1. Model for the quantification of benchmarks
A graduated approach was adopted for the benchmarks, starting on a simple basis and being refined and increasing in complexity over time, as data collection on buildings and relative processes becomes more complete and precise.
The benchmarking of buildings is thus an evolving process as illustrated in Figure 1, and the main steps of approach are:
Step 1) Definition of objectives and scope
Step 2) Data collection
Step 3) Quantification of benchmarks
Step 4) Setting of benchmarks
In this project, benchmarks were developed, based on the statistical analysis of the sample of buildings collected in the project.
Moreover, ‘conventional’ practice is assumed to be given by the median value of the environmental performance of the buildings; while, ‘best practice’ is assumed to be given by the value of the environmental performance that is achieved by only 25% of the buildings, i.e. the upper limit of the first quartile, as illustrated in Figure 2.
2. Limit state of sustainability
The European standards for structural design are based on the limit state concept, which consists on the definition of structural and load models for relevant ultimate and serviceability limit states.
In this project, a performance-based approach for sustainable design was introduced, which enables to assess the efficient use of resources in buildings throughout the complete life cycle of the building, and complies with the Eurocodes.
In this case, a structure shall be designed in such a way that it will with appropriate degrees of reliability, in an economical way and with low environmental impacts, attain the required performance’. Therefore, the aim of the proposed approach is the pursuit of a building design with a lower environmental performance than a reference value, represented by the average performance of the same type of buildings, in a given area.
Hence, in this model two variables are defined: (i) the environmental performance of the building being assessed (E) and (ii) the reference value (R) of the environmental performance of a set of buildings, in a given area. In this case, taking into account the goal of the approach, the condition that should be satisfied is E ≤ R.
In this case, a limit state function may be defined by S = E – R and therefore, S = E – R ≤ 0.
However, both variables are quantified based on a life cycle approach and therefore, they are subjected to a high degree of uncertainties and variabilities not only due to the long life span of buildings but also due to the inherent uncertainties in life cycle approaches. These uncertainties are taken into account in the analysis and therefore, both variables are defined by respective probability density functions, as represented in Figure 3.
In this case, the probability of achieving a good environmental performance, i.e. the probability of achieving an environmental performance better than the reference one, is given by P{f(S)≤0}.
This new limit state is denominated sustainability limit state and it is complementary to the ultimate and serviceability limit states referred above for structural design.
3. Results of the project
The calculation of benchmarks was based on the statistical analysis of the buildings collected in the scope of the project.
It is important to highlight that the quality and robustness of benchmarks, based on a statistical analysis, is strongly dependent on the quality and representativeness of the sample in relation to the ‘basic population’. However, the number of buildings collected in this project is reduced and does not enable a proper statistical analysis. In spite of this limitation, the set of values obtained in the project was used to demonstrate the approach for sustainable design.
Focussing on the results of the initial sages (modules A1-A3) and the results of the complete life cycle (A1-D), the range of values are indicated in Figure 4 and Figure 5, for the impact categories of GWP and PE, and for residential and office buildings, respectively.
