Sistering beams - which is stronger (if any)

[senconailgun]

The question:
Is bolting 2 - 9x3 beams togther with dog tooth plates and bolts the same strength
as bolting 3 - 9x2 beams togther with dog tooth plates and bolts ?

On paper, the 9x2 is better grade timber because is rated at C16
So that will make it slightly stronger.
9x3 tends to be a little rougher and most do not have C16 classification

 

[Dereekoo]

Yes and no. Whichever way you go you end up with a 9 x 6 beam with wood of strength X. Except that when wood is graded to be on the safe side its strength value is assumed to to be at the lower end of the statistical probability, however if wood is made up different pieces the chance that some pieces of wood will be at the higher end of the statistical probability increases and therefore the strength value of X may be enhanced. Of course if you just had one piece of wood this could well be stronger than the X value given to it. This is why the allowable stress values in floor joists can be increased by 10% if 4 or more floor joists are used in conjunction with each.

 

[^woody^]

I think you're asking the wrong question, unless this is for a test paper.

A beam of any sort only needs to be adequate for the loads, so that is what you need to work out.

If we are in to hypotheticals, then fixings, more fixings and charring or rotting slender timber could create more risk of failure.