Tension Slenderness ratio Kl/r < 300 for Canada but L/r < 300 for US 1

[matty54]

Hi all,
I am a little confused about these slenderness recommendations. I am wondering why in the Canadian code it actually specifies the Slenderness ratio (including the effective length factor) to be KL/r < 300 whereas in the US code is recommends just L/r <300.

From my understanding the tension slenderness ratio is just to limit the Deflection during handling of the member in the fab shops BEFORE it has been incorporated into the structure, which is, from what I understood, is the US code it does not include the K value.

Does anyone have any insight into this and why the Canadian code would be including the K value, but the American code is not?

 

[EngDM]

I'm not sure if it has changed in the new S16 code, but in S16-14, clause 10.4.1 reads "The slenderness ratio of a member in tension shall be taken as the ratio of the unbraced length, L, to the corresponding radius of gyration." This same clause specified to use K for compression members. Further, for truss assemblies tension ratios need not be limited as per clause 16.5.7.5.

TL;DR - K isn't used for tension slenderness ratios in CSA.

Edit: It's the same clause in S16-19, in case that is what you are using.

 

[matty54]

ah thanks for clearing that up for me EngDM. I was mistaken about that.
I guess my question then becomes why does S16-19 specify the slenderness ratio based on the unbraced length and the most recent AISC 360 specifies it as the fabricated length? It's a big difference in some cases whether you are basing the slenderness on the fabricated length or unbraced length. Should we be expecting S16-19 to be updating their statement in the future to specify the fabricated length as well?

 

[CDLD]

The commentary states:

Clause 15:

The K value would affect the "flutter", so it makes sense that it is included in the calculation.

A good reference for calculating the flexibility of a bottom chord is "the importance of tension chord bracing" by James M. Fisher.

 

[phamENG]

AISC eliminated the member slenderness requirements for both tension and compression members, but retained them as "user's notes" and "guidelines"...largely because they have been used so much in the past and it's likely that as a fall back it has saved more than a few engineers' skins (whether they were aware of it or not). Note that I'm NOT referring to component slenderness to determine compactness for local buckling considerations.

But AISC looks at fabricated length because it's now primarily a constructability concern. Unless you're working with cable or wire rope, you expect the members to have enough rigidity to handle and put in place. If you exceed these limits, it starts getting difficult to handle the members and put them in place without them being damaged under their own self weight.

 

[dik]

From S16-14 the following:

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[matty54]

AISC: "For members designed on the basis of tension, slenderness ratio of the member as fabricated - taken as the fabricated length of the member divided by the least radius of gyration of the section ..."

CISC: "The slenderness ratio of a member in tension shall be taken as the ratio of the unbraced length, L, to the corresponding radius of gyration"

So I take this to mean that in Canada it's ok to design a structure with say W6x15 @ 60ft long acting as a tension member as long as it's unbraced length in the structure is <~36ft. I'm not quite sure what situation this might occur in, but if it does, there is no recommendations against doing this for CISC. But AISC would not recommend this because it is using L=60 for its slenderness calculation where CISC is using L=36