| | | | | | | | | | | | | | | | | | | | | | | | | | Speed-strength |(0.014)|(0.005)| 0.012 | 0.033 | 0.014 | 0.026 | 0.049 | 0.026 | 0.037 | 0.053 | 0.033 | 0.038 | 0.049 | 0.014 | 0.035 | 0.038 | 0.006 | 0.025 |(0.023)|(0.004)|(0.014)| | | modulus, T | | | | | | | | | | | | | | | | | | | | | | |-----------------------------------------------------------
--------------------------------------------------------------------------------------------------------------------------------------| | NOTE.--The usual speeds of testing at the U.S.  Forest Service laboratory are at rates of fibre strain | | of 15 and 10 ten-thousandths in. per min. per in. for compression and bending respectively. | |-----------------------------------------------------------
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BENDING LARGE BEAMS

Apparatus:  A static bending machine (described above), with a special crosshead for third-point loading and a long platform bearing knife-edge supports, is required. (See Fig. 29.)

[Illustration:  FIG. 29.—­Static bending test on large beam.  Note arrangement of wire and scale for measuring deflection; also method of applying load at “third-points.”]

Preparing the material:  Standard sizes and grades of beams and timbers in common use are employed.  The ends are roughly squared and the specimen weighed and measured, taking the cross-sectional dimensions midway of the length.  Weights should be to the nearest pound, lengths to the nearest 0.1 inch, and cross-sectional dimensions to the nearest 0.01 inch.

Marking and sketching:  The butt end of the beam is marked A and the top end B.  While facing A, the top side is marked a, the right hand b, the bottom c, the left hand d.  Sketches are made of each side and end, showing (1) size, location, and condition of knots, checks, splits, and other defects; (2) irregularities of grain; (3) distribution of heartwood and sapwood; and on the ends:  (4) the location of the pith and the arrangement of the growth rings, (5) number of rings per inch, and (6) the proportion of late wood.

The number of rings per inch and the proportion of late wood should always be determined along a radius or a line normal to the rings.  The average number of rings per inch is the total number of rings divided by the length of the line crossing them.  The proportion of late wood is equal to the sum of the widths of the late wood crossed by the line, divided by the length of the line.  Rings per inch should be to the nearest 0.1; late wood to the nearest 0.1 per cent.