The Victorians built long-lasting structures with limited foundations yet our perception of acceptable risk has changed completely and we are now overdesigning. Anna Beckett wonders if there is a better balance to be struck.
As anyone who as ever worked on a Victorian terrace will know, they usually have barely any foundations. In some cases the wall extends about half a metre into the ground and then there is nothing below it. Not exactly the soundest of footings…
And yet, these buildings have lasted for well over a hundred years relatively unscathed. If there is some cracking evident, it is usually relatively minor and, so long as it is not a sign of something more sinister, we are willing to see past it.
When we design a modern building, we test the ground before we start building and then we apply a factor of safety of three to those values because of the inherently variable nature of the soil. Then we design a suitably sized lump of concrete to ensure that there will not be any movement because the small amount of cracking we accept in a Victorian building would be unacceptable in a new one.
Why could the Victorians build with limited foundations but today our foundations need to be considerably larger? We are applying similar loads to similar ground conditions and yet we are designing something much bigger despite the massive amount of evidence that something smaller would work.
Obviously, this is hugely simplified and there is a lot more involved, but maybe one of the main reasons is because our perception of an acceptable risk has changed completely. Historically you might have expected things to go wrong sometimes – cheap houses might be a bit wonky and occasionally a building might have fallen down. Today we look at things a bit differently.
In structural design we are so risk averse that we are actually hugely overdesigning many structural elements. If we consider a standard office building, BCO guidance suggests that we should design for 2.5kN/m2.
As structural engineers we treat this load as a best guess and then apply a factor of safety to that, just in case we have got it wrong.
As we look to reduce the amount of materials we are using, perhaps it is time to look more carefully at the design loads that we are applying
But if we consider realistically the way that we currently use offices, most people have a desk with a chair, a computer and if (like me) they are not particularly tidy, some paperwork they should have filed six months ago. Unless your desks are made of lead or you are all sitting extremely close together, the loading is probably less than 1kN/m2 for most of the space. So we are designing for more than double the load that the space is likely to be used for.
How do we find the right balance between an acceptable level of risk and over-designing the structure? As we look to reduce the amount of materials we are using, perhaps it is time to look more carefully at the design loads that we are applying.
In most modern buildings the “stuff” we are putting into the building weighs considerably less than it did 50 years ago. Our offices are not filled with filing cabinets of paper anymore and that Ikea wardrobe in your bedroom definitely weighs less than the hardwood one that your grandmother had.
So what is stopping us from taking a bigger risk and pushing everything a little bit farther? Well the idea that something might be wrong with a building is completely unacceptable – and in lots of ways that is fair enough. Loss of life, unsafe buildings and damage to property are all clearly unacceptable.
But indicators that something is wrong with a building are often misunderstood or are judged by someone without the correct experience. Cracking in a house could be a sign of subsidence, but it could also be caused by seasonal variations in the water table or by someone removing a brick to fit a vent 25 years ago. And your insurer might not see it that way.
Secondly, we frequently design for future scenarios “just in case”, and maybe in some cases that is necessary. But in most buildings – even when the building changes use – it still doesn’t come close to the design loading. Plus, most organisations are pretty terrible at keeping record drawings so it is often not even possible to tell what it was designed for.
Doing a more detailed loading assessment and proving it is acceptable is much more involved for your engineer, and there is still the question of “what if”. But, if we can make a reasonable assessment of the acceptable loading in Victorian houses, then I am pretty sure the engineers that follow us will be able to do it for the buildings we are designing as well.
Reducing the loads is a bigger risk, but we have managed to use and re-use buildings. So why can’t we trust the designers of the future to do the same?