Return to index: [Subject] [Thread] [Date] [Author]

Re: Column K factor.

[Subject Prev][Subject Next][Thread Prev][Thread Next]
If I am understanding correctly what you are designing, it sounds like you 
are providing a pin-connected frame that only resists vertical forces.  The 
lateral forces I am assuming are resisted by tilt-up shear walls.  The base 
connection sounds like it is pinned but the top connection I am assuming is 
pinned but free to translate laterally based on the amount of shear 
deformation in the roof deck.  

Assuming I am picturing this frame correctly, i do not see it as K=2.0 but 
also it is potentially greater than 1.0.  My understanding of the K-chart is 
that these values model the length a column will act as even though the 
measured length is some other value.  The chart does not allow for P-delta 
effects, that is another area of design.   A pin-pin with translation is not 
shown as an option.  If I take "d" on the chart and allow the top to 
translate, I would have to project the dashed line farther to get it to 
vertically align with the base again.  For this reason, I think the value is 
something greater than 1.0  The amount I must project the line would be based 
on the amount of drift actually present.  I may not be able to convince the 
engineer it is 1.0, but I think I could possibly get him to agree it is no 
larger than 1.5.  The last paragraph on page 5-134 discusses the use of 1.5 
on part "f".  Even though your design may utilize a pin at the top, it has 
some flexural stiffness since it is not a true pin.  

I am familiar with companies designing gravity load only frames as K=1.0, but 
I have to admit that a pin-pin with translation is not a value shown on the 
chart.  To correctly figure the amount of projection needed to return to a 
vertical alignment with the base would require knowing the curvature when 
buckled.

Hope this helps.

Ron Martin
Tuscaloosa, AL