How Engineers View Foundation Performance

Serving the purpose intended?

If you have been a real estate agent or a real estate inspector for very long, you have heard or read this phrase repeatedly: (Fill in the blank) was (or was not) performing the function intended. There was a time long, long ago that the standard sales contract was intimately tied to the TREC inspection report. The buyer was allowed to ask the seller to repair only items reported as failing to perform as intended.

This created a situation where the TREC inspector was sometimes pressured in one way or another to report that the foundation was in need of repair or not. 

Where serving the purpose intended works

If we are talking about electrical and mechanical equipment, there are well-established, clear, and specific rules about how the equipment is supposed to run, how to test it for proper operation and how to repair or replace it should that be necessary.  

Why it doesn’t work for structural components & systems

When it comes to structural components, there are no hard and fast rules concerning performance other than obvious. Let me give you a specific example: a sagging floor. If you look at the codes, what you find is a rule for designing the floor joists which is emphatically a performance standard for at least four reasons. First, it makes no claim to be a performance standard. Second, the design rule is for restricting the calculated deflection under live load only. There is no code mandated restriction for dead load. Third, the calculated live load deflection does not include any deflection due to creep, a material property. It is deflection due to the passage of time. creep deflection can easily exceed load deflection once the joists are older than ten years. Fourth, the codes specify that the floor joists be designed for uniform live loads. For instance, a bedroom floor is normally designed for 30 lbs per square foot. If there are concentrated loads, it is possible, that the concentrated loads are causing the objectionable floor sag.    

How engineers assess foundation performance

Consider the following from a standard engineering text

A common misconception, even among some engineers, is that foundations are either perfectly rigid and unyielding, or thet are completely incapable of supporting the neccessary loads…All engineering products, including foundations have varying degrees of performance that we might think of as various shades of gray. The engineer must determine which shades are acceptable and which are not. (Foundation Design Principles & Practices, Donald P. Coduto, PE GE, Prentice Hall, Englewood Cliffs, N.J. 07632 Copyright 1994

Notice that performance is judged against a subjective standard: the expectations of the engineer evaluating the performance of a specific foundation. Contrast this with the standard “serving the purpose intended”. The question of whose purpose comes immediately to mind. This is clearly subjective as well, but there is a significant difference: no one knows whose purpose is in play. In the case of the engineer’s assessment, we know who the person is: the engineer. 

A concrete example  

Imagine a brick veneer house that has several cracks in the brick veneer. An engineer might reason like this: The calculated deflection that will cause the brick veneer to fracture is much less than the calculated deflection for the concrete foundation. This is sufficient for the engineer to conclude that the cracked brick is within his or her expectations and thus the foundation is performing as intended.

On the other hand, using the “serving the purpose intended” standard an inspector might conclude that no one would intend for the brick to crack and thus recommend foundation repair.            

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