SUPERSTRUCTURES. 563 



doors and window-frames, for the flooring and wainscoting of 

 the different stones (though even these may be made partly of 

 stone and cement, supported by steel girders), and for stair- 

 cases and roofing. 



Wooden scaffolding used during the construction of build- 

 ings of all kinds also requires a large quantity of round 

 timber and some planking ; work-sheds and other similar 

 constructions are made usually with a wooden framework. 



Beams, scantling, planks, etc., and round timber for 

 scaffolding, are the usual forms in which wood is used in 

 superstructures, while the properties which timber should 

 possess for use in superstructures may be considered under 

 the headings shape and dimensions, strength, durability 

 and weight. 



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

 the construction of staircases and of half-timbered houses, 

 curved wood is not excluded, 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 are classed in uniform sizes. They 

 are seldom thinner at centre than 4 to 6 inches, or thicker 

 than 1 to 1J feet. 



(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 purpose 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 thickness therefore should be placed in the direc- 

 tion in which the load is applied. In order to provide 

 sufficient stiffness asw r ell as strength, the depth of beams, etc., 

 is made from T ^th (in short beams) to ^jth of the length or 

 span (in beams 20 feet long), and to give lateral stiffness, the 

 width is about Jrd of the depth in short beams, and Jth in 



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