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  • Writer's pictureSi Shen

Reliability

My previous post http://www.si-eng.org/blog/universal-engineering-principles touched upon the subject of ‘reliability’ as one of the universal engineering principles. This blog post expands on this topic.


Reliability is the consistency in performance. This is typically a much desired thing generally in life, be it a person, an organisation or a material. In this blog post, let’s talk about reliability from an engineering material perspective.


The following properties ensure good reliability :

· No statistical ‘fat tail’, performance is concentrated in a particular region. Low deviation from its average performance means it is easier to get a grip on its properties.

· Fore warning before failure. Any sudden movement makes people nervous. A ductile material gives more fore-warnings than a brittle material hence is more reliable in that it is less likely to lead to a disastrous failure. Things could be done by the time it shows signs of distress.


A lack of reliability gives you uncertainty in engineering properties. What do we do when facing uncertainty? We are forced to take a more conservative stance by making extra allowance on top of the ‘typical’, or ‘average’ outcome. This can be represented by a reduction factor.


It is not difficult to observe a pattern in engineering materials - engineered materials are typically more reliable than natural ones. For example, loosely speaking, steel is more reliable than concrete. The raw material of both concrete and steel comes from the nature. But steel is a highly extracted material with higher purity and therefore the properties of its content is better controlled. Statistically speaking, the profile of steel’s performance has much less extremities compared to that of concrete. This can be reflected in the different material factors – 1.15 for steel and 1.5 for concrete.


Tensile strength of concrete is unreliable, especially in the ultimate limit state, as its failure is sudden, catastrophic and without warning. This is why the tensile strength of concrete is discounted in all ultimate limit state checks – this can be understood as a factor of safety of 0 is applied to concrete tensile strength.


Some natural materials like rocks are even stronger than most concrete, why don’t they get used as often in the modern world compared to concrete? Rocks are raw materials that come from the nature. Mother nature has highly variable temperament so these raw materials are non-homogeneous, which means their properties are highly variable. Therefore they are highly unreliable materials as you have to re-study their properties individually at each location, so it is much harder to apply prior experience onto a new project.


To manage the variability of engineering materials so as to consistently reach certain level of reliability in design, the concept of ‘characteristic value’ is invented. What we specify on drawings are usually ‘characteristic’ strength of concrete. This is a conservative number as it is based on a level that only 5% of all test data is lower than this. The mean tensile strength is much higher than this. The standard deviation of concrete strength is about 8MPa, which means typically the mean strength is 4MPa higher than the characteristic value.

Lastly, the same principle holds true for people. The more consistent your performance is, the more predictable you are, the more reliable you are, the more attractive you are (as a team member or service). When managing a low reliability person or organisation, we have to deduct their average performance down, until we reach a level above which we feel the person or organisation can consistently achieve. Consciously or sub-consciously, we are using the ‘characteristic’ expectation to manage performance. Being a high-performer a few times is easy, and can be done by any lay-person. However, being a professional is about being a high-performer consistently, with high reliability.

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