Cost and Consequence
About once a month my train to work is delayed. There are signal failures, over running works, unexpected weather; the litany of excuses is endless. Only last week I sat in Waterloo and watched hours of trains get canceled because of ‘a lot of rain and some lightning‘. Every time something like this happens my unplanned ‘time to reflect’ on a busy station platform is punctuated with a prerecorded voice that somehow tries to make the current predicament sound like an unpredictable act of God that just couldn’t have been avoided.
As an engineer, I know that’s not true.
Infrastructure is a game of cost and consequences; the built environment- a giant gamble. When my train is canceled due to flooding on the track; it is more correctly canceled because there was more rain than we felt it was worth paying to defend against. The signals fail because of conditions considered too unlikely to be worth the cost of coping. The network grinds to a halt because the rewards of running at a high capacity most of the time are thought to outweigh the penalty of being able to recover quickly when something happens.
Civil engineering is a compromise. We are the insurers against nature. At some point, someone decided that fortifying our rail network against anything above a 1:x storm would be an unjustifiable expense. Engineers took that brief and designed a drainage system that would fail after an acceptable threshold had been met. It would not have been impossible for my journey last week to have been completely unaffected by the storm the night before; just more expensive.
And it’s not just the nature of events that we have taken a punt at. Our structures our built from materials and to loads laced with uncertainty. We expect our steel to be ~5% weaker than it is; our loads to be ~50% more than we specified; the resulting product to be ~25-50mm eccentric from our drawings. Sure, some concrete will be even less than the ~75% of specified strength we’ve accounted for; but we’re satisfied that it’s a risk worth taking- the point where further mitigation is no longer worth it.
What fascinates me the most, however, is that we do very little to verify our forays with chance. Have you ever checked back to see if your 1:50 year drainage scheme survived the last storm? Can you tell me just how underutilised one of your structures was during its life? Is your road already filled with traffic despite it only being 10 years old? And if you had that knowledge; what would you do with it?
Engineers deal in cost and consequence; but in the built environment we rarely investigate either. In many cases the scope and mitigation for risk is mandated to us- through codes; through specifications. A lot of the time this is useful, it saves us a lot of justification. But it also robs us of the chance to learn and adapt. It discourages us from looking further- building risk and reward profiles for our clients: Yes, said track meets the 1:50 year storm requirement- but it wouldn’t be much harder to meet the 1:100 year storm; What’s that worth to you?
Because it might just be worth two hours of my time, every couple of months…