column effective length factor K

[johndeng]

Sorry if this is too beginner question.
For a steel framed building, all connections are pinned, what's the k value for columns, 1.0 or 1.2?
column length is from connection of girder to top of base plate, or to top of slab on grade?

 

[KootK]

I like K=1.0 from the top of the base plate. With conventional slab on grade detailing, there's usually a bit of movement allowed/encouraged where the columns pass through the slab on grade.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[johndeng]

Thanks, but the building column is unbraced, should we use 1.2?
Actually we always use 1.2 in our firm for all building columns.

 

[jayrod12]

it may be unbraced directly, but is the building itself not braced? If the diaphragm at the top of the column is adequately braced down to the foundation, then I would be using K=1.0 I think that K=1.2 might be overly conservative.

 

[johndeng]

The building might be braced at end bay, but the mid bays are not. Will the interior columns/bay sway?

 

[KootK]

The interior bays will ride along with the braced bays with regard to drift. As such, the gravity only columns will indeed pick up a bit of moment and be subject to some P-delta effects. In my experience, engineers do not consider this explicitly. Based on typical connections, most gravity column connections probably justify K < 1. In a way that is not typically quantified, that somewhat offsets the fact that every column is a bit of a sway column, even in a braced building.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[JoshPlumSE]

Back in the effective length design days, I would have used a K=1.2 for the situation you describe. Though, the code suggests that 1.0 would have been okay. Nowadays, with the direct analysis method, we can use k = 1.0 for almost all cases.

 

[Lomarandil]

"[Effective length isn't] I'm not dead yet!"

Agreed. If truly pinned, I'd use 1.2 from the top of the base plate (if pinned, what base plate?). With realistic connections, 1.0 from the top of the base plate is reasonable.

 

[KootK]

If truly pinned, I'd use 1.2 from the top of the base plate

If truly pinned, I'd actually be 100% certain of K=1.0. Truly pinned = no end moments = no P-delta.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[rowingengineer]

I think we should all now discuss Kx verus Ky and how this differs depending on girts and bracing.

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"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[JoshPlumSE]

Yes, 1.0 is the theoretical correct value as KootK suggest.

Common practice for some of us would be to up that to 1.2. I would also always use Cb = 1.0, and Cm= 1.0. Lots of little conservative assumptions for the "preliminary design" phase. Which was before we knew exact loads or locations or such. Then later, if necessary, during the final design phase, we could sharpen our pencils and back away from the conservative assumption.

 

[Lomarandil]

Ah, good point KootK.

 

[johndeng]

Thanks for all the input and no laughers at my beginner question.

What if I change the beam to column connection from pin to moment connection, what will be the k value?
Will the column capacity be increased or reduced? k may be smaller, however, it take moment too.

 

[KootK]

In that case your K value will need to be calculated based on the relative stiffness of the members coming into the joints. Regarding the interplay of moments and K values:

1) In a slender column that buckles elastically, I would expect capacity to increase.

2) In a squat member that doesn't buckle at all, I would expect capacity to decrease.

3) For your usual, intermediate column, I find it tough to call. I'm guessing a slight capacity decrease.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.