4

• THE NEW OMAHA BRIDGE.

first time. On the same day Mr. Duryea turned the bridge over toMr. V. G. Bogue, Chief Engineer of the Union Pacific Railway ; andmy connection with the structure ceased. As a preliminary to thistransfer, on the third day of November, 1887, I addressed a letter toMr. T. J. Potter, showing in detail what remained to be done. Thisletter may be found in Appendix G.

II.

GENERAL DESCRIPTION.

The new Omaha Bridge was designed to accommodate both rail-road and highway traffic. It is a double-track railroad bridge; thetwo tracks being placed 12 feet between centers, and the clearancebetween trusses being 26 feet. The highways are carried on cantileverarms projecting outside the trusses.

The new bridge is really a reconstruction of the old bridge, thoughvery little of the old bridge remains. The same approaches are used,and the location is entirely unchanged, while the dimensions of the oldstructure fixed the dimensions of the new structure.

The old bridge was 2,750 feet long, but for several years little orno water had flowed through the four easterly spans. A clear widthof 1,000 feet affords ample water-way on this section of the Missouri .In many places where the channel has remained stable for a long seriesof years, the river has reduced its width between high-water banks toless than this. It was farther decided to limit the water to the spacebetween Piers I. and VI., or to a width of 1,250 feet, the additional 250feet more than balancing the obstruction offered by the old piers andthe piles of riprap around them. The length of the new bridge is,however, 1,750 feet; the 250 feet west of Pier I. and that between PiersVI. and VII. being occupied by an iron structure, though the floodwater is entirely excluded therefrom.

The cylinders of the old piers are 8-| feet in diameter, having across-section of about 57 square feet; the weight of the superstructurebeing carried directly on the masonry filling and not borne by the ironshell. The maximum weight of superstructure and moving load whicheach of these cylinders was required to bear was at least 300 tons, pro-ducing a pressure of about 75 pounds per square inch on top of the

cylinders, and of over 200 pounds at the base. These pressures wereconsidered as large as it was wise to use on this kind of work.

The old piers had been protected by heavy piles of riprap aroundthem, which 'made it seem expedient to build the new piers as far fromthe old piers as possible, where the obstructions to be encountered wouldbe a minimum. This fixed the location of the new piers half-way be-tween the old piers. With the possible water-way of the new bridgelimited to the space between Piers I. and VI., and with the location ofthe new piers fixed midway between the location of the old piers, twoarrangements were possible: three new piers and two spans of 500 feeteach, or five new piers and four spans of 250 feet each. If you hadbeen building an ordinary single-track railroad bridge the former arrange-ment would have been preferable; but for the heavy double-track struc-ture which has been erected, the latter arrangement was found a littlemore economical, besides giving very much better facilities for handlingthe work independently of stages of water, the short spans of less than125 feet between the old and new piers making it possible to raisewithout danger at all times.

The structure as now built extends from the old masonry pier toPier VI., — a distance of 1,750 feet. The easterly and westerly spans ofthis length were subdivided by building two new piers. As these piersare outside the limits within which the flow of the river is confined, itwas not thought necessary to go to the expense of putting in deepfoundations, and they are founded on concrete at shallow depths. Thebridge, therefore, consists of four through spans measuring 1,000 feetbetween centers of piers, and at each end three deck spans measuring375 feet between centers of piers, — making the total length, as alreadystated, 1,750 feet.

The east approach is the old approach, which your company havewidened for a double track; and the embankment when completed —which I strongly advise be done as soon as possible — will extend toand surround the old Pier VII. The west approach is the old approachwidened. Both approaches are on the same tangent as the bridge, andthe grade is level from the west end of the west approach to Pier VII.,— the east end of the east approach, — from which point a uniformgrade of T 6 T of one per cent descends towards Council Bluffs .

The river is now in quite a stable condition, the only danger com-ing from the erosion of the east bank, three or four miles above, whichtends gradually to throw the bend above the city further down stream.

The vested interests in Council Bluffs and elsewhere will, however, prob-ably prevent this erosion continuing to a dangerous extent. The westshore is thoroughly protected from the smelting works to the bridge line.The general direction of this shore makes an angle of about no° withthe bridge line. It would be very desirable to have this shore line forcedforward until it be at right angles with the bridge. The expense ofdoing this would be pretty large, but it would reclaim from the river anamount of very valuable land worth several times the cost of the work.I fear, however, that this contemplated scheme has since been blockedby the construction of the highway bridge at the foot of DouglasStreet.

The rectification of the river here is a matter of large importance,not only as regards the bridge, but as regards many other interests. Asat other places on the Missouri , the bottom-land slopes lightly awayfrom the river. At the place where the river has been cutting aboveOmaha , the bottom-land is lower than it is at the bridge site, and thesurface of the water is a few feet higher. The danger of overflow tothe lower portion of Council Bluffs comes from this place, as a rise ofwater which would be perfectly harmless at the bridge line would makean overflow three or four feet deep at this place. A location was madefor a line of levee from Pier VI. to the east bluff above Council Bluffs .This levee is the only sure way of protecting the lower portion of Coun-cil Bluffs from the overflow, and it would seem that the united interestsof the railroad and the property owners ought to build it.

During the conduct of the work all levels were referred to the da-tum used during the construction of the old bridge, and this datum wasthe assumed low water, which, however, is considerably below any lowwater which has actually been observed, and six feet lower than the lowwater recently established by the Missouri River Commission. To avoidthe use of negatives, this datum was called 100. Therefore, in theelevations used in this report, 100 corresponds to o in the publishedreports of stages of water, that zero being according to the levels of theMissouri River Commission, 545.61 feet above the St. Louis city direct-rix, which is 413.05 feet above mean tide in the Gulf of Mexico . Torefer the levels of this report to tide-water, it will be necessary to add858.66.