... hmmmm ... so it seems there may be a trend. I don't know if this could be a rule of thumb, or not ... but it seems with our dimension lumber we are getting joist depths that are about 1/16th the span, and the design is governed by flexure (bending stress). For our glulam beams we were getting depths of about 1/20th (or so) the span, and deflection controlled.
Some thoughts: assuming we are using the same wood `resource', glulam is `more efficient'; we place the higher strenght pieces where they do more good, and put the lower strength where strength is not as much needed. The result is that our DF glulams are about twice (or more) as strong as the `sawn' ... but with near the same stiffness (E).
Another thought is this: generally we are using glulam for the longer spans ... and deflection governs for longer spans, irrespective of strength.
(Short spans: shear or bearing; medium spans: flexure (or maybe deflection); long spans: deflection (sometimes flexure); really long spans: deflection.)
... flexure (F_b) is a `strength' issue ...
... deflection is a `stiffness' issue (E) ...
... flexure deals with `safety' ...
... deflection deals with `serviceability'.
...
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