Effin Bridge, the railway lift-bridge over the Royal Canal on the seaward side of Newcomen Bridge in Dublin, has caused some little annoyance to boating folk. It is raised on a small number of days each year to allow boats through; many staff must attend and Waterways Ireland must pay Iarnród Éireann, the railway company, for each lift, as well as paying its own staff for attending.
Perhaps a more modest structure might work. Something like this.
Description of a new Lift Bridge for the Midland Great Western Railway, over the Royal Canal at Newcomen Bridge, Dublin. By Bindon B Stoney, MA, MInstCE
This bridge carries a short branch of the Midland Great Western Railway of Ireland across the Royal Canal immediately below Newcomen Bridge, at the very oblique angle of 25 degrees and, though the canal is only 15 feet wide, the bridge carrying the railway requires to be nearly 40 feet long on the skew.
The trains run over this bridge at about two feet above ordinary water level, and whenever a boat is passing along the canal the bridge is lifted from 8 to 13 feet, according to the height of the deck load, so as to permit the boat to pass beneath. The bridge is formed of two strong single-plate girders of the usual type, which lie underneath the rails, with cross girders and side brackets over which the platform is laid. This bridge is lifted by means of a lever 40 feet long, formed of two plate girders braced together horizontally, and attached rigidly at right angles to the centre of the bridge, and this lever is itself balanced at its centre on blunt steel knife edges like the beam of a pair of scales. The weight of the bridge at one end of the lever is counterpoised by an equal weight of metal attached to the other end, so that the whole structure turns freely on the knife edges, which work in steel pillow blocks on the top of metal standards, one on either side of the lever. The opening and closing motions are regulated by a small crab-winch and chain worked by hand; the ends of this chain are attached to the lever at several feet on either side of the knife edges, and its centre is wound on or off from the barrel of the winch, which is itself bolted down to a mass of concrete extending beneath the metal standards.
The man in charge works this arrangement with the greatest ease, and it is so regulated that the bridge is opened or closed in about one minute. It might be moved much faster than this, as the friction is reduced to a mere trifle by the knife edges, but it is not convenient to put so large a mass in rapid motion when there is nothing to be gained by so doing. It was essential that the bridge should be erected speedily and so as to interrupt the traffic as little as possible, and the first engine passed over it in about twelve weeks after the contractors, Messrs Courtney, Stephens and Bailey, of Dublin, got instructions to proceed with the work and the traffic was interrupted for only about one week during erection. The lever sloping upwards has a somewhat singular appearance when the bridge is in position for trains to pass over and, on the other hand, the bridge itself has a singular effect when it is tilted up into the air for canal boats to pass beneath; but the author has successfully obtained what he aimed at — namely, simplicity of design, strength, ease of working, little aptitude to go out of order and last, but not least, very moderate cost.