GRAPHIC STATICS 



233 



Let / = unit stress specified, and which will be used for dead 



load, or for total quiescent load. 

 S m = maximum load on the member. 

 S p = minimum load on member, 

 then 



Working Stress - ^-= 

 l.o 



When one load is compressive and the other is tensile replace 

 the positive (+) sign by a negative (-) sign. 



Example. The maximum and minimum loads in a certain 

 member are respectively 105,000 Ibs. tensile and 49,000 Ibs. ten- 

 sile. What is a proper working stress? 



16,000 A 49,000 



Ans. Working stress = 



1 + 



= 13,200 Ibs. 



= 9600 Ibs. 



1.5 V ' 2 X 105,000, 

 per sq. in. 



Example. The maximum and minimum loads are respectively 

 105,000 Ibs. tension and 23,000 Ibs. compression. What is the 

 proper unit working stress? 



16,000 /, 23,000 



Ans. Working stress = -j-^- (l - 2 x 



per sq. in. 



The reduced stress found by the above range formula is used 

 for members in tension. The compressive stress is determined 

 by an appropriate column 

 formula, but it cannot 

 exceed the range stress. 



Snow Load 

 The snow load is al- 

 ways included with the 

 dead load. It varies with 

 the latitude as well as 

 with the slope of the 

 roof. In Fig. 155 is 

 shown the snow load to 



Latitude in Degrees 



Fig. 155. Snow Load on Roofs for Different 

 Latitudes 



use according to the recommendation of Professor Ketchum in 

 "The Design of Steel Mill Buildings" in 1903. 



English text books state that an allowance of 5 Ibs. per sq. ft. 

 of horizontal projection is common for Great Britain. Ketchum 

 recommends the minimum ice and sleet load for all slopes of 



