PROBLEMS IN DESIGN OF BEAMS 91 



Substituting in the formula n = 1.5, find h = 12.6 ins. 



2 x 12.6 

 o = = 8.4 ins. 



Use a commercial size beam 9 ins. X 13 ins. 



Third. Design so the breadth equals three-quarters the depth. 



Substituting in the formula n = 1.33, find h = 12.08 ins. 



, 3 x 12.08 



o = - = 9.06 ins. 



4 



Use commercial size beam 9.5 ins. x 12.5 ins. 



The student will have noticed that in all cases the exact size 

 computed cannot be used and it is necessary to take a commercial 

 si/<- enough larger so the loss in dimensions through cutting will 

 give a beam the size of the computed beam, or slightly larger. 

 Si i Kill beams will run from j in. to f in. smaller than nominal size, 

 but beams of the size here considered will seldom run less than 

 \ in. smaller in each dimension than the nominal size, and if the 

 superintendent of construction is not careful the loss will be 

 even greater. The writer is acquainted with designers who use 

 the nominal size always in their designs, assuming that the maxi- 

 mum fiber stress allowed is really less than the wood can stand. 

 It is not good practice. 



Assuming that the fiber stresses are based on the use of wood 

 freely exposed to weather, then the following increases in fiber 

 stress are allowable for long-leaf yellow pine : 



Class A (moisture contents, 18 per cent). Structures freely 

 exposed to the weather, such as railway trestles, uncovered bridges, 

 etc., let allowable stress equal 1 X /. 



Class B (moisture contents, 15 per cent). Structures under 

 roof but without side shelter, freely exposed to outside air, but 

 protected from rain, such as roof trusses of open shops and sheds, 

 covered bridges over stream, etc., let allowable stress equal 1.15 x /. 



Class C (moisture contents, 12 per cent). Structures in build- 

 ings unheated, but more or less protected from outside air, such 

 as roof trusses of barns, inclosed shops and sheds, etc., let allow- 

 able stress equal 1.4 x /. 



Class D (moisture contents, 10 per cent). Structures in 

 buildings at all times protected from the outside air, heated in 

 the winter, such as roof trusses in houses, halls, churches, etc., 

 let allowable stress equal 1.55 x/. 



