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WOOD: "Apparent Rigidity"

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I have come across this term in perusing the WFCM, and I cannot figure out exactly what it means.

 

It occurs in particular in Tables 2.13A-B (CEILING FRAMING CAPACITY REQUIREMENTS). Footnote (1) to this table states:

 

“Apparent rigidity capacities shall include the effects of both bending and shear deflections. Apparent rigidity capacities have been adjusted for solid-sawn lumber to account for these effects. Contact the I-joist manufacturer for apparent rigidity capacities to be used for I-joists in this table.”

 

And Footnote (2) says:

 

“Tabulated apparent rigidity requirements assume single span conditions. For continuous span conditions, tabulated apparent rigidity requirements shall be permitted to be multiplied by 0.75.”

 

The units of “apparent rigidity” are in.^2/lb., and a typical value is between 5 and 30 MILLION.

 

So not sure how to use this information.

 

Interestingly, the rafter span tables give you selection values in f-sub-b for load and E for deflection. Don’t know why the difference.