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  • Writer's pictureSi Shen

Design life – what differences does it make?

Heavy civil structures typically have an intended ‘design life’ of 50 to 120 years. But what does ‘design life’ mean? What if you were to reduce its design life down to 1 year? What differences would it make? What design criteria could be relaxed? What money could be saved?

Truly understanding the meaning of design life and its influence over design decisions can help engineers optimise economy whilst providing the necessary functionality of the structure within the given scope. In simple terms: no straw houses, but no bullet proof phone cases either.

Functionality and serviceability

First of all, this question of course only a hypothetical one. It never pops up in real life. A 1 year structure is likely to be a piece of temporary works, which will have fundamentally different design requirement compared to permanent works. In real life, a structure designed to last for 1 year must have different functional requirement compared to another that lasts for 100 years. The 1 year structure is most likely to have no permanent use requirements (serviceability requirements), such as aesthetics, cracks and water tightness.

What the structure will go through

Overall speaking, the engineer needs to consider what scenarios the structure is likely to experience during its useful life. What loadings is it required to resist? What environmental change is it likely to experience? What changes in properties the construction material and the surrounding nature is likely to experience over time? What risks are likely to arise during the intended useful life of the asset?

Some loadings or risks only exist in the long term. For example, you may wish to design against sea level rise if you were to design a marine structure to 100 years design life, as it is a likely scenario given the current trend. It almost definitely won’t happen for a 1 year structure. You may also choose to disregard of a 100 years return period earthquake if you were to design a structure with a 1 year design life, as it is unlikely to happen.

The ‘behavioural problem’ over the long term

Some construction materials exhibit time-dependent behaviour, such as creep and drying shrinkage of concrete. The effects from such behaviours only start to become significant in the long term. Creep and drying shrinkage of the concrete depends on several factors including the ambient temperature, humidity, concrete strength, the time at which loadings are first applied.

Natural materials such as soil exhibit time-dependent behaviour too, including consolidation, stress relaxation and creep. Consolidation of soil is the drainage of pore water when the ground is being squeezed. If the ground is relatively impermeable whilst the loading from the structure only applies over a short period of time, then the ground behaviour is closer to ‘undrained’ condition, rather than ‘drained’, as there is not enough time for pore water to escape under the pressure. Stress relaxation is the ‘internalisation’ of the deformation of soil particles, i.e. some elastic deformation turns into plastic deformation. Creep is the slow and gradual increase of deformation when subject to the same loads over long term. I think I will write another blog on stress relaxation and creep later.

Durability of different parts

Not all parts of the structure are born equal. Some auxiliary parts of the structure may have much shorter design life compared to the main part of the structure. For example, joint sealants may have a design life of 20 years. Bridge bearings may have a life span of 20-45 years. If the whole structure is designed to 120 years, the design may be required to eliminate the use of these, or have a service plan to inspect and replace them. If the structure is designed to 1 year only, you may choose cheaper alternatives or simply ignore them if you can.

Durability of the main structure is also part of the equation. This may be reflected in thicker cover to reinforcement for concrete structures to make allowance for ‘wear and tear’, or the number of cycles for fatigue calculations for steel structures under dynamic loadings.

End-of-life considerations

Since the structure is intended to be used for 1 year only, the designer ought to take into consideration the imminent decommissioning of the structure. Reusable materials with ease of dismantle such as metals, rather than concrete, are more likely to be used to achieve best economics. The consideration on decommissioning would be a much less serious consideration for a 120 years structure as your guess on the state of available technology (or the existence) of human civilisation can be wildly off.

It’s just like designing a phone case

At the end of the day, all of these requirements will translate into the ‘quality’ of the structure. ‘Quality’ here does not just mean workmanship; a trendy buzzword for it is ‘resilience’ - what the structure is designed to survive. If you were to design a phone case, you probably won’t need it to be bullet-proof, but it is probably adequate to design it to survive a drop from a height of 1.5m, which is the scenario the phone is likely to experience (slipping out of hands when using the phone). There is a spectrum of possible design qualities of phone cases, ranging from surviving meteor impact down to broken by a gentle touch. Where your design lands on the spectrum depends on the engineer making sound discretionary decisions, on a case-by-case basis. How critical is the structure? How much impact there is likely to be if it failures? Answering these two questions will give you an indication on the risk tolerance and influence how much ‘resilience’ is required on the structure, which all translates into the ‘quality’ of the structure at the end of the day.

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