Now, in the case of a masonry arch (Fig. 26), which has the weight all on top, there is no necessity for the tie-rod to hold the ends of the arch together, for the reason that the ends of the arch are always built so as to abut against heavy masonry which will withstand the horizontal thrust, and thus without the intervention of any tie members we have a perfect bridge by means of the
arch, all the weight coming upon the top being passed from one stone to the other in the arch until it reaches the two abutments the same as in a framed truss.
The suspension bridge (Fig. 28) is nothing more or less than the arch turned upside down. In the arch, as we have seen, the
only strain that comes upon it is compression; in the suspension bridge, on the other hand, the only strain that comes upon the sustaining member or cable which is stretched between the points of support is tension. In the arch bridge all the weight comes above the arch and presses down upon it; in the suspension bridge all the weight comes below the suspension chains and simply
