INFLEXIBILITY 165 



III. PRINCIPLES GOVERNING THE CONSTRUCTION OF 

 THE MECHANICAL SYSTEM. 



One of the most powerful of the considerations that influence an 

 engineer (or architect), in deciding upon the plan of construction of a 

 bridge (or roof), is the desire to economise material. In other words, a 

 design is not satisfactory unless it ensures the maximum of strength 

 and solidity with the minimum expenditure of material. To this end 

 the resistant elements of the structure must be arranged in a particular 

 manner in accordance with approved mechanical principles. An 

 organism is confronted with a precisely similar problem when it is 

 called upon to provide itself with a sufficient degree of mechanical 

 strength. In this case also the desired result must be obtained with 

 the smallest possible expenditure of material ; hence the principles of 

 construction to which the human builder adheres are precisely those 

 which control the morphogenetic activities of organisms. The very 

 same mechanical principles that are embodied in a modern railway 

 bridge of bold and elegant design, were expressed perhaps with even 

 greater perfection hundreds and thousands of years ago in the skeletal 

 systems of the plants of former geological periods. 



In the succeeding sections of this chapter we shall become more 

 closely acquainted with the most important of the principles that 

 govern the construction of the mechanical system. 



A. INFLEXIBILITY. 96 



If a straight girder be supported at both ends and weighted in the 

 middle, it will bend to a greater or less extent according to the magnitude 

 of the load; as a result of this curvature the upper side of the girder must 

 be slightly shortened, while the lower side is correspondingly lengthened. 

 The shortening, of course, produces a state of compression, the lengthening 

 a state of tension, in the corresponding halves of the girder ; the effect, 

 in either case, is most pronounced at the upper and lower surfaces, 

 while on approaching the centre of the cross-section from either side 

 the corresponding tension gradually diminishes, then falls to zero and 

 finally passes over into the tension of opposite sign. The layer of zero 

 tension is known as the neutral surface. 96 * In order, therefore, that a 

 girder may possess maximum inflexibility, the available material must 

 be concentrated in the regions of greatest tension, that is to say, near 

 the upper and lower faces. A typical girder thus comes to consist of 

 upper and lower flanges, which are firmly joined together by a con- 

 necting piece, or web. The cross-section of such a girder usually 

 resembles an I or a combination of two T's (I), the horizontal strokes 

 representing the flanges and the vertical line the web (Fig. 55 a). 



