Modelling Column Stiffness

[bjr]

Whilst modelling a flat slab I am trying to take into account the stiffness of the columns above and below. Can someone outline in simple terms the best method/equation to use to achieve this so I can put a spring stiffness in my model to represent the column stiffness.

Many Thanks

 

[phuduhudu]

I believe the normal method is to model the column as pinned at mid height between floors and then use a stiffness of half the gross section stiffness of the concrete. If you want to be more accurate in your model you'll have to work out a cracked section stiffness allowing for the steel in the column. But there won't be any point in doing that unless you are modelling your slab with the same accuracy. Carl Bauer

http://www.bauerconsultbotswana.com

 

[Qshake]

My suggestion would be follow the necessary stiffness method used in formulating the DOF for the column element. This seems appropriate, pending the direction of forces, as the slab deformations under gravity loading will effect the axial and lateral deformations and pending the supporting layout of the columns and any lateral loadings, torsion deformations will be effected.

As carlbauer noted, this is only as accurate as you method of modelling the slab.

 

[Ingenuity]

bjr,

For 2-D flat slab analysis for vertical gravity loadings I use the "equivalent frame method" (Ref ACI 318) where the column stiffness and the stiffness of the transverse torsional member is summed via their inverse to give an "equivalent column stiffness". I usually do a full length 2D frame with full column height above (fixed at far end) and full column height below (fixed at far end, unless it is the foundation level and i may pin it).

Having said that, i occasionally use the net column stiffness, or sometimes the "enhanced column stiffness" to account for the infinite stiffness of the column over the slab/beam depth.

The variation between all 3 methods is seldom very large, for "typical" arrangements, so basically you can use any "rational" approach.

At end columns i usually reduce the column stiffness substantially as it is difficult to develop the calculated moments, and it they are indeed not developed the first interior span will have increased positive moments and deflections.

HTH

 

[JAE]

bjr...we do what ingenuity stated.