Wood exhibits its greatest strength in tension parallel to the grain, and it is very uncommon in practice for a specimen to be pulled in two lengthwise.  This is due to the difficulty of making the end fastenings secure enough for the full tensile strength to be brought into play before the fastenings shear off longitudinally.  This is not the case with metals, and as a result they are used in almost all places where tensile strength is particularly needed, even though the remainder of the structure, such as sills, beams, joists, posts, and flooring, may be of wood.  Thus in a wooden truss bridge the tension members are steel rods.

The tensile strength of wood parallel to the grain depends upon the strength of the fibres and is affected not only by the nature and dimensions of the wood elements but also by their arrangement.  It is greatest in straight-grained specimens with thick-walled fibres.  Cross grain of any kind materially reduces the tensile strength of wood, since the tensile strength at right angles to the grain is only a small fraction of that parallel to the grain.

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|                           TABLE II                           |
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|   RATIO OF STRENGTH OF WOOD IN TENSION AND IN COMPRESSION    |
|           (Bul. 10, U. S. Div. of Forestry, p. 44)           |
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|               |       Ratio:     |  A stick 1 square inch in  |
|               |                 |      cross section.        |
|               |      Tensile    |                            |
| KIND OF WOOD  |      strength   |   Weight required to--     |
|               | R = ----------- +----------------------------|
|               |     compressive | Pull apart | Crush endwise |
|               |      strength   |            |               |
|---------------+-----------------+------------+------------

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| Hickory       |        3.7      |     32,000 |     8,500     |
| Elm           |        3.8      |     29,000 |     7,500     |
| Larch         |        2.3      |     19,400 |     8,600     |
| Longleaf Pine |        2.2      |     17,300 |     7,400     |
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| NOTE.--Moisture condition not given.                         |
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Failure of wood in tension parallel to the grain occurs sometimes in flexure, especially with dry material.  The tension portion of the fracture is nearly the same as though the piece were pulled in two lengthwise.  The fibre walls are torn across obliquely and usually in a spiral direction.  There is practically no pulling apart of the fibres, that is, no separation of the fibres along their walls, regardless of their thickness.  The nature of tension failure is apparently not affected by the moisture condition of the specimen, at least not so much so as the other strength values.[3]