The following assumptions are adopted in calculations of spread footings with two columns.
- Only analysis of rectangular foundations is allowed when a line passing through points of pier axes is parallel to the foundation edge.
- In soil calculations the foundation is treated as infinitely rigid, whereas the soil as linearly elastic. It is assumed that stresses in the soil can be described by means of a plane.
- In static analysis the foundation is calculated as a single-span beam with cantilevers supported by columns and loaded with soil pressure. Calculations are performed independently for both directions
- Work of the foundation treated as plate is not taken into account in calculations
- The foundation is designed for the maximal span moment and support moment; these moments are calculated considering the support width. The occurrence of top reinforcement depends on that, whether the tensile moment acting on top fibers exists (it may as well be forced in the Reinforcement Pattern dialog box)
Once the above calculation principles are adopted, the foundation with two columns should satisfy the following requirements:
- Foundation thickness, in relation to its dimensions, should be large enough to allow the user adopting the assumption that the element is undeformable; for elements with small stiffness, for which deformation is greater than approx. 0.5% of element length, the solution obtained may be inconsistent with the real element work; for design of such elements in the Robot program the Continuous Footing Design module is used
- The relation of foundation length to foundation width should not be less than 2.0; it concerns mainly foundations with large dimensions and relatively small stiffness; in such elements effects connected with work of the foundation treated as a plate may lead to overestimation or underestimation of moments for the element calculated independently in both directions.