ON BRIDGES — BEAMS. 



147 



their ends would close up, and tlic upper edges would touch. Now if, 

 when in this condition, we unite in any firm way these upper edges, 

 when we take away the support from below, the beams cannot sink to 

 their original position, since the triangular spaces cannot open, and 

 the tensional strain thus brought upon the upper edge over the pier 

 will tend to neutralize the compressive strain always existing on the 

 upper edge of a beam. In a wrought-iron structure this may be very 

 easily done by raising the ends A and C until the gap at B is closed, 

 and then riveting the upper plates together. Upon letting the ends 

 A and C down again the deflection between them is diminished. 

 This was most successfully done in the case of the great Britannia 

 Bridge. 



Professor Gillespie has determined that with a flexible beam on 

 three supports, each support bears the portion of a uniformly distrib- 

 uted load indicated by the fractions in Fig. 39, and on four supports 

 as in Fig. 40. 



Fig. 39. 



It is evident that a flexible beam with a uniformly distributed load 

 may be so placed on four supports that two of them will not bear any 

 part of the weight, as in Fig. 41. 



A few words upon the practical considerations involved in the use 

 of iron in engineering structures, will not, perhaps, be amiss. In 

 this country where timber is abundant, and labor and carriage dear, 

 wood has been used to a great extent for bridges, and when iron has 

 been resorted to, wrought has usually been preferred. In England, 

 however, where the engineering taste is decidedly for the ponderous, 

 cast iron has been used to a considerable extent, and ample opportu- 

 nity has been afforded for a comparison of its merits with those of 

 wrought iron. 



Cast iron is crystalline, hard, brittle, and non-elastic ; it bears a 

 crushing strain up to from 80,000 to 100,000 pounds per square inch, 

 and a tensile strain of about 15,000 pounds.* 



Its principle advantage is the ease with which it can be cast into 

 any required form, and for heavy masses, or for pieces of nearly equal 



Eaton Hod<rkinson. 



