Link zur deutschen Version: Leistungsbasierter Vergleich von ein- und zweiachsig tragenden Geschossdecken
As floor slabs generally account for the largest proportion of a building’s mass, this is one of the greatest potentials for reducing CO₂emissions in structural design [1]. The more efficient the cross-sectional shape or the load-bearing structure is, the less material is required to meet the structural requirements. It should be noted that requirements relating to acoustic insulation, fire safety, architecture, building services and comfort can have a significant influence on the dimensioning of structural elements. Structurally inefficient floor slabs are often not due to oversizing, but result from these requirements. If these requirements are not met by a design that is material-efficient from a structural point of view, the structural heights must be increased or additional non-load-bearing layers must be incorporated into the structure. Both measures negatively impact the design’s environmental sustainability.
To investigate the impact of these additional design requirements for floor slabs on their GWP, an in-depth comparative analysis of the construction height and emission values of various reinforced concrete floor slab systems (configurations comprising a load-bearing structure and a superstructure) was conducted. This analysis took into account both the load-bearing structure (structurally effective design, planned by civil engineers) and the finish (non-load-bearing floor layers, planned by architects or building physicists).
Loads and requirements
The study is based on load assumptions typical in Switzerland, as well as the relevant requirements for structural safety and serviceability set out in the applicable standards and regulatory framework. As the construction of new residential space is currently being prioritized over office space in Switzerland, the loads specified in SIA 261:2020 for residential buildings were used.
Table 1 sets out the underlying design and optimization criteria. It illustrates the specific requirements and boundary conditions according to which the individual floor systems were designed and optimized to achieve a minimum GWP.
Fundamentals of comparisons
The floor systems investigated are summarised in Table 2. These are single- and double-axis load-bearing systems that are widely used on the market. In order to cover the entire range between a simply supported system and an idealised central span of a continuous system, simply supported systems with freely rotating and fully fixed supports, respectively, were investigated. These were compared in terms of their structural efficiency, material usage and environmental impact over their entire life cycle for spans ranging from 3 m to 12 m. Particular attention was paid to minimising the GWP, which is also used here as a key parameter for assessing environmental sustainability.
To ensure a fair comparison of the different floor systems, all functions a typical floor slab must fulfill were considered. The cross-sections, minimum dimensions and structures used as a basis for this can be found in Table 2 and Table 3. These were defined in collaboration with an experienced building physicist to ensure that all floor systems met the same sound insulation requirements.
The cross-sections have been reduced to the required minimum dimensions. Embedded pipes, cables and other installations would increase the minimum cross-sectional dimensions and thus lead to an unnecessary increase in the CO₂ emissions of the floor systems. In order to minimise the GWP, it is therefore assumed that, in variants without fill material, pipes will be routed outside the floor system.
Some floor systems already meet the sound insulation requirements simply through their own mass, possibly in combination with the screed, which is, in any case, a necessary prerequisite, whilst other floor systems require an additional fill layer on top of the load-bearing structure. The ‘standard superstructure’ requires a reinforced concrete slab with a minimum thickness of 16 cm. In the ‘standard superstructure + fill’ configuration, the minimum slab thickness is 13 cm, with 60 mm of gravel fill required in addition to the standard superstructure. These minimum requirements are derived from the sound insulation requirements for a concrete slab alone, without additional fill.
The GWP was minimized using an optimization algorithm. The minimum and maximum values of the respective building material emissions were used for this purpose. The mean value is shown in the subsequent diagrams. The variables listed in Table 2 are optimized to minimize the total emissions of the floor systems. It is interesting to note that, when optimizing all the systems investigated, the reinforcement content tended to be very low (average values for all solid slabs investigated: 78 kg/m³ for spans of 4 m–6 m, and 97 kg/m³ for spans of 8 m–12 m) compared with empirical values from practice. This suggests that the contribution of the reinforcement to the GWP of the floor system is crucial for achieving the lowest possible GWP.
Comparison of single-span floor systems
The analysis in Figure 1 shows that, for small spans, the differences between the floor systems are relatively small. For a span of 4 m, the GWP for the entire floor system ranges from around 60 to 67 kgCO₂-eq. The larger the spans, the more pronounced the influence of the load-bearing system or the cross-section on the GWP, with the system’s influence via the cross-sectional shape dominating. The continuous solid slab proves to be more efficient than a simply supported ribbed plate with an efficient cross-section. The difference in structural height must also be taken into account. The ribbed plate requires a significantly greater structural height to achieve comparable environmental efficiency, see Figure 1.
The non-load-bearing layers make a significant contribution to a functional floor system. However, as Figure 1 illustrates, they also play a significant role in CO₂ assessments. For smaller spans, they account for around half of the floor system’s total GWP, see Table 4. The choice of non-load-bearing floor layers is therefore just as important for reducing the GWP as the choice of the load-bearing structure. A comprehensive comparison of entire floor systems must therefore take into account the non-load-bearing layers.
Table 4: Structural heights and GWP of the load-bearing and slab systems for single-axis reinforced concrete slabs for selected spans (minimum values per span shown in green).
A detailed analysis of small spans in the range of 3 m to 6 m shows that, by using a superstructure with fill, the slab thicknesses can be reduced to the minimum slab thickness of 13 cm (required for sound insulation with 60 mm of gravel fill). Consequently, the systems with the standard construction featuring infill have the lowest GWP values for small spans (4.5 m for the solid slab, single-span, simply supported; or 6 m for the solid slab, single-span, continuous; and the ribbed slab, single-span, simply supported).
Detailed analysis for smaller spans
From a span of 6 m onwards, the structurally required slab thickness aligns with the minimum slab thickness of the slab system with the standard construction, making the slab strip fixed at both ends more efficient with the standard construction, see Figure 2.
Table 4 presents the relevant data from Figure 1 for spans of 4 m, 8 m and 12 m for comparison.
Comparison with two-axis load-bearing floor systems
The potential to optimize biaxial load-bearing systems, such as slabs supported at all four edges, whether simply supported or fixed, was also investigated. Biaxial load transfer can significantly reduce both deflections and design moments. For the sake of simplicity, the same span was assumed in both directions. For comparison, the following figures also show the most efficient uniaxial load-bearing system, i.e. the continuous solid slab.
As can be seen from Figure 3, for small spans of 3 m to 5 m, the unidirectionally supported continuous slab is just as efficient as the bidirectionally supported simply supported slab. As the minimum slab thickness is the determining factor, there is no reduction in slab thickness due to the support conditions. The thinnest slab thickness is also achieved with the standard construction using fill. The biaxial continuous slab can even be designed with a minimum thickness of 13 cm for spans of up to 8 m. The relevant data from Figure 3 are given in Table 5.
The potential for selecting an efficient structural model in terms of reducing the GWP is also clearly evident here. The freely rotatably supported solid slab requires a greater slab thickness even under biaxial loading and also has the highest GWP.
Table 5: Design heights and GWP of load-bearing and floor systems comprising two-axis load-bearing solid slabs, compared with the continuous, single-axis load-bearing solid slab for selected spans (minimum values per span shown in green).
The impact of requirements for impact sound insulation and acoustic insulation
The floor systems presented here meet the minimum requirements for impact sound insulation and airborne sound insulation in accordance with SIA 181:2020, Tables 2 and 4. If higher standards are required, the minimum slab thickness for the standard superstructure must be defined in consultation with a building physicist. For the selected floor structures, the required slab thickness increases from 16 cm to 24 cm.
Figure 4 shows that a floor thickness of 24 cm is structurally necessary for single-axis load-bearing slabs only from a span of around 6.5 m (single-supported) or 11 m (continuous), and for biaxial load-bearing slabs only from 9.5 m or significantly more than 12 m. The choice of sound insulation requirements therefore has a decisive influence on the structural height and thus on the GWP of the load-bearing system, and should be chosen with due care (no excessive requirements where not strictly necessary).
Variation in the CO₂ emissions of building materials
Figure 5 combines the results shown in Figure 1(c) and Figure 3(c) and supplements them with the variation in CO₂ emissions for the building materials. A detailed analysis of the variation in CO₂ emissions can be found in a previous blog post. It can be seen that taking this variation into account results in significant overlaps in the GWP values of the various structural systems. The choice of manufacturer or products within a building material group therefore has a correspondingly strong influence on the GWP of the load-bearing system. If, for a given building material group, a product with a high GWP is used in an efficient structural system, the total GWP may still be higher than in a less efficient system in which, however, a product with a low GWP was used.
This finding suggests that, wherever possible, a low-GWP product should be specified for a given group of building materials, or incentives should be created to encourage its preferential use. This can be achieved, for example, through appropriate tender criteria [2]. However, as long as the specific product is not known at the design stage, the assessment of different design variants must also take into account the existing variation within each building material. The sustainability indicators for each building material available in recognized databases (including [3]) should be supplemented to account for the details on the variation, ensuring the reliability of the sustainability comparisons based on them. This is particularly important given that many stakeholders and decision-makers in the construction industry – such as architects, civil engineers or building owners – do not have in-depth training in sustainability assessment, yet must make decisions based on this data – typically in early planning phases, when no specific products or even manufacturers can yet be identified, but only the basic choice of materials is being made. As CO₂ emissions vary considerably between different materials, life cycle assessments of structural systems that take into account only the average GWP value for the material in question do not adequately reflect reality and thus make it impossible for clients and planners to make informed decisions.
Conclusion
Public and private decision-makers rely on sound technical expertise and advice from planners in order to make informed decisions regarding low-emission and resource-efficient construction in the early project stages. It should be noted, however, that when designing a structural system, emissions from the selected building materials can only be roughly estimated. The variations within the respective material groups presented in this project show that manufacturer-specific parameters can significantly influence the sustainability of the products used. It should also be borne in mind that sustainability cannot be assessed solely at the material level, as differences in the mechanical performance of building materials significantly influences material consumption and, consequently, the GWP of the structural system.
By understanding the variation within a single material group, procurement departments can establish maximum permissible environmental impacts for a specific material, thereby actively managing the actual CO₂ emissions generated.
A comparison of structural and floor systems highlights the need to consider complete systems that meet identical requirements, as the structural requirements are often not the determining factor in the design of the overall system. These comparisons must be carried out in the early stages of a project, as this is when the GWP reduction can be most effectively influenced. To effectively reduce greenhouse gas emissions, optimized component dimensions and materials, structurally optimized load-bearing systems (within the limits permitted by the other requirements), and a holistic assessment of all components, including non-load-bearing layers, are necessary.
Funding
The project is largely financed by project partners. The financing status is currently around 80%, and we are looking for additional partners from the construction industry to cover the remaining 20%. We would be particularly pleased to welcome a partner from the timber construction industry to the project.
A big thank you goes to our existing partners, whose contributions have made this project possible:
- Anliker AG
- Debrunner Acifer AG
- Holcim (Schweiz) AG
- Implenia AG
- Marti Bauunternehmung AG
- TFB AG
- Walo Bertschinger AG
- armasuisse Immobilien
Research team: Sustainability in Structural Design
Referenzen
- Bischof, P.; Mata-Falcon, J.; Kaufmann, W. (2022) Fostering innovative and sustainable mass-market construction using digital fabrication with concrete. Cement and Concerte Reserach 161, 106948.
- Empfehlung Nachhaltige Beschaffung von THGE-relevanten Baumaterialien – Beispielhafte Anwendung am Bewehrungsstahl, KBOB, 2026.
- Ökobilanzdaten im Baubereich, Version 8.02, Bundesamt für Umwelt (BAFU), KBOB, Amt für Hochbauten Stadt Zürich und Verein ecobau, https://www.ecobau.ch/de/instrumente/oekobilanzen, 16.03.2026.