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Flexible Diaphragm Deflection Calculations

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Once again, reading FEMA 273 confirms conformance with the 97 UBC blocked diaphragm deflection criteria. However, the unblocked deflection criteria suggested in 273 uses Formula suggests using the deflection calculations for single straight sheathed diaphragms with a modification. Section defines a straight sheathed diaphragm deflection as:

D = v L 4 / (G b 3 ) (8-5)

Section for Wood Structural Panel Sheathed Diaphragms suggests using formula 8-5 with Gd for unblocked diaphragms as:

Unblocked Chorded Diaphragms:     Gd = 8000,000 lb/in

Unblocked UnChorded Diaphragms Gd = 400,000 lb/in.

Has anyone compared with with the APA recommendation of using 2.5 times the Blocked diaphragm deflection in the current '97 UBC? Does this provide a more accurate and acceptable alternative to determining an Unblocked plywood diaphragm deflection?

BTW, this is recommended for New Construction in FEMA 273.

Dennis S. Wish, PE
(208) 361-5447 Efax