Horizontal shear failure, in which the upper and lower portions of the beam slide along each other for a portion of their length either at one or at both ends (see Fig. 17, No. 6), is fairly common in air-dry material and in green material when the ratio of the height of the beam to the span is relatively large.  It is not common in small clear specimens.  It is often due to shake or season checks, common in large timbers, which reduce the actual area resisting the shearing action considerably below the calculated area used in the formulae for horizontal shear. (See page 98 for this formulae.) For this reason it is unsafe, in designing large timber beams, to use shearing stresses higher than those calculated for beams that failed in horizontal shear.  The effect of a failure in horizontal shear is to divide the beam into two or more beams the combined strength of which is much less than that of the original beam.  Fig. 18 shows a large beam in which two failures in horizontal shear occurred at the same end.  That the parts behave independently is shown by the compression failure below the original location of the neutral plane.

[Illustration:  FIG. 18.—­Failure of a large beam by horizontal shear. Photo by U. S, Forest Service.]

Table XI gives an analysis of the causes of first failure in 840 large timber beams of nine different species of conifers.  Of the total number tested 165 were air-seasoned, the remainder green.  The failure occurring first signifies the point of greatest weakness in the specimen under the particular conditions of loading employed (in this case, third-point static loading).

|------------------------------------------------------

-----|
|                         TABLE XI                          |
|-----------------------------------------------------------

|
|          MANNER OF FIRST FAILURE OF LARGE BEAMS           |
|             (Forest Service Bul. 108, p. 56)              |
|-----------------------------------------------------------

|
|                  | Total  | Per cent of total failing by  |
|   COMMON NAME    | number |---------+-------------+-------|
|   OF SPECIES     | of     | Tension | Compression | Shear |
|                  | tests  |         |             |       |
|------------------+--------+---------+-------------+-------

|
| Longleaf pine:    |        |         |             |       |
|   green          |    17  |    18   |      24     |   58  |
|   dry            |     9  |    22   |      22     |   56  |
| Douglas fir:      |        |         |             |       |
|   green          |   191  |    27   |      72     |    1  |
|   dry            |    91  |    19   |      76     |    5  |
| Shortleaf pine:   |        |         |             |       |
|   green          |    48  |    27   |      56     |   17  |
|   dry            |    13  |    54   |             |   46  |
| Western larch:    |        |         |             |       |
|   green          |    62  |    23   |      71     |    6  |