Bending stiffness
The initial stiffness is usually determined using proportional loading of the detail with constant eccentricity e= MEd/NEd= const. Sometimes, stiffness for constant axial force is used (Bowman, 1989).
The analysis model uses the simplification mentioned in previous chapters. This simplification deals with the centre of compressive stress under the column flanges. The dependency of deformations δt, δc of particular components on inner forces is based on stiffness kt of part in tension and stiffness kc of part in compression. Following expressions are used:
Analysis model
The rotation of the foundation is calculated using expression:
Therefore, initial stiffness is
Non-linear part of moment-rotation dependency is based on the shape factor
The tangential bending stiffness is calculated using expression
Rotation-moment diagram for proportional loading with constant eccentricity
The linear part of the curve represents bolts in tension and second flange in compression. Non-linear part describes plastic deformation of a component (end plate, bolts in tension or concrete in compression).
Following figure shows the effect of simplified model on resistance determination.
Comparison of simplified model with active area under the column flanges (curvature changes are caused by the activity of bolts) and model with active area under the all cross-section
References
Bowman L P, Gresnight A. M., Romeijn, A: Reserch into the connection of steel base plate to concrete foundation, holandsky, Stevin Laboratory Report No.: 25.6.89.05/c6, Delft 1989.