A. Frank Woeste, P.E., a professor of wood construction engineering at Virginia Tech, responds: It is true that the allowable Fb value decreases as the depth increases. This phenomenon has been proven by extensive testing. The current span tables — not only for Southern Pine, but for all lumber species — reflect that testing.
As to "why" this is so, there is no conclusive answer, but several theories have been offered over the past three decades. My own theory is based on probability — the probability that, under ASTM test protocols, more knots and other imperfections are likely to occur in wider pieces of lumber.
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