STEEL FRAMEWORK OF BUILDINGS 7 



Earth, ordinary : 1 J tons 



" tamped or naturally solid 3 " 



Clay, soft 1 " 



" dry or mixed with dry sand. 2 " 



" hard pan 3 " 



" hard and mixed with coarse sand. . . 4 " 



Sand and gravel mixed, when dry and 



coarse 6 " 



Rock, broken or partly disintegrated 5-25 " 



Rock, solid bed. 200 " 



21. Bearing Power of Piles. Wooden piles shall be spaced 

 not less than 30 in. c. to c. 



When the piles are driven through wet and loose soil to a 

 good bearing the pressure shall not exceed 300 Ibs. per sq. in. 

 of their average cross-section. When driven through a firm soil 

 this pressure may be increased to 600 Ibs. (Pars. 270, 277.) 



DESIGN OF TEAMING 



22. Roof Trusses. Roof trusses shall preferably be of such 

 type as will allow the purlins to be placed only at the -panel 

 points. Wherever this is neither practicable nor economical, 

 the top chord shall be designed for both bending and direct 

 stresses. 



Roof trusses shall be riveted throughout, except in special 

 cases for long spans where the field splices and main joints may 

 be pin-connected. 



23. Spacing of Roof Trusses. The spacing of roof trusses 

 shall preferably be such as to allow only the use of single rolled 

 shapes as purlins (Par. 26). Whenever this limit be exceeded 

 intermediate jack-rafters may be used. For ordinary roofs the 

 most economical spacing of trusses will generally be as follows: 



For spans up to 40 ft. 16' 0" c. to c. 



" from 40-55 " 17' 0" 



" 55-70 " 18' 0" 



" 70-85 " 19' 0" 



" 85-100" 20' 0" 



