cementitious backerboard underlayment structural

huangyhg

cementitious backerboard underlayment structural?
i have read many claims from manufacturers that cbu are not structural panels and must be placed with a solid ply subfloor min 5/8".  when looking at the span tables published by the american plywood association, it will be found that a single layer of plywood 19/32" (15 mm) on joists spaced at 16" o.c.(400 mm) the maximum amount of weight that can be applied is 368 lbs (167 kg) before the deflection succeeds l/360 (uniform load distribution).  if a concentrated load is applied between the joists using a 3"(75 mm) wide disk with 200 lbs (91 kg) applied to the same piece of plywood, the plywood can deflect, up to approximately .078" = 1/12.5" or 2mm. this calculation indicates that the single layer of plywood can have approximately twice the deflection acceptable for the installation of tile.
my question if backerboard panels are not structural how can a 1/2" cbu be placed over 5/8" plywood and still be accepted under manufacturers warranty.
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cbus won’t rot, swell or deteriorate, and decrease the deflection by incrasing the section modulus.
the minimum subfloor specified is a 5/8" exterior grade plywood or 3/4" exterior grade osb (oriented strand board) subflooring with a deflection not greater than l/360 (i use l/480 for my designs) for live & dead loads. over joists that do not exceed a min. span of 16" on center.  
see ansi a137.1 specifications for ceramic tile  and ansi a108/ansi a118/136-1999 specifications for the installation of ceramic tile