compression load calculation- resistance factor 3

[ENGR_2321]

So as I am calculating the compression load (design-LRFD)

equation as we all know is øPn = ø Fcr Ag

per the steel manual, AISC the value of the ø which is the resistance factor , is 0.90

but I am looking at 2 design book examples and they use 0.85 ! why ?

they are calculating the fcr differently too. They use Fcr = 0.658 ^ sigma squared FY
but I had learned it a different way. The way I did it, and according to another book I have , they do the whole KL/r check with the 4.71 √E/Fy and then just use fcr = (0.658 ^fy/fe )Fy since KL/r < 4.71√E/Fy . They check the slenderness different too. They are using a whole different equation with sigma. I thought you checked it with the KL/r < 200 ?

is this a code update? I have the 14 edition aisc manual.


I've been trying to figure out all night why but I really can't find it the explanation to why this is.
any help would be appreciated. thanks!

 

[Ingenuity]

Check out the AISC 360-10 Commentary for Chapter E commencing on page 16.1-290 of the 14th MSC.

The resistance factor (ø) was increased from 0.85 to 0.9 in the 2005 Specification.

They use Fcr = 0.658 ^ sigma squared FY

They (you?) probably mean lambda, λ, not sigma.

Lambda, λ, is a slenderness parameter and equal to:

AISC 360-10 has a user note on page 16.1-34 of the 14th MSC, explaining both methods:

I thought you checked it with the KL/r < 200 ?

Equation E3-2 is applicable for INELASTIC buckling where the slenderness is less than 4.71√(E/Fy) = 113 for 50 ksi grade steel.

If you slenderness is greater than 113, then you are in the ELASTIC buckling range (Euler), and equation E3-3 is applicable:

 

[ENGR_2321]

Thank you Ingenuity! You are a life saver. This definitely answered it. I think the design books I picked are a bit old and I completely overlooked the commentary section. THANK YOU

 

[Ingenuity]

You are welcome, ENGR 2321.

Your design books may be a bit 'old' (relatively - mine are 30 years old ), but interestingly, the AISC 1923 column curve (extended in 1946) is plotted alongside the LRFD column curve (neglecting ø factors etc.), for F[sub]y[/sub] = 30 ksi, with extraordinary results. 'Old' is still relevant - even if nearly 100 years old.

 

[ENGR_2321]

Haha yes that's about how old some of mine are but that's true. Some concepts never change

 

[IDS]

'Old' is still relevant - even if nearly 100 years old.

Well they both come from Euler's buckling formula, which is a good bit older than 100 years! (260 years old this year)

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[Ingenuity]

Well they both come from Euler's buckling formula, which is a good bit older than 100 years! (260 years old this year)

Agree for L/r > 100 or so (elastic Euler buckling), but for intermediate L/r ratios where INelastic buckling controls, then Euler's 260 year old theory is not appropriate...but I get your point...and Euler was a rockstar!