146 



LECTURE 



to resist the direct crushing strain, but that it is sufficiently great, 

 compared with their length, to avoid a sidewise flexure and conse- 

 quent failure. Hodgkinson, in his elaborate experiments, has shown 

 that, in practice, when the length of a post is less than thirty times 

 its diameter it is not apt to break without it is absolutely crushed ; but 

 in such cases the ends should be square and' well fitted, and the strain 

 should be central, and not on one side. Posts with rounded ends are 

 much weaker than those with flat ones. 



When a post, subjected to an axial crushing strain, is inclined, as 

 in the case of a main brace in a bridge truss, we must bear in mind 

 that its deflection from its own weight will tend to weaken it as a 

 strut, since it commences the flexure to the side which is the ultimate 

 cause of the failure of a strut. For this reason, if the cross section of 

 such a strut is not a square, and if the length is at all great, the 

 greatest side of the cross section should be vertical, as in the case of a 

 beam or joist. If a timber strut seems to be too flexible it may be 

 much stiffened without adding much ,to its weight by spiking to the 

 upper or lower side a fin of narrow plank, deep in the middle and 

 tapering off towards the ends. 



Cast-iron struts should either be tubular or have a cruciform 

 section, as in Fig. 36, so that the material being disposed at 

 the greatest distance from the neutral axis may act with the 

 preatest effect in preventing what we may call the initial 

 Wrought iron may be used in both these forms with great 

 economy of material, a piece of ordinary gas pipe forming the best of 

 struts, and the cruciform section being readily got in the rolls of the 

 mill. In fact these remarks apply to all pieces subjected to a com- 

 pressive strain, such as posts, struts, and the upper chords of framed 

 beams or bridges, the tubular or the cruciform section being necessary 

 where economy of material and lightness of the structure are desired. 

 Since, in practice, it is not always convenient or possible to span a 

 chasm by one single beam, intermediate supports (piers) must be made 

 use of, and, in an iron structure at least, advantage may be taken of 

 them to assist in relieving the strain at other points of the beam 

 beside those immediately over them. If the spaces are spanned by 

 unconnected beams, as in Fig. 37, each one will act independently, as 



there shown, but if the whole beam is continuous 

 will behave differently. 



as in Fig. 38, it 



Fig. 38. 



If, by any means, in Fig. 37, we were to raise the middle points of 

 the deflecting beams into a straight line the triangular spaces between 



