SUPERSTRUCTURES. 113 



(b) Shape and Dimensions of Timber Used. — Although in the 

 construction of staircases and of half-timbered houses, curved 

 wood is admissible, the carpenter requires straight, cylindrical 

 logs for most of his pieces. The length and diameter of the 

 pieces depend, of course, on the size of the building for which 

 they are required, but the pieces used for any particular building 

 will be classed in uniform sizes. They are seldom thinner 

 at centre than 4i to 6 inches, or thicker than 1 foot. The 

 usual transverse dimensions of squared timber for constructions 

 is from 7 to 9 inches, for which, allowing for bark and sap- 

 wood and average cylindrical shape, trees measuring 1 foot to 

 14 inches in diameter are required. As regards length, the 

 carpenter usually prefers the longest pieces, provided the fall- 

 ing-otf in cylindrical shape is not too great. 



(c) Strength of Material. — Timber is subjected to loads which 

 when applied transversely to the length of the pieces tend to 

 cross-break them. In such cases, the timber serves the pur- 

 pose of a beam, as for instance, the joists for supporting floors 

 and rafters for roofs. 



The strength to resist bending is proportional to the width of 

 the beam and the square of its depth. Two beams of half width 

 have the same strength as one of whole width, but two beams of 

 half depth superposed one on the other have only half the 

 strength of one of whole depth. The greater transverse thick- 

 ness therefore should be placed in the direction in which the 

 load is applied. In order to provide sufficient stifiness as well 

 as strength, the depth of beams, &c., is made from yVth (in 

 short beams) to ^rVth of the length or span (in beams 20 feet 

 long), and to give lateral stiffness, the width is about ^rd of the 

 depth in short beams, and ^th in long beams. For spans 

 exceeding 20 feet, iron and steel girders are generally used. 



When the load is applied in the direction of the length of the 

 piece of timber, the latter acts as a strut or column, when the 

 load thrusts, or as a tie when the load pulls. Timber is very 

 rarely exposed to a strong pull on account of the difficulty of 

 getting secure attachments to the ends of a piece, by merely 

 butting the ends, however, unlimited thrust may be applied. 

 Long struts or columns are liable to yield not by direct crushing, 



VOL. v. I 



