How long do buildings last?

I recently made a realisation, after being told by my local auto shop that its more expensive to repair the body of my car than to buy a new one (and no I’m not driving a vintage Ferrari), that we are now in an era where the art of repairing things is slowly dying. Despite the thousands of dollars we throw into electronics and cars, most of them are replaced after 5-10 years (or we get cornered to go for ‘upgrades’).

Fortunately, there are still things that have a relatively long life span, such as buildings or houses. But how long exactly do these structures last?

Majority of today’s modern structural design standards such as concrete, steel, masonry, etc. have factors calibrated to ‘normal’ conditions, which translates to a 50-year design life. Houses and many buildings fit into this category. However, certain special buildings and structures are designed for more a 50-year design life, such as major infrastructure works. Conversely, there are buildings that can be built with less than 50-year design life, like temporary structures or shelters.

While engineers can provide a design suitable to the desired life span of the structure, the quality of work during construction has a huge effect on the durability of the building once completed. Builders must follow the requirements set out by the standards. There are many issues on this particular topic namely concrete curing, protection of reinforcement, admixtures to concrete, durability requirements of external timber, and many more. This topic by itself is a vast and important, which will not be discussed in detail today.

Whilst design and construction set outs the durability of the structure, the key to prolonging the serviceability of a building is good maintenance. As it turns out not every single piece of the building is made to last 50 years and maintenance is required. A few examples are: Steel sections and connections, particularly on exposed structures, shall be inspected to ensure that surface protection for corrosion is intact. Cracking on concrete shall be repaired (filled or grouted). Timber shall be checked for decay and replaced when required. Maintenance is a must to keep the structure in good condition.

But what does a 50-year design life mean?

Does it mean after 50 years the building will collapse? Before we answer this question let us take a step back and remember that buildings are designed for various types of loads including natural phenomenon i.e. wind, earthquake, snow, floods. These events vary in severity. Based on historical data and statistics, the various magnitude of these conditions can be drawn out and be used in structural engineering, taking into consideration the importance of the building. As an example houses under ‘normal’ conditions are designed for wind and earthquake forces that has a 10 percent probability of being exceeded within 50 years. For more important buildings such as hospitals, a lower probability is used (2.5%), while a less important structure such as sheds, uses a higher probability (20%).

To answer the question above, if a building was designed to the minimum requirement will it collapse after its design life? Most likely no, unless a very rare and severe event occurs i.e. strong earthquake or wind.

Remember that the forces calculated by the engineer for these types of scenarios are taken probabilistically and has a likelihood of being exceeded even during the design life of the building. Yes, there is an element of chance in the engineer’s design (like the majority of things in life), but the odds are tremendously in favour of safety.

The only thing that is certain is that once the building reaches its design life, the likelihood of it to encounter forces beyond the minimum required by the standard is higher compared to the years during its design life. Personally, my preference is to always provide a reasonable buffer between my design and what is required, just to give that extra capacity to the building during an extreme event, as they say ‘It's better to have it and not need it than to need it and not have it’.

Let me know your thoughts (or questions) and leave a message on the comments section below.