K factor for sidesway--gravity column
K factor for sidesway--gravity column
(OP)
What is the correct way to assign the K factor for a pin-pin gravity column that is experiencing sidesway from an adjacent moment frame? In other words, I have 4 columns along a column line. The first 2 columns are a moment frame rigid connections at the top. The last 2 are just typical pin-pin gravity columns that are displacing at the top from the sway of the moment frame.
By looking at the AISC alignment chart, p.16.1-192 of LRFD 3rd, if the girder stiffness is zero, the K factor would go to infinity for a sidesway uninhibited column. If you go by the footnote on p.16.1-191, G can be taken as 10 for pinned ends at the foundation.
Is this what you do for pinned end columns experiencing sidesway from adjacent moment frames? Use G=10 at the bottom(per note) and G=infinity at the top, and get K=4.5 from the chart?
By looking at the AISC alignment chart, p.16.1-192 of LRFD 3rd, if the girder stiffness is zero, the K factor would go to infinity for a sidesway uninhibited column. If you go by the footnote on p.16.1-191, G can be taken as 10 for pinned ends at the foundation.
Is this what you do for pinned end columns experiencing sidesway from adjacent moment frames? Use G=10 at the bottom(per note) and G=infinity at the top, and get K=4.5 from the chart?






RE: K factor for sidesway--gravity column
RE: K factor for sidesway--gravity column
RE: K factor for sidesway--gravity column
RE: K factor for sidesway--gravity column
But as they do this, they (and the whole diaphragm) deflect laterally, and this results in secondary effects (pdelta forces). So if the whole diaphragm deflects laterally, say 1", then we sometimes add to the original lateral force a force equal to the sustained dead load x 1" divided by the diaphragm height. For flexible diaphragms, this can be sometimes substantial. For seismic, there are provisions given in the code (I'm thinking IBC here) where this pdelta effect can be neglected if the lateral deflections are small enough.
RE: K factor for sidesway--gravity column
Instead of increasing K value, can we perform P-Delta analysis for the structure and design the gravity columns for the second order moments with K = 1.
Thanks
Murali
RE: K factor for sidesway--gravity column
RE: K factor for sidesway--gravity column
RE: K factor for sidesway--gravity column
Can this P-Delta effect capture the global effect (sway) on column instability?
The local column instability due to slenderness can be captured based on its size?
Please explain on this or site some reference for the same.
Thanks
Murali
RE: K factor for sidesway--gravity column
http://www.eng-tips.com/viewthread.cfm?qid=151341
Murali27
P-Delta capture some (may be not all) stability issues, like in ACI-318 chapter 10.13.6 where the magnified moment from P-Delta limited to be sure the column is stable.
That was for concrete column.Iam not sure if similar limits avaliable for steel columns.
Regards
Ahmed
RE: K factor for sidesway--gravity column
RE: K factor for sidesway--gravity column
Chapter C, which deals with stability and Pdelta issues offers three analysis methods and within that, two analysis/design types - very convoluted and I've not yet had time to study it all in depth. But what I did take out of it was that you can perform a 3D analysis via computer but the program must be able to provide for both P(DELTA) forces and P(delta) forces....overall frame DELTA and member curvature delta. Most programs only do the DELTA part.
Usually, the second order interation that I described above (manually placing pdelta forces in addition to the main lateral forces) takes into account a very large percentage of the total. Subsequent interations would add to that but they are usually much smaller and we have usually added some conservatism to the results to account for that.
RE: K factor for sidesway--gravity column