oints; and some simply brings out the old and over-worked
argument which can be paraphrased about as follows: "The structures
stand up and perform their duty, is this not enough?"

The last-mentioned argument is as old as Engineering; it is the
"practical man's" mainstay, his "unanswerable argument." The so-called
practical man will construct a building, and test it either with loads
or by practical use. Then he will modify the design somewhere, and the
resulting construction will be tested. If it passes through this
modifying process and still does service, he has something which, in his
mind, is unassailable. Imagine the freaks which would be erected in the
iron bridge line, if the capacity to stand up were all the designer had
to guide him, analysis of stresses being unknown. Tests are essential,
but analysis is just as essential. The fact that a structure carries the
bare load for which it is computed, is in no sense a test of its correct
design; it is not even a test of its safety. In Pittsburg, some years
ago, a plate-girder span collapsed under the weight of a locomotive
which it had carried many times. This bridge was, perhaps, thirty years
old. Some reinforced concrete bridges have failed under loads which they
have carried many times. Others have fallen under no extraneous load,
and after being in service many months. If a large number of the columns
of a structure fall shortly after the forms are removed, what is the
factor of safety of the remainder, which are identical, but have not
quite reached their limit of strength? Or what is the factor of safety
of columns in other buildings in which the concrete was a little better
or the forms have been left in a little longer, both sets of columns
being similarly designed?

There are highway bridges of moderately long spans standing and doing
service, which have 2-in. chord pins; laterals attached to swinging
floor-beams in such a way that they could not possibly receive their
full stress; eye-bars with welded-on heads; and many other equally
absurd and foolish details, some of which were no doubt patented in
their day. Would any engineer with any knowledge whatever of bridge
design accept such details? They often stand the test of actual service
for years; in pins, particularly, the calculated stress is sometimes
very great. These details do not stand the test of analysis and of
common sense, and, therefore, no reputable engineer would accept them.

Mr. Turner,