140 



LECTURE 



Fig. 17. 



In later examples of wrought iron beams rolled in one piece, the two 

 flanges are made of the same size, to avoid warping in cooling, hut in 

 beams made of pieces riveted together, this proportion should be 

 observed. 



A wrought-iron beam may be modified in another way. 



It is sometimes advisable to divide the web into two plates, 



putting one on each side, as in Fig. 17, and then we have 



the box form, identical in principle with the usual form, 



but in some cases more convenient to manufacture, and 



possessing more lateral stiffness. 



The flanges themselves may be made of several parts, and made 



even tubular, as we shall see in the description of the Britannia bridge 



in a succeeding lecture. 



So far we have only considered cases in which the web is a solid 

 plate, but it will frequently be desirable, and often necessary, to make 

 the web of pieces, or to frame it ; if we use wood this can hardly be 

 avoided. 



We must be able to arrange the parts in such a way as to insure 

 strength and stiffness, with economy of material, for we shall thus not 

 only save in first cost, but relieve the structure of much dead weight 

 of material which would only load it to its injury. 



In using any material in the form of rods or posts we must endeavor 

 to direct the strain through the axis of the piece, since all material 

 bears a direct strain of compression or extension better than any 

 other. 



If a piece of timber projecting from a wall, as A B, 

 in Fig. 18, is to be strengthened so as to support a 

 weight, W, we can best do it by putting an inclined 

 piece under it, with its lower end, C, fastened firmly in 

 the wall. Now the triangle is the only straight sided 

 figure, the angles of which cannot be altered without 

 changing the length of the sides, and the point D can- 

 not sink unless A draws out of the wall, or D be- 

 comes shorter, since we have supposed the end, C, to be 

 immovable. 



If it should not be convenient to place a brace 

 under the beam, we may substitute for it a tie or ten- 

 sion rod above it, as in Fig. 19 ; this tie will be sub- 

 jected to a tensile strain only, and may therefore be 

 a rod of wrought iron, or even a rope or chain. 



If we have, therefore, to construct a simple bridge 

 over a stream, the width of which is too great to 

 permit us to use a single beam, which would deflect 

 too much, or perhaps break, we may 

 shorten the actual span of the beam 

 by introducing braces or struts, as in 

 Fig. 20, where the clear span of the 

 beam is reduced, from A B to C D, the 

 points C and D being firmly supported 



Fig. 18. 



Fig. 20. 



by C E and D F. 

 If, for any reason, 



it is not convenient to have such framing under 



