Reduced reinforcement through reduced material partial factors

Appendix A of Eurocode 2 permits a reduction in material partial factors for reinforcing steel and concrete. Designing elements with the reduced material partial factors reduces the amount of reinforcement steel required, hence a reduction in embodied carbon and energy, without any loss of performance.

Material Partial Factors in Eurocode 2
To calculate the design strength of a material, the characteristic strength (above which 95% of samples are expected) is divided by a material partial factor. The ‘standard’ material partial factors are: γs = 1.15 for reinforcement and γc = 1.5 for concrete (EC2 § 2.4.2.4); these correspond to a “normal level of workmanship and inspection”.

Appendix A of Eurocode 2 specifies scenarios of enhanced quality control which enable the material partial factors to be reduced:

  • If the quality control system can reduce deviations of the concrete cross-section and the reinforcement location to those shown in Table A.1 then γs can be reduced to 1.10
or (mm) Reduced deviations (mm)
Cross-section dimension
h, Δb (mm)
Position of reinforcement
c (mm)
≤ 150 5 5
400 10 10
≥ 2500 30 20
Note 1: Linear interpolation may be used for intermediate values
Note 2: +Δrefers to mean value of reinforcing bars or prestressing
tendons in the cross-section or over a width of one metre (eg slabs and walls)

Table A.1 Reduced deviations

Note: The value of γs, red1 for use in a country may be found in its National Annex. The recommended value is 1.1.

  • If γs is reduced to 1.10 and the ‘coefficient of variation’ of the concrete strength can be reduced to 10%, then γc can be reduced to 1.4
  • Further reductions to γs and γc are possible if deviations can be further reduced – these require maximum permitted deviations to be explicitly included in design calculations
 

Quality control
As stated above, reducing material partial factors is premised on geometric and concrete strength control in excess of that deemed “normal”. This requires a quality control system capable of delivering and recording these outcomes and therefore the agreement and support of the concrete contractor. Precast production environments and processes may lend themselves more readily to such controls.

The contractor must understand why the quality control is required and be confident it can be delivered. This can be achieved by engagement during the design process or by explicit inclusion in relevant Employer’s Requirements and tender documents.

Material, carbon and cost savings potential
Columns with reduced material partial factors have been deployed on projects in the UK. These have achieved significant reductions in steel area, leading to a reduction in column embodied carbon and column cost. Designs not limited by strength, but instead minimum detailing / robustness / deflection / etc. requirements will result in lesser, or even no, savings.

Note that – instead of reinforcement reduction – material partial factor reduction could be used to slightly reduce overall concrete dimensions; this results in much less cost and carbon saving and more congested (hence less buildable) elements.

Deployment checklist

  • Assess opportunity for significant material savings – which elements are predominantly strength-governed / what proportion of structure are these
  • Engage with contractor to explain aspiration, work out level of quality control regime achievable, how to deploy and assess wider impacts
  • Agree on maximum deviations and include in design to reduce material partial factors
  • Monitor quality controls and measure savings
 

References
[1] British Standards Institution (2008) BS EN 1992-1-1:2004 Eurocode 2: Design of concrete structures – general rules and rules for buildings, London: BSI
 

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