This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
A live load of 200 pounds per square foot over the entire arch would unquestionably increase the thrust over the entire arch, especially at the abutments. The stress due to shortening will of course be increased in proportion to the increase in the thrust. The stress due to moment cannot be accurately predicted. Of course such an examination and test for full loading should be made in the case of any arch to be constructed, and should be worked out precisely on the same principles and in general by identically the same method as was used above.
To test the arch for a concentrated loading such as would be produced by the passage of a road roller, or, in the case of a railroad bridge, by an especially heavy locomotive, the test must be made by assuming the position of that concentrated load which will test the arch most severely. Ordinarily this will be found when the concentrated load is at or near one of the quarter points of the arch. The only modification of this test over that given above in detail, is in the drawing of the load line, but the general method is identical.