Strength of Timbers

As this was a much greater weight than was generally to be hoisted to the warehouse in ques tion, my two carpenter friends felt quite satis fied when our calculations showed that their guess at the size of the cantilever had been on the safe side, and that it was strong enough for any emergency.

A

Braced Cantilever Beam. On inquiring as to the reason why the point of suspension for the load was so far out, it was explained that bulky packages were carried up and down, and it was therefore necessary that they should swing well clear of the stories below. That led to the sug gestion that a strut or brace could be used under the cantilever; and, as this would increase its strength very materially, it was decided to cal culate just what would be gained by so doing.

Taking the sketch of the strutted cantilever (Fig. 163, B), we found that the amount over hanging the end of the strut was 2 feet. But the under side had been weakened at the joint where the strut had been let into the cantilever, and the effective depth of the beam was thereby re duced by 11/2 inches, the depth of the shoulder. sum, therefore, was changed, reading as at C; and gave us for result cwts. as the breaking weight of a pitch pine beam 2 feet long, by 5 inches. Dividing this by 4 (neglecting

the odd cwt.), we obtained our breaking weight for the cantilever cwts. Again dividing this by 8, we obtained as our safe load cwts., approximately, thus showing a considerable increase of strength over the cantilever without the strut.

The foregoing example of a cantilever used for permanent hoisting purposes is not, perhaps, met with so commonly to-day as formerly, ele vators inside of buildings having largely super seded them. For purposes of temporary work, however, such as the raising of some heavy article to the upper floor of a building, the piece of timber projecting from some opening is still frequently in evidence. The nearest carpenter is generally called upon to rig up the affair, and the formula worked out here (see Fig. 162) is an exceedingly useful one to have at hand in such cases. For, unlike beams, girders, joists, etc., the sizes of which are calculated in most cases by the designer of the building, these tem porary rigs are left to the skill and ingenuity of the carpenter, who may be called upon at a moment's notice to supply something upon which the lives of some of his fellows and the safety of some valuable piece of work may depend.