[Footnote 57:  Bul. 108, U. S. Forest Service:  Tests of structural timbers, pp. 53-54.]

MOISTURE DETERMINATION

In order for tests to be comparable, it is necessary to know the moisture content of the specimens at the zone of failure.  This is determined from disks an inch thick cut from the timber immediately after testing.

In cases, as in large beams, where it is desirable to know not only the average moisture content but also its distribution through the timber, the disks are cut up so as to obtain an outside, a middle, and an inner portion, of approximately equal areas.  Thus in a section 10” x 12” the outer strip would be one inch wide, and the second one a little more than an inch and a quarter.  Moisture determinations are made for each of the three portions separately.

The procedure is as follows: 

(1) Immediately after sawing, loose splinters are removed and each section is weighed.

(2) The material is put into a drying oven at 100 deg.  C. (212 deg.  F.) and dried until the variation in weight for a period of twenty-four hours is less than 0.5 per cent.

(3) The disk is again carefully weighed.

(4) The loss in weight expressed in per cent of the dry weight indicates the moisture content of the specimen from which the specimen was cut.

MACHINE FOR STATIC TESTS

The standard screw machines used for metal tests are also used for wood, but in the case of wood tests the readings must be taken “on the fly,” and the machine operated at a uniform speed without interruption from beginning to end of the test.  This is on account of the time factor in the strength of wood. (See SPEED OF TESTING MACHINE, below.)

The standard machines for static tests can be used for transverse bending, compression, tension, shear, and cleavage.  A common form consists of three main parts, namely:  (1) the straining mechanism, (2) the weighing apparatus, and (3) the machinery for communicating motion to the screws.

The straining mechanism consists of two parts, one of which is a movable crosshead operated by four (sometimes two or three) upright steel straining screws which pass through openings in the platform and bear upward on the bed of the machine upon which the weighing platform rests as a fulcrum.  At the lower ends of these screws are geared nuts all rotated simultaneously by a system of gears which cause the movable crosshead to rise and fall as desired.

The stationary part of the straining mechanism, which is used only for tension and cleavage tests, consists of a steel cage above the movable crosshead and rests directly upon the weighing platform.  The top of the cage contains a square hole into which one end of the test specimen may be clamped, the crosshead containing a similar clamp for the other end, in making tension tests.