. Rankine states the load of a dense crowd of people at 120 pounds per square foot; Stoney names from 145 to 149 pounds as the weight of a dense crowd of men, but adopts 100 pounds per square foot as the standard load for public bridges.

It seems proper to state in this connection that the loads and strains recommended by the committee of the Society of Civil Engineers were not adopted by the society and must be considered only as the recom mendation of the engineers who sign the report. The recent fall of the bridge over the Missouri River at Saint Charles, built under the direction of one of the very able engineers who sign this report, together with the fact that during the year 1881 there were no less than fortythree bridge "accidents" in the United States, would seem to indicate that the conditions which enter into the strength and durability of bridges have not been sufficiently provided for, and that the standard generally adopted is too low.

The Corrugated Metal Company proposed a width of roadway of 20 feet and two sidewalks of 5 feet each.

The Little Falls Bridge has no sidewalks, and at the site of the Three Sisters, I think that one sidewalk would be ample for the accommodation of all foot travel likely to come on the bridge. By omitting one sidewalk and thus reducing the total width to 25 feet, retaining at the same time the load of 2,250 pounds per linear foot of bridge for the 187foot spans, there would result an increase of working strength of 20 per cent., or a load per square foot of 90 pounds, which would provide to some extent, at least, for dynamic strains. In the event of the construc

tion of the bridge, I would recommend that the width be reduced to 25 feet.

Strain diagrams.-The strains shown on the strain sheets submitted by the Corrugated Metal Company were obtained by the graphical method. They have been calculated in this office by the analytical method given "Shreve on Bridges and Roofs" with somewhat different results. The principal difference is in the chord strains, which are at a maximum when the bridge is fully loaded. The strain sheets show these strains as decreasing from the center to the ends of the truss while the formulæ of Shreve give strains which increase from the center to the ends of the truss. I also make the length of the lower wind truss less than shown on the strain sheets. By reason of the illness of the chief engineer of the Corrugated Metal Company, these and other differences have not yet been explained.

The company, however, waive any errors in strain calculations, and are willing to make any changes which may be required, agreeing to the following section of the specifications, on page 27:

In case the sizes of iron or members herein specified or stated on the strain sheets shall be found by the engineer to be insufficient to meet the strains arising from the specified loads, the sizes of such iron or member shall be increased so as to meet the required strain per square inch.

This clause was inserted in the specifications before the completion of the calculations, it being assumed that, in case of change, the proper sizes could be inserted in the specifications for a contract.

Width of Piers.-The width of piers specified in your letter to the Corrugated Metal Company of December 2, is as follows:

Eight feet for 300 feet spans and 7 feet for smaller spans.

As the greatest span on the modified plans is 187 feet, the width of piers at the top should be 7 feet. The Corrugated Metal Company, in their letter of February 13 (inclosure B), adhere to the original width of piers proposed by them, 6 feet, but in a postscript to their letter they give the width of piers at 7 feet, measured on top of coping. As the coping would project about 6 inches on either side, this would give an available width of only about 6 feet. I understand, however, from their later letter of February 16 (inclosure C), addressed to the Chief of Engineers, that they are willing to subscribe to the width named in the specifications of seven feet.

Height of Piers.-The original plans of the bridge submitted by the Corrugated Metal Company with their bid (3556, inclosure 4, River and Harbor, 1881), showed the masonry carried up to the height of the center of the trusses. Subsequently, however, the company modified their plans so as to provide for a pier built of masonry only to the level of 27 feet above low-water, or about the level of the bottom of the truss. Above this level the truss rests on iron posts, these being introduced to save the expense of carrying the masonry up the proper level. At first the plans showed a vertical end post of iron without any inclined braces, but, as this was shown to be insufficient, the inclined brace was subsesequently introduced. These end posts (if of sufficient strength) will support the weight of the bridge, fully loaded, but the objection to the end posts arises from the longitudinal strains which come upon the floor system of the bridge from the movement of loads, as, for instance, heavy teams, the marching of troops, or from a drove of cattle. A high wind blowing nearly in the direction of the bridge will also induce severe longitudinal strains.

These strains cannot be precisely determined as to amount, but they will have a very serious effect if the bridge is not fully braced to resist them.

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