Page:Final Report - The Columbia River Interstate Bridge.pdf/40

 required for sinking these piles, an equipment which proved to be adequate and which accomplished the work, so that the piers as they stand are on piles which on the average extend into the river bed 25 ft. deeper than the bottoms of the piers of the railroad bridge adjacent. The conditions of the Columbia River and of the Oregon Slough were deemed to be particularly f avor— able to the erection of practically the entire bridge by building the spans on shore and ﬂoating them to position. The plans were made with this in view and the speciﬁcations indicated a preference for this method of construction, although the ordinary method of driving piles and building a temporary trestle on which to erect the steel work was not debarred. Diﬁ'erent con- tractors bid upon different methods, but the bid taking advan- tage of the opportunity for ﬂoating the spans into place was $50,000 lower than the lowest bid for erecting on falsework. The exceptional opportunities for constructing the approaches by building permanent embankments by the hydraulic ﬁlling process also seemed to your Engineers to ﬁx this as the proper design of structure, for there were abundant supplies of sand in Oregon Slough; cheap electric power, little difﬁculty with dis- posal of excess water, and cheap lumber for bulkheads; and the speciﬁcations so provided for dredging, although no limitation was made on the construction of the embankments by steam shovel and train haul, or other methods. Even though the length of embankment was beyond any precedent for this kind of work that your Engineers could ﬁnd, the correctness of this decision was also veriﬁed when the bids for placing the work by hydraulic dredging were found to be less than two-thirds of the lowest bid contemplating train haul.

In the preparation of the plans, minute care was taken to secure every possible advantage which a duplication of items would aﬂ'ord. In the steel work there are, over the Columbia River, only two different truss spans, and the ﬂoor systems even of these are duplicated. Over the Oregon Slough there are like— wise two patterns of longitudinal girders with one pattern of ﬂoor system, and over the Columbia Slough the girders are all duplicates and the floor system is the same as on the Oregon Slough. It was, therefore, possible for the structural shops to make only a few sets of patterns and to duplicate the parts many times, and this contributed very materially to the low price for manufactured steel. The same duplication was followed as far as possible in the design of the substructure so that the coli'er- dams and forms for the concrete could be moved from one pier to another and used repeatedly. The concrete ﬂoor was designed

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