Ian G wrote:
Bolting through the neutral axis will have negligible effect on the
stiffness of the beam
A 12 mm hole is about an eighth of your depth, Stiffness proportional to
depth cubed, so about 0.2% reduction..
There is no need to glue the beams together (as confirmed by my structural
engineer)

You can stick these on each bolt between the beams:

http://www.screwfix.com/sfd/i/cat/50/p1704550_x.jpg

(and flat plate washers either side)


--
Cheers,

John.

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