FIRE-BOX GIEDER STAYS. 2B3 



When L = length of girder in feet. 



„ d = depth of girder in inches. 



„ ^ = thickness of girder in inches. 



,, c = 500 when the girder is fitted with one support- 

 ing bolt. 



„ c = 750 when the girder is fitted with two or three 

 supporting bolts. 



„ c = 850 when the girder is fitted with four support- 

 ing bolts. 



then — ^ =»= W.P, = Working pressure. 



{W-P)DxL 



For wrought-iron the " Resistance to cross-breaking," or 

 " Modulus of Rupture," is taken = 54,000 lbs. per square 

 inch, when all the dimensions are expressed in inches ; but 

 as the length of the girder in the Board's Formulse is ex- 

 pressed in feet, 



the " Modulus of Rupture " becomes = — , 



12 



and the " Factor of Safety " is taken = 6. 



Assuming, as is almost uniformly the case, that the sup- 

 porting bolts are pitched at equal distances along the girder, 

 so that the weights are symmetrically disposed about the 

 centre, then the " Moment of Rupture " will be greatest at 

 the centre of the girder ; and in the case of girders with two 

 or four supporting bolts, will be constant along that part of 

 the girder contained between the two supporting bolts which 

 are situated immediately on either side of the centre, so that 

 in all cases where the above conditions obtain, it is suffi- 

 cient to consider the ''Moment of Rupture" at the centre 

 of the girder which is equal to the sum of the " Moments of 

 Rupture" produced at that point by each of the weights 

 separately. 



In a beam of this character it is not necessary to consider 



