By F. A. Talbot
ONE of the most perplexing situations in railway building is the necessity to cross a busy waterway at a low level, owing to the physical characteristics of the banks on either side.
In order that no serious restriction may be imposed upon navigation, it is imperative that the bridge shall be provided with some means of opening so as to permit vessels to pass up and down easily. When this handicap is associated with a busy city, spreading over the opposite banks, the problem becomes aggravated, as the engineer is cramped for space in which to achieve his object.
Many ingenious methods have been elaborated to meet such conditions. There is the swing bridge, in which a span of the bridge is able to swing round in a semicircle upon a pivotal pier, thereby providing two channels—one on either side of the support. Another expedient, which has come into vogue extensively during recent years, is the bascule bridge, in which the moving section of the structure, being hinged at one end, is raised and lowered like a drawbridge. A third method, which, however, is not employed very freely, is the vertical lift bridge, wherein the span is raised bodily in conjunction with counterweights, between two supporting towers, this system being similar in its principle and operation to the sash-line window.
Recently a new and ingenious development in connection with the last named method has been introduced. It is the patent of two American engineers, Waddell and Harrington, and it possesses many interesting features. The Oregon-Washington Railroad and Navigation Company desired a new entrance into the city of Portland, Oregon. The Willamette, which divides the city in twain, is a broad, deep river, enabling large vessels to reach this point from the sea.
When the railway first entered the city it built a large steel bridge with a swing span, which it was concluded would meet all the requirements of navigation and yet at the same time would not hinder railway operations. But the trade of the port has increased amazingly during recent years, with the result that the swing span was constantly having to be opened and closed. On the yearly average this occurred 70 times a day—practically once every 20 minutes—while in one interval of 24 hours it has been opened as many as 134 times.
Such frequent manipulations handicapped the railway traffic very seriously, so the company decided to build a new bridge across the waterway, 700 feet above the original swing bridge. This decision was seized as an opportunity to improve the vehicular and pedestrian traffic between the two banks, so a double deck bridge was adopted, the lower deck being for the railway, and the upper level for public use. But the question of protecting the navigation interests arose. The various methods of operating a moving span economically and expeditiously were investigated at length. In this case the problem was complicated by the top deck used by the public, which it was essential should be kept open as much as possible so that inconvenience to vehicular and pedestrian intercommunication might be reduced to the minimum. Obviously, both the swing and bascule systems had to be ruled out of court, because it was impossible to place the upper deck at such a level as to be beyond interruption at intervals. It was a peculiar problem which demanded a special solution. The railway engineers thereupon investigated the Waddell and Harrington invention, and finding that it met the situation very completely, it was adopted forthwith.
The river channel is crossed by three spans—a fixed span 287 feet in length on either side, flared at the shore ends to provide for the necessary curvature of the railway lines, and a centre movable span 220 feet long. The public deck of the bridge being nearly 50 feet above the level of the railway, suitable approaches at the ends had to be provided to secure an easy grade. That at the east end is by means of a viaduct 305 feet in length; on the west bank there is a highway approach 512 feet long. Thus the over-all length of the structure is 794 feet for the railway, and 1,611 feet for the public use.
The moving span, which rises vertically between two towers by means of counterweights, is telescopic. That is to say, the moving span is so built that the lower deck may be raised independently of the upper deck. This is a distinct advantage, inasmuch as this vertical travel amounts to 46 feet. Accordingly, when the railway deck is raised until it touches the floor of the upper deck, a clearance of 72 feet above low water is secured. This is sufficient headroom for the greater proportion of the river traffic, so that there is no necessity to disturb the public highway. When, however, a large vessel with tall masts approaches, demanding a greater clearance, the bottom deck first is telescoped, and then, together with the higher level of the span, is lifted until the whole moving part comes to rest near the top of the towers, whereby a clearance of 165 feet at low water is given. By this provision all but the very largest craft coming to Portland are able to proceed through the bridge.
The independent movement of the bottom deck is accomplished by supporting the railway floor system and lateral truss on hanger posts from the upper movable span. Each hanger is connected to four cables, and is designed to move vertically inside the corresponding vertical post of the upper movable span. Both railway and public decks are counterbalanced with concrete weights. The lower moving section has eight of these counterweights, disposed four on each side, each section of four having an aggregate weight of 212 tons. The upper deck has two similar weights, each weighing 866 tons.
The river piers were sunk by the open caisson method. These were landed on cement gravel at a depth of 123 feet below low water. The six dredging wells were excavated to a depth of 10 feet below the cutting edge of the caisson, and the concrete was deposited under water by means of bottom dump buckets. In the completion of the sub-structure work some 30,000 cubic yards of concrete were used.
The steel viaduct carrying the highway approach on either side was erected by means of a 30-ton skid derrick, having a boom 45 feet in length. This plant also set the upper floor system and laterals for the fixed spans, as well as the lower members of the two towers, the heaviest single piece which it was called upon to handle weighing 29 tons, represented by a beam for the upper deck. The two fixed spans were erected by the aid of timber false-work, which was built upon the double bent system with 22 piles. For the raising and placing of the truss members and the lower floor a barge, or scow, derrick was devised. This had sheer legs 110 feet in length, fashioned from four heavy piles. By means of this apparatus a chord section weighing as much as 54 tons was set in position.
The building of the vertical lift span, and the movable railway deck, had to be carried out in such a manner that no interference was offered to the river traffic, and also in such a way as to be as independent as possible of the level of the water in the river. A clearance of 116 feet above low water was considered to be sufficient for the purpose. Four wooden cantilever brackets were put together on barges, and by means of the floating sheer-leg derrick were hoisted in position on the adjacent ends of the fixed spans. The bases of these brackets rested on the piers, while the tops were anchored to the fixed spans by means of iron rods 2-1/2 inches in diameter, four rods being used for each bracket. The two skid derricks then were rigged with booms 110 feet in length, and by their aid four Howe trusses, measuring 120 feet long, and weighing 40 tons each, were lifted and set upon the timber brackets. The iron required for this false-work was fashioned from the scrap of old spans removed from the railway company's line several years previously.
In the erection of the two towers carrying the counterweights, as well as the sheaves, a gallows-frame, 152 feet in height, was built and lifted into position on top of the Howe truss false-work. This was anchored back to the truss and hinged at the base, so as to obtain sufficient rotation to set any members of the towers into the requisite position. The heaviest lift which this gallows-frame effected was the section of a tower post weighing 42 tons. The steel used in the erection of the towers was loaded on to cars which were run out to the site over the railway deck. They were hoisted through a hole in the upper deck, this having been obtained by leaving out the stringers of two panels of the latter, sufficient clearance thus being provided to permit the passage of the largest sections of steel.
The members of the lift span were handled by a traveller 96 feet in height. A construction tramway was laid on the upper deck of the east span and viaduct, over which a "creeper" car travelled. The steel was brought up on cars to the viaduct, and a derrick hoisted the sections therefrom on to the creeper car, which was then run out to the erecting point.
While the lift span was being built the casting of the concrete counterweights proceeded. These were prepared in their ultimate positions within the towers. The forms whereby these weights were moulded rested on boxes containing a layer of sand 18 inches in depth, and the moulds were placed 18 inches above the position they were to occupy ultimately. By this arrangement sufficient play for the stretch of the ropes, and to provide requisite ease in connecting the cables, was ensured. After the concrete had set, and directly the lift-span was completed, the sand was permitted to run out of the boxes, so that the weights gradually descended to their requisite level and took up the ropes, thereby swinging, or taking up the weight of the lift span, so as to enable the false-work to be demolished.
The lower or railway deck of the movable span was erected in sections on barges, a floor beam and two hangers first being riveted up, and then raised into position. The concrete counterweights for this deck were cast separately for each panel joint, and they were previously cast and supported on false-work on the upper deck. So soon as a panel joint was in place it was connected to its cables and supported by the counterweight. All the hanger posts, with the floor beams, were set first, the stringers and lower lateral truss being filled in subsequently.
By these ingenious arrangements the construction of the most difficult section of the bridge—the moving span—was completed without any material interference to navigation. All river traffic was able to pass to and fro without the slightest hindrance. The methods adopted furthermore secured rapid construction.
The machinery house for the operation of the moving sections is placed centrally on the deck above the highway level of the bridge. The mechanism is operated electrically. A large gauge is provided on either side of the bridge extending from the top of the piers to the point corresponding with the level of the telescoping railway deck, so that the navigator can determine the clearance available both when the lower deck and when the two spans are raised to the full limit of their respective travels. When the public level is lifted, the two descending counterweights block the thoroughfare on either side of the opening.
The total weight of the bridge is 8,585.8 tons, divided up as follows :— West highway approach 687 tons ; East highway approach 398.8 tons ; Four 80-feet deck railway girders 103 tons ; East riveted span 2,230 tons ; West riveted span 1,948 tons ; Lift span with lifting deck 1,594 tons ; Towers 799 tons ; Tramway poles, balustrade, gas and water mains, etc. 342 tons ; Machinery and motors. 484 tons while its total cost was £383,000.
The bridge was completed for railway traffic on July 23rd, 1912, while the public highway was finished a fortnight later. Since it was opened, the chief engineer of the railway informs me that the bridge has been working with perfect and entire satisfaction. The time occupied in raising both decks to the maximum clearance above low water—165 feet—is 1-3/4 minutes, while lowering requires a similar interval. The result is, that when a sea-going ship with high masts desires to pass through the bridge, the public traffic on the upper deck is interrupted for five minutes. As the bulk of the navigation, however, demands only the moving of the lower or railway deck, the interference offered to vehicular and pedestrian traffic is so slight as to be insignificant. The unique success of this installation doubtless will lead to the wider adoption of the idea where similar conditions prevail, and where it is often essential to utilise a bridge both for public and railway services.