I 



284 Cilley — Fundamental Propositions in the 



is raised in certain cases by overstrain. Doubtless other factors 

 are prominent also in this phenomenon, especially in the case 

 of simple tension bars, although even there annular stresses 

 thus engendered may play an important part. This part of 

 the subject is pregnant with practical and valuable suggestions 

 for the engineer. 



It would be of great interest to enter into an analysis of some 

 of the primary stresses engendered by temperature changes on 

 the one hand, and those due to the tightening of parts in built- 

 up constructions on the other hand. But the former in the 

 case of stresses due to the unequal cooling of castings, is a very 

 difficult subject, and in the case of ordinary temperature 

 stresses due to unequal expansion, is already well known. As 

 to the latter it presents many most interesting problems which 



resent time and space unfortunately forbid our considering. 



t may be remarked however that in a majority of instances 

 the primary stresses thus engendered are local, as is also true 

 of most of the primary stresses due to cold working (shearing, 

 punching, etc.) of metals. A case in which primary stresses 

 introduced through construction are not local and are compar- 

 atively easy of consideration, is that of indeterminate frame- 

 works. The writer has elsewhere* presented some special 

 studies in this connection showing the desirability of avoiding 

 constructions in which primary stresses may occur, and the 

 necessity of considering these stresses where they may occur. 



Probably few questions in structural engineering have caused 

 more controversy than the theory of the masonry arch. 

 Volumes have been written on the true position of the line of 

 resistance, methods innumerable have been developed for its 

 determination and the greatest diversity in practice and opinion 

 still rules in this connection. Apparently the present best 

 view tends to the treatment of the masonry arch on the same 

 lines as the metal arch, that is as an elastic arch, but with this 

 distinction, that the axial contraction be taken into considera- 

 tion as well as the flexure, an addition necessitated by the 

 usually considerable relative thickness of a masonry arch ring. 

 This is excellent as far as it goes but it is not sufficient. From 

 the manner of construction, flexibility of centers, employment 

 of mortar, decentering, etc., primary stresses (horizontal com- 

 ponent of reaction and moments at the abutments) of consid- 

 erable importance are almost certain to be introduced. They 

 should be studied and determined by observation as closely as 

 possible. Nowhere is better exemplified the fact that the true 



* " Some Fundamental Propositions Relating to the Design of Frameworks " in 

 the Technology Quarterly, June, 1897, Boston, Mass, 



" The Kxact Design of Indeterminate Frameworks " in Transactions of the 

 American Society of Civil Engineers, June. 1900, New York. 



