These are usually made of wrought iron. This metal, when of excellent quality, may be safely trusted with 12,000 pounds per inch sectional area. But it is usual, for good work, to compute the area at only 9000 pounds per inch, and, as ordinarily made, these rods ought not to be loaded with more than 7000 pounds. The strain divided by this value per inch of the metal will give the sectional area of cross-section. For example, the strain in the rod D G, Fig. 84, is 867 pounds (Art. 225); therefore -
A = 867/7000 = 0.124;
or, the sectional area required is only an eighth of an inch. By reference to the table of areas of circles in the Appendix, the diameter of a rod containing the required area, as above, will be found to be a little less than half an inch. A rod half an inch in diameter will therefore be of ample strength. For appearance's sake, however, no rod in a truss should be less than 3/4 of an inch in diameter.
The rod FE has to resist a strain of 3192 pounds. For this, then, we have -
A = 3192/7000 = 0.456.
A reference to the table of areas shows that a rod containing this area would be a little more than 3/4 of an inch in diameter; it would be of ample strength, say, at 7/8 of an inch in diameter.
The rod C B, at the centre, has to carry a strain of 10,167 pounds. For this, then,- we have -
A=10167/7000=1.452. A' reference to the table of areas shows that this rod should be 1 1/2 inches in diameter.