Redesigning for Disassembly and Carbon Footprint Reduction: Shifting from Reinforced Concrete to Hybrid Timber–Steel Multi-Story Building
2. A 9-Story RC Residential Building
3. Re-Designing for Disassemble and Carbon Footprint Reductions
3.1. Building Structure and Layout Distributions
- The foundation and basement (underground) remain in RC;
- The ground floor is considered commercial area;
- The mezzanine floor is considered office area, yet live load is assumed equal to residential floors;
- Seven floors are considered as residential areas;
- Vertical force resisting system is designed in timber;
- Lateral force resisting system is designed as steel concentrically braced frames;
- Fire design strategies are considered.
3.1.1. Vertical-Force Resistant System
3.1.2. Seismic-Force Resistant System of the Hybrid Timber–Steel Building
3.1.3. Fire Resistant Design
3.2. Envelope, Finishing, and Building Components
- Maximizing the use of standard components for increasing potential for disassembly;
- Minimizing energy consumption by reducing post-production of components (e.g., avoiding in situ modifications and/or cutting-offs);
- Minimizing waste generation by adjusting design demands to standard components.
3.2.1. CLT Envelope and Partition Walls
3.2.2. Openings and Building Components
4. Life Cycle Assessment of the Buildings
- The original RC basement is kept the same in the new design. Because of the topography, three sides of the basement floor are underground; hence, it is very likely that this underground area will be still designed in RC even when the upper structural frame is timber. Since both basements are the same, their total carbon footprint would also be the same. Therefore, their carbon emissions are not considered in calculations.
- For consistency, foundations are also kept the same in the new building. However, had a new foundation been designed, a reduction of embodied carbon can be expected . This is because the reduced weight of a hybrid timber–steel structure, compared with that of the RC building, would require smaller foundations, reducing the amount of concrete and steel bars and thus bringing in a reduction in the overall carbon footprint.
- The contribution of the embodied carbon of the insulation materials for the walls is not considered in the RC residential building. This is an effect of the old Turkish codes, which did not require buildings to have insulated facades. However, they were added in the timber building to assess their environmental impact in terms of carbon footprint and to serve as a design example of a timber building’s façade.
- The embodied carbon produced by the production and use of elevators in the buildings is not considered. The reason is that in the existing building there is only one, whereas in the alternative building there are two, resulting from the application of current Turkish norms.
5. Effects of Early Design Decisions on the Seismic and Functional Performance of the Building
5.1. Seismic Performance
- Structure was modelled with and without infills;
- Equivalent C18 and S220 materials were used for concrete and rebar, respectively;
- Plastic hinges were defined at the end of beams and columns; shear hinges were applied to shear walls.
5.2. Functional Performance
6. Conclusions and Future Research
- Underground construction is not considered in these analyses. However, it should be noted that using CLT instead of RC can potentially reduce embodied carbon due to the comparatively reduced weight of CLT structures. This can result in the use of smaller foundations and thinner shear walls, requiring less concrete and RC bars and thus contributing to an overall reduction in embodied carbon.
- Elevators were not considered in the total carbon footprint to avoid an unfair comparison between the existing building, which has one elevator, and the alternative design, which includes two.
- Since the RC building was built according to the old building codes, no insulation was applied on the facade. Consequently, insulation materials were excluded from the computation of the RC building’s carbon footprint. In contrast, the timber building design incorporated insulation materials to highlight their impact on carbon emissions. It is worth noting that the addition of insulation materials in the RC building may result in a greater difference in embodied carbon between the two designs.
- The existing building’s roof was inaccessible for a detailed evaluation. For consistency, the same timber roof structure was applied in both buildings.
- According to the results of the LCA, the RC residential building emits two times more carbon than the hybrid steel–timber residential building. When examined in terms of structure, the carbon footprint resulting from the production of concrete beams and columns is approximately six times higher than the production of timber columns and beams. This shows that if the building construction industry does not move away from reinforced concrete, there will be six times more CO2 emissions, leading to more extreme versions of global warming. As in similar studies , LCA is a reliable tool to evaluate and optimize architectural and structural engineering design choices to reduce the environmental impact of buildings.
- The fundamental role of designing for disassembly becomes clear when considering that, if timber elements are reused, the hybrid building has almost no carbon footprint. From the lessons learned from the design process, the relevance of considering the carbon footprint in combination with the design decisions seems to be the key to introducing circular projects in seismic regions such as Italy, Greece, etc. This is because not all decisions are based on achieving the lower embodied carbon factor but rather on those that increase the potential for disassembly throughout the lifespan of the building.
- Since the study focused on upgrading an existing building, a broader perspective is needed to validate the large reductions in embodied carbon when shifting from RC to timber buildings. For example, current building codes demand a larger number of elevators and skylights and the compulsory use of insulation, which cannot be simply compared to those resulting from past codes. Similarly, pre-existence entails several modifications performed by users through time, which are neglected in these studies for simplicity. However, users’ preferences and actions should be considered as part of the whole assessment for circularity potentials.
- The complexity and entanglement of several disciplines when transitioning from demolition to disassembly seem to indicate that a holistic design approach is indeed required. In this regard, key aspects need to be researched. For instance, following usual practice when designing the lateral force-resisting system, GLT frames are assumed unable to transfer moment forces as they are connected with simple shear tab connectors, available in the market. However, these connections may behave semi-rigidly, and, thus, the contribution of the GLT frames to the seismic capacity of the hybrid building system could be also included.
- Fire safety and protection is an import topic considering the relationship between fire performance levels and carbon footprint assessment. A timber design practice reported in  pointed out that increasing the sections will not largely affect the total amount of embodied carbon of a timber building.
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Conflicts of Interest
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|Story||S235 Braces||S355 Columns||S355 Girders|
|G–1||Tube 180 × 8||HE 280 M||HE 180 B brace-intercepted girders|
HE 200 B main girders
HE 200 M roof girder
|2–3||Tube 170 × 8||HE 240 M|
|4–5||Tube 170 × 6||HE 180 M|
|6–7||Tube 140 × 6||HE 120 M|
|8||Tube 115 × 4||HE 100 M|
|Staircase Type||Material Distribution||Mass (kg)||Carbon Factor (kgCO2e)||Embodied Carbon (kgCO2e)||Total Embodied Carbon (kgCO2e)|
|Hybrid Timber–Steel||Timber Tread||137||0.493||67.6||31,095|
|Steel Only||Steel Tread||314||2.46||772.44||101,579|
|Timber Only||Timber Tread||137||0.493||67.6||16,911|
|Stage||Carbon Emissions (kgCO2e)|
|Reinforced Concrete||Hybrid Timber–Steel|
|End-of-life Disposal (C1–C4)||244,004||1,033,557|
|Total Embodied Carbon||1,692,256||803,104|
|Building||Configuration||T (s)||Base Shear (kN)|
|RC||With infill walls||0.62||7357|
|Without infill walls||0.67||7282|
|Number in Plan (Figure 12)||Residential||Office|
|2||Dining room||Waiting area|
|3||Living room||Meeting room|
|5||Suite bedroom||Private office|
|6||Dining room||Working area|
|7||Studio room||Waiting area|
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Morales-Beltran, M.; Engür, P.; Şişman, Ö.A.; Aykar, G.N. Redesigning for Disassembly and Carbon Footprint Reduction: Shifting from Reinforced Concrete to Hybrid Timber–Steel Multi-Story Building. Sustainability 2023, 15, 7273. https://doi.org/10.3390/su15097273
Morales-Beltran M, Engür P, Şişman ÖA, Aykar GN. Redesigning for Disassembly and Carbon Footprint Reduction: Shifting from Reinforced Concrete to Hybrid Timber–Steel Multi-Story Building. Sustainability. 2023; 15(9):7273. https://doi.org/10.3390/su15097273Chicago/Turabian Style
Morales-Beltran, Mauricio, Pınar Engür, Ömer Asım Şişman, and Gizem Nur Aykar. 2023. "Redesigning for Disassembly and Carbon Footprint Reduction: Shifting from Reinforced Concrete to Hybrid Timber–Steel Multi-Story Building" Sustainability 15, no. 9: 7273. https://doi.org/10.3390/su15097273