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KL/r<200 how to make SAP2000 omit this requirement on auto-select lists?

KL/r<200 how to make SAP2000 omit this requirement on auto-select lists?

KL/r<200 how to make SAP2000 omit this requirement on auto-select lists?

(OP)
KL/r<200 how make SAP2000 omit this requirement on auto-select lists?

RE: KL/r<200 how to make SAP2000 omit this requirement on auto-select lists?

Why would you want to omit this requirement?

RE: KL/r<200 how to make SAP2000 omit this requirement on auto-select lists?

Just one point of order: KL/r<=200 has only been a "preference" since the 2005 AISC Specification. Not saying it's invalid, and I do typically adhere to it, but it's not a requirement.

And to answer the OP's question: I don't know of a way, and doubt there's a way. Hopefully someone will prove me wrong.

RE: KL/r<200 how to make SAP2000 omit this requirement on auto-select lists?

I guess that depends on region. In Canada, CSA S16 has had the limit of KL/r < 200 for compression members from the print version I have (2000) from school. I also looked at older structures and had to dig into past codes and I saw that this requirement dates back to at least the 1965 edition.

Also, from the sounds of it, he's trying to use the automated design feature where the software selects the optimum member size, which would only be used for new designs, and I have trouble seeing how to waive the slenderness requirement.

RE: KL/r<200 how to make SAP2000 omit this requirement on auto-select lists?

My 1989 AISC ASD Manual, Section B7 says "KL/r preferably should not exceed 200."

I don't know the history as well as I thought I did. LOL. I thought this was a requirement until 2005.

RE: KL/r<200 how to make SAP2000 omit this requirement on auto-select lists?

Yeah, it's been a preference for as long as I can remember. I'm not sure the actual reason, but my think of it along these lines:
1) We have a similar L/r >300 "preference" in the code for single angle members. The code commentary gives the rationale that members like this are very easily damaged (during construction / fabrication or such). Therefore, I would NEVER allow a KL/r approaching 300 for a compression members.

2) The AISC code commentary for section E2 states that the KL/r<200 was MANDATORY in the 1978 specification. But, since 1999 (3rd edition LRFD) that limit was eliminated completely in 2005. But, at various points it's been a "preference" or "User Note". It also suggests that the members will sag or deflect under self weight and look unsightly. And, these bending stresses will reduce the available strength left for compression.

3) I don't ever like using KL/r > 200. But, if it's an existing member I don't have a problem with it. For new construction, I MIGHT allow it to be slightly higher. Like 205 or 210. Maybe, I try to see if there's a way I can make it slightly pretensioned....

4) If it's a tension only brace member, obviously, I'll allow it. But, I get worried if it's over 250 and it's not a rod, strap or cable. And, I definitely prefer to have my tension only members pretensioned in some way. Easy with Rods and Cables. With X braced single angles or such, I'll allow a KL/r > 200 for the compression one, but I'll analyze the structure as if they're both a tension only braces.

But, to get back to the OP's question, I don't know of a way to turn this off in SAP2000 for AutoSelect. You can manually set the K and L/r ratios, of course. But, I don't think you can manipulate whether the AutoSelect picks a member with a KL/r > 200 or not. Also, you can manually select members with a KL/r>200 if you want to use them. You'll be warned of that fact, but you should still get a code check.

RE: KL/r<200 how to make SAP2000 omit this requirement on auto-select lists?

(OP)
Yes, the problem is that you shall select manually sections with KL/r>200 ... auto select lists don't work if that condition exists. SAP2000 will discard any section even if it's a beam (not compression).

RE: KL/r<200 how to make SAP2000 omit this requirement on auto-select lists?

Should the beams have compression? If not, then you could use Assign, Frame, Tension/Compression Limits to make the members not have compression.

I've done this before with slender rod bracing and it worked OK.

RE: KL/r<200 how to make SAP2000 omit this requirement on auto-select lists?

If the beam doesn't have any compression, then just set the weak axis K or L value to something smaller..... And, you'll get members auto-selected for you again.

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