Author: Muiris Moynihan
16 October 2019
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This guidance provides practical advice on reducing waste regarding:
The sections below outline discrete steps an engineer can take to implement each of these concepts, with references to useful supporting material.
Revisit default assumptions
Reducing load values, partial factors and deflection limits can deliver significant reductions in material at the design stage, but must be consistently applied and agreed/coordinated with the client and supply chain.
Load values for commercial office in the UK are significantly larger1 (often imposed load of 4kN/m2 ) than the Eurocode values2 (2.5 kN/m2). A significant amount of material can be saved if the lower value can be agreed with the client.
Similarly, using the common deflection limits3 (eg span / 500) can lead to overdesign in many circumstances - instead it should be understood what is driving the deflection criteria (eg facade tolerance requirement), where the limits can be relaxed (eg away from facade) and/ or what alternatives/trade-offs exist to change the limits (eg different facade systems).
Partial load factors for dead loads can be reduced by 5% If the dead weight of the structure is better controlled than typical4, for instance if it is manufactured off-site.
Partial material factors for concrete can be reduced to 1.4 (from 1.5) if a higher level of accuracy and control can be demonstrated in its construction5. This can be achieved through on- and off-site methods (for instance increased supervision or more rigorous quality assurance processes) but requires collaborating with the supply chain to ensure there is a clear strategy to do so.
Design for maximum utilisation
Designers should avoid the waste of overdesign by targeting 100% utilisation in every member; research indicates this could result in 30% less material in structures6. This is often entirely within the control of the structural engineer, provided they apply best practice:
Consider ‘non-standard’ sections
For many projects the standard set of sections/ sizes for steel/ concrete/ timber will be appropriate, however if the project requires (either architecturally or structurally) bespoke sections then there may be opportunities to remove material by optimising section geometry or by varying the section along the length:
Both of these strategies require collaboration with the supply-chain to ensure the design intent can be delivered.
Harness potential within the supply chain
Certain designs can be built with less waste, if they consider and align with common practice and products within the supply chain. Equally some site conditions impose requirements on the design. Better understanding up and down the supply chain will lead to more efficient use of material:
Staging and dependencies
All the above actions cannot be implemented in isolation - changing assumptions, particularly if dependant on a precast solution, has client and commercial implications, whilst contractor/supply-chain involvement is crucial for many of the others, again with potential commercial, architectural and other impacts. Whilst the engineer cannot make such decisions unilaterally, they can:
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