Roger gives an excellent description of moment (bending) capacity for a beam. But, I think the question is about bearing capacity of wood (compressive strength in the radial or transverse direction to the grain). If I'm understanding the op right the compressive strength perpendicular to grain is considered to be the same per square inch whether the lumber is up on its side or flat on its' face. The compressive strength is a good bit higher parallel to grain, longitudinally, standing on end. Remember that in any given board we have no control over which face is radial and which is tangential , or somewhere in between. The saw made that choice.
Then there are the real world ceaveats. Short chunks of end grain cutoffs jammed under a beam will shear. split and crumble under load. A tall narrow piece of wood under a beam is likely to tip over. Bearing on the end grain, say of a stud or post provides great bearing strength as far as the stud is concerned but it is supporting the side grain of the beam, check the bearing capacity of it perp to load.
Going all the way back behind the NDS to the USFPL's "Wood Engineering Handbook" is this comment, talking about loading perp to grain;
Stresses perpendicular to the fiber (grain) direction may be at any angle from 0 degrees (tangential) to 90 degrees (radial) to the growth rings. Perp to grain properties depend somewhat upon orientation of annual growth rings with respect to the direction of stress. Compression perp to grain values are derived from tests in which the load is applied parallel to the growth rings (tangential direction). Proportional limit in compression increases from 0 degrees (T) to 90 degrees ®, the total increase amounting to about one third. For some softwoods in compression the values at ) and 90 are about the same, with the value at 45 about 2/3 of the others.
