CLASTIC PROPERTIES OF MATERIALS 17 







Besides these elements of uncertainty every construction is 

 attended 1\ its own peculiar circumstances, such as the duration 

 to be given t. it. the gravity of an accident, etc-., which requires a 

 special determination of the factor of safety. 



all these reasons it is impossible to definitely fix a factor of 

 which will tit all cases, and the only guide that can be given 

 as to its cho to say that it will lie between certain limits. 



According to Resal,* th- factor of safety for iron, steel, and ductile 

 metals should be 4 or 3, and never less than 2^; f..r heterogeneous 

 materials, such as cast iron, wood, and stone, the factor of safety 

 should lie b JO and 10, and never be less than the latter. 



Problem 20. In r StatM u'-'Vernment tests of rifle-barrel steel it was 



found that for a certain sample the unit t at tin- elastic limit was Tl.ooo 



.-, ami that tin- ultimate tensile strength was 118,000 lb./in.-. What must 

 be in order to l-rim: the working stress within the elastic 



Problem 21. In t! - :: nt tests of concrete cubes made 



of A thus cement in t he proportions of 1 part of sand to 3 of cement ami < of broken 

 inmate comprefwive strength of one specimen was 883 lb./in.-. and .if 

 another specimen was 82"-' If the working stress is determined from the 



ultimate strength of the first specimen by using a factor of safety of 5, what factor of 

 >ed to determine the same working stress from the other specimen '.' 

 Problem 22. An elevator cab weighs 8 tons. With a factor of safety of h<>w 

 large must a steel cable be to support the cab ? 



22. Physical constants. The following brief tables of physical 

 mts are the results of a comparati\< .study of the N////7 //>,,/// 



i erett's C.G.S. >>- 

 tern of Units, and the United States Government Reports on 



In ' 'nluiiin of the taMrs th- limiting values of tin- < ii- 



ii. \vhi.-h serves to indicate the wide range of variation 

 to whi- h th.-.se so-called constants are subject 



:;mts are tabulated in the second 



luniii and agree closely with the values ordinarily given in engi- 

 neers' handbooks. Th-v me n..t t<> U* regarded as more correct than 

 alae0 tabulated in tin- tii>t luinn. hut are inserted for emiv-ii- 

 in solving th- jii-ohli-ins in this book. 



, R**i*tanct de* MaUriaux, p. 



