CHAPTER I. 



STRESSES AND DEFORMATIONS WITHIN THE ELASTIC 



LIMITS. 



1. Object of the Study of Stresses. In order that an engi- 

 neer may be able to design structures to resist forces to carry 

 loads he must know how forces are transmitted, and the proper- 

 ties of the materials of construction for transmitting tiiein. The 

 subject consists of two distinct parts : 



1. The stresses must be calculated ; this part is based upon 

 that branch of mechanics called statics, which treats of the equi- 

 librium of forces. 



2. The amount and distribution of material in a structure 

 must be determined so as to resist the stresses safely and econom- 

 ically ; 'this part is based upon the strength of materials as de- 

 termined by experiments. 



In what follows, only so much of the second part of the 

 subject will be considered as is necessary to bring out the practi- 

 cal applications of the first part. 



2. Kinds of Structures. The kinds of structures to be 

 considered are beams, girders and trusses, singly and in com- 

 binations forming railway and highway bridges, roofs, buildings, 

 cranes, etc. 



A beam, in general, is a single piece or a combination of 

 pieces of material which carries loads in cross bending between 

 or beyond supports. 



An engineer's definition of a simple beam is one consisting 

 of a single piece, as a stick of timber or an I beam, but in building 

 construction an architect's definition is any member which 

 directly carries external loads in cross bending. 



The engineer's definition of a girder is a beam which is 

 built up of a number of pieces rigidly held together, while the 

 architect terms a girder any member in cross bending, which 

 carries the ends of other beams, whether it be composed of 



