Effective Length of Sway Uninhibited Multi-level Columns 3

[matty54]

Hi, I've been trying to understand how one would go about assigning effective length factors to columns in sway frames when all the intermediate floor beams are pin connected to the column? Would it be like I have shown here?:

Would I split the column up into sections and use G=10 and G=1 for the joints or would I just ignore the intermediate members completely if they are all pinned and use the entire length of the column with K=1.2?

 

[lexpatrie]

The whole reason why this is being discussed is because you seem to not acknowledge that k<1 is a realistic possibility and that using k<1 can significantly increase your calculated capacity.

I don't seem to not acknowledge. I full on do not acknowledge.

I don't agree with the "significantly" increase. And it's not all that realistic, either. As I've stated before, perhaps not as clearly as possible, the effect of end restraint on a pin-pin column is not dramatic. Not for realistic scenarios. Even if you went with inelastic G factors. Even with a "fixed" base (that's not justified by any research).... I don't see a huge difference between k= 0.85 and 1.0, (particularly not when some newer engineer is going to see this thread and think a pin-pin column can realistically be justified for K= 0.5 when it can't. ever. happen.)

I'd also suggest that the more you shave/cheat the K factor on the pin-pin columns in the story, by using a K that's less than 1, the more you need restraint from the framed columns/lateral system/diaphragm, and that's potentially beyond the notional loads we impose to account for those forces. Use of K< 1 also doesn't seem justified (from the 14[sup]th[/sup] Steel Construction Manual).

The key issue with the third option, is "columns whose flexural stiffnesses are considered to contribute to the lateral stability and resistance to lateral loads. These gravity only columns don't seem to satisfy that requirement. That would seem to indicate that the gravity columns have to be in the lateral system, at which point you might be able to computationally justify a K less than 1.0 (I believe this has shown up, analytically in the Ziemian thesis.)

I have some doubts here, that a) K<1 is allowed in the analytical procedures, b) K<1 produces meaningful steel savings (even so, "savings" that may be eroded by stiffeners being needed). This approach seems to be outside the specification.

I was going to put in an example, here, but I think that's not relevant until a) is overcome. I also have some questions regarding that example to resolve, unrelated to K factor.

 

[Smoulder]

I use K=1 if analysed the column as pin-pin. Otherwise bending moment isn't consistent with capacity calculation. Also K<1 ionly if K=1 result is a real problem.

@Lexpatrie K<1 does need more stiffness and strength from the structure but the notional loads increase if you actually use that extra capacityso it works out.

For K<1 see slides 31& 34 from one of the guys who write the codes. He's talking about columns that are part of moment frames not pin-pin gravity columns.

https://seawi.org/images/downloads/..._direct_analysis_method_of_design_handout.pdf

 

[lexpatrie]

Yeah Don White has been at the forefront of this for about 30 years. Note there appears to be a typo on slide 31 and 34 (K=0), I was going to say I thought Don White was Ron Ziemian's thesis advisor, but based on the publications since, I think it was Bill McGuire, another large figure in the direct analysis movement.

I can follow these guys up to a point, but once they get off the groomed trails I start to struggle, it's not one of my main areas of current practice. I just know to stay on the marked trail. I dislike close encounters with boiling hot water from geysers.

Much of these discussions are "off the specification" meant more for the LeMessurier/TT/SOM design practice on 40-50 story buildings and up. But they have the theoretical chops (or are ethically obligated to develop the necessary theoretical chops) to address it, and are partly the source of the limitations on the provisions such as they stand (i.e. White, Ziemian, McGuire, Galambos, Tide, and many others are behind the development, directly or indirectly, of that portion of the code).

When it comes to stability, everything is fine, especially if the design loads never happen. I suppose the southern parts of the U.S. are more likely to survive a stability issue because they are usually seeing lower roof/snow loads, but then again, if the capacity is also reduced, the margin for error may be smaller (or larger).

 

[Smoulder]

@Lexpatrie That K=0 isn't a typo. It means you can use the section capacity if you modelled initial sweep of column or if the nonsway p delta is small because the load is small compared to buckling load.

 

[lexpatrie]

Ah, I suppose, it's just a wonky way of saying P[sub]y[/sub]A[sub]g[/sub]? Because when somebody says K=0 to me that suggests infinite capacity.

So, offhand if anybody is interested I have the Ziemian thesis in the garage... I mean if you want a figure or an excerpt.