Page:Cassells' Carpentry and Joinery.djvu/56

40 various pieces, and for the effect of shakes, knots, etc. For example, the case of a balk 13½ in. square and 10 ft. 6 in. span gave 1,114 cwt. as the result according to Tredgold, and 1,120 cwt. according to Barlow. The same size and quality of timber tested by the Mersey Dock Engineers

Fig. 162.—Beam with Wrought Iron Strap.

gave a result of 610 cwt. only, against the preceding figures. It appears from the latter example that the constant should be reduced to, say, 2·6 or 2·3, or say 2·5 to 3 for Memel timber. The formula then is

where is the breaking weight in cwt. in the centre of the span. We then have for

Fig. 163.—Cup-shake in Log.

Fig. 164.—Cup-shake in Balk.

Fig. 165.—Heart-shake in Log.

a beam 12 in. wide, 11 in. deep, and 24 ft. span, a strength of $$\frac {12 x 11^2 x 3} {24}$$ = 181·5 cwt. = 9 tons 1½ cwt. Kirkaldy's experiments with a beam of this span and size showed the strength to be 10 tons. It is considered that timber has a set with only one-fifth of its breaking load, and is really safe when loaded to only one-sixth of the breaking load. If the beam is fixed at both ends, it is stronger than when only supported at the ends as 3 is to 2. Some qualities of timber are stronger in tension than in compression, whilst others have just the opposite qualities. Experiments show that Dantzig fir is crushed before it is torn asunder; or, in other words, that its ultimate compressive stress is less than its ultimate tensile stress as 4 is to 5. An experiment was made some time ago by Kirkaldy on the strength added to a beam by the fixing on the top of the beam of a flat iron bar. The span of the beam was 24 ft., and the depth and width were 14in. and 12 in. respectively. According to the above formula, with a constant of 3, the central breaking load should be $$ \frac {14^2 x 12 x 3}{24}$$ = 294 cwt. = 14¾ tons, or with constant of 2·5 $$ \frac {14^2 x 12 x 2.5}{24}$$ = 245 cwt. = 12¼ tons. When the experiment was made, however, the beam snapped suddenly with a central load of 10 tons, showing that the above constants were too high for this case. A similar beam (Fig. 162) was then provided with a wrought-iron strap fixed on the top, and it was then found that the beam failed slowly and gradually under a load of 13 tons—an experiment which showed that added strength was given to the beam by the addition of the iron bar. In other kinds of timber possibly the iron strengthening bar should be on the other side of the beam.

Fig. 166.—Heart-shake in Balk.

Cup-shakes.—These are cracks extending circumferentially at one or more places, caused by the separation of the annual rings, as in Figs. 163 and 164.