Bruhn Crippling Equation

[mathlete7]

Hi,

I'm looking at my copy of Bruhn, pg. C7.2, equation C7.6. This equation is missing the "g" term (which are present in C7.4 and C7.6) and I'm guessing that this must be an error. Is anyone familiar with this?

Thanks much.

 

[rb1957]

have a look at figure 7.6 on page C7.4

also the gerard method is written up in lots of places (Miachael Niu, NACA TNs, etc

 

[mathlete7]

thanks for the reply rb. right, i saw figure 7.6 which shows you how to calculate g. however I was actually asking why the "g" term isn't shown at all in equation C7.6 (for z, j, and channel sections). however it IS in equations C7.4 and C7.5. I was wondering if this was an error in the book.

i have a copy of niu so i'll check what form he presents this equation in. thanks for the tip.

 

[rb1957]

oh i see now (!) ... none so blind as those that will not see ...

yes, there is an error in eqn C7.6, it should be "g*t^2" (not "t^2"), as in the other expressions. probably another one to add to the list ... i started a thread awhile back for an erratum for bruhn (no slight intended) but it's pretty much died out.

 

[rb1957]

dammnit ... i really shouldn't shoot from the hip !

thinking for 1 msec, the factor "3.2" is so different to the other sections "0.67" that i thought maybe something is up. Michael Niu has the same equations (ie no "g" factor for 2 corner sections, maybe because with the limited geometry (2 corner sections) then "g" is the same so it's already included in the equation.

that sounds good, but since Niu might have read Bruhn and copied directly (oh crap, here come the lawyers) i'm going to go look for gerard's original work; sigh.

 

[rb1957]

bruhn and Niu are alittle different (disguised by their presentations !) ...

Bruhn eqn C7.6 B = 3.2, m = 0.75 (using Niu's format)
Niu eqn 10.7.2 B = 4.05, m = 0.82

Bruhn eqn C7.5 B = 0.67, m = 0.40
Niu B = 0.58, m = 0.80

Bruhn eqn C7.4 B = 0.56, m = 0.85
Niu B = 0.58, m = 0.80

maybe it doesn't matter That much ...

 

[mathlete7]

yeah, i kind of was thinking that perhaps "g" is already wrapped into the equation for the z, j, and channel sections since that factor is so high (3.2). I was leery to run with this however cause, while it seems like both a "z" and a channel section would have the same "g" (g=5 per the method in Figure C7.6) a "j" would be different (g=6 if I'm thinking about this correctly)...

 

[edbgtr]

Hi Stressers, Why don't you cross-check the final results of these formulas using Bill McCombs' modified Needham formulas (In his Bruhn Supplement) for one end and no end free. I've cross correlated various formulas from company manuals and found Bill's formulas to be spot on, or very close. You must use the Fcy cut-off value for any "stable" members. This way you should be able to sort out the "g" factor.
Ed.

 

[rb1957]

Needham tends to be more accurate for formed sections and Gerard for extrusions, tho' either way don't think the results should be significantly different and i'd be splitting hairs to say that the section is good or bad 'cause it passe/fails one or other of these allowables. i gess if t passed both it'd be good, if it failed both it'd be bad, and if it fails either one it'd be marginal !?