Distortional buckling of C channels without lips

[tramyer]

Hi,

Do C-Channels without lips exhibit distortional buckling?
And if does what's the formula to use?
And how do I use the AISI (Eq. C3.1.4-6) formula to non-lipped channels? Is it applicable?

kind regards,

Tramyer

 

[msquared48]

By "lips", I assume you mean flanges?

If that's the case, then you are just dealing with a vertical plate.

Are you a student?

Mike McCann, PE, SE (WA)

 

[Jerehmy]

Mike, lips are the little ~ 3/4" long pieces that are parallel to the web but on the opposite side of the flange.

Tramyer - The commentary for C3.1.4 in 2007 states "Distortional buckling is an instability that may occur in members with edge stiffened flanges..."

Therefore, it is not something you need to check for channels without lips. Based on the shape of the member during distortional buckling, it appears to be an instability form the lip itself under compression. So it makes sense you don't check it when there are no lips. There aren't usually fasteners attached to the lips so they have to rely on the connection to the flange and how often the flange itself is braced to restrain the buckling pattern.

 

[Celt83]

to me the AISI is not the easiest specification to follow.

I'd say for a channel without lips you'd be outside the scope of C3.1.4 as it's written, and would fall back to section B limiting the effective width of the unstiffened flanges.

You could always try and run thru C3.1.4b and equation C3.1.4-10, using the full section properties since the stiffening effect still technically exists at the the flange to web interface.

Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

 

[rb1957]

The commentary for C3.1.4 in 2007 states "Distortional buckling is an instability that may occur in members with edge stiffened flanges..." ... I think this is saying edge stiffening flanges (or lips) can but not will reinforce the flange and prevent crippling (or "distorsional buckling"). If the lips are small then they're not good at reinforcing the crippling of the flange.

another day in paradise, or is paradise one day closer ?

 

[Jerehmy]

Rb, I'm going to have to disagree with you interpretation. The first sentence of specification section C3.1.4 clearly states "The provisions of this section shall apply to I-, Z-, C_, and other open cross-section members that employ compression flanges with edge stiffeners, with exception of members that meet criteria of......."

Distortional buckling simply does not occur without lips. The lip buckles out-of-plane at its free end causing rotation of the flange. This does not occur when there aren't any lips, i.e. edge stiffeners. There is no reason to check this limit state when there are no edge stiffeners.

 

[rb1957]

so a flange without lips doesn't buckle ? I don't understand, but you know your code better than I so I'm ok.

another day in paradise, or is paradise one day closer ?

 

[Jerehmy]

The flange does buckle. You use a reduced, or effective, flange width based upon whether the flange is stiffened at both ends (by the web and a lip) or just on one end (by the web). In the first case, your effective flange width is much greater since it is stiffened at both ends. But, you must also check for distortiinal buckling due to out of plane buckling of the lip.