« ÖncekiDevam »
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
limit for a short-time load. This may be illustrated to some extent by a test like that of the three oak specimens shown in Fig. 4 with diagrams in Fig. 16. This, of course, cannot be carried out before the class on account of the long time required, but, if explained in object and plan, and kept going during the week, will arouse much interest and discussion. The underlying motive in all this testing in connection with the class
room work should be to arouse the student, to make him see facts and relations clearly, to make him think. To this end also some simple problems in designing may be introduced from time to time, illustrating the practical use to be made of these test results. For instance, following the timber tests, the design of a low railroad trestle proves both interesting and instructive, requir
RESULTS OF TESTS OF WHITE PINE-SERIES B-PROPERTIES OF
ing as it does a discussion of factors of safety and allowances to be made for knots, seasoning checks and decay.
Now taking up the different materials as cast iron,
mwa Tons of white Pine Sonno - Jirengrm in Iwper a inshow according to painon A piece in plan, ond under three condimront Dylce---specimens dried over biter an. ..
s} Wer :