TABLE 87. 

 PROPERTIES OF PLATE GIRDERS. 



Some specifications require that plate girders be proportioned by the moment of inertia of 

 their gross section and some by the moment of inertia of their net section. The moment of inertia 

 of the gross section can be obtained by direct addition from Tables 3, 5 and 33. The moment of 

 inertia of the net section is obtained by subtracting the moment of inertia of the holes from that 

 of the gross section. The moment of inertia of the holes can be calculated by the formula / = AJP, 

 the moment of inertia of the holes about their own axis being negligible, A Q being the diametral 

 area of the hole and h the distance from the neutral axis to the center of the hole. 



The method of calculating the moments of inertia of plate girders will be illustrated by a typical 

 example. 



Example: Determine the moment of inertia and section modulus of a section consisting of 

 4 angles s"x3^"x^", long legs out, 24!" back to back, i web plate 24"x|", 2 cov. plates I2"xf". 



Moment of Inertia and Section Modulus of Gross Section. 



Moment of Inertia of Rivet Holes (" Rivets, i" holes). 



The Moment of inertia of the net section is 4872 973 = 3899 in. 4 , and the section modulus 

 is 3-899 -h 12.875 = 302.8 in. 3 . 



Approximate Methods. 



The use of the moment of inertia of the net section in proportioning plate girders, requires 

 that holes in the compression flange be deducted as well as those in the tension flange. This only 

 approximates the true condition so that great accuracy in calculating the moment of inertia of the 

 net section does not seem warranted. The following approximate solutions give results which are 

 sufficiently accurate for use in design. 



ist Approximate Method: 



Net / of Angles = Gross I X ^ '_ A ^_ = 2074 X ^ = 1556 Table 33. 



Gross Area 



Net 7 of Web PL = Gross 7 of Net Depth = 7 of 22" X f " PL = 333 



Net 7 of Cov. Pis. = Gross 7 of Net Width = 7 of 2 - 10" X f " Pis. = 1972 



Total Moment of Inertia of Net Section = 3861 in. 4 



2d Approximate Method: 



^N^fet Arcs ^2 *7C 



Net 7 = Gross 7 X . = 4872 X ^^ = 3989 in. 4 

 Gross Area 40.00 



This method gives more accurate results for sections without cover plates. 



204 



