Characteristic failures of simple beams.
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 formulæ for horizontal shear. ([See page 98] for this formulæ.) 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.
Figure 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) | ||||
| COMMON NAME OF SPECIES | Total number of tests | Per cent of total failing by | ||
| Tension | Compression | Shear | ||
| 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 |
| dry | 52 | 54 | 19 | 27 |
| Loblolly pine: | ||||
| green | 111 | 40 | 53 | 7 |
| dry | 25 | 60 | 12 | 28 |
| Tamarack: | ||||
| green | 30 | 37 | 53 | 10 |
| dry | 9 | 45 | 22 | 33 |
| Western hemlock: | ||||
| green | 39 | 21 | 74 | 5 |
| dry | 44 | 11 | 66 | 23 |
| Redwood: | ||||
| green | 28 | 43 | 50 | 7 |
| dry | 12 | 83 | 17 | |
| Norway pine: | ||||
| green | 49 | 18 | 76 | 6 |
| dry | 10 | 30 | 60 | 10 |
| NOTE.—These tests were made on timbers ranging in cross section from 4" × 10" to 8" × 16", and with a span of 15 feet. | ||||
TOUGHNESS: TORSION
Toughness is a term applied to more than one property of wood. Thus wood that is difficult to split is said to be tough. Again, a tough wood is one that will not rupture until it has deformed considerably under loads at or near its maximum strength, or one which still hangs together after it has been ruptured and may be bent back and forth without breaking apart. Toughness includes flexibility and is the reverse of brittleness, in that tough woods break gradually and give warning of failure. Tough woods offer great resistance to impact and will permit rougher treatment in manipulations attending manufacture and use. Toughness is dependent upon the strength, cohesion, quality, length, and arrangement of fibre, and the pliability of the wood. Coniferous woods as a rule are not as tough as hardwoods, of which hickory and elm are the best examples.