Slenderness on Compression Member Problem

[1rv4n]

Hi,all... I need your opinion.
I'm debating with my co-worker about the slenderness effect on compression member. My opinion is that we should fulfill the slenderness requirement for compression member which is k*l/r > 200. Neglecting the slenderness may cause sudden failure for the component due to local buckling or failure in plastic mode.

But my coworker insist that the slenderness of the component may be neglected as long as the stress ratio (from SAP with AISC ASD89 design code checking) is acceptable. She said that the SAP already taken consideration on the slenderness while calculating the member's capacity. Btw, the stress ratio of the member is 0.645 with AISC-ASD89 and unfactored load combination.

Personally I would rather not messing with code requirement, so I insist that we should follow the requirement. But on the other hand, I cannot give reasonable reason too.

Please give me your opinion.

Thank you very much.

 

[ishvaaag]

The question is that the code being used needs to be met ... point. Being outside the code specs you are not complying with the implied reliability warranted by the code, in whatever the way; this is irrespective of the normative procedures specified within them. There are not dismissable specs in the code; all are there for a purpose.

So looking the specific code:

"B7. LIMITING SLENDERNESS RATIOS
For members whose design is based on compressive force, the slenderness ratio Kl/r preferably should not exceed 200. If this limit is exceeded, the allowable stress shall not exceed the value obtained from Equation (E2-2).

For members whose design is based on tensile force, the slenderness ratio Kl/r preferably should not exceed 300. The above limitation does not apply to rods in tension. Members which have been designed to perform as tension members in a structural system, but experience some compression loading, need not satisfy
the compression slenderness limit."

So you see there that your colleague may be right if the stress does not exceed that from E2-2.

 

[JAE]

I believe that the current AISC limit of L/200 is a "preferable" limit and not an absolute. The values provided for columns with longer L/200 still work. Read the commentary on this section in AISC's spec.

 

[ToadJones]

I beleive Ishvaaag is quoting AISC 360-05 or 360-10 and the OP is using 1989 ASD.

What JAE said is correct.

Regardless of code required or not, compression members with KL/r approaching 200 will have a very low allowable stress (ASD 1989) and probably indicates a poor choice of section.

 

[ishvaaag]

The quote is from

Specification for Allowable Stress Design and Plastic Design
June 1,1989 with Commentary

 

[ToadJones]

ahh, yes

 

[271828]

As JAE pointed out, KL/r<=200 is preferable.

Imagine what happens if the column has a huge KL/r ratio and the load is at a slight eccentricity and/or the member has a little sweep or other out-of-straightness. The first order moment, that might not even be in the analysis, is small. However, if the P - little delta amplification (B1 in the AISC Specification) is large, then the second order moment might be significant.

In other words, if you start to get outside the "normal bounds," you have to use engineering judgment and take into account things you normally wouldn't.

 

[delagina]

i would check columns for slenderness since you are still using ASD 9th edition. i dont see any reason why not since you are computing K anyway.

Now if you are using Direct Analysis Method where you dont need to compute for K anymore, I probably wont coz it's only "preferable" and I'm kinda lazy also. hehe.

 

[delagina]

I just checked my green book, it did say if kl/2 200 is exceeded, it will use Eq E2-2. So i guess you are still ok per code even on ASD 9th.

 

[MiketheEngineer]

Just make sure you can explain it to the judge and jury!!

 

[1rv4n]

I have found some answers, but please let me know if I made mistakes in my conclusions.

After I discuss with some of my friends, maybe I could conclude some point about the slenderness limit rule.

1. As long as the structure's failure is not decided by earth quake, especially in large seismic area, the slenderness limit (KL/R>200) may be neglected with careful consideration.
2. The strength of the slender member must be proven to be strong enough to bear the load. In my opinion, it would be better it the ratio between the load vs capacity is less than half.
3. For supporting member such as stiffener beam or compression bracing, the slenderness may be neglected.

Please revise me if I conclude wrong things.

If you don't mind, I also would like to know your opinion about slenderness limit for tension member (L/R>400) according to AISC ASD89.

Thank you very much for your helps.

 

[ishvaaag]

I do not agree with point 3. Some bracing member where the load is assumed to be taken in compression certainly needs the ability to stand such compression and every specification for compression members is applicable to it. Primary, secondary are just labels convenient to understanding, the physical behavior is what matters. There's no mention to any such stiffener/bracing thing in B7 that rules the slenderness of members in ASD.

Respect the slenderness bigger than L/400 where the only structural action required to get proper functioning of the structure is tensile, the code allows for it even if under L/300 is preferred.

 

[DamsInc]

Just wanted to add to this thread, the "Limit States Design in Structural Steel" (Kulak, Grondin) states that:

"For extremely slender members the maximum strength is highly sensitive to changes in end conditions and to initial imperfections, and such members cannot safely be designed according to Equation 4.21 (Cr as per 13.3.1 in CISC 16-01). For this reason, Clause 10.4.2.1 of S16-01 limits the slenderness ratio for compressive members to KL/r=200 (which corresponds to lambda=2.7 for Fy=350MPa).