On Some Common Errors in Iron Bridge Design

48 and grow out of that which is scientific and useful, has led many designers into extravagancies and absurdities. For example the old Church Street Bridge, Richmond, near Melbourne, originally had footpath brackets made of thin round rods bent into the curves of a raw’s horn, an absurd and excessively weak form, while the recently constructed Swing Bridge at Footscray has the webs of its footpath cantilevers made of circular rings of L iron, a costly and unscientific arrangement, the exact strength of. which it is impossible to compute. It is suggested that such brackets be carefully examined, tested, and if showing any signs of weakness strengthened, and that in future structures, scientific and rational forms be adopted instead of these unsatisfactory pseudo-ornamental abortions. 26. Parapets too Iotv, too weak, or too open.—These are of not uncommon occurrence, and as sources of danger to the public are frequently of serious import. An examination of such parapets on bridges in. or near Melbourne has revealed the fact, that some are less than 3 feet in height, while others approach 5 feet. The former are certainly dangerously low, the latter needlessly high. It is recommended that no bridge parapet should be less than 3 feet 6 inches, which is the height of the elbow of a man of medium size, while 4 feet may be taken, as a maximum beyond which it is unnecessary to go. The same rule .should, in the writer’s opinion, be applied to the balustrades of staircases, landings, and balconies in buildings. These are usually too low, and have repeatedly been the cause of serious accidents. No rule has been to the writer’s knowledge generally accepted for the strength of parapets. He therefore proposes that to provide for the pressure of a dense crowd, they be made strong enough to endure a horizontal pressure of 100 lbs. per foot in length, applied at the top with a safety factor of not less than. 3 for metal and 5 for timber. This strength may be provided by the resistance to bending of the uprights or standards of the parapets. But if this be found to involve an. undesirable amount of material, lighter standards may be employed, with sloping struts outside. These struts should be straight, inclined at an angle of not less than. 20 deg. to the vertical, and extend from the projecting end of the foot- ■ MH