Paul Sweet said:
As GHRoberts and Brudgers pointed out, you can't substitute directly because timbers have lower strengths than 2-bys. For instance, #2 Southern Yellow Pine has a bending strength of 1200 PSI for a 2x8, 975 PSI for a 2x12, and only 850 PSi for a timber (
http://www.southernpine.com/designvalues.shtml )I like to use the Western Wood Products Association Span Calculator to size joists & beams.
http://www.wwpa.org/store/product1.asp?SID=3&Product_ID=13
Codewonk:
Solid hewn beams has essentially the strength of a #3 grade and is as good as the weakest link. This is because of the internal make up of a piece 10" x 12" has grade variability ranging from #1/SS to #3 all within the same beam. When you slice beams down into thinner pieces such as 2x members, you can easily learn that the beam that the grade can change dramatically from the exterior face to the most inner wood.
A built-up is all controlled grade. Giving better design value option. Solid hewn can not be given as high a design value and you have to go with the lower grade even if the particular piece might happen to be completely clear/straight grain with no knots. You have to be able to determine precisely what the grain make-up is and determine the lumber grade.
At 2x dimensions, it is much easier to determine but in a 10x or 12x, it is alot harder. There isn't much visual difference between SS grade and #1 grade. #2 just has a few knots and little bit more wavy grains. #3 has more knots and more crappy quality.
Design values does have some factor of safety built-in for the relatively statistical variability of one grade lumber from another of same grade.
When designing, you need to have that understanding to properly size the member. Certainly, you can base your built-up based on the same figure for same dimensioned timber frame but you may end up using a larger built-up member then you need. For example: A 7-ply 2x12 Built-up beam may be stronger and serve the same load as a 12x12.
This is because of the controlled-lumber grading of each ply. However, Ply thickness doesn't really matter.
Brudgers: you misunderstood my post earlier. The bottom line: it needs to be properly calculated and cross-section of built-up beam means more then ply thickness. Depending on state laws, an unlicensed person may perform the calcs but must be done correctly and the math must be shown to show how you arrived to the specifications such as beam dimensions and spans. So an unlicensed person would need to do the math the old-fashion way.