Environmental Impact of Earthquake-Resistant Design: A Sustainable Approach to Structural Response in High-Seismic-Risk Regions

2.1. Case Study Building

The case study building was stablished in the district with the most mid-altitude buildings in Quito (Table 1) [16]. The dimensions of the building were chosen after conducting a statistical analysis with an 80% confidence level of the types of buildings in the area (Table 1). Additionally, it is important to note that the building's foundation is on a type D soil [17,18].

2.2 Structural Design Alternatives

The case study building consisted of three structural systems, i.e. Optimised Frame System (OFS) (Figure 1a), Optimised Dual System (ODS) (Figure 1b) and Equivalent Frame System (EFS) (Figure 1c). An optimised system considers the Ecuadorean National Standards [19] to define the minimum cross-sectional area of the structural elements to provide an adequate mechanical response to specific structural demands. However, the OFS presents a drift of 1.75%, which is at the limit of what is permissible according to the Ecuadorian construction norm, set at 2%. This is important because high drifts near this limit contribute to the failures of non-structural elements such as masonry. These detrimental outcomes in a building were observed in a recent earthquake in Pedernales – Ecuador in 2016 where masonry debris involved deaths [20].

The OFS constitutes a more flexible structure; therefore, it has a greater floor drift compared to the ODS; in other words, these systems do not exhibit the same level of structural behavior against seismic actions, since the maximum floor drift of the OFS was 1.75%; while for the ODS, it was 0.83%. Given that the ODS is characterized by its structural rigidity, it is possible that it requires less material to achieve the same resistance and stability as the OFS, which relies on flexibility to absorb and dissipate seismic energy. This means that constructing the ODS could imply less extraction of natural resources and a lower generation of CO2. Additionally, the reduction in the amount of material used could also have a positive impact in terms of energy associated with the production and distribution of those materials.

Therefore, from an environmental perspective, it is argued that the ODS is more sustainable than the OFS due to its potential to minimize the use of construction materials and thus reduce its overall environmental footprint. Since these systems are different, they cannot be directly comparable to each other. For this reason, the research was extended to a third structural system known as the Equivalent Frame System (EFS). In this system, the same level of performance was achieved in terms of floor drift as the ODS (0.83%). Therefore, a structurally equivalent system to the OFS was attained to enable a comparison between them and observe the differences between CO₂ emissions and energy consumption.

The three systems considered continuum sections, symmetric columns and beams for the framed systems (OFS and EFS) and proportionality in the length of the structural walls of the dual system. The Ecuadorean National Standards [19] were considered in this study to compute the optimum dimensions of the structural components per each structural system. The dead load of the whole structural system was 0.45 tonnes/m² while the superimposed dead load of 0.22 tonnes/m², which corresponded to permanent load (e.g. mezzanines and subfloors) and 0.08 tonnes/m² for the roofs. Moreover, we considered the postulates of the NEC-15 [21] to include a live load of 0.20 tonnes/m². Finally, the Yield stress (fy) of the steel, compressive resistance (f’c) of the concrete and allowable compressive strength of soil were assumed as 240 MPa, 28 MPa, and 18.34 tonnes/m² respectively (Table 2).

Structural components such as stairs, vehicular ramp, retaining walls and roof slabs were the same in the three structural systems. The staircase consisted of roof slabs of thickness of 15 cm, tread of 25 cm and risers of 19 cm. The vehicular ramp had a slope of 12.5% constructed in a deck system with a thickness of 15 cm. The retaining walls varied in thickness across the three subsoils with 35 cm, 40 cm and 45 cm.

Roof slabs consisted of a two-way slab with a compressive layer of 5 cm, reinforced with structural joists of 10x20 cm and voided slabs made of merged concrete blocks of 40x40x20cm.

In general, a structure can be calculated using procedures for obtaining lateral forces, either static or dynamic. The dynamic response analysis, using the SRSS method for combining different dynamic responses, were based on the structural configuration, allowing the incorporation of torsional effects and vibration modes other than the fundamental. The seismic load was defined considering the seismic hazard of the structure's site based on the Ecuadorian standard [21]. The dynamic effects of seismic activity were modelled using a design response spectrum that considers a fraction of the structure's critical damping [21]. The seismic-resistant design of the three structural systems aims to prevent structural collapse during earthquakes, with the objective of safeguarding the lives of occupants. To achieve this design philosophy, all three systems were designed to have the capacity to resist seismic forces, maintain storey drifts below the allowable limit specified in Ecuadorean standards (2%) [21], and dissipate inelastic deformation energy (Table 3).

1.3. Structural design

Reinforce concrete elements sections and Steel-reinforcement were quantified using the [22] and [19] and the Load and Resistance Factor Design (LRFD) approach as it correlates the mechanical resistance of each component with the necessary resistance according to load combinations (Equation 1). The equation shows that Rn is the nominal resistance, $\varphi$ is the resistance factor, $\varphi .R_n$ is the resistance design and $\sum \left({\lambda }_i.Q_i\right)$ is the most unfavourable stress (Table 4).

$$\sum \left({\lambda }_i.Q_i\right)\le \varphi .R_n$$

(1)

It is worth clarifying that the three structural systems share a similar design in the foundation beams due to the presence of multiple basement levels, as these contribute to reducing seismic moments on the foundation due to the lateral confinement provided by the soil.

2.4 Life Cycle Assessment of the Structural Systems

The life cycle assessment entails the evaluation of all the carbon emissions during the different phases to construct a building. In this study, we conduct a life cycle assessment in the three structural systems described above, which are the OFS, ODS and EFS, to identify the system with the lowest carbon emission and energy consumption.

The most important environmental variable to evaluate the impact of human activities in nature is the emission of greenhouse gases (i,e. CO₂ emissions) into the atmosphere. CO2 emissions in the construction industry are not only limited to the construction phase of the building but also to the manufacturing, production, transportation, and distribution processes of the construction materials. These stages were followed to agree with the PAS 2050:2011 [23] standard. A detailed view of the processes behind constructing a building is described in the process workflow (Figure 2).

2.4.1. Materials Quantification

Each of the three structural systems, comprising beams, columns, stairs, and other elements, has a direct impact on the total carbon emissions during the building's design, material provision and construction. In this study, the components of each structural system, such as walls, stairs, and slabs, were fabricated with materials such as concrete, rebar steel, and wood (Table 5). In addition to the concrete elements, other sources of carbon emissions include excavation, soil disposal, and activities related to backfill material in foundations (Table 6).

2.4.2. Emission and energy consumption factors per material

The CO₂ emission factors (in tonnes of CO₂ per material unit) and energy consumption factors (in MJ per material unit) quantify the pollutants released during the fabrication of building materials, such as ready-mixed concrete or wooden formwork, and their impact on the environment. The assessment of these factors considered all stages involved in the fabrication of a building material, including the extraction of raw materials from the quarry and their transportation [24]. The EF and CF data for the calculation of gas emission and consumption factors were obtained from various sources (Table 7) [25,26,27].

2.4.3 Emission and consumption Factors in Machinery and Equipment

The construction industry relies heavily on machinery and instruments for various processes involved in building construction, such as materials transportation, soil compaction, and excavation. Most of the machinery used in construction runs on fossil fuels and electricity, which makes them significant sources of CO₂ emissions. The fuel efficiency of machinery was measured in litres of fuel per hour (Table 8) [9,28].

Sources of CO₂ emissions were classified based on their use in building construction, such as transportation of equipment (e.g., trucks), construction machinery (e.g., concrete mixer machine), and electric instruments (e.g., electric circular saw) [10,29] (Table 8 and Table 9).

It is noteworthy that the contribution of CO₂ emissions from electric instruments is much lower than those produced by fossil fuels (approximately eight times lower than fossil fuels) [10]. The gas emission factor of diesel machinery ($E{F}_{mach}$) was calculated by multiplying the fuel efficiency of machinery ($n_{mach}$) (Table 8 and Table 9) with the gas emission factor of diesel ($E{F}_{diesel}$) (Table 10) (Equation 2). The machinery consumption factor ($F{C}_{mach}$) was calculated by multiplying the fuel efficiency of machinery (Table 8 and Table 9) with the diesel consumption factor ($F{C}_{diesel}$, refer to Table 10) (Equation 3). The resulting factors are presented in Table 11.

$$E{F}_{mach}=n_{mach}\left[{\displaystyle \frac{L}{km}}\right]\ast E{F}_{diesel}\left[{\displaystyle \frac{tonnes of {C{O}_2}_{eq}}{L}}\right]$$

(2)

$$C{F}_{mach}=n_{mach}\left[{\displaystyle \frac{L}{km}}\right]\ast C{F}_{diesel} \left[{\displaystyle \frac{MJ}{L}}\right]$$

(3)

2.4.4 Materials transportation

Material transportation (Table 12) involves the movement of construction materials from the distribution depot to the building site [9]. In this study, the transportation analysis was conducted for ready-mixed concrete, steel reinforcement bars, and wooden formwork. Additionally, the transportation of waste from the building site to the construction material ºwaste disposal site was also considered. The total distance travelled by the materials was computed by multiplying the number of trips (Equation 4) by the distance travelled.

$$Numberoftravels={\displaystyle \frac{Quantityoftherequiredmaterial\left(units\right)}{Loadingcapacityofthetransportationmedium\left(units\right)}}$$

(4)

2.4.5 Assessment of CO₂ Emissions and Energy Consumption per Activity

CO2 emissions and energy consumption were evaluated from the fabrication of materials stage to the construction of the building (Figure 2). To evaluate the emission factor (EF) and consumption factor (CF) of each activity, we propose a new methodology that considers the inputs of the sources of pollution, such as machinery, materials, and transport to convert the units of the different polluting sources into a single activity work unit (Equations 5 and 6).

$$E{F}_{activity}=\left(E{F}_{machinery}+E{F}_{material}+E{F}_{transport}\right)\left[{\displaystyle \frac{tonnes of {C{O}_2}_{eq}}{u}}\right]$$

(5)

$$C{F}_{activity}=\left(C{F}_{machinery}+C{F}_{material}+C{F}_{transport}\right)\left[{\displaystyle \frac{MJ}{u}}\right]$$

(6)

The calculation of total emissions and energy consumption is based on the multiplication of the FE and CF per activity by the total number of activities performed, respectively (Equations 7 and 8, Table 13).

$$F{E}_{activity}=E{F}_{activity}\left[{\displaystyle \frac{tonnes of {C{O}_2}_{eq}}{u}}\right]{quantity}_{activity}\left[u\right]$$

(7)

$$C{F}_{activity}=C{F}_{activity}\left[{\displaystyle \frac{MJ}{u}}\right]quantit{y}_{activity}\left[u\right]$$

(8)

3. Results

3.1. Overall Results

The evaluation of CO₂ emissions showed that the EFS system produced the highest emissions compared to the OFS and ODS systems. The emissions in the EFS system were approximately 39% higher (around 480 tonnes) than the emissions in the OFS and ODS systems (Figure 3). Between the OFS and ODS systems, the emissions of CO₂ in the ODS system were approximately 5% higher (around 43 tonnes) than in the OFS system (Figure 3). Energy consumption (Figure 3) showed a similar trend in comparison with the EFS, OFS, and ODS systems, with the highest consumption in the EFS system at around 58% (around 6 million MJ) higher than in the OFS and ODS systems. Similar to CO₂ emissions, the ODS system consumed slightly more energy (around 6%) than the OFS system.

3.2. A detailed Description of Results Per Building Stage

The life cycle assessment showed that the EFS system had the highest CO₂ emissions and energy consumption compared with the OFS and ODS systems (Section 3.1). The assessment considered the stages of material fabrication, transportation, and building construction, with the fabrication stage accounting for the largest CO₂ emissions and energy consumption (Figure 4). The differences in CO₂ emissions and energy consumption between the OFS and ODS systems were insignificant across stages, except for the fabrication stage where the ODS system had 5% higher emissions and consumption than the OFS system (Figure 4). When comparing the OFS and ODS systems with the EFS system, the fabrication stage accounted for over 40% of the total CO₂ emissions and energy consumption.

3.3 Influence of the Material on the Emission of CO₂ and Energy Consumption

The influence of materials on the emission of CO₂ and energy consumption considers the impact of ready-mixed concrete, steel rebar reinforcement, wooden formwork, and soil-related activities in the three structural systems (OFS, ODS, and EFS). Results showed that ready-mixed concrete and steel rebar reinforcement are the main contributors to CO₂ emissions and energy consumption, accounting for more than 50% and 40% of overall emissions, respectively (Figure 5). Wooden formwork and soil removal activities have a relatively small impact, contributing to less than 1% and 5% of emissions, respectively. Notably, in the EFS system, steel rebar reinforcement and ready-mixed concrete have similar CO₂ emissions, with steel rebar reinforcement emissions being slightly higher (around 0.5%) than ready-mixed concrete emissions (Figure 5). In contrast, in the OFS and ODS systems, ready-mixed concrete was the main source of CO₂ emissions, accounting for more than 27% more emissions than steel rebar reinforcement. Based on the overall contribution of materials to CO₂ emissions and energy consumption, the structural systems can be ranked from highest to lowest environmental impact as EFS, ODS, and OFS.

3.4 Environmental Impact of the Structural Elements Used in Each Structural System

The structural elements that contribute the most to CO₂ emissions and energy consumption in the OFS and ODS systems are the foundation, beams and slabs (Figure 6). In the OFS system, foundation beams account for around 22% of the total CO₂ emissions and 22.7% of the total energy consumption. In the ODS system, these figures increase to approximately 26.4% and 26.3%, respectively. In the EFS system, foundation beams contribute to around 16% of the total CO₂ emissions and 17% of the total energy consumption (Figure 6 and Figure 7). Notably, in the EFS system, columns and beams, rather than foundation beams, are the main sources of CO₂ emissions and energy consumption, which are roughly two times higher than those produced by foundation beams. This difference in results is due to the modifications made to the columns and beams in the EFS system to reduce drift and achieve a similar structural response to the ODS system, allowing for a better assessment of the impact of materials and energy on the EFS system's environmental impact.

Further analysis of the components of the structural elements in terms of the materials used showed that steel reinforcement in columns (maximum reinforcement ratio elements) accounts for approximately 64% and 54% of CO₂ emissions and energy consumption, respectively, in all three structural systems. In contrast, ready-mixed concrete is the main source of emissions in minimum reinforcement ratio elements, such as slabs, contributing to around 58% and 68% of CO₂ emissions and energy consumption, respectively (Figure 6 and Figure 7). Therefore, steel is the primary source of emissions in maximum reinforcement ratio elements, while ready-mixed concrete is the main source in minimum reinforcement ratio elements.

4 Discussion

The design and construction of buildings is a complex process that requires consideration of various technical factors, including mechanical resistance, environmental impact, and sustainability [9,10,32,33]. While traditional building design in developing countries has focused primarily on structural design, it has largely neglected environmental factors such as emissions of CO₂ and energy consumption[34,35]. This led to buildings that are likely to be environmentally inefficient. The study discussed in this article is the first of its kind in Ecuador, comparing the carbon footprint of different structural systems (OFS and ODS) with an equivalent structure (EFS). The results of this study provide evidence of how structural decision made into the design of building structures, could incurred in higher or lower levels of pollution; focusing on the context of developing country with a high seismic risk.

There is a notable impact of the materials utilized on CO₂ emissions and energy consumption within the construction industry. The use of different materials such as concrete, steel, and wood can have different environmental impacts. For instance, concrete has a high carbon footprint due to the production of cement, which is one of the major contributors to CO₂ emissions. In contrast, wood has a lower carbon footprint due to its renewable nature and the ability to store carbon. In addition to the type of material, the origin of the material also plays a role in the emission of CO₂ and energy consumption. Materials that are sourced locally have a lower carbon footprint compared to materials that are transported from distant locations. This is because the transportation of materials consumes energy and emits CO₂, adding to the overall environmental impact of the building. Furthermore, the durability and maintenance requirements of the materials used can also impact the emission of CO₂ and energy consumption. Materials that require frequent maintenance and replacement can have a higher environmental impact compared to materials that are durable and require less maintenance. Research by Resch et al. (2020) [36] found that transportation of materials significantly contributes to CO₂ emissions in building construction, averaging around 6%. Overall, the choice of materials used in construction can have a significant impact on the emission of CO₂ and energy consumption. Sustainable and eco-friendly materials, sourced locally and with low maintenance requirements, can reduce the environmental impact of the building.

The life cycle assessment conducted for each structural system (Figure 3) indicated that the OFS system had the lowest CO₂ emissions and energy consumption among the three alternatives. However, the earthquake-resistant performance of the OFS system is unsatisfactory, as it shows a story drift of 1.75%, which may be deemed excessively high. This drift has recently been associated with non-structural damage, resulting in numerous deaths in recent earthquakes [20]. The ODS system demonstrated the best balance between environmental impact and structural behaviour. While CO₂ emissions and energy consumption were slightly higher than those of the OFS system by 5% and 6%, respectively, the ODS system's short drift (0.82%), which less than half the storey drift generated in the OFS, it substantially reduced the impact of earthquakes on the building's non-structural components, preventing deaths from these causes. Therefore, the ODS system is the most suitable alternative that offers both environmental efficiency and earthquake-resistant performance.

There is a direct relationship between the need to create more rigid structures to mitigate the effects of seismic forces and pollution levels. In this sense, the EFS is a clear example of an ill-advised way to generate a rigid structure, as it incurs in high levels of pollution. A similar finding was reported in a life cycle assessment of buildings in Atlanta, USA, where energy consumption and CO₂ emissions increased in more complex, heavier and rigid structures compared to lighter structures [9]. Therefore, it is necessary to seek a balanced approach to forming a rigid structure with low pollution levels, as achieved with the ODS. Regardless of the chosen structural system (EFS, OFS, and ODS), the maximum CO₂ emissions and energy consumption (about 90%) were recorded during the materials fabrication stage. This can be attributed to industrial processes involved in material fabrication, such as steel melting and heat treatment, or cement clinker production. Previous studies examining the environmental impact of steel and concrete used in residential [33] and commercial buildings in Singapore [37] reported similar findings. It is worth noting that the location of the building has a significant impact on the materials fabrication stage. For instance, countries in the Circum-Pacific belt, known for their high seismic activity, require more robust structural systems, which requires more concrete, and steel, leading to higher CO₂ emissions and energy consumption during fabrication compared to areas with lower seismic activity such as the East Coast of the United States.

The production of concrete and steel, which are the primary materials used in each structural component that provides mechanical resistance, contributes significantly to carbon emissions and energy consumption. In the OFS and ODS systems, the major contributor to CO₂ emissions and energy consumption was the ready-mixed concrete. However, in the EFS system, despite the abundance of ready-mixed concrete, most of the emissions and consumption were caused by steel. This outcome can be attributed to the fabrication process of steel, which involves several heating processes, including melting and heat treatments, resulting in significant pollutant emissions [38]. In contrast, fabrication of ready-mixed concrete requires only one heating process [39]. Moreover, the EFS system has large, robust structural elements that require substantial amounts of steel, altering the weight ratio between steel and ready-mixed concrete. The ratio is higher in the EFS than in the OFS and ODS systems, making steel reinforcement the primary contributor to CO₂ emissions.

Another notable difference between the optimised systems (OFS and ODS) and the EFS was the environmental impact of the structural elements. In the OFS and ODS systems, foundation beams and slabs accounted for the majority of emissions and energy consumption, while in the EFS, beams and columns were the main contributors of pollutants into the environment. This discrepancy could be explained by the balanced number of structural elements in the optimised systems, which resulted in no significant differences in emissions between them. However, in the EFS system, CO₂ emissions and energy consumption of beams and columns were at least ten times higher than those of other structural elements (eg. slabs). This highlights the fact that the EFS system requires more columns and beam elements (approximately 419% more columns and 190% more beams) to achieve a similar storey drift compared to the ODS system, which has a similar structural response with fewer columns and beams.

This study examined the structural behaviour of three different systems and their impact on the environment. In developing countries, there is limited information available on how the structural system of buildings affects the environment, with most studies focusing on architectural design or the type of material used [11,12,14]. By incorporating environmental considerations into the design of structures, we offer a new methodology for creating sustainable buildings that significantly reduce the carbon footprint compared to traditional design approaches. Furthermore, we also assessed the structural behaviour of these systems, with a focus on storey drift as the primary structural variable. The ODS system, which combines the benefits of lower CO₂ emissions and lower storey drift, underscores the significance of storey drift in the structural response of the system and its environmental impact. This system offers the advantage of combining the low CO₂ emissions and energy consumption of the OFS system with the mechanical strength of the EFS system.

As the field of environmental impact assessment for buildings in Ecuador is still in its infancy and our study is the first to integrate earthquake-resistant design and environmental impact analysis, there is limited information available on the factors that contribute to CO₂ emissions and energy consumption during the construction process. To overcome this limitation, we relied on published databases to quantify the environmental impact of structural elements and materials in each of the three structural systems [25,26,27]. Furthermore, a more comprehensive life cycle assessment that takes into account the role of building occupants will provide additional insights into the environmental impact of new constructions.

Table 14. compares the results obtained in the different stages of the life cycle, as can be seen in each of the different studies in scope, some cover more stages, others less. However, in most studies [8,9,40] the results (maximum and minimum) are within the same order of magnitude, except for Suzuki's study et al., 1995 [33] conducted in Japan.

Author	Location	Embodied carbon of concrete structures [tonnes of CO2 eq.]
		Manufacture phase	Transportation phase	Construction phase
Moussavi & Akbarnezhad, 2015	USA	548-847	37-58	50-66
Suzuki et al., 1995	Japan	2232-3034	-	-
Cole, 1998	Canada	-	33	29
Kua & Wong, 2012	Singapore	871	-	-
This study	Ecuador	796-1313	7-11	40-43

Table 14. compares the results obtained in the different stages of the life cycle, as can be seen in each of the different studies in scope, some cover more stages, others less. However, in most studies [8,9,40] the results (maximum and minimum) are within the same order of magnitude, except for Suzuki's study et al., 1995 [33] conducted in Japan.

Author	Location	Embodied carbon of concrete structures [tonnes of CO2 eq.]
		Manufacture phase	Transportation phase	Construction phase
Moussavi & Akbarnezhad, 2015	USA	548-847	37-58	50-66
Suzuki et al., 1995	Japan	2232-3034	-	-
Cole, 1998	Canada	-	33	29
Kua & Wong, 2012	Singapore	871	-	-
This study	Ecuador	796-1313	7-11	40-43

5 Conclusion

This study compared three structural systems—EFS, OFS, and ODS—and evaluated their environmental impact in terms of CO₂ emissions and energy consumption. Results showed that concrete and reinforcing steel fabrication contributed to a greater proportion of carbon emissions and required substantial energy. Ready-mixed concrete was the primary contributor to the OFS and ODS systems, while steel dominated in the EFS system.

In terms of emissions and energy consumption distribution within each system, foundation beams and slabs were significant in the OFS and ODS systems, while beams and columns played a major role in the EFS system. This disparity can be attributed to the balanced number of structural elements in the optimized systems, where emissions and energy consumption of beams and columns in the EFS system exceeded the others by at least ten times.

The study also found that the ODS system that CO₂ emissions were related to a lower storey drift, highlighting the importance of storey drift in the structural response and environmental impact. It emphasized the need for a new methodology to design sustainable structures that consider the carbon footprint and reduce environmental impact.

Overall, the study emphasizes the importance of considering environmental impact in the design and construction of building structures to minimize negative effects. As the first study in Ecuador to quantify and consider environmental impact when designing new buildings, it highlights that approximately 90% of overall CO₂ emissions and energy consumption occur during materials fabrication. To reduce the environmental impact of construction materials, new guidelines for more efficient production should be implemented. The study suggests the ODS system as the best option in designing new buildings in Quito, Ecuador, considering mechanical performance during earthquakes and identified CO₂ emissions and energy consumption. The ODS system highlights the potential to achieve buildings with highly efficient structural performance and reduced environmental impacts.