Designing Leaner Temporary Works

21 Jan

Welcome back to Tony’s Technical Blog

It’s certainly been a while, a year to be exact since my last post, where has the time gone?

Anyway, to get back in the saddle so to speak, I thought I would reproduce a 5-minute “Viewpoint” article that I presented back at last October’s GE Basement Conference. As long term sponsors of this event, we are entitled to plan out one of the themed breakout sessions. The theme we settled on was “Designing Leaner Temporary Works”.  To start the session we had three 5-min viewpoint presentations; contractor, consultant and supplier i.e.me, on how this could be achieved. This was how I see it from a designer/supplier’s perspective.

Perhaps sadly, I have been involved in designing and specifying proprietary shoring solutions for over 30 years, so forgive me if I have a slightly biased approach here. As a supplier, all be it with a design facility, by default we tend to find ourselves at the end of the supply “food chain”, where we are expected to come up with a utopia type solution in very short timescale. So in order to remain in business we are constantly striving to do things better more efficiently and yes cheaper but with the same high standards of safety and design assurance that the industry expects.

Designing Leaner Temporary Works

Providing leaner and competitive T/works solutions has been one of the main drivers behind the meteoric growth of below ground proprietary propping solutions over the last 20 years. Prior to the introduction of high strength hydraulic props in the late 90’s. Proprietary equipment had been limited generally to trench boxes, smaller hydraulic braces and lightweight waler systems. The rental market for this equipment was hugely successful, but limited in scope, so an upscale of products was necessary. The first generation of high capacity props had a working load of 50 tonnes; we have recently launched one capable of resisting 750 T. Customer driven growth indeed!

Any below ground T/wks propping has to operate in a harsh environment, so the equipment itself needs to be robust, durable and capable of sustaining additional unplanned loading during the construction process. The design of the propping system itself also needs to be robust so as to resist potential additional accidental loading without failure and consequent risk of progressive collapse.  This leads to the question: “does a robust design need to be overdesigned”? Providing that risks are assessed fully and managed on sites adequately, personally I don’t think this should necessarily the case despite the traditional temporary works engineer mentality of using big structural sections and member redundancy. As designers we are all guided by codes of practise and complimentary information, ignore these at our peril. However there is very little design guidance specifically on temporary wks design, so we do need to apply engineering judgement at times and assess risks fully and realistically, bearing in mind the relatively short lifespan of temporary works designs.

As mentioned earlier, proprietary equipment is designed and constructed to last. A rental market by definition requires the products to be robust, re-usable and consequently sustainable. Rental rates are determined in part by equipment life so this must be the case if hire rates are to be kept at competitive levels. It is industry practice when designing temporary works equipment for standard material properties to be down-rated by 10% (γm = 1.1) to allow for a gradual deterioration of the product due to re-use over time. So having built additional durability into the resistance, side of the load / resistance balancing equation, there is, in our opinion, no need to be over-conservative in the determination of the loads i.e. the geotechnical analysis. We will frequently design down to the equipment limits in order to provide a competitive solution knowing that the equipment has plenty in hand before reaching risk of failure.

In the major project arena, our biggest competitor is probably the welded structural steel solution. We are often presented with a previously designed structural steel solution and asked to provide a competitive alternative. This process seems to me to be hugely wasteful in design resources. We have been talking about early involvement with designers and contractors for years yet little seems to change.

The one thing you will be guaranteed to get with a proprietary solution is a competitive one. At the last count there were 17 shoring equipment suppliers in the UK alone, perhaps though only three or four who genuinely operate in the major project or large propping arena. Assuming that all designed solution options are safe by design, the customer’s decision on their chosen supplier will inevitably come down to price. Note that the rental vs purchase (or proprietary vs welded steel) is an age old debate that is not for now.

To finish, I feel there are three things that can done to improve design efficiency, firstly, better quality design briefs will lead to better designs through greater confidence, secondly early involvement in the design process will lead to better and quicker optimisation of a design solution without wasted effort and expensive design time and lastly, perhaps the biggest gains in economy are to be had with better use of observational methodology. The availability nowadays of high quality data monitoring systems provides a much better picture of what is actually going on in terms of wall deflections and strut loading. We rarely see loads in monitored props anywhere near the calculated design loading, thermal loading for example is often overestimated by out of date empirical calculation.

Anyway, I hope that provided some food for thought.

 

Until next time….

Tony Gould
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