Monday, April 18, 2011

Cognative Dissonance

Cognative Dissonance – psychological conflict resulting from incongruous beliefs and attitudes held simultaneously.

In engineering, it may be expressed in other terms.
Example, Beliefs
  1. Engineering is important; to do construction correctly is valued; I make my meager living as a “Professional Engineer”.
  2. Construction should be conducted as per the Alberta Building Code. 
  3. Some cases the Alberta Building Code is grossly wrong.
  4. Now what? We are forced to create an “artificial value” for an Ultimate Limit State soil strength, that has no physical significant, and calibrate it to the desired outcome of the building code.
Most foundation should be designed for the Service Limit State. In 40 years, I have not run across a condition that I would employ a Ultimate Limit States soil strength for design. Many of the methods of evaluation of the soil strata strength do not generate a ultimate value, largely due to the time required in clays, and the gross amount of movement that occur in such tests.
Pile load tests are as close to ultimate as we would ever come, and usually those are limited to something like 50 millimetres of movement before it is declared failed. The true pile capacity may be 1.01 to 2.0 times that amount, for discussion.
For a typical project there is a wide range of soil strengths, and all this soil data must be reduced to something that is manageable for design. That is more of an art than a science, since the reliability and variation of the data, the amount of data, the geology of the formations, and the type of foundation all play major roles in the final value. After we go through all that, then we have to fudge a number so that the final result will be correct, due to government insistence on a system of design that matches the structural concepts of how materials should behave, not reality.

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