TASMANIAN TIMBERS. 65 



The values given in the preceding tables for S, the trans- 

 verse strength of a beam supported at the ends, and loaded 

 in the centre of the clear span, are for breaking-weights, but 

 the working load should never exceed one-third of this for 

 static loads or one-sixth for moving loads; it is usual prac- 

 tice to take one-fourth for static and one-eighth for moving 

 loads. The practice for railway bridges is one-fifth for static 

 and one-tenth for moving loads. 



To find the deflection that any weight in the centre of 

 span will cause in a rectangular beam of any of the woods 



given in the table, supported at the ends — o — 4 b d a e 



Multiply the weight in pounds by the cube of the 

 length in inches, and divide the product by the product 

 of four times the breadth, by the cube of the depth and by 

 the value given for E in the table. 



To find the breaking-weight in a rectangular beam of any 



4 b d a S 



of the woods given, loaded in the same way, W = l 



or multiply four times the breadth by the square of the 

 depth by the value given for S, and divide the product by 

 the length in inches. Or, by using the column S ^, multiply 

 the breadth by the square of the depth in inches by S^, and 

 divide the product by the length of span in feet; or, by 



formula W = i 



Again given the span in feet, the load in pounds, and the 

 breadth in inches, of a beam, to find the depth in inches, so 

 that the beam shall not bend more than one-fortieth of an 

 inch to a foot, or one four-hundred-and-eightietb of 



its span, the formula is 7" — - — , or multiply the 



square of the length in feet by the load in pounds 

 by the value given for A, and divide the product by 

 the breadth in inches; this will give the cube of the depth, 

 and the cube^root will be the result required. 



It must be remembered to add half the total weight of 

 the beam itself to the load for the total centre load upon the 

 beam in all casies. 



