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Unread 11-25-2020, 08:57 AM   #9
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Join Date: Dec 2015
Location: Berkeley, CA
Posts: 1,107
Doubling of I joists, joined by solid filler in the web with glue and screws
This is the one proposed method that would double the stiffness of an I joist (plus a bit more for the solid filler). The other proposed methods are going to increase stiffness less than 100%.

Halving the span with a beam would increase the deflection resistance by at least a factor of 8.

If the bottom of the joists are clear and you didn't mind closing off the joist bays, adding a layer of plywood as a ceiling would certainly improve to the composite action of the joists to resist point loads. It could also create something like a box beam. I'm unclear on whether a single strip of plywood 4' wide or 8' down the middle would give you most of the benefit of a full ceiling.

Cheers, Wayne
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