This guidance provides practical advice on reducing waste regarding:
- Revisiting default assumptions
- Targeting maximum utilisation
- Considering opportunities outside ‘standard’ sections
- Harnessing supply-chain potential
The sections below outline discrete steps an engineer can take to implement each of these concepts, with references to useful supporting material.
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.
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:
- Design each beam resistance equal (or close) to applied load (ie Rd = Ed ), using only the safety factors in the codes, ie avoiding adding extra material (using the next size up) 'just in case'
- Leave contractors/ fabricators to ‘rationalise’ the design for construction/ manufacture. These parties know their own commercials and processes intimately, and will suggest changes to optimise designs for their benefit
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:
- Optimised sections, eg dimensions chosen as the minimum required for structural resistance, are common for large fabrications but are increasingly economic with off-site and automated manufacture of sections with dimensions variable by the milimeter. In precast concrete, robotically-welded reinforcement meshes can varied to match the stress profile exactly, giving reductions of up to 30%7
- Varying the section along the element length allows the mass of a beam to be reduced by up to 40% as the material can better match the stress profile8. This can also give more architecturally ‘interesting’ sections such as those detailed in The Structural Engineer article Concrete structures using fabric formwork.
Both of these strategies require collaboration with the supply-chain to ensure the design intent can be delivered.
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:
- Many boards (eg plaster, ply) are produced in ~1.2m dimensions (or multiples), therefore grids that align with this should result in less board waste. If off-site methods/products (eg bathroom pods) are being used then there is potential to use off-cuts from one job on another
- Temporary states can govern design, eg rebar top mesh provides a working surface/ propping determines composite deck thickness. Collaboration with the site team to find alternative solutions (eg removable boards or increased propping) can reduce or remove these instances of materials
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:
- Highlight the opportunities at an early stage to ensure designs do not preclude them
- Articulate benefits and persuade other parties of them
- Keep a record of potential options through the project and reminding others of them so that they can be implemented at the appropriate stage
- Engage with suppliers and contractors in a timely manner so that they are aware of the opportunities and are primed to take advantage of them
References
[1] Minimising energy in construction: survey of structural engineering practice, University of Cambridge, 2018
[2] B1 General office areas, Table NA.2, UK NA to BS EN 1991-1-1:2002
[3] Concise Eurocode 2, Concrete Centre, 2006
[4] Common rules for precast concrete products, BS EN 13369:2013
[5] Eurocode 2: Design of Concrete structures, general rules and rules for buildings, BS EN 1992-1-1:2004. See also Sustainability Panel note Reduced Reinforcement through Reduced Material Partial Factors
[6] Moynihan, M. and Allwood, J, Utilisation of structural steel in buildings, Proceedings of the Royal Society A, 2014
[7] Going on a metal diet, WellMet2050, University of Cambridge, 2011
[8] Orr et al (2011), Concrete structures using fabric formwork, The Structural Engineer, 89 (8), pp. 20-26