Introducing Sustainability Concepts in the Design of Structures (An Application to Low-Cost Residential Houses in the Philippines)

Introducing Sustainability Concepts in the Design of Structures

(An Application to Low-Cost Residential Houses in the Philippines)

Andres W. C. ORETAProfessor, D. Eng.Department of Civil EnggDe La Salle University,Manila, [email protected]@dlsu.edu.ph

Janelle Kathryn P. ONGCivil EngineerBSCE, De La Salle University,Manila, [email protected]

Nicolas Ryan D. ARCILLACivil EngineerBSCE, De La Salle University,Manila, [email protected]

SummaryThis paper assessed the environmental impact for manufacturing and disposal of the structural systems of housing units in the Philippines using a Life Cycle Inventory. Based on the assessment, a “Structural Sustainability Index (SSI)” was derived considering the various environmental impact indicators. The SSI is a score derived from the weighted average of five environmental impact categories – Global warming potential, Ocean acidification, Human toxicity, Abiotic material depletion and Energy use. A survey of experts on the environment was conducted to derive the weights used for the SSI computation. The concept was applied to low-cost housing units in the Philippines, specifically on four models with 60 sq.m. floor area. The SSI for the various housing units was compared and an analysis of the critical indicators that affect the SSI were determined. The critical environmental impact indicators may be used by structural designers to improve their designs and to make their structures more “environmental friendly.”

Keywords: Life cycle assessment, green building, sustainability, environmental impact, structural sustainability index, Low-cost house, Philippines

1. IntroductionIn designing a house, or any structure, there are three things commonly considered by the structural engineer. Namely: safety, serviceability and cost. Safety and serviceability ensure that the structure can fulfil its intended purpose by satisfying code requirements on strength, ductility and deflections. Addressing economy, on the other hand, requires value engineering to produce an optimum design with reasonable cost. There is now an increasing concern about the environmental impact of structures (Kang et al 2007). Sustainability is a concern that must also be addressed by structural engineers. Structural engineers must be able to discriminate as to which materials and processes would have a lesser impact to the environment, and to coordinate with the other stakeholders of the structure. The concept of the study is to enable the structural engineer to analyze the sustainability of structural systems in a quantifiable manner. But what parameter may be used to guide structural designers to make their structures “greener”?

This paper proposes the use of a “Structural Sustainability Index” or SSI, a single-score based on the Life Cycle Assessment (LCA) framework. The SSI was derived from five environmental impacts based on the manufacturing only of the structural materials, whose respective weights were determined from a survey of professionals. The SSI can be used for ranking houses based on environmental impact and can be used as a parameter to guide structural engineers in comparing various design alternatives and selecting “greener” designs.

2. Environmental Impact Assessment

Life Cycle Assessment (LCA) is a method of evaluating a product, in this case a house, through its life cycle from cradle to grave. The life cycle of a building as specified in ISO 14040 throughout the implementation process follows five stages: (a) product stage - raw material extraction and manufacturing of all its materials, (b) transport - refers to the raw material extraction and manufacturing of all its materials, (c) construction process, (d) use stage, (e) disposal - depending on circumstances and condition of the structure, it could either be reused or demolished and recycled. Each stage has an impact on the environment. For this paper, only the product stage or manufacturing and disposal of the materials is considered since the choice of materials is one of the most important aspects for structural designer. Hence, only the environmental impact of the materials in the production stage was assessed using a life cycle inventory provided by Ecoinvent. Five environmental impacts and their equivalent units are considered: (1) Human Toxicity (HT) in kg 1,4-DCB-Eq, (2) Ocean Acidification (OA) in SO2-eq, (3) Global Warming Potential (GWP) in CO2-equivalent, (4)Abiotic Material Depletion (AMD) in ELU units, and (5)Energy Use (EU) in points.

3. Structural Sustainability Index

An application of the SSI was done to four types of low-cost houses shown in Figure 1. The houses denoted as (a) Full Modular, (b) I Beam, (c) Conventional and (d) Modified based on their structural system. The floor areas of the houses are 68, 55, 60, 60 m2. respectively. For

comparison purposes, the masses were converted for a floor area of 60 m2 . The brief descriptions of the houses are:

(a) Full Modular: No columns were used. Load bearing concrete hollow blocks were used for exterior walls. Conventional pouring was used for loft beam while beam blocks were used for roof beams. T-Joists were used for slabs. 4 bedrooms, 2 Toilet and bath

(a) Full Modular (b) I Beam

(c (c) Convnetional (d) Modified (Fig

. Figure 1.Low-Cost Houses in the Study

(b) I Beam: Conventional concrete pouring and molding method for slabs. Hand packed concrete was used for exterior walls. I-beams were used for beams and columns until column footing. 3 bedroom, 1 Toilet and bath, Balcony, Carport

(c) Conventional: Conventional concrete pouring and moulding method for beams, columns, slabs, and footings. Load bearing Concrete hollow blocks were used for exterior walls. Beam blocks were used for the roof girder. 3 bedrooms, 1 Toilet and bath

(d) Modified: Vertical wall stiffener and Horizontal wall stiffener were used. One column + footing was used. Non-load bearing concrete hollow blocks was used for exterior walls. Prestressed T-joists were used for slabs. Beam blocks were used for the roof girder. 3 bedrooms, 1 Toilet and bath

For each housing unit, the structural system was broken down into seven major parts, namely the wall footing, wall, beam, slab, column footing, column, and roof. For each component, the volumes of concrete and steel were estimated. The estimated component volumes (cubic meters) were converted into mass (kg) of cement, sand, gravel, and steel. They were computed based on the obtained or assumed material densities, mix percentages for concrete, and details for precast and prestressed members. From the computed mass of the materials, the impacts were assessed using the Ecoinvent inventory database.

Aggregating the five environmental impact values into one score is then derived as a “Structural Sustainability Index” or SSI. The five impact values have different units and to be able to combine them a normalisation method was done to transform them to a single unit to enable comparison. The normalisation used relative comparison, assigning the greatest impact value the highest reference value of 1. The impact values of each house for the five parameters were then divided by the greatest impact value among the houses for each parameter. The normalised values will be between 0 and 1.

A survey of experts from the academe, industry and, suppliers was conducted to determine the relative importance of the environmental impacts using Analytic Hierarchy Process (AHP). In the survey, each of the five environmental impacts was pitted against the one other, resulting in a 10-questionnaire survey. The survey resulted to the following weight factors (GWP = 0.36, EU=0.26, AMD = 0.155, HT = 0.122, OA = 0.103).

The normalized values were used in the SSI computation, in conjunction with the weights from obtained from a survey of experts. To compute the SSI, we take the sum of the product of the weight factors and the corresponding normalized impact values as shown in the formula:

Table 1 shows the summary of the structural sustainability indices of the four houses. The SSI is an index between 0-1, with 1 being the “most harmful” to the environment. The index is a relative value that compares houses against each other for ranking purposes. Table 1 shows the comparison of the SSI for the four houses. Applying the SSI to a study of four houses, the I Beam structure was found to be have the smallest SSI (“most environment-friendly”) while the Conventional structure has the largest SSI. The main difference is in the use of material as shown in Figure 2. The I Beam model used the largest amount of steel, but used the least in all other categories. On the other hand, the conventional house had the highest use of cement, and second-highest of sand and gravel. The SSI scores of the houses show that steel is preferable to concrete.

Figure 3 shows the distribution of the normalized environmental impacts of the four models in a radar chart. The closer a value to the center, the “less harmful” it is to the environment. The greatest disparities can be observed in the Human Toxicity and Global Warming categories, with the I Beam scoring significantly lower. These categories are related as CO2 emissions, which are the measure of GWP, which is toxic to humans. The I Beam, while having the lowest SSI at 0.679, had the comparatively highest AMD value of 1 in a 0-1 scale. This indicated that usage of steel as the primary structural reinforcement increased the Abiotic Material Depletion greatly while lessening all other impacts. Processing steel required extracting more non-renewable resources, specifically iron ore, than the limestone and other minerals for cement. Based on the weighting of scores, steel is preferable. If AMD

Figure 2. Material Distribution (Mass converted to 60 m2 floor area)

Table 1. SSI of Four Types of House

NormalizedImpact

Full Modular

I Beam Conventional Modified

OA 0.756 0.679 1.000 0.899

GWP 0.754 0.561 1.000 0.897

EU 0.901 0.747 1.000 0.899

HT 0.705 0.496 1.000 0.893

AMD 0.875 1.000 0.908 0.838

SSI 0.805 0.682 0.986 0.888

were given greater priority however, this may not be the case.

4. Conclusion

A Structural Sustainability Index or SSI was derived through a Life Cycle Analysis using five weighted environmental parameters. These were Global Warming Potential, Abiotic Material Depletion, Energy Use, Human Toxicity, and Ocean Acidification. The SSI was able to present a single assessment of the sustainability of a house model’s structural system. The proper weighting was determined through a survey of in the Philippines, and then processed through AHP. The survey revealed that GWP was given the greatest significance, which indicates a heightened awareness of the negative effects of global warming. These include shifts in the weather patterns such as extreme heat and extreme rain. The SSI and LCA framework were applied to Low-cost housing in the Philippines. Four model units with different structural systems were considered. Their environmental impacts were analysed through the manufacturing and disposal stages. Among the life cycle stages considered in the study, the manufacturing stage contributed the most damage, especially in Global Warming Potential and Human Toxicity. On weighted percentage, about 77.4% of the impacts were incurred in this stage, so priority should be given in reducing Manufacturing impacts.

With the SSI and LCA framework, structural engineers would be able to quantify sustainability concerns and incorporate sustainability as a factor in choosing among alternative designs.

REFERENCES

ISO 14040 (Environmental management - Life cycle assessment - Principles and framework) (2006)

Kang, G. S., Ap, L., Kren, A., Co-chairs, P., Cho, C., Corotis, B., Goodson, E., et al. (2007). Structural Engineering Strategies Towards Sustainable Design.

Figure 3. Radar Chart