high steel ratio for edge columns at roof level

[chekre]

Hi guys,

when analysing a roof slab, edge columns will absorb unbalanced moment which will result in high ratio of steel since the axial force will be small to counteract the tensile forces caused by thr moment. If we look at the below floors, the ratio will decrease since the moment will be smaller and the axial force higher.
Do u generally increase the reinforcement in the roof floor or do u assign releases for the columns (Not very prudent)?

 

[chekre]

in continuation to the above, what i do is that while designing the roof slab, i pinn the edge columns so no negative moment will occur.
however, there will be some minimum top reinforcement at the column edges and this reinforcement will transmit some moment To the columns.
Therefore, i assign 0.35 as property modifiers to the edge columns in my 3d model (reinforcement will be decreased) and i put the required steel into the column so the column can sustain some moment transmitted from the slab.
any comments or other suggestions ?

 

[hokie66]

Columns near the top of buildings commonly require more reinforcement due to the reason you gave.

 

[chekre]

Thanks hookie for ur reply.
there are some cases where the floor below requires the minimum 1% (aci 318) and the floor directly above jumps to 3 or 4 %. will u jump directly to this high ratio in ur columns ?
The second method i wrote about in the 2nd message doesnt seem for u a good alternative rather than putting a big amount in the top of the buildings ?

 

[structural87]

interesting !! I would really know more about it ! What do other members think ?