Frame foundation

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Columns belonging to frames i.e. clusters of columns and beams and the behaviour of their foundation depends upon the interaction between the frames and the soil. The simplest frame consists of two columns.

When no seismic loads are applied, the soil pressures have a slight trapezoidal form and the spread footing is subjected to severe rotation.

When seismic loads are applied, the two spread footings are heavily over-stressed and they are forced to ‘work’ partially thus leading to large soil pressures. When the earthquake forces shift direction, there is a symmetrical change in the spread footings’ over-stressing.

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In case of an earthquake, a large part of the footing cannot function in the required way. All spread footings have almost the same behaviour (boundary spread footings are slightly over-stressed), as shown at the figure.

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Generally, it is useful to place connecting beams at the foundation because they carry the horizontal shear forces and reduce differential settlements. However, in earthquake resistant structures, they are not just simply useful but absolutely necessary because apart from everything else, they reduce the spread footings stress eccentricities and ensure the fixed-end support condition of the columns to a high degree.

When no seismic loads are applied, the soil stresses have an almost orthogonal form and the spread footings together with the connecting beam are almost undeformed.

During an earthquake, both spread footings work in a satisfactory level. The one is over-stressed thus creating larger soil pressures while at the same time, the other one is relieved. When the earthquake shifts direction, the stress conditions reverse. The connecting beam is subjected to large and continuously changing deformation and stresses.

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In a multi-bay frame with spread footings and connecting beams, the behaviour of the former, in a seismic event, is satisfactory. The boundary spread footings are over-stressed (or slightly under-stressed depending on the earthquake’s direction). The earthquake causes alternating stresses at each point of a connecting beam thus applying almost the same tension at the upper and the lower fiber.

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In case of more than two connected spread footings and consequently more than one connecting beams, the reinforcement follows the same rules mentioned above.