Sayfadaki görseller
PDF
ePub

properties. For the strength tests, a clear, straightgrained, well-seasoned, white pine plank, 16 inches broad and 13 inches thick, having a little sap-wood in two outer corners, was used (Fig. 4). Specimens from this plank were tested under three conditions of moisture, viz.: "Dry," specimens dried over a boiler one week; "Normal," specimens tested as cut from the plank; and "Wet," specimens kept under water one week. There being six laboratory divisions, the plank was cut so as to give six endwise compression specimens in its width (Fig. 4). Thus, specimens Nos. 1 Dry, No. 7 Normal, and No. 13 Wet, representing the three moisture conditions, and all from similar positions with respect to the center of the tree, were tested in the first laboratory division (Fig. 5). The next three specimens, in the second division (Fig. 6), and so on (Figs. 7, 8, 9 and 10). These three tests, with full data as to loads and shortenings, were easily finished within the hour. The shortening was measured with a Brown and Sharpe test indicator as shown in Fig. 11. This instrument reads directly to thousandths of an inch, but ten-thousandths may be estimated with fair degree of accuracy. The method of measuring between compression blocks proves sufficiently accurate if a small initial load be applied before taking the first reading.

In same manner, during the laboratory periods of the following two weeks, the remainder of the tests (Fig. 4), consisting of across grain compression (Figs. 12, 13 and 14), endwise tension, across grain tension, and along the grain shear, were completed; each student following a strip of the plank throughout its length through all these tests.

[graphic][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][merged small][merged small][subsumed][subsumed][ocr errors][subsumed][ocr errors][merged small][subsumed][subsumed][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][ocr errors][ocr errors][ocr errors][ocr errors][merged small][ocr errors][ocr errors][merged small][ocr errors][merged small][merged small][merged small]
[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][ocr errors][ocr errors][merged small][merged small][ocr errors][ocr errors][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][ocr errors][merged small][ocr errors][ocr errors][merged small][merged small][ocr errors][merged small][merged small][ocr errors][merged small][merged small][ocr errors][ocr errors]
[graphic][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][ocr errors][ocr errors][ocr errors][subsumed][subsumed][merged small][merged small][merged small][ocr errors][merged small][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][merged small][subsumed][graphic][subsumed][subsumed][ocr errors][subsumed][ocr errors][subsumed][subsumed][subsumed][merged small][subsumed]
[graphic]

All the results were now tabulated and reported to the students on mimeographed sheets in the form shown. in the table. These results were then plotted by the students with respect to position of the test specimens in the plank, to exhibit more clearly the variations due to difference in the structure of the wood formed at different periods of the tree's growth (Fig. 15). On the whole these results are consistent, though but a

single specimen represents each actual condition, and illustrate quite forcibly some of the variables in the strength of timber. There is another variable, however, which must be recognized before results like these can be used intelligently in designing, and that is the "time effect." The strength of timber for permanent loads is only about fifty to sixty per cent. of these values; or, in other words, the ultimate strength of timber for a permanent load is the same as the elastic

« ÖncekiDevam »