Slab design and foundation performance

Foundation design & slab-on-ground foundation performance

For engineered slab-on-ground foundations on expansive soils, there are two code approved design protocols that may be used, one published by the Post-Tensioning Institute (PTI) and the other published by the Wire Reinforcement Institute (WRI).

The Post-Tensioning Institute

The 1996 edition of the PTI book Design and Construction of Post- Tensioned Slabs-On-Ground includes the following:

Application of these recommendations results in slab designs similar to those that have exhibited satisfactory performance.

The Wire Reinforcement Institute

The WRI publication, Design of Slab-on-Ground Foundations – An Update, includes similar wording. They only promise that their design protocol is intended to provide foundation designs that have been foundation to give acceptable performance in the past.

What These Design Protocols Do Not Promise

Neither code approved design protocol promises that the actual deflection of engineered slab-on-ground foundations will be less than some stated amount. They do promise that actual foundation performance in terms of superstructure distress will be “satisfactory” and the distress “minimal.”  The levelness of the slab surface is not addressed by either design protocol.

Who Decides How Much Distress is Acceptable?

Both of these protocols are intended to result in foundation designs that have performed “satisfactorily in the past”. That raises the questions: satisfactory to who and what past are we talking about? The answer of who decides how much distress is acceptable is simple; the authors of the design documents, mostly Professional Structural Engineers and a few engineering professors, the large majority of whom work for, or with,  home builders.

The Bottom Line

I think it is fair to say that slab-on-ground foundations designed and constructed in accordance with either design protocol will not deflect enough to cause structural (load-bearing) damage to the superstructure.  There is likely to be some degree of cosmetic distress and some minor door problems.

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