developed in different structural forms to those developed in the small clear specimens is shown for each factor in the column headed “Ratio to 2” X 2".”  Tests to determine the mechanical properties of different species are often confined to small, clear specimens.  The ratios included in the tables may be applied to such results in order to approximate the strength of the species in structural sizes, and containing the defects usually encountered, when tests on such forms are not available.

A comparison of the results of tests on seasoned material with those from tests on green material shows that, without exception, the strength of the 2” X 2” specimens is increased by lowering the moisture content, but that increase in strength of other sizes is much more erratic.  Some specimens, in fact, show an apparent loss in strength due to seasoning.  If structural timbers are seasoned slowly, in order to avoid excessive checking, there should be an increase in their strength.  In the light of these facts it is not safe to base working stresses on results secured from any but green material.  For a discussion of factors of safety and safe working stresses for structural timbers see the Manual of the American Railway Engineering Association, Chicago, 1911.  A table from that publication, giving working unit stresses for structural timber, is reproduced in this book, see Table XXII.

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| TABLE XVIII                                                TABLE XVIII                                                            |
|-----------------------------------------------------------

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|                                                  BENDING TESTS ON GREEN MATERIAL                                                  |
|-----------------------------------------------------------

------------------------------------------------------------------------|
|                 |      Sizes      |       |       |       |   F.S. at E.L.  |     M. of R.    |     M. of E.    |   Calculated    |
|                 |-----------------| Num-  | Per   | Rings |                 |                 |                 |      shear      |
|     Species     |          |      |  ber  | cent  |  per  

|-----------------+-----------------+-----------------+-----------------|
|                 |  Cross   | Span | of    | mois- | inch  | Average | Ratio | Average | Ratio | Average | Ratio | Average | Ratio |
|                 | Section  |      | tests |  ture |       | per sq. | to 2” | per sq. | to 2” | per sq. | to 2” | per sq. | to 2” |
|                 |          |      |       |       |       |  inch   | by 2” |  inch   | by 2” |  inch   | by 2” |  inch   | by 2” |
|-----------------+----------+------+-------+-------+-------

+---------+-------+---------+-------+---------+-------+---------+-------|