FIRE-BOX GIRDER STAYS. 235 



Kesistance " is expressed by the equation R = , in 



which 



c = the " Modulus of Rupture " of the material, 

 d = the depth of the girder, 

 t — the thickness of the girder. 



So that taking the " Modulus of Rupture" of wrought-iron 

 = 54,000 lbs. per square inch, we have the equation 



1/ _ ^v xl _ Gx d^ xt _ „ 

 . ' 4 ~6 



But as the length of the girder in the formulae is expressed 

 in feet, and the Factor of Safety is to be = 6, the equation 

 becomes 



T^f _ wxl _ cxd"xt_p 

 4 6 X 12 X 6 ~ ' 



or, M=iv xl = '^'' cxd^xt ^ j^ 



6 X 12 X 6 



And substituting the proper " Modules of Rupture " for r,^ 



.^ , 4x54000xc?^x^ n 

 M = 10 X I = = Jx, 



6x12x6 



^r 7 216000 xd^xi p 

 or, 31 — 10 X I = = M. 



432 



or, 31 = IV X I = 500 xd'xt = E, 



bOO xd'xt - 



and IV = 



I 



Now IV ~ the total load on the supporting bolts, and is 

 equal to the area of the fire-box supported by the bolts x by 

 the working pressure ; and when 



P = the pitch of the supporting bolts, 



W = the width of the fire-box. 



