How do I model a nominally rigid base in Tekla Portal Frame Designer?
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Tekla Portal Frame Designer
Environment
British Standard
Nominally rigid bases should be modelled in accordance with clause 5.1.3.2 of BS5950-1:2000 which states
"If a column is rigidly connected to a suitable foundation, the following recommendations should be adopted:
a) In elastic global analysis the stiffness of the base should be taken as equal to the stiffness of the column for all ultimate limit state calculations. However in determining deflections under serviceability loads, a base may be treated as rigid.
b) In plastic global analysis any base moment capacity between zero and the plastic moment capacity of the column may be assumed, provided that the foundation is designed to resist a moment equal to this assumed moment capacity, together with the forces obtained from analysis. In elastic-plastic analysis the assumed base stiffness should be consistent with the assumed base moment, but should not exceed the stiffness of the column."
In Tekla Portal Frame Designer you can specify the ULS Rotational Stiffness and also the Base Capacity but the values should be consistent.
"If a column is rigidly connected to a suitable foundation, the following recommendations should be adopted:
a) In elastic global analysis the stiffness of the base should be taken as equal to the stiffness of the column for all ultimate limit state calculations. However in determining deflections under serviceability loads, a base may be treated as rigid.
b) In plastic global analysis any base moment capacity between zero and the plastic moment capacity of the column may be assumed, provided that the foundation is designed to resist a moment equal to this assumed moment capacity, together with the forces obtained from analysis. In elastic-plastic analysis the assumed base stiffness should be consistent with the assumed base moment, but should not exceed the stiffness of the column."
In Tekla Portal Frame Designer you can specify the ULS Rotational Stiffness and also the Base Capacity but the values should be consistent.
If you want to limit the base moment:
- Check maximum moment that can be passed to the base by setting ULS rotational condition to ULS 100% stiffness and 100% Mpr base capacity
- View the Bending Moment diagrams
- If the pad base is to be designed the carry the full moment - Fully Nominally Rigid - change the base moment capacity to match the bending moment diagram and set the SLS rotational condition to Restrained
or - If the pad base is to be designed the carry the less than the full moment - Partially Nominally Rigid - leave the base capacity as 100% Mpr and adjust the ULS rotational stiffness until the desired moment is displayed on the bending moment diagram. Once the required stiffness has been found use this value in both the ULS and SLS conditions and then set the base capacity to the desired value.
If you want to limit deflections:
- Change the Base Rotational Stiffness until you achieve the required deflections
- View the Bending Moment diagrams
- Set the Base Capacity moment = de-factored ULS base moment