end releases of columns

[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?

 

[slickdeals]

What is your framing? Do you have a beam running perpendicular to the span at the end? If so, try assigning the torsional stiffness modifier to 0.1 (compatibility torsion).

 

[chekre]

No it is a post tensioned slab. I have assigned 0.5 as modifiers for slab. if u want to increase the column size, it will become stiffer and will have more moment coming into top and bottom. Any ideas?

 

[slickdeals]

What is your slab thickness? For a 14m span PT slab, you should be in the 300mm + range. Does your column work for punching shear? 1200x350 seems small.

 

[chekre]

yes, the column is fine. actually, it is residential so the loads are not that big. shear reinforcement will be provided. Even if u are going to increase the 350mm of the column, it will absorb more moment since it will become stiffer. I am thinking of assigning releases to the column although i dont like but if the slab is designed in order not to transfer any negative moment, releasing may be acceptable. Right ?

 

[chekre]

Any ideas or suggestions ?

 

[asixth]

Have you looked at increasing the stiffness modifier for the column. If it cracks the moment will re-distribute back to the slab.

 

[chekre]

yes, i have assigned instead of 0.5modifiers 0.35 for the column (i cant go below), still the reinforcement is coming near 4%. what i did is that i have released the top and bottom of the column in order to carry just axial forces and the slab will be designed as pinned connection. the columns that have been assigned wont participate in resisting lateral loads (the shear walls will do). i think it is something acceptable.