HEAVY TIMBER MILL CONSTRUCTION BUILDINGS 



Where maximum strength requirements are desired the quality 

 dense pine alone should be specified and used, but where the 

 strength requirements are not of the highest character the 

 grade sound pine will be found sufficient. 



The size, character and distribution of knots may materially 

 affect the strength of any good timber, as indicated in the ap- 

 pendix to the suggested Building Code of the National Board 

 of Fire Underwriters. 



"The weakening effect of knots also depends upon their position, as well 

 as their soundness, tightness and the amount they distort the grain of the 

 wood from a straight line. A comparatively small knot near the lower edge 

 of a beam may be more harmful than a large knot located elsewhere. For 

 example, a series of tests made upon loblolly yellow pine beams by the 

 U. S. Forest Service, showed that the average strength of such beams with 

 knots located in the bottom quarter of the middle half of the beams, was 

 reduced 25% below that of similar beams with knots located in other 

 portions. In such cases a knot near the neutral plane may act as a pin 

 and serve to strengthen the beam against failure by horizontal shear. 



"The number, character and location of defects in timber has much to 

 do with its strength value. Checks and shakes in beams reduce the area 

 which resist horizontal shear. Such defects are most harmful in the middle 

 half of the height of the beam, as they are then comparatively near the 

 neutral plane where their effect is greatest. The best place to judge of the 

 effect of such defects is on the ends of the timber." 



It should be noted that most timber specifications have ex- 

 planatory clauses relating to the size of timbers as specified. 

 The American Society for Testing Materials and the American 

 Railway Engineering Association have adopted a clause which 

 is used more or less universally, according to which rough sawed 

 timbers should not be more than 14 inch, nor dressed timbers 

 more than % i nc ^ scan t of nominal size; that is, a nominal 

 12"xl2" timber should not be less than ll%"xlj%? when 

 sawed, or n.^"xll^>" when dressed. 



The lasting power of timbers will be determined both by 

 the kind and the quality of the wood used and by the con- 

 ditions which obtain in the building in which it is employed. 

 The timbers may have sapwood and heartwood in varying per- 

 centages. Sapwood is usually short-lived and where conditions 

 are favorable to decay, will usually decay very rapidly. Heart- 

 wood, of practically all species, on the other hand, is compara- 

 tively long-lived. In buildings where the humidity is low such 

 timbers usually last 25 and 30 years and longer when practically 

 all-heart timbers are used. It is therefore of the utmost im- 



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