Fig. 100. The King Rod Bridge. ends of the spars were nailed together and their lower ends were firmly planted about four feet apart in the banks of the brook. A stick nailed to the apex of each pair of spars served temporarily to brace them apart. The center cross beam of the bridge was now suspended from the spars by means of heavy galvanized iron wire (No. 14, I should say). The beam was hung high enough to allow for stretch of the wire, making the roadway incline upward from both sides to the center. Aside from carrying the floor of the bridge, this beam was used to brace the inclined spars when the temporary crosspiece was removed. The ends of the beam projected about thirty inches beyond the bridge at each side, and they supported braces which extended diagonally upward to the crossing ends of the spars. When this was done the temporary crosspiece above referred to was removed. As the span between the center cross beam and the banks was a little too long to provide a steady floor, a couple of intermediate cross beams were suspended from the inclined spars. The floor beams were then laid in place and covered with a flooring of slabs.
Stiffening the Bridge.
The bridge was a pretty good one, except for a slight unsteadiness between the center and either end. When Uncle Ed saw it he showed us at once where the trouble lay. Our intermediate cross beams were hung from the center of the spars, and consequently made them bend, because the strain came across their length, while at the center of the bridge there was no chance for the spars to bend, because the strain was exerted along their length, that is, it tended merely to push the ends of the spars deeper into the banks. To remedy the trouble he proposed propping up the center of each spar with a brace running from the center crosspiece. The dotted lines in Fig. 100 show how these braces were applied. They made the floor perfectly solid throughout, and gave the bridge a much better appearance. Uncle Ed told us that the structure might be called a “king rod truss,” except that in place of rods we had used wires.
The King Rod Bridge.
The Bridge over Cedar Brook Gorge.