I understand that the Size Factor modifies the Fb bending stress for 2,3,and 4 inch thick sawn dimension lumber (5" and thicker, the Size Factor = 1.00), and the size factor is based on edgewise use (load applied to narrow face).

Now, for flatwise use, the Fb is modified by the Flat-use factor.

This may sound contradictory, but do I modify Fb by BOTH the size Factor AND the Flat-use factor for flatwise use? Or, do I use ONLY the Flat-use factor?

The reason I ask is I have an old Code book which reads thusly:

"Flat-use Factor....When dimension lumber is used flatwise...the bending design value Fb, shall also be multiplied by the following flat-use factors:"

(Table follows, with factors ranging from 1.0 to 1.2)

I use both

Both. See Breyer et all.

Thanks Mike & frv

Just remember, you can only use the Flat factor when the lumber is actually used in the flat orientation.  Seems some people - fellow idiot engineers don't understand that!!!

"Just remember, you can only use the Flat factor when the lumber is actually used in the flat orientation.  Seems some people - fellow idiot engineers don't understand that!!! "

LOL, I remember a custom house where a very long span glulam beam  (it was deep mainly for deflection control) was hanger-supported to a short glulam header (the Bottom of Beam elevations were same).

The header glulam was designed much shallower than the supported beam, considering shear and bending were OK, but just the amount of nails thru the hanger appeared to put the header in danger from splitting the wood.

The jobsite workers had even started a betting pool where they guessed what date the header would collapse!

So in other words, the designer of that header should have realized it was stupid to try to save few dollars in a 4 foot long header, he should have spec'd a much deeper header.