Professor Howe made some recent tests on checking up the elastic behavior of arches.  He reports[X] that “a very slight change at the support does seriously affect the values of H and M.”  The arch tested was of 20-ft. span, and built between two heavy stone walls out of all proportion to the magnitude of the arch, as measured by comparison with an ordinary arch and its abutment.  To make the arch fixed ended, a large heavily reinforced head was firmly bolted to the stone wall.  Practical fixed endedness could be attained, of course, by means such as these, but the value of such tests is only theoretical.

Mr. Mensch says: 

“The elastic theory was fully proved for arches by the remarkable tests, made in 1897 by the Austrian Society of Engineers and Architects, on full-sized arches of 70-ft. span, and the observed deflections and lateral deformations agreed exactly with the figured deformation.”

The writer does not know of the tests made in 1897, but reference is often made to some tests reported in 1896.  These tests are everywhere quoted as the unanswerable argument for the elastic theory.  Let us examine a few features of those tests, and see something of the strength of the claim.  In the first place, as to the exact agreement between the calculated and the observed deformations, this exact agreement was retroactive.  The average modulus of elasticity, as found by specimen tests of the concrete, did not agree at all with the value which it was necessary to use in the arch calculations in order to make the deflections come out right.

As found by tests on blocks, the average modulus was about 2,700,000; the “practical” value, as determined from analysis of a plain concrete arch, was 1,430,000, a little matter of nearly 100 per cent.  Mansfield Merriman, M. Am.  Soc.  C. E., gives a digest of these famous Austrian tests.[Y] There were no fixed ended arches among them.  There was a long plain concrete arch and a long Monier arch.  Professor Merriman says, “The beton Monier arch is not discussed theoretically, and, indeed, this would be a difficult task on account of the different materials combined.”  And these are the tests which the Engineering Profession points to whenever the elastic theory is questioned as to its applicability to reinforced concrete arches.  These are the tests that “fully prove” the elastic theory for arches.  These are the tests on the basis of which fixed ended reinforced concrete arches are confidently designed.  Because a plain concrete bow between solid abutments deflected in an elastic curve, reinforced concrete arches between settling abutments are designed with fixed ends.  The theorist has departed about as far as possible from his premise in this case.  On an exceedingly slender thread he has hung an elaborate and important theory of design, with assumptions which can never be realized outside of the schoolroom or the designer’s office.  The most serious feature of such theories is not merely the approximate and erroneous results which they give, but the extreme confidence and faith in their certainty which they beget in their users, enabling them to cut down factors of safety with no regard whatever for the enormous factor of ignorance which is an essential accompaniment to the theory itself.