STRENGTH OF MATERIALS 



247 



8. A bar of wrought iron 2^ in. in diameter ruptures under a 

 tension of 271,000 Ibs. What is its ultimate strength? 



FACTOR OF SAFETY 



For convenience in working the following problems we will use 

 values given in the table below, unless otherwise specified. These 

 are average values which have been established by actual test. 



Material 



Brick 



Stone '. . . 



Timber 



Cast Iron. 

 Wrought Iron 

 Steel. . 



E. L. U.T.S. U.E. U.S.S. U.C.S. 



3,000 10,000 .015 3,000 



6,000 20,000 .005 20,000 



25,000 55,000 .20 50,000 



50,000 100,000 .10 70,000 



2,000 



6,000 



8,000 



90,000 



55,000 



150,000 



E.L. = Elastic limit 



U.T.S. = Ultimate tensile strength 



U.E. = Ultimate elongation 



U.S.S. = Ultimate shearing strength 



U.C.S. = Ultimate compressive strength 



The above is given in pounds per square inch, except "ultimate 

 elongation" which is given in inches per linear inch, ''linear" mean- 

 ing "inch of length." 



1. A piece of steel shows a tensile strength of 85,000 Ibs. per 

 square inch, and is used in a bridge where it is subjected to a steady 

 stress of 17,000 Ibs. per square inch. What is the factor of safety? 



2. If a piece of wrought iron has a tensile strength of 42,000 Ibs. 

 per square inch, find the load that would be needed to break, in 

 the testing machine, a piece of the same material % in. in diameter. 



3. A wrought iron bar ^ in. in diameter is pulled apart at a 

 load of 4970 Ibs. What would be the tensile strength of this iron? 



4. What would be a safe load for the bar in problem 3 if it 

 were to be subjected to a varying stress? 



