Page:Professional papers on Indian Engineering (second series).djvu/77

 No. CCXCI.

SCANTLINGS OF DEODAR TIMBER FOR FLAT ROOFS.

[Vide Plate.]

Communicated by the Secy. to Government Punjab, P.W. Department. Extracts of Circular No. 44, dated Lahore, 30th November, 1877.

The calculation of the scantlings of deodar timber for flat roofs has been subject to uncertainties and inaccuracies from various causes.

It had appeared that the coefficient of strength in ordinary use deduced from experiments on deodar made at Attock in 1856, and at Roorkee in 1858, was too large. And the results of the experiments recently made show that this was the case.

One of the chief causes of the erroneous results obtained from the old experiments referred to,-a probable cause of error in most experi- ments of the kind, not in India only, is the small size of the specimens with which the experiments were made. It was with reference to this defect that a set of five experiments were made at Chatham a short time ago on pieces of Memel Fir of large dimensions, the results of which were published in the Royal Engineer Journal, March 1st 1876. The nature of the error is, generally, that the strength, deduced from exper- iments on small pieces, is too great.

Again, scantlings calculated from the Strength formula, dependent on the coefficient obtained from breaking weights, even if correct, are not always sufficient to secure the required stiffness. And it was necessary to calculate the scantlings likewise by the Deflection formula, and adopt the larger result. The mode in which the use of the strength formula only has been made to answer the same purpose, according to very usual prac- tice, was to apply such a factor of safety as ensured its covering the result given by the other method. But this was not an accurate procedure though, considering the very various results of even the best experiments, its inaccuracy may not have been greater than that of the experimental data it assumed. 45