80 CAMBRIA STEEL. 



EXAMPLE. 



What is the proper size of I-Beam without other lateral support 

 than the usual tie rods, corresponding to a total fibre stress of 

 16 000 pounds per square inch under the following conditions? 

 The beam is 18 feet between end supports and carries a tile arch 

 on one side having a nominal depth of 9 inches, effective depth of 

 6.6 inches, a span of 5 feet, designed to carry a superimposed load 

 of 75 pounds per square foot in addition to the weight of the arch 

 and other floor materials. The hollow tile arch weighs 36 pounds 

 per square foot and the other materials, including plastering, 

 weigh 14 pounds, making a total load, exclusive of the weight of 

 the beam, equal to 125 pounds per square foot. 



For tie rods of f " diameter the spacing between them would be 

 5.9 feet, as shown by the table of Spacing of Tie Rods on page 77 

 in which the safe stresses in the rods only are considered. 



Substituting the proper values in the formula for lateral thrust 

 of arches, given on page 75, this will be 



AA 



J X o.o 



Substituting this value for w in formula (2) page 78 and assum- 

 ing a 10" beam 25 Ibs. per foot, the moment of inertia of which is 

 6.89, as given in the Tables of Properties of I-Beams, page 182, 

 we have 



, 710 X 4.66 X 5.9 2 oero ., 

 P = - -- = 8 358 lbs ' per Sq ' m ' 



Therefore p = 16 000 - 8 358 = 7 642 lbs. per sq. in. 

 Hence the safe load as determined by the consideration of 



vertical loads only, should be reduced to , or approximately 



.48 of the amount given by the Tables of Safe Loads in case the 

 spacing of the tie rods is not changed. 



The safe vertical load for a 10" beam, weighing 25 lbs. per foot, 

 18 feet long between supports, for fibre stress of 16 000 lbs. per 

 square inch, is 14 470 lbs. uniformly distributed, including the 

 weight of the beam as given in the Tables of Safe Loads, on page 

 109, or!4 020 exclusive of the weight of the beam, and .48 of this 

 is 6 730 lbs., which is the vertical load it can safely carry in order 

 that the total stress due to it and the lateral thrust shall not 

 exceed 16 000 lbs. per square inch. 



