Electric Railway Bridge Over The Clackamas River Near Portland, Ore.
Street Railway Journal, Vol. XXI. No. 19, May 9, 1908. Pg 790
Last March a bridge over the Clackamas River was completed for the Oregon City line of the Portland Railway, Light & Power Company of Portland, Ore. This bridge is a 260-ft. span steel structure of ten 26-ft. panels with Warren riveted trusses. It replaced a combination bridge of the same span erected in 1892 which had become unequal to the increased loads of the rolling stock passing over it. The lower chord, made of bolted timbers, was overstrained and in 1906 it was decided to relieve this by driving piles in groups of four at each panel point to form a trestle which would support a part of the loads until the bridge could be replaced by a steel structure.
The first plan for the new bridge called for two 130-ft. spans with a pier at the center of the river supported on piles. Preliminary plans were drawn, but when borings were made at the center of the river it was found that there was a gravel formation as far as the boring proceeded or at least 75 ft. It was then decided to leave out the pier in the center of the river the make the bridge a single span of 260 ft.
The bridge does not accommodate highway traffic or pedestrians, being used exclusively by the Portland Railway, Light & Power Company for electric traffic, including both freight and passenger cars. The carloads having greatly increased in the past few years, it was decided to design the new bridge for a moving load of 4000 lb. per foot of bridge with an additional excess load of 30,000 lb. for one panel. This loading is as heavy as that of many steam railroad bridges and should satisfy the electric traffic for many years to come.
The location of the bridge on the Clackamas River made the erection difficult, as this stream is subject to very sudden rises of from 10 ft. to 12 ft. During such a rise the current is very rapid and the drift of logs, timber and trees quite considerable.
The bridge is very near the mouth of the Clackamas River, where it joins the Willamette River and consequently it gets the full effects of such sudden freshets. However, when the river reaches its highest point, which is about 30 ft. above low water, there is little or no current, as the water is backed up from the Willamette River. After the pile trestle was put in a period of high water carried out two bents of piles, but did no other damage.
The contract for the new bridge was signed Oct. 9, 1907, and in the latter part of the same month preparations for erection were made by driving three more piles in each bent, one on the downstream side and two on the upstream side, and diagonal bracing was put in. The combination span was taken down with the exception of the bottom chord, which was left to make a runway for the traveler.
The original piers for the combination span were 3-ft. 10-in. diameter steel tubes which ran down into hard gravel. These were thought to be sufficiently strong for the new bridge and the increased loads, but as an additional precaution new steel bracing was put in and the tubes surrounded by concrete in a rectangular form for a height of 6 ft. from the bottom of the tubes and a width of 9 ft. and a length of about 26 ft., which completely surrounds the tubes; above this the concrete is extended from 9 ft. with each tube surrounded by concrete in the form of an octagon, and the octagons, whose widths across are 9 ft., are connected by a web 2 ft. thick in which old rails were put diagonally to make reinforced concrete bracing.
The bridge was swung Jan. 9 with a drop of ¾ in. from the camber of 5 ½ in. when the falsework was in, to a camber of 4 ¾ in. when the bridge was swung. The erection took place during a bad time of the year, as the winter rains had commenced and everything was so wet the erection did not go on as rapidly as was expected; each riveting crew of four men arranged about 175 rivets a day. The members were very accurately laid out in the shop, everything fitted with great precision and the holes matched so that almost no reaming was done in the field.
The unusual points about the bridge are the length of span for a riveted connection bridge of heavy loading and the fact that it is entirely a Western product, as the bridge was designed by J. B. C. Lockwood, of Portland Ore., and fabricated in Portland by the Northwest Bridge Works, formerly J. R. Bowles. The erection was done by Robert W. Wakefield without any interruption of traffic.