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The vertical posts which carry the load from the bridge floor to the truss will bave 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 tbree 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 recommendation 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 aujout 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 leve). 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.
In an ordinary quadrangular truss, these strains are taken up by the chord at each panel point and transmitted directly to the points of support, 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 objection 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 pot 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.
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 Corrugate i 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 (81,500) 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.
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.
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 mas nry 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 bave 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: Masonry
15, 000 Approaches .
6,000 Superstructure.. Contingencies.
201, 000 140,000
Balance .. The cost of a bridge with the same class of masonry as now proposed would be as follows: Masonry
15, 000 Approaches
14,000 Total .....
188,500 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