How do I design a base with an irregular geometry, or having more than two columns?
The Concrete Base Design module design rectangular footing with one or two columns. Any base with a more complex geometry (e.g. circular, triangular, etc.) and/or more than two columns should be evaluated using the Sumo or Frame Analysis modules:
- Model the concrete footing as a grid of shell finite elements, or using a grillage of beam elements. The advantage of using shell finite elements is that you will have the option to perform a reinforcement concrete design (analysis output will include reinforcement contour diagrams).
- Model the soil as an elastic medium using spring supports at every slab/grillage node. One approximation for determining the stiffness of the spring supports is using the foundation modulus – the spring stiffness is set equal to the foundation modulus multiplied by the bearing area that the spring represents. Foundation modulus values for various soil types are given in the the KnowledgeBase article: http://support.prokon.com/helpdesk/kb_cat.php?id=2.
- If you are concerned about uplift, you can avoid tension forces in the spring supports by using specifying gap behaviour (i.e. footing will lift up when rotated sufficiently instead of being held down by springs).