# Frame foundation

The structural frame
The construction
The reinforcement I
The reinforcement II
Quantity/Cost estimation
Detailing drawings
Introduction >

Wind and Seismic Forces >
Structural model and Analysis
Slabs
Seismic behavour of frames
Appendix A
Appendix B
Appendix C
Appendix D
Introduction >
Modelling slabs

Materials
To be continued >
Introduction

## One-bay frame with concentrically placed spread footings

< project: foundation110>

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.

## Behaviour of one-bay frame with concentrically placed spread footings (no seismic loads applied)

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

## Behaviour of one-beam frame with concentrically placed spread footings (under seismic loads)

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.

## Multi-bay frame behaviour under seismic loads

<project: foundation115>

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.

## One-bay frame with spread footings and connecting beam

<project: foundation120>

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.

## Frame behaviour (no seismic loads applied)

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.

## Frame behaviour when seismic loads are applied in +x direction

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.

## Multi-bay frame behaviour under seismic loads

<project: foundation125>

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.

## Foundation reinforcement of one-bay frame

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## Reinforcement of a continuous connecting beam

<project: foundation130>

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