4 6 4 



THE POPULAR SCIENCE MONTHLY. 



inches wide. We now wish a bridge wider than this, but with 

 no more material in it except what may be needed in order to 

 make the floor. Suppose we take the top and bottom plank of 

 this box girder, cut them in two lengthwise, making out of each 

 two planks six inches wide instead of twelve, and we fasten these 



upon each of the vertical planks, as shown in the drawing. We 

 then have what is technically known as an I-bar, or flanged girder 

 (Fig. 6). Provided these flanged girders are so braced as to pre- 

 vent their bending sideways, the two flanged girders are of exactly 

 the same strength as the box girder, and, as you see, can be placed 

 at any distance apart, and the floor simply placed on top of them 

 or on the lower flange, and we have a bridge as wide as we wish, 

 with the strength of the box girder. 



This I-bar, or flanged girder, is one of the most generally used 

 forms of construction for bridges of short spans. 



So far we have considered the material used to be simply wood, 

 but I-bars are now made of iron or steel, and within the last few 



Fig. 6 A. 



years entirely of steel, owing to the fact that the improved method 

 of making steel has rendered it even cheaper than wrought iron. 



Let Fig. 6 represent a side-view of the flanged girder, or I-bar, 

 of which Fig. 6 A is an end-view. 



Suppose this beam to be supported at each end and a load 

 placed upon the center. Then the tendency of that load would 

 be to bend the beam down in the shape of the dotted line, and, in 

 case the load is sufficient, it would break in that way. 



Before the breaking-point is reached the top and bottom of this 

 beam are subjected to totally opposite classes of strain, as you will 

 see. If you bend the beam, the tendency in the bottom of it is 

 to pull the beam apart, or, in technical language, the bottom 

 flange of the beam is in tension tension being the force which 

 tends to pull apart the particles of the beam. Thus, if you take 

 a string fastened at one end, and hang a weight on the other end, 

 the string is in tension, the action of the weight being to pull the 

 particles of the string apart. 



The top flange of the beam is in compression that is, the 



