AN ACCOUNT OF CERTAIN TESTS OF THE TRANSVERSE STRENGTH AND STIFFNES OF LARGE SPRUCE BEAMS.

BY

GAETANO LANZA,

Professor of Applied Mechanics, Massachusetts Institute of Technology,

BOSTON, MASS.

I BEG leave to call your attention to the results of certain tests of the transverse strength and stiffness of full-size spruce beams, carried on by members of my classes in my laboratory at the Massachusetts Institute of Technology. The machine with which they were made is a 50,000-pound machine, and is capable of testing beams twenty five feet long and under, as well as many of the framing joints used in practice.

It consists, as shown in the cut, of a compound lever, hung in a cast iron frame, to which is connected, by means of a steel rod and turn buckle, one end of a lever, of equal arms, placed below, this lever having a 12-inch leverage, and being connected at its other end by means of a chain with the yoke shown in the cut. Two hard pine beams, each 20 inches deep, 10 inches wide, and 26 feet long, are laid across the timbers of the machine in such a way that the chain already referred to is midway between them. Two common jack-screws, each in a pair of wrought iron stirrups, are placed at a distance apart depending upon the span of the beam to be tested, the latter being placed, as shown in the cut, upon the jack screws, and under the yoke. The jack-screws are then screwed up and the beam to be tested is thus raised at its two ends, and hence loaded at the point where the yoke is attached.

It was in operation about two months last session, and has been in operation about one month this present session. During that time we have tested about thirty specimens for breaking strength and about fifteen for deflection. The breaking has been effected as rapidly as could be done, consistently with the determination of the deflections, and the deflections under various loads were measured within a very short time after the application of the loads. In short, no experiments have thus far been carried on to determine the effect of time upon these quantities, though some will be made.

very soon. It was also deemed best to keep to one kind of timber, until we should have a sufficient number of tests to warrant us in drawing a conclusion as to the average value of the modulus of

rupture, and of the modulus of elasticity of that kind of wood; and spruce was selected, as a wood that is very much used in building. Experiments are now in progress in my laboratory to determine the same quantities for yellow pine.

The results that were obtained last session are shown above the double line in the accompanying table, and those obtained this present session, below.

At the beginning of this session these tests were continued, with the following objects in view:

1. Inasmuch as the lumber had, last session, been selected either by a carpenter, without reference to testing, or had been simply ordered at the yard, directions being given to the dealer to send merchantable stock, it seemed best that I should go to the yards myself and select from the piles some of the best and some of the average of what was on sale as merchantable stock, and thus that we should be able to speak with certainty about the values of the modulus of rupture and modulus of elasticity of such lumber.

2. Inasmuch as last year we had had only three determinations of the modulus of elasticity, it was desirable to obtain more values, and thus to be able to determine an average value.

3. It was desirable to see how far, in the light of what had already been done, we should be able to judge of the modulus of rupture by simply inspecting a piece of timber, and to endeavor also to train the students, to some extent, to have this ability.

That the values of the modulus of rupture which we have obtained should differ very considerably from those given in our text-books and engineers' handbooks, and deduced from tests of small pieces, I need not tell a company of engineers; but, as you may not carry in your minds the precise figures given by different authorities for the modulus of rupture of spruce, I will place them here:

Trautwine advises for use to deduct one-third in the case of

knotty and poor timber.