Slab-On-Ground Foundations: The Relation Of Analysis / Design To Performance Assessment

I anticipate that this article will be at least somewhat controversial. It will definitely be more technical than some readers might like. Engineers should have no problem understanding the article. TREC inspectors, I suspect, will be able to understand all the salient points. Real estate agents are likely to believe they have more profitable ways to spend their time. I do not think they will have problems understanding the main points I make. It is just that it is not likely to help them sell more houses, but as with everything else on this site, agents are welcome to plunge in.

From an engineer’s perspective, not all is lost. These documents specify, for the most part, that a reinforced concrete beam be designed so it is stiff enough that the calculated overall deflection is limited to 1/360. As an example, this means simply that the calculated deflection due to soil movement should not exceed 1 inch when the length of the slab is 30 feet.

This seems at first glance to be pretty specific and concrete, but that first glance is misleading. You might think that if an engineer uses a deflection ratio of 1/360 in his design, then if the slab was constructed properly, it should not deflect more than 1 inch across 30 feet. Unfortunately, that is just not true.

It turns out that a reinforced concrete beam subjected to a design load may deflect in excess of 30% more than what calculations indicate. Even under laboratory conditions, graduate students will find that reinforced concrete beams that they build under tightly controlled conditions will deflect between 20 to 30% more than what the calculations indicate. Under field conditions, the deflection is likely to be even more than under laboratory conditions.

I do not expect TREC inspectors to be familiar with this, but any Professional Structural Engineer should be.

US Army Corps of Engineers

Design/analysis of slab-on-ground foundations

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Slab-on-ground foundation performance

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Primary deflections of slab-on-ground foundations

 

Secondary deflections of slab-on-ground foundations

 

An overview of the design/analysis process

 

Why slab-on-ground foundation analysis is not a good predictor of performance

 

 

Stiffness versus deflection

 

Stiffness calculations

 

Construction tolerances for levelness

 

 

The effects of creep

 

 

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