Slab Foundation Distortion Modes

What is a distortion mode?

Every structural element such as beams, columns, grids, foundations, beam-columns, etc. has one or more distortion or failure mode(s).

In the case of slab-on-ground foundations, the way they are designed makes use of an engineering model: specifically a beam on an elastic foundation. The beam is assumed to fail or distort by bending excessively.                                                       

Slab foundations distort in two modes

Center lift distortion mode

Center lift is where the elastic foundation (or supporting soil) swells into the shape of a mound or an upside-down bowl. The pressure against the beam forces the beam into an upside-down bowl shape: a mirror image of the mound shape.

The center lift distortion mode is also known as the long-term distortion mode. The reason is that the large majority of slab-on-ground foundations are distorted into an upside-down bowl shape within around seven years.                                 

The long-term distortion mode

The center lift distortion mode is also known as the long-term distortion mode. The reason is that the large majority of slab-on-ground foundations are distorted into an upside-down bowl shape within around seven years.             

The short-term distortion mode

The short term distortion mode is the reverse of the center-lift distortion mode. In the short-term distortion mode, the perimeter of the foundation is pushed upward relative to the center area. It is always possible that the foundation was built in this shape, but the more common reason is that the perimeter was pushed upward because the drainage near the foundation is poor.   

Why the distortion mode is important

If the foundation is in an edge-lift distortion mode, the appropriate repair, assuming repair is warranted, is to correct the drainage. 

 

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