chris wise engineer

Fighting the flab in buildings

In the early 90’s Paul Cross, one of Arup’s clever men, built a spread-sheet tool for our Commerzbank Tower project in Frankfurt…..an “Optimiser”. He made sure that the 18,000 tonnes of steel in the tower were all working at more than 90% of capacity. In the modern idiom, he ensured there was less than 10% wastage. The financially savvy client immediately tried to buy the Optimiser for 1.5 million Deutschmarks.

Today, Clement Thirion, one of our own bright young things, is doing an Engineering Doctorate at University College London on the subject of “Putting the Material in the Right Place”. Its aim is broadly the same as Paul’s spreadsheet 20 years ago, but Clement is looking much wider. His research began with some simple case studies, looking at the material efficiency of typical buildings. The results are bewildering and mask an iceberg of wastefulness in the industry. To take one simple example: in a 4 storey concrete frame building, less than 40% of the concrete in the columns is actually working. The received wisdom behind this is that it is “cheaper to make all the columns the same size”. But the old timber shuttering system implied by this remark would be familiar to those Romans who first used concrete in buildings 2000 years ago.

In the same case-study building, Clement found that 20% of the concrete in the floor slabs could be removed and they would still comply with the code. Much more interesting, way beyond the code, only 50% of the remaining concrete is actually useful….the rest is in a tension zone where it does diddly squat except add to the weight of the building and boost the load on the columns and foundations, so they also get bigger. So, all in all, only about 40% of the concrete in the building is actually working. The rest is just ballast and that’s a polite way of putting it.

So here’s a simple suggestion, a sort of “fight-the-flab” for engineers. From now on you will only get Building Regs approval if you demonstrate that all of your structure is acting at say 90% of its capacity. Prove that your structure sits in the Goldilocks zone where it is just strong enough, but not too strong. Overdesign would become a professional offence. Overnight, this will sort it out. Welcome to a new culture which will help those apprehensive engineers who are not quite sure exactly what they are doing, so they simply stuff in more concrete, or more steel, or more safety factors, to make sure that their “thing” won’t fall down.

I’d go further. We have another Eng D running at UCL, called “What’s the point of Serviceability?” “Serviceability” is just a name for the set of rules that prescribe how buildings should behave in use. It runs through every aspect of building design. This Eng D is challenging the received wisdom on, say, how far a building may move. For example, many codes require that, under load, the middle of a floor should sag by less than say L/360….that’s less than 30mm over a 10m span. On façade lines, this requirement is often even stricter, say no more 20mm over 10m, usually driven by an aesthetic desire to keep the joints between the façade panels as thin as possible. In lift and escalator shafts, we’ve often been given movement limits of no more than 1 in 1000. To hit all of these often arbitrary limits, engineers add even more material to something that is already too strong. I for one cannot even see a floor sagging by 30mm over 10m, and anyway it will never sag that much because it will never see its over-conservative design load. And although I love beauty and refinement of detail as much as the next person, does it really matter if the joints on the skin of a skyscraper are 50mm wide when most of the time they are looked at from half a mile away? Rules like these reflect the value system of a bygone age and bygone technologies and people who persist in using them need to bring themselves up to date. They wouldn’t watch a television designed with the technology of the 1950’s, so why design a building that way when it is so wasteful?

There are many culturally accepted homilies behind these anomalies….for example, you can’t let the floors sag too much because they will “crush” the partitions; it’s “cheaper” to use standard sized components; “Labour is more expensive than materials”. None of these stand up to even a moment’s scrutiny. Isn’t it about time we stopped turning a blind eye to cultures of overdesign that, in real terms, bring us absolutely no benefit. Instead, roll on intelligent action on the way we use our limited supply of materials. More power to Paul and Clement.

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