end releases of column

[chekre]

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?

 

[paddingtongreen]

Try turning the columns 90^o to reduce their stiffness in that direction.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[chekre]

The columns layout is imposed by the architectes dude. i cant change it like that

 

[BAretired]

14m is a pretty healthy span for a slab. How thick is the slab? Perhaps beams on the column lines would help.

BA

 

[chekre]

32cm post tensioned slab. I have tried to put beams, it didnt help that much. 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. If I increase the columns sections, it will become stiffer and will attract more momemts so the final result will remain the same.

 

[hokie66]

You need to totally reexamine the structural scheme. A 320 slab is much to flexible for a 14 metre simple span. Suggest you seek assistance from an engineer experienced in long span concrete design.

 

[chekre]

it is residential slab hookie, the loads are not that much. I received the 320mm thickness from a sub-consultant who is expoert in post tensioned slabs. Cant i release the columns from taking moments ?

 

[paddingtongreen]

I haven't done any post tension work (it looks like a black art to me) so forgive me if this is silly but will not the act of tensioning the cables put reverse bending into the slab and then into the columns? Is this an offset? Will the forming be in place when the cables are tightened? It looks to me to be analogous with a steel beam that is cambered after being rigidly fixed to the columns at both ends.

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

 

[BAretired]

The live load may not be much, but the dead load is substantial. If column moment is a problem, you can increase the level of prestress in the slab so that you are balancing a greater portion of dead load and reducing column moment. With such a hefty slab, you could also consider voids to reduce dead weight. Beams on column lines, if nothing else, should reduce the thickness and weight of slab so it is not clear why you said "I have tried to put beams, it didnt help that much."

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

 

[chekre]

Hi BA retired,

attached 2 screenshots of the Etabs model showing the columns layout.

 

[chekre]

1 screenshot, excuses

 

[BAretired]

Wow!

BA

 

[BAretired]

I think there is merit in using a couple of wide beams between columns and cutting the slab depth down a bit. You might want to shift the two middle columns a tad to the left or add a couple of columns to get a more economical arrangement.

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

 

[chekre]

Lool yeah man, i love them specially when they begin to give u solutions which will never work.

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

 

[BAretired]

Where are the shear walls?

BA

 

[chekre]

hi, i have attached the vertical elements layout

 

[BAretired]

I think that the columns shown in green will play a significant role in resisting lateral forces. The shear walls shown in red are too remote from the c.g. of the lateral force to rely on them alone.

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

 

[chekre]

ok, thanks BA retired. I run the model and the columns are not participating that much in resisting lateral loads (the acting moments vary between 2 and 10T.m) (specially after assigning the modifiers 0.7 in the strong direction, and 0.35 in the weak). Anyway, i'll see with the architect how the column layout may be modified to accomodate more columns and better placement.

Thanks again for your help.