K-Value for (steel) Girders 1

[WARose]

Whenever you figure the K-Value for a girder in a steel frame do you just use an idealized value (like in Table C-C2.2 (p16.1-240 of the 13th edition))? Or do you use the alignment charts? The thing about it is: every time I have seen an example of an alignment chart used: it’s for the column portion of the steel frame. If you did use an alignment chart, would you use it the way you always do, or would the column and beam terms switch places (since you are using it for another direction)?

One of the reasons I ask is: in that same section of the 13th edition I mention above, they state that one of the assumptions (#9) is that “no significant axial compression force exists in the girders”….so I’ve always wondered this about alignment charts.

Thanks in advance.

 

[JoshPlumSE]

It's the same concept for Girders as it is for columns. Just flip the beams and columns when calculating the G values. Since the columns may have significant axial force you might apply a "stiffness reduction factor" to the column stiffness. Not sure if this concept still exists in the 13th and 14th edition manuals. But, if you go to the older LRFD manuals (like the 2nd edition I used in school) I know it will be there.

That being said, if you're using the 13th edition and the "direct analysis" method then you can ignore all this alignment chart stuff and just use a K value of 1.0.

 

[WARose]

Thanks Josh.....this is one of those fundamental things that you forget from time to time. I like the direct method too.