General requirements (columns seismic: ACI 318)
End Fixity
Reinforced concrete columns assigned to Moment Resisting Frames have their end fixities at the base of the building limited to:
- Fixed base;
- Pinned base;
- Spring base (foundation flexibility).
Minimum factored axial force
Members experiencing axial compression forces higher than the minimum threshold in the code from any of the load combinations are required to be checked for flexural strength and to consider flexural detailing within the strong column - weak beam design philosophy according to their assigned SFRS type.
If SFRS Type = Ordinary Moment Frame, then no No axial compression load requirement applies.
If SFRS Type = Intermediate Moment Frame, or Special Moment Frame
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Pmin | = | Ag * f’c / 10 |
| where | |||
| Pmin | = | Minimum required axial compression | |
| Ag | = | Gross area of the concrete section | |
| f’c | = | Specified compressive strength of concrete | |
|
The check passes and the member is designed for seismic provisions as a compressive member if Pu > Pmin
|
where | ||
| Pu | = | Largest factored compressive axial force at the top of the stack from any load combination. | |
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Note: This check is no longer required in ACI 318-14.
Maximum recommended axial force
ACI 318 allows for the maximum design axial load to be as high as 0.8*ɸ*Pn,max, where Pn,max is the maximum compression resistance of the section composed of concrete and steel. However in the event of a severe earthquake a full height beam yielding mechanism could occur inducing higher compression strain on the columns than the one predicted by elastic design.
Good practice recommends that the maximum compressive strain in a column part of a Special Moment Frame should remain below the balanced value.
Note: The current release of Tekla Structural Designer does not check if the compressive strain is below the balanced point.
Minimum cross-section dimension
ACI 318 limits the shortest cross-sectional dimension of a column that is part of a Special Moment Frame to a lower fixed value in any direction measured in a straight line passing through the section centroid and also to a fraction of the length of the perpendicular dimension.
If designing to ACI 318-14, there is an additional limit of half the height of the deepest beam connecting at the joint.
These minimum dimension restrictions are calculated and applied accordingly when the column that is part of a Special Moment Frame.