The Post-Tensioning-Institute & Slab Foundation Performance

What is the Post-Tensioning Institute (PTI)?

PTI is a trade association that publishes information of interest to its members all of whom are in the business of designing and constructing structures made of post-tensioned concrete.   Specifically, they publish guidelines for engineering, designing and constructing post-tensioned slab foundations. They also publish a document titled:  PTI Guide for Performance Evaluation of Slab-on-Ground Foundations.

The PTI Guide for Performance Evaluation of Slab-on-Ground Foundations  

The PTI guide for performance evaluation is written s0 that it applies to conventionally reinforced slabs in addition to post-tensioned reinforced slabs.

What to expect from a slab-on-ground performance evaluation

The visual assessment

Any performance evaluation starts with a visual assessment. What the engineer is looking for are irregularities in the finish materials that are consistent with the way wood frame structures react to slab foundation distortion.

The elevation assessment

An elevation assessment consists of making elevation measurements on the finish floor and any exposed slab. The measurements are analyzed by a Professional Engineer to make an estimate of how much the foundation has bent and tilted.

Elevation assessments have their limitations.  has distorted. 

The purpose of a slab-on-ground foundation

Slab foundations serve at least three purposes:

1. The top surface serves of the slab as a floor for the house.

2. The slab foundation transmits the weight of the foundation and the house and all live loads to the soil without causing the soil to become overloaded

3. The slab should be stiff enough that the predicted soil movement will not cause excessive cosmetic distress, excessive functional distress, and no structural distress.

What constitutes acceptable & unacceptable performance 

TREC requires home inspectors to render an opinion of the performance of the foundation. Should Professional Engineers do this? My answer is: yes, especially if the report is for a real estate transaction.

One way to do this is to report the foundation performance as “acceptable”, “unacceptable”, or “marginal” based on observing various performance indicators as listed in the TREC SOP.  

Three types of distress

Cosmetic distress: Irregularities that are noticeable but which does not adversely affect the ability of the slab foundation or the frame structure to transmit loads. Virtually all houses in expansive soil areas will exhibit cosmetic distress during the life of the house. 

Functional distress: Irregularities that affect how the house is used. Examples include doors that are noticeable but which does not adversely affect the ability of the slab foundation or the frame structure to transmit loads.  

Structural distress: This is a more serious type of distress. Structural distress negatively affects the ability of the frame structure to safely transmit loads. Examples include rafters that are being pulled away from the ridge.

Mitigating slab foundation movement

 

Evaluating slab foundation movement

 

Slab movement & distress

 

Slab foundations are never built level and flat

 

Elevation differences but no visible distress

 

Larger than normal elevation differences

 

Cosmetic distress is normal and expected

 

What causes slab foundation movement

 

Slab foundation movement during construction

 

Initial settlement before occupancy

 

Initial movement after occupancy

 

Seasonal slab foundation movement

 

Slab foundation movement & unanticipated issues

 

Older slab foundations & movement

 

Local cosmetic and functional distress

 

Misuse of local slope measurements

 

 

 

 

 

 

 

 

 

 

 

 

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