A =Treble-sling Bridge= has three slung transoms, one being slung from the standards on either side between the fork and road-bearing transoms. The process of construction is similar to that of a single-sling. If the frames are very long and heavy, it is well to brace them above as well as below the road-bearing transoms to prevent their getting racked out of shape in launching. The upper braces, if they interfere with the headway, can be removed after the frames are locked. The great length of the standards in single-and treble-sling bridges, and the heavy transverse strains brought upon them by the weight of the roadway, will generally necessitate their being stiffened by ties which should be carried over vertical frames anchored to earth anchorages, or stout bollards on the bank, and twisted up taut with a handspike. Can be used for spans up to 70 feet. ESTIMATE OF TIMBER. ------------+------+------+--------------+--------------------------- Kind of |No. of|Length| Diam. in In. | Bridge. |Spars.|in Ft.| | | | +-------+------+ Purpose. | | |At Tip.| Mean.| ------------+------+------+-------+------+--------------------------- Single-sling| 4 | 37½ | 6 | |Standards. (50 to 60 | 3 | 15 | | 10 |Road-transoms. Ft. Span). | 3 | 15 | | 6 |Top and fork transoms. | 4 | 15 | |4 to 6|Ledgers and shore-transoms. | 4[10]| 18 | 3 | |Braces. | 10 | 30 | | 6 |Balks. | 4 | 30 | 3 | |Ribbands. ------------+------+------+-------+------+--------------------------- For a treble-sling bridge standards from 50 to 60 feet long will be required; also additional spars for transoms, road-bearers, and vertical frames. THE STRINGER BRIDGE. If the span does not exceed 25 feet, abutments good, and proper timber near at hand, this bridge can be easily built. TABLE OF TOOLS AND MATERIALS FOR SINGLE-AND DOUBLE-LOCK AND UNSTIFFENED SINGLE-SLING BRIDGES.