Conservative K value for partially braced frame?

[StructureMan44]

What is a conservative K value for a partially braced frame like in the attached picture? It’s somewhere between sideway inhibited and non-inhibited.

 

[canwesteng]

I would call the chevron braced portion rigid, and use K=1.2 (assuming the base is fixed as shown) for the unbraced segment. If the base is pinned you could conservatively say K=2

 

[BAretired]

I agree with canwesteng but I'm wondering why the columns are not rotated 90^o so that the major axis is resisting the frame bending moments.

BA

 

[PaulG93]

You could calculate G and use sidesway uninhibited alignment chart. Conservatively ignore the braces and the strut.

 

[JLNJ]

2.0 with L being taken from the point of the lower strut to the foundation would be conservative in my book.

 

[KootK]

Pinned base = 2X distance between base pl and truss CL. Fixed base = 1/2 that. These are upper bound approximations that assume sway frame action and members of infinite axial rigidity. The real values will be less than these approximations but more than 2X/1x the base plate to low strut distance which serve as lower bound approximations. I think this is as good as it gets without diving deep into the math. I’d also temper these values with a judgement factor as we do with other effective length situations where we know our assumptions are imperfect.

You could also use the alignment chart with the column length taken to truss CL and the truss stiffness subbed in for the girder stiffness. This is effectively column and girder with a wildly stiff girder.




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.

 

[PEFLWI]

Using the direct analysis method K is always 1.0. The direct analysis method involves using a reduction on E, second order analysis and notional loads (minimum horizontal loads). It looks like you are using RISA. They have very good online videos and customer service to help you figure this out. See the following link for an explanation of the direct analysis method.

https://www.youtube.com/watch?v=dcKQ0SD-rp8