12 



flections. The upper dotted zig-zag line similarly indi- 

 cates the limits of deflection for a modulus of elasticity of 

 750,0'00 Ib. per square inch. 



The lower full zig-zag line indicates the limit of allow- 

 able shearing stress along the axis corresponding to the 

 allowable intensity, for yellow pine, of 150 Ib. per square 

 inch. 



Similarly, the upper full zig-zag line indicates the limits 

 for shearing along the axis for white Oak based on an 

 allowable intensity of 200 Ib. per square inch. 



BEARING AT POINTS OF SUPPORT. 



Care should be taken in designing to provide sufficient 

 bearing at the points of support so that the allowable in- 

 tensity of compression across the jirain is not exceeded. 



This may be obtained, where necessary, by the use of 

 corbels or bearing plates of harder wood arranged so as 

 to give a large bearing area against the softer beam. 



The following statements are made in Bulletin No. 12, 

 U. S. Department of Agriculture, Division of Forestry: 

 RECOMMENDED PRACTICE. 



"Since the strength of timber varies very greatly with 

 the moisture contents (see Bulletin 8 of the Forestry Divi- 

 sion), the economical designing of such structures will 

 necessitate their being separated into groups according to 

 the maximum moisture contents in use. 



MOISTURE CLASSIFICATION. 



"Class A (moisture contents, 18 per cent.) Structures 

 freely exposed to the weather, such as railway trestles, 

 uncovered bridges, etc. 



"Class B (moisture contents, 15 per cent.) Structures 

 under roof but without side shelter, freely exposed to oui- 

 side air, but protected from rain, such as roof trusses of 

 open shops artd sheds, covered bridges over streams, etc. 



"Class C (moisture contents, 12 per cent.) Structures in 

 buildings unheated, but more or less protected from out- 

 side air, such as roof trusses of barns, enclosed shops and 

 sheds, etc. 



"Class D (moisture contents, 10 per cent.) Structures 

 in buildings at all times protected from the outside air, 

 heated in the winter, such as roof trusses in houses, halls, 

 churches, etc. 



"For long-leaf pine add to all the values given in the 

 tables, except those for moduli of elasticity, tension and 

 slir:irin.LT, for Class B, 15 PIT cent; for ClfUM (', 40 per 

 cent ; for Class D, 55 per cent. For the other species add 



