412 



CAMBRIA STEEL. 



UNIT STRESSES FOR STRUCTURAL TIMBER. 



(Expressed in Pounds per Square Inch.) 



NOTE. These unit stresses are for a green condition of timber and are to 

 * Partially air-dry. 



The above table gives the ultimate and safe unit stress values for structural 

 timber as adopted by the American Railway Engineering and Maintenance 

 of Way Association, upon recommendation of their Committee on Wooden 

 Bridges and Trestles, Convention of 1909 ; and published in the Association's 

 "Bulletin No. 107," 1909, and "Manual," 1911. 



They state that the working unit stresses given in this table are intended 

 for railroad bridges and trestles. For highway bridges and trestles, the 

 unit stresses may be increased twenty-five (25) per cent. For buildings 

 and similar structures, in which the timber is protected from the weather and 

 practically free from impact, the unit stresses may be increased fifty (50) 

 per cent. To compute the deflection of a beam under long continued loading 

 instead of that when the load is first applied, only fifty (50) per cent, of the 

 corresponding modulus of elasticity given in the tables is to be employed.f 



The safe unit stresses were determined by carefully considering both the 

 average ultimate stresses, which represent the best results now available, as 

 well as the unit stresses which have been in use in designing wooden bridges 

 and trestles, and have been demonstrated by extensive practice to be safe. 



t Timber has no well-defined modulus of elasticity. ED. 



