CAMBRIA STEEL. 81 



The actual vertical load on the beam under consideration is as 

 follows: 



-|x 18 X 125 = 56251bs., 



which is less than the allowable amount, 6 730 Ibs., as figured 

 above, so that a smaller beam may suffice. 



Therefore, assume a 9-inch beam, weighing 21 Ibs. per foot, the 

 moment of inertia of which about an axis coincident with center 

 line of web is found in the Table of Properties, on p. 182, to be 5.16. 



In this case 



Substituting this in the formula for p we have 



p = 16 000 - 10 370 = 5 630 Ibs. per sq. in. 



Therefore the safe vertical load will be r- m , or approximately 



io ULMJ 



.35 of the tabular safe load. 



The safe vertical load for a 9" 21 Ib. beam, 18 feet long, for a 

 fibre stress of 16 000 Ibs. per square inch is 11 180 Ibs., as given in 

 the Table of Safe Loads, on page 109, and .35 of this, after deduct- 

 ing weight of the beam, is 3 781 Ibs., which is less than the actual 

 amount, 5 625 Ibs., as calculated above, so that the 9" 21 Ib. beam 

 will not suffice. 



If the spacing of the tie rods at the center be reduced from 5.9 

 feet to 3.25 feet, it may be found, in a manner similar to that used 

 in the above calculations, that the safe vertical load for an 8" 

 I-Beam, weighing 18.0 Ibs. per foot, is reduced to .74 of its tabular 

 value of 8 430 Ibs., or 6 328 Ibs., and as this amount is greater than 

 the actual load as above, namely, 5 625 Ibs., the 8" beam would 

 answer the purpose, under the changed conditions as to spacing 

 of tie rods. As this beam might deflect beyond the limit for 

 plastered ceilings, it should be examined in accordance with the 

 rule or formula given for obtaining safe deflections in the explana- 

 tion of the Tables of Safe Loads, and elsewhere herein. 



Calculating this by the rule given on page 102, the safe load for 

 the allowable limit of deflection is 



which is greater than the actual amount, 5 625 Ibs., so that the 8" 

 beam is sufficient and proper if the spacing of central tie rods be 

 changed to 3.25 feet, as assumed in the last case. 



