CHAPTER VI. 



STRESSES IN BEAMS AND GIRDERS. 



66. Classification of Beams. According to Art. 2, beams 

 are of three kinds, simple beams, girders, and trusses. Simple 

 beams and girders may be called solid beams to distinguish them 

 from trusses, which are trussed beams. 



Simple beams usually have a uniform cross section from end 

 to end. Timber beams usually have a rectangular cross section 

 and steel beams an I-shaped one. 



Girders may have a uniform or variable cross section; the 

 variation may be due to a variable depth or to a difference in 

 length of flange plates. 



I beams, channels, Z bars, and girders have webs and flanges. 

 See Fig. 1, Art. 2. 



Beams may be classified in accordance with the manner in 

 which they are supported. 



1. Supported at both ends (commonest case) simply rest- 

 ing on the supports so as not to interfere with elastic bending. 



2. Fixed at both ends so that the axis at these points does 

 not change direction when the beam bends. 



3. Fixed at one end and free at the other (cantilever). 

 Case 2 becomes case 3 when one support is removed. 



4. Fixed at one end and supported at the other. This case 

 is intermediate between cases 1 and 2, or 1 and 3. 



5. Continuous over three or more supports. 



In practice the condition of fixity at the supports is not met 

 with except when the beam is continuous over the supports and 

 loaded beyond them; in other cases the fixity is only partial and 

 the beam is often considered as simply supported. " Fixed 

 ends " implies freedom to change length due to elastic deflection 

 and changes of temperature, which is the usual condition in the 

 other cases. 



Beams may have straight axes or curved axes as in arches. 



91 



