MECHANICS 



Built-Up Section. Fig. 4 shows a section of the rafter 

 member of a large roof truss formed of a f"X16" web-plate 

 and a f"X 12" flange plate, the two joined by two 4"X4"X$" 

 angles. It is desired to know the distance from the neutral 

 axis of the section to the top edge of the flange plate. By 

 means of the principles given, the centers of gravity of the two 

 rectangular plates are easily located as shown. The centers 

 of gravity of the angles might also be located by applying the 

 rule given in the preceding paragraph; this, however, is 

 unnecessary because the position of the center of gravity 



FIG. 4 





can be obtained directly by referring to the tables of .the 

 properties of rolled sections furnished by the various steel 

 manufacturers. By referring to any of these tables, the 

 center of gravity of a 4"X4"X^" angle is found to be 1.18 

 in, from the back of a flange, thus giving the distance 1.18 + 

 .375 = 1.555, or about lif, in. from the top edge of the flange 

 plate to the axis through the centers of gravity of the angles. 

 From the same tables it is also found that the area of the 

 section of a 4"X4"X V angle is 3.75 sq. in. 



The area of the section of the flange plate is f X12 = 4.5 

 sq. in., and of the web-plate, |X 16 = 6 sq. in.; the area of the 

 whole section is therefore 2X3.75 + 4.5+6 = 18 sq. in. 



