Using a uniform tension intensity of 18,000 lbs. for eye-bars and 16,000 lbs. for built members will strain the metal up to nearly one half of the elastic limit shown by specimen tests, and probably somewhat higher than one half of same shown by tests of full-size members. So long as the greatest actual intensity of working stress is kept in the neighborhood of one half of the elastic limit, sufficient precaution has been taken against all possibility of failure by load even in the far-distant future.

The impact formula for highway bridges given in Chapter XVI., viz.,

was established to fit the author's practice. Its correctness is not likely to be ever determined by experiment.

MEDIUM STEEL.

The reason for using this metal almost exclusively and barring out soft steel, except for rivets and adjustable members, is because the two kinds of the raw material cost almost exactly the same per pound, while medium steel is the stronger of the two by fully fifteen per cent, and is in every particular just as reliable and satisfactory for use as soft steel. The only advantage claimed for the latter is that it requires no reaming, which, in the author's opinion, is a fallacy, hecause, in order to obtain proper matching of rivet-holes in the various component parts of a piece, reaming is a sine qua non.

HIGH STEEL.

The use of this material is limited to those portions of very long spans where the impact is small, and where there is neither reversion of stress nor any condition even approaching same. The specifications bar out its employment for intermediate posts  of  Simple  trusses,  because  in  modern,  long-span