FARM STRUCTURES 



405 



Factor of Safety. The factor of safety is the ratio of the 

 ultimate strength to the working stress of a material. 



i-'S 



s 

 when S = ultimate strength, s = working strength, f = factor of safety. 



The engineer in charge of design is called upon to decide 

 the factor of safety to be used. 



The factor of safety should (1) be much below the elastic 

 limit, (2) be larger for varying loads, (3) be larger for non- 

 uniform materials. 



Factors of safety for various materials. 



Materials 



For steady 



stress. 

 Buildings 



For varying 

 stress. 

 Bridges 



For shocks. 

 Machines 



Timber 



Brick and stone 



8 

 15 

 6 

 4 

 5 



10 



25 



15 



6 



7 



15 

 30 



Cast-iron 



20 



Wrought iron 



Steel 



10 

 15 



This table is taken from an architect's handbook, and the 

 factors of safety here recommended are nearly twice as large 

 as are commonly used in designing farm structures. 



QUESTIONS 



1. Define mechanics. Define statics. 



2. In what two ways does a force acting on a body tend to produce 

 motion? 



3. What are the two conditions for equilibrium? 



4. Define moment of force. 



5. When does an equilibrium of moments exist? 



6. Define stress. Define strain. 



7. Describe a tensile stress. A compressive stress. A shearing 

 stress. A complex stress. Define unit stress. 



8. Explain what is meant by the elastic limit of a material. 



9. Define ultimate strength. Working stress. Factor of safety. 

 10. Upon what conditions will the size of the factor of safety depend? 



