Thursday, January 27, 2011


A majority of structural failures and damages costs in ordinary building ( some estimates range as high as 80%) occur as a result of errors in planning, design, construction and use rather than stochastic variability in resistance and load. Errors in concept, analysis and execution and other unforeseen circumstances occur even when qualified personnel are involved in design and construction and when accepted methods of qualified assurance and control are employed. Such errors result from human imperfections and are difficult to quantify. Modern code safety checks, such as load and resistance factor design (LRFD), are not developed with such errors in mind. Design and construction errors do not simply change the statistics in load or resistance that are used to derive probability-based design criteria; rather, they change fundamentally the load and resistance models or the relevant design limit states. This sources of ‘abnormal’ load or resistance is better dealt by the engineer recognizing that things can go wrong, though a consideration of hazard scenarios, and through improvements in quality assurance and control. Such performance – oriented thinking is essential to progressive collapse mitigation in general.

Range of design errors
  1. Structural systems or concepts, and load paths
    • There must be a continuous and sensible load path from the point of application of a load all the way to the foundation. Although this seems evident, load path issues do arise when the design analyses and detailing lose sight of the overall structural concept and become inconsistent with it. They also arise when the structural concept itself is inconsistent with the loads it needs to resolve and resist.
  2. Incorrect loads or incorrect assessment of load effects
    • Determining the correct design loads is not just a question of extracting values from a building code. It is a critical element of the design. Even building code load values have and continue to evolve as our understanding of natural events, such as winds and earthquakes, and their effects improves and as the statistical foundation for their frequency and magnitude are enhanced.
  3. Structural analyses and calculations
    • With the prevalence of terms such as computer-aided engineering, it is understandable if people think that computers can do all the work. They can, but they cannot do the thinking. The adage ‘garbage in, garbage out’ remains true, except it occurs faster and faster. Structural analysis, whether computer-aided or not, is fundamentally a mathematical representation of physics. It remains a designer’s responsibility to ensure that the complete structural behavior of a given system is captured, and to ensure that the analysis results are properly interpreted. The problem is actually exacerbated when engineers fail to appreciate the underlying assumptions and limits of applicability of the analysis software they sometimes use as a design aid. 

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