NDS Column Interaction Formula Clarification

[dhoward26]

I'm working on writing my own program for wood column design and have spot I need clarification on.

NDS Section 15.4 general equation...the first term is where I'm hung up:

(fc/F`c)(fc/F`c)

What value do you use for F`c if you have a compressive load with an eccentricity in both the x and y directions? Do you use the value that gives you the worst case of fc/F`c for this scenario?

 

[woodman88]

You should be getting only one answer for fc/F'c in the equations 15.4-1 or 15.4-2 in your pdf. Which will be the axial load in the column based on from all the axial loads being applied to the column.

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.

 

[dhoward26]

I agree that it would be the same if the column were say a 4x4, 6x6, 8x8, etc.

But for cases where you have a 6x10, your column stability factor Cp would be different base on different slenderness ratios, thus giving you a different F'c. I have had architects in the past specify 4x10's and 6x10's for exterior, load bearing, un-braced columns for architectural purposes. It's been a conundrum for me and I have always deferred to WoodWorks for the design...and having my own spreadsheets in my calc package saves me time in the long run.

 

[dhoward26]

woodman88:

Never mind. I think I may have answered my own question based on your comment and a quote from NDS:

"For solid columns with rectangular cross section, the slenderness ratio, Re/d, shall be taken as the larger of the ratios le1/d1 or le2/d2 where each ratio has been adjusted by the appropriate buckling length coefficient..."

Thank you for your help.

 

[woodman88]

Has the Cp calculation changed since the 2001 NDS? In Section 3.7.1.3 of the 2001 NDS it states that the Cp shall be based the larger of the slenderness ratios. So you would still get only one answer.

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.

 

[Michel60]

Good afternoon dhoward,

I believe woodman is correct for all member geometries. The axial load/capacity ratio fc/F'c is to be the one associated controlling case for axial load. The remaining portions of the interaction equation then addresses the strength and stability considering the bi-axial bending which may have differing stress limits. Also I'm not sure which version of NDS you have referenced but the 2005 version did have an errata for this particular series of equations that corrected errors that could lead to O/S in flat-wise bending. It should be available on the AF&PA website.

regards,
Michel

 

[nac521]

As per 2005 NDS, Cp is still based on the larger l/d ratio. One answer, as woodman stated.