Repetitive Member, Cr, Increase 2

[kennyb04]

IBC 2006 Table 2306.2.1 Allows an increase for bending of up to 1.5 in lieu of the 1.15 that the NDS shows. The 2012 IBC finally references the 2008 Special Design Provisions for Wind and Seismic which shows this same table for the Cr increase in Table 3.1.1.1.

The problem I have is with the 2009 Code. I can not find that table in the 2009 IBC, yet the 2009 IBC has not adopted the Special Design Provisions yet. Does anyone know if there is a reference anywhere to these values for the 2009 code cycle to do I just end up using the values because they are still current and hope we don't get asked for the reference in the exact code year the city is currently using?

 

[manstrom]

I always use the 1.5 factor. Never been questioned.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[Triangled]

Sounds like typo as 50% increase for repetitive doesn't seem to make sense..... at least to my way of understanding the reason for the allowable increase. Gonna research this...

 

[msquared48]

With 1.5 / 1.15 = 1.30, I wonder if this is related to the old 1.33 factor that was eliminated.

Mike McCann, PE, SE (WA)

 

[mike20793]

The repetitive member factor is different for wind loading or gravity loading. The 1.5 value (for 2x4's) is for out of plane wind loading, that is why it is given in the SDPWS. For gravity loading, the repetitive member is 1.15, that is why it is given in the NDS.

Section 3.1.1 of the 2015 SDPWS:
The reference bending design value, Fb, for sawn lumber wood studs resisting out-of-plane wind loads shall be permitted to be multiplied by the repetitive member factors in Table 3.1.1.1, in lieu of the NDS repetitive member factor, Cr = 1.15.

 

[Triangled]

The Msquared combined with the other Mike, yes Mcubed, is making sense to me

 

[Miner99er]

IBC's increase of 1.5 for 2x4s is for wind loading only and takes into account specific sheathing and drywall panel requirements to contribute to the composite and load sharing of lateral wind loads. NDS repetitive member factor covers a broad range of loading (gravity, wind, seismic, combined) and is more general with the sheathing requirements.

 

[msquared48]

miner99er has it - composite action allowance.

The 2012 IBC defers to the NDS and the 1.5 factor is found in section C3.1.1 of the 2012 NDS.

Mike McCann, PE, SE (WA)

 

[RWW0002]

IBC 2009 does reference the 2008 version of the SDPWS - (NDS 2005 but SDPWS 2008). Check out the Referenced Standards attached.