dations only, and that the piers and abutments therefor should never rest on piles. While it is true that bed-rock is the ideal foundation for such structures, pile foundations can be made entirely satisfactory, provided that the masonry of the bases is carried well down into firm material which is capable of resisting properly the horizontal thrusts. It is not legitimate to count upon the horizontal resistance of the piles; and it would be criminal practice to rest the piers and abutments of an arch bridge on piles the tops of which below the pier bases act like stilts because of passing through soft material before reaching a hard bearing.

magnify image

Ninth. As stated by Mr. F. H. Frankland in his discussion, "in respect to arch designing, the proper determination of the true economics calls for more judgment on the part of the designing engineer than with any other type of bridge."

The paper and the résumé combined contain twenty-six diagrams, of which only ten have been reproduced in this chapter; hence the reader who desires to utilize the results of that investigation, is referred to the paper itself, which, with all the discussions and the résumé, is soon to be published in the "Transactions" of the American Society of Civil Engineers.

As previously indicated, the comparative economics of steel arch-bridges