end releases of column
end releases of column
(OP)
Hi guys,
i am having some problems with a concrete slab spanning one way 14 meters and supported on 1200x350mm concrete columns( the 350mm is perpendicular to the span) . The reinf't in these columns is coming too much due to the portal frame action(the moments are coming mainly from gravity loads (unbalanced moment)). I have run the model assigning 0.7 for the major direction and 0.5 for the minor but still, ratio is exceeding 4%. I was thinking of assigning end releases at the bottom and top of these columns and the slab will be designed as it was resting on pinned supports
Is this acceptable noting that these columns are not participating not that much in resisting lateral loads and as mentioned earlier, the high moments are due to gravity ? and if it is not acceptable, what do u generally do in such cases?
i am having some problems with a concrete slab spanning one way 14 meters and supported on 1200x350mm concrete columns( the 350mm is perpendicular to the span) . The reinf't in these columns is coming too much due to the portal frame action(the moments are coming mainly from gravity loads (unbalanced moment)). I have run the model assigning 0.7 for the major direction and 0.5 for the minor but still, ratio is exceeding 4%. I was thinking of assigning end releases at the bottom and top of these columns and the slab will be designed as it was resting on pinned supports
Is this acceptable noting that these columns are not participating not that much in resisting lateral loads and as mentioned earlier, the high moments are due to gravity ? and if it is not acceptable, what do u generally do in such cases?






RE: end releases of column
Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin
RE: end releases of column
RE: end releases of column
BA
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RE: end releases of column
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RE: end releases of column
The sequence seems to be critically important here.
Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin
RE: end releases of column
In your first post you say "the 350mm is perpendicular to the span". Does that mean the 1200mm dimension is parallel to the span? If so, why are you now saying "The moments are coming mainly due to the big span and to the weak inertia of the columns which is placed perpendicular to the span." It seems to me that the moments are coming from the long span combined with the strong axis bending of the columns. Have I got it right? Which way are the columns oriented? Perhaps a framing plan would help.
BA
RE: end releases of column
attached 2 screenshots of the Etabs model showing the columns layout.
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RE: end releases of column
BA
RE: end releases of column
Post tensioning cables in the beams would then be running at an angle to the Y axis (approximately thirty degrees). Cables in the other direction could be fanned with the exterior cables parallel to the edges of slab.
At the right end of the plan, perhaps you could place a column at each edge and one in the middle with a two span beam connecting them.
Architects...don't you love'em?
BA
RE: end releases of column
Anyway, i cant change the columns layout. Releasing the columns might be the only solution (despite the fact that i dont like to release columns). but i am saying to myself that, in case of lateral loads, shear walls will take care of controling the system stability. columns will be there just to carry axial loads. What do u think ?
Thank you
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BA
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RE: end releases of column
I do not agree with the idea of creating a hinge at the top and bottom of columns. It would be more prudent to allow the columns to participate in lateral force resistance even if it results in column cracking. Cracking can be minimized by using wide beams between columns, heavily post-tensioned to balance dead load and thinning down the slab as much as possible to reduce dead load.
The column locations selected by the architect are not particularly well chosen from a structural point of view and the exterior shearwall should be extended a bit more to reduce the span to the rightmost columns.
BA
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Thanks again for your help. :)