compression load calculation- resistance factor
compression load calculation- resistance factor
(OP)
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! :)
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! :)






RE: compression load calculation- resistance factor
The resistance factor (ø) was increased from 0.85 to 0.9 in the 2005 Specification.
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:
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:
RE: compression load calculation- resistance factor
RE: compression load calculation- resistance factor
Your design books may be a bit 'old' (relatively - mine are 30 years old
RE: compression load calculation- resistance factor
RE: compression load calculation- resistance factor
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/
RE: compression load calculation- resistance factor
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!