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aluminum slenderness limits

aluminum slenderness limits

(OP)
I wrote here yesterday and I'm afraid I didn't phrase my problem very well.
I am useing the 2000 edition of the aluminum design manual (allowable stress method).
In section 3.4.14 I am having trouble calculating S1.
I am useing equation 3.4.14-4 and useing the buckling constants from table 2-1 and Fcy=16ksi(my aluminum is 6063-T5 <.50)my result is 127.345....now I compare that to the value in design guide Table 2-22 and here the limiting value S1 is given as 204.
If I multiply my result by 1.6 S1 would round to 204 but I am not sure that this is correct.  It doesn't seem like it would be since my value for S2 useing equation 3.4.14-5  seems to round to the expected number of 3830 without any additional use of a saftey factor.
Thanks for any help in this matter.
Replies continue below

RE: aluminum slenderness limits

I have a book titled, "Aluminum Structures, A Guide to their Specifications and Design" by Kissell and Ferry which addresses this issue.
It states that in the Tables the Slenderness Limits are calculated based on the intersection of the expresseions given in Table 2-2 for allowable stress for slenderness less than S1 and allowable stress for slenderness between S1 and S2.
Find the intersection as
9.5=10.5-0.070(x)^0.5
solving for x, gives x=204.
Also mentions not to be alarmed by such a difference (127 compared to 204) since when allowable stress are calculated the differences are small.
10.5 - 0.070*(127)^0.5 = 9.7 ksi
10.5 - 0.070*(204)^0.5 = 9.5 ksi
Hope this helps. This book is very helpful if you do a lot of aluminum design.

RE: aluminum slenderness limits

(OP)
Thanks Wbrus...you are correct.  I sent an e-mail regarding this issue to the aluminum association and none other than Randy Kissell answered my question...I assume it's the same guy who authored the book you mentioned.  Thanks for your reply!

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