TIES AND STRUTS 



209 



compressive force to do any clainage to bone which is as it should be (Table 

 XXXII.). 



The engineer plans his structures to give the niaxiniuni strength 

 with the minimum weight. Examination of the girders holding a 

 roof will show that those running from wall to wall have a cross- 

 section like the capital letter " I." This girder is a tie rod and has 

 to withstand a stretching force. How has the engineer arrived 



BEAM UNLjOADED 



BEAM LOADED 



Fig. 49. — To show lines of compression (darli) and lines of tension (dotted) in a loaded 

 rectangular beam. The clear space between the strut-lines and the tie lines indicates the 

 neutral zone. 



at this form ? Fig. 49 represents a beam of square section. 

 When such a beam is loaded midway between its supports it 

 is slightly bent to give a concave upper surface. The upper 

 surface is compressed while the lower is stretched. Therefore 

 midwav between the upper and lower surface lies a neutral zone 

 or line of no stress and in its neighbourhood the material needs 

 to have little strength. 



LOAD 



Fig. .50. — A simple triangular roof truss. 



The girder maker can 

 therefore quite safely 

 cut away the centre of 

 his beam, leaving only 

 the upper and the 

 lower surfaces, and of 

 course some connec- 

 tion between them 

 which may be almost 

 as thin as he likes 

 without destroying the 

 strength. In other words, if the engineer can map out the 

 lines of stress or directions of compression and tension in the 

 loaded structure, all the manufacturer has to do is to see that these 

 lines lie in his material ; all the rest may be cut away. 



By means of the truss (Fig. 50) the simple girder becomes a tie 

 between two struts. The horizontal member of the truss under- 

 goes tension only, while the sloping beams are compressed. Such 

 a structure permits of the use of two kinds of material — matter 

 with a high tensile strength for the tie and matter able to bear 

 up under compression for the struts. 



The two principal connective tissues are fibrous tissue and 



