Repetitive member factor, Cr

[BadgerPE]

I am working on a retrofit to an exiting post frame building and I am having a bit of trouble getting the posts to pass. I have partial calcs from the original design and as far as I can tell, the designer used a Cr value of 1.35 for the built-up (3) ply posts per NDS 2001 standards. I have not been able to locate that factor in the 2001 NDS. Does anyone know where this factor may have come from? NFBA? I highly doubt I will be able to prove that the new posts work if I have to reduce the Cr factor to 1.15 per current NDS standards.

 

[TehMightyEngineer]

What about some of the other factors such as load duration, size factor, etc. Could those be part of the 1.35 factor?

Maine EIT, Civil/Structural.

 

[BadgerPE]

Not that I have seen so far. I am backing all of the numbers into the existing calcs to see where they are at, but it looks like Cr=1.35 by itself.

Another thing that is kind of throwing me for a loop it that the buckling length coefficient, Ke=0.65. The post is embedded at the base and the top is knee braced to the truss, but I would think that would be a fixed-translation free/rotation fixed condition and Ke=1.2

 

[manstrom]

Off the top of my head, Cr applies to built up members applied to bending only - Fb, not compression. Still may apply to part of the calc.

 

[BadgerPE]

Manstrom,

You are correct. Cr does only apply to bending members. The specific issue I am seeing with the posts is a beam-column check with both lateral and gravity loads applied to it.

 

[Archie264]

Looks suspicious. Are you sure he didn't use a K value of 2.0 without the Cr factor? I think that would get you about the same result and would make sense if he didn't have confidence in the post's "embedment".

I've never seen or used a Cr of 1.35...only 1.15. It's helpful when designing joists but you have to make sure to not leave it entered into your software when you then design the beam.

 

[RobertHale]

There is a provision in IBC (2306.2 in IBC 2006) as well as Table 3.1.1.1 in Special Design Provisions for wind and seismic that allows for increased repetitive member factors on studs. 2x6's can use 1.35.

 

[BadgerPE]

Archie,

The existing calcs specifically call out Ke=0.65 and Cr=1.35. Not sure how to justify this, but it may be an industry thing that I am unaware of.

Robert,

I was not aware of that provision, but in this case I have a built-up (3) ply 2x8 post. Thanks for the info.

 

[woodman88]

The 1.35 (for 2x6 or 1.25 for 2x8) is a system factor to replace the Cr. It requires 1/2" Gypsum wallboard and 3/8" wood structural panels plus a minimum nailing to use.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[Triangled]

woodman.... quite interesting. could you pls provide a reference? thank you!

 

[woodman88]

Triangled

I was expanding on RobertHale post

"There is a provision in IBC (2306.2 in IBC 2006) as well as Table 3.1.1.1 in Special Design Provisions for wind and seismic that allows for increased repetitive member factors on studs. 2x6's can use 1.35."

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[BadgerPE]

An update for all. Per Table 7.3 of the 1999 NFBA "Post-Frame Building Design Manual" for a visually graded (3) ply post, the repetitive member factor can be increased to 1.35. There are a series of stipulations that go with using this increase, but it appears that was the basis for the original design.

However, I still haven't found any justification for the use of Ke=0.65 for an embedded post/knee brace truss configuration.

 

[jayrod12]

Just tossing out this answer, not saying it's correct. The only time I have seen a K of 0.65 used is when the designer assumes an approximate fixed-fixed condition.

I personally could not fathom how a wood post could be considered fixed-fixed. I do however understand how it could be considered fixed-semi-pinned as the roof will provide some resistance it might even provide enough to consider it fixed-pinned.

When I had to design a few of these post-frame buildings I started with fixed-free and quickly realized that was never going to work. The more I thought about the deflected shape of the posts I moved closer to a fixed-pinned (hence semi-pinned). You may want to investigate that train of though.