More to the point, this is such an extreme condition that it does not reflect a serviceability (deflection) limit state. Calculation of deflections at the ultimate limit state is not pragmatic and probably not realistic, since even the best tools at our disposal cannot reliably predict deflections (strains) that occur in concrete as it is being crushed. We know that the likelihood of a sufficiently designed member ever reaching this failure state is extremely small and that, if it does happen, the member will fail “in a certain way” so that its behavior is controlled, manipulated and even predictable ductile is another descriptor that comes to mind. When evaluating a “strength” limit state – e.g., flexural design of a reinforced concrete beam – we are considering the ultimate failure state of the member. Why is this necessary? The answer is simple and grounded in the basic theory of limit states design. For instance, when the deflection of a concrete beam comes into question, we revert to methods drawn from the working stress design theory. However, for most of us, limit states design has always been the norm, though some remnants of working stress design have endured. Perhaps you are seasoned enough to remember the days when working stress design of reinforced concrete was the norm, and limit states design was a fairly new concept. Limit states design – also known as ultimate strength design or load and resistance factor design (LRFD) – is largely supplanting the traditional methods of allowable stress design for most structural materials.
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