Unbraced length Lu

Question

Unbraced length Lu

aiden (Structural)

(OP)

26 Dec 06 13:14

i am developing a program for steel design using CISC/CSA16.1-94. It is almost complete except that I could not find any formula to calculate Lu (limiting unbraced length) this is equivalent to "Lp" of AISC 360-05, (Lp values from AISC 360-05 are too conservative).

I tried the iteration procedure using Galambos (SSRC) formula for Mu by substuting Mp (plastic) for Mu to obtain Lu, but yeilds a rather too high limiting values.

can anyone help?

dik (Structural) · Answer 1 · 2006-12-26 14:02:19

Current code is S16-01 for 2001... quite a change in torsional component for columns...

Dik

aiden (Structural) · Answer 2 · 2007-01-06 17:32:12

still waiting for my copy of S16-01, i still dont expect it contains the information i need. its not in 3rd to 6th edition of CISC handbook either.

i think im going to mail the code committee (once i get their address).

MALKOBANI (Structural) · Answer 3 · 2007-03-19 16:50:52

Hi Aiden,

Just like you, I thought Lu is calculated by equating Mu to Mp. Looks like it is not the case. Have you figured out how to calculate it?

Thanks

strainman (Structural) · Answer 4 · 2007-03-20 13:53:36

aiden, "Crawley and Dillion Steel Buildings Analysis and Design" state Lu= 545\(12*d/Af), but when I worked this out to check Lu in the AISC 9th Edition Manual the numerator back figures to be closer to 555 than 545. But, this should give you a good starting point. The Lu values are rounded in the manual to one place past the decimal point so the exact numerator is hard to pinpoint.

Good Luck,

strainman (Structural) · Answer 5 · 2007-03-20 14:56:02

Add on to above, the 555 is for ASTM A36 steel only. Lu values very for different grade steels.

MALKOBANI (Structural) · Answer 6 · 2007-03-27 12:58:53

Strainman,

Can you plese define the variables and the units in this equation?
Thanks

strainman (Structural) · Answer 7 · 2007-03-27 13:24:55

Lc an Lu are stated in feet in the AISC Manual.

12, in/ft;
d, in;
Af in^2

kbarnett (Structural) · Answer 8 · 2007-04-03 17:23:52

Lp = 1.76 ry (E/Fyf)^.5

This is only part of your concern. You also have to check lateral torsional buckling, web local buckling, and flange local buckling, to find out if any of these control L.

aiden (Structural) · Answer 9 · 2007-04-04 15:05:04

dik,
ive got my new S16-01, most of torsional-flexural requirements for compression members are applicable only to asymetric, singly symmetric and cruciform sections which are rarely used compared with standard rolled sections.

kbarnett,
that formula you wrote is from AISC360-05 and the results obtained from that are nowhere near the values in CISC tables. and indeed, i have all those cases covered in the separate program i developed for AISC steel design from case F2 to F7.

malkobani,
using an iteration procedure and applying a lateral-torsional buckling coefficient of 2.5 (maximum value permitted in AISC ASD 9th ed.) to Galambos ultimate moment "Mu" for lateral torsional buckling would yeild a fairly similar value.

since all the methods i tried have 5-10% difference from CISC Lu values, i went on with the hard way - i.e., prepare a database for all Lu values for sections identified in CISC handbook. For sections that are not given Lu values, i treat them as laterally unbraced members and i got good results for Mr.

thanks for all the replies.

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Unbraced length Lu