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Unread 12-28-2019, 04:00 PM   #13
cx
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Join Date: Jul 2001
Location: Boerne, Texas
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The between-joist blocking (X bracing is actually the better method) serves to keep the joists from twisting under load when the unsupported span is long. That does help in allowing the vertical joists to display their full deflection potential, and may actually transfer some point loads to adjacent joists, but it does not add anything to the inherent stiffness of the joist itself. The overall design deflection of the joists remains exactly the same as it would without the blocking. The application of a ceiling to the bottom of the joists provides a similar result in keeping the joists upright, but doesn't increase the design characteristics of the structure.

The rigidity of the sawn wood joist is determined by the species, grade, and condition of the wood itself. The design deflection of the structure will be the result of the size, shape, spacing, and unsupported span of the individual joists. It is beneficial to keep the joists in position and vertical under load to maintain that design deflection, but any framing units that merely keep the joists in place do nothing to increase their capabilities.

The same is true of engineered wood joists except that the characteristics are determined more from the design than the inherent characteristics of the individual components. You will frequently see between-joist blocking specified as a requirement for a particular floor design, especially in long spans, and that is precisely because part of the design is that the joists remain vertical under load and some of those joists, by their design, are prone to twisting under load. The design is for the entire structure, often including a glued and mechanically fastened subfloor of a specific type and size. The design deflection is for the entire package and that deflection is not expected to be exhibited unless all the components are included and properly installed. If you were to install additional between-joist blocking to one of those floors, you could expect to achieve nothing at all beyond the original design specifications.

Perhaps someone else will find you an authoritative article or source for your answer, but I'm afraid I don't know one offhand. And then perhaps you'll be able to say with authority that I was wrong. It will not be my first time.

My opinion; worth price charged.
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