Eco-friendly Mid-rise Apartments Using CLT Panels

Abstract

This paper represents a series of design alternatives for a mid-rise apartment complex, inspired by the configuration of Habitat 67, where the prefabricated modules seem to be stacked chaotically. Unlike Habitat 67, where prefabricated reinforced concrete panels are used for construction, the apartment complex in this project is to be built by structural Cross-Laminated Timber (CLT) panels. In this study, characteristics of Habitat 67 are considered to evaluate the performative characteristics of the generated solutions. The form exploration process in this study is based on a GA+TRIZ hybrid method [1]. First, the design objectives are set, and the Theory of Innovative Problem Solving (TRIZ) is used to define parametric modeling. Then, a genetic algorithm (GA) is employed to generate a wide variety of design solutions. Through the iterative process of form generation, the structural performance, the heating energy demand throughout November to March, the shell cost, the environmental and life cycle impact of the design alternatives are studied. The suitable solutions are explored using Pareto sets and TRIZ matrix of contradictions. Finally, performative characteristics of Habitat 67 are compared with those of the generated solutions, and it is explained how the design objectives and sustainable development of our environment are sought within this project.

Full text

Proceedings of the IASS Annual Symposium 2019 – Structural Membranes 2019
Form and Force
7 – 10 October 2019, Barcelona, Spain
C. Lázaro, K.-U. Bletzinger, E. Oñate (eds.)
Copyright © 2019 by <Anahita KHODADADI>
Published by the International Association for Shell and Spatial Structures (IASS) with permission.
Eco-friendly Mid-rise Apartments Using CLT Panels
Anahita KHODADADI
* Portland State University
Portland, OR
Anahita2@pdx.edu
Abstract
This paper represents a series of design alternatives for a mid-rise apartment complex, inspired by the
configuration of Habitat 67, where the prefabricated modules seem to be stacked chaotically. Unlike
Habitat 67, where prefabricated reinforced concrete panels are used for construction, the apartment
complex in this project is to be built by structural Cross-Laminated Timber (CLT) panels. In this study,
characteristics of Habitat 67 are considered to evaluate the performative characteristics of the generated
solutions. The form exploration process in this study is based on a GA+TRIZ hybrid method [1]. First,
the design objectives are set, and the Theory of Innovative Problem Solving (TRIZ) is used to define
parametric modeling. Then, a genetic algorithm (GA) is employed to generate a wide variety of design
solutions. Through the iterative process of form generation, the structural performance, the heating
energy demand throughout November to March, the shell cost, the environmental and life cycle impact
of the design alternatives are studied. The suitable solutions are explored using Pareto sets and TRIZ
matrix of contradictions. Finally, performative characteristics of Habitat 67 are compared with those of
the generated solutions, and it is explained how the design objectives and sustainable development of
our environment are sought within this project.
Keywords: Form exploration, mid-rise apartment complex, Cross-Laminated Timber (CLT) panels, genetic
algorithm, TRIZ, Habitat 67, life-cycle assessment
1. Introduction
In 1967, Moshe Safdie designed Habitat 67 as the Canadian Expo Pavilion. Habitat residential complex
gained worldwide admiration as it was an outstanding integration of architecture and structure and could
break the traditional form of orthogonal high rises and introduce a new housing topology. Fifty years
later, Habitat 67 still seems a spectacular residential project. However, today, the industrialization of
building construction is no longer a major concern for architects and engineers. Designers are to find
solutions for sustainable development of human-built environment and reduction of building
construction impact on global warming, CO2 emission, ozone depletion, and consumption of natural
resources. Therefore, if such an innovative project as Habitat 67 were to be built today, the design
approach would be different.
Habitat 67 is constructed from 354 identical prefabricated modules stacked in various geometrical
configurations to provide 158 apartments (Figure 1). There are 15 models of apartment units that include
one to five modules. The units look similar, but the interior space of each house differs from the others.
The apartments are soundproofed and heated by a central heating and air-conditioning system. The
geometrical and material properties of Habitat 67, along with the cost estimation, are described in Table
1 [2, 3, 4, 5]. The manufacturing site of Habitat 67 was located 300 m away from the site of
implementation. The complete modules, whose weight varies between 70 and 90 tons, were hoisted by
a giant crane, set up and secured to other units using post-tensioning cables. (Figures 2 and 3) [2].

Proceedings of the IASS Annual Symposium 2019 – Structural Membranes 2019
Form and Force
2
Figure 1: Habitat 67, Montreal, Canada [6]
Table 1: The design properties of Habitat 67
Geometrical properties
354 modules 38×17 ft2 [7] (12.5×5.7×3.2 m3 external measurement)
Total Height: 36.576 m (120 ft) | 12 stories
15 different modules with an area of 55 m2 (624 ft2)
Floor plans range from 1 to 4 bedroom apartments and 1 to 3 stories
Weight of each module = 70 to 90 tons
The area of terraces range from 20-90 m2 (225 to 1000 ft2)
Material
Exterior walls of the units: sand-blasted concrete
Window frames: brown anodized aluminum
Cost
140,000 CAD per unit Total cost = 140,000 × 158 = 22,120,000 CAD
Heating/cooling cost for a one-bedroom apartment of Habitat = 400-600 CAD per month throughout the year
[8]. (Heating/cooling cost for a one/two-bedroom apartment in Montreal = 200-300 CAD per month
throughout the year )
Figure 2: Floor plan of an apartment and constructional Figure 3: Prefabrication of the apartment units
details [6] in the manufacturer’s site [9]

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Form and Force
3
2. Configuration processing of the CLT mid-rise residential tower
Habitat 67 includes three clusters of stacked modules. With the same fashion, the residential complex
in this project is to be composed of three same clusters of apartment units (Figure 4). In order to reduce
the simulation time in this project, only one of the clusters is parametrically modeled and analyzed.
Therefore, performative characteristics such as the shell cost, thermal energy demand, weight of
structures, and life cycle impact are compared to one-third of the corresponding values in Habitat 67.
The configuration of each cluster of apartment units is defined by creating a stack of 12×16×11
(width×length×height) modules whose number of floors is reduced regarding the location of four
attractor points on the XY plane (Figure 4). Then, 250 to 500 modules from this basic model are removed
randomly to shape the roof gardens and private balconies (a seed value controls the reduction pattern).
Each module is 5.8×5.8×3.4 m3. Then, columns and required structural supports are added to the model
where units are not supported by the walls of the lower floors. The geometrical properties of the
residential complex are described in Table 2.
Figure 4: The configuration processing of mid-rise residential complex
Table 2: The geometrical properties of the mid-rise residential complex
Geometrical properties
Habitat module dimension = 12.5×5.7×3.2 = 228 m3 Habitat maximum height = 36.576 m | 12 stories
Assumption: Each residential unit includes two geometrical modules, floor to floor height = 3.4 (11 ft)
 Geometrical dimension of each module = 5.8×5.8 m2
Height of each apartment module = 3.4×11=37.4 m | 11 stories
Area of each apartment module: 5.8×5.8 m2 (19×19 ft2) Height of each apartment module: 3.4 m (11 ft)
Maximum number of floors: 11
3. Structural and thermal analyses and life cycle impact assessment
The walls, roofs, and floor slabs are all to be made of structural panels. The roofs are expected to be
used as roof gardens. Therefore, the appropriate combination of live load and snow load is considered
in the structural analysis. Total weight of the structure, its natural frequency, and maximum
displacement are obtained through the simulation carried out in Karamba a plugin for Grasshopper [10].
Thermal analysis is carried out for assessment of the Heating Ideal Zone Load for five months, from
November 1st to March 31st. In Montreal, heating cost and maintaining the comfortable temperature
within these five months are critical while cooling energy demand throughout the year is not important.
The thermal analysis is carried out using ArchSIM plugin for Grasshopper [11].
In this design case study, the Life Cycle Assessment (LCA) is carried out to evaluate the magnitude and
significance of the potential impacts of the residential building complex on the environment. In this
study, the same input data, including the type of occupancy, location, and gross floor area, are truncated

Proceedings of the IASS Annual Symposium 2019 – Structural Membranes 2019
Form and Force
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from the model for practical purposes and quick estimation. The Product Declarations of the structural
materials, provided by the suppliers, are used to compute the LCA. The LCA assessment in this design
project includes global warming potential, acidification potential, ozone depletion potential, total used
energy through the life cycle, non-hazardous waste, hazardous waste, and consumptions of freshwater,
non-renewable and renewable material.
4. Form exploration of the mid-rise residential complex using the GA+TRIZ method
In this project, the main design objectives are minimizing the environmental impact, reducing the
heating energy demand in the cold season, providing a pleasant residential space for the tenants, and
maximizing the number of units and total floor area of the apartments. This form exploration method in
this study is based on a GA+TRIZ hybrid method [1] (Figure 5).
Figure 5: The plan of work in the GA+TRIZ design method [12]
According to the GA+TRIZ method, the design objectives are synthesized into their basic factors and
checked for probable conflicts. As a result, the contradiction between the life cycle impact of the
building and the amount of structural material or the amount of floor area was revealed. Design
alternatives with a greater number of units and amount of total floor area require more consumption of
material resources and, as a result, they have greater potential to increase global warming. Referring to
the TRIZ inventive principles and the matrix of contradiction [13, 14, 15, 16, 17] made the designer opt
for an eco-friendly material such as CLT panels instead of prefabricated reinforced concrete. There are

Proceedings of the IASS Annual Symposium 2019 – Structural Membranes 2019
Form and Force
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five reasons for choosing CLT panels as a replacement of prefabricated reinforced concrete in this case
study. The first reason is the renewability and low environmental impact of the wood, in particular, its
low carbon footprint. Second, CLT panels can be employed in the prefabricated building industry where
panels are cut to the required dimensions, engineered and treated to be thermal and fire resistant, and
finally erected in-site relatively quickly. Third, the presence of wood can provide the occupants with a
pleasant environment and improve the quality of interior space. Fourth, the reason for choosing CLT
products and not, for example, light-frame wood construction is to compare two massive constructions
that have two different carbon footprints. Fifth, in Canada, in particular in Quebec, forestry is a huge
part of the economy, and several companies supply the CLT products for the building industry. Also,
the required building codes are provided for architects and engineers to employ CLT products. There
are successful examples of CLT residential buildings all over the world, as well as in Canada, that have
pushed the boundaries of the CLT industry furthest [18, 19, 20, 21]. Structural specifications of the CLT
products used in this design project are provided by Nordic Structures Company [22] in Montreal.
Furthermore, in Habitat 67 prefabricated reinforced concrete panels were used without any thermal
insulation. In this project, design analysis suggested thermal insulation of the building for better
environmental performance. Moreover, the application of GA+TRIZ form exploration method implies
consideration of two separate variables to define the thickness of CLT panels and the thickness of
thermal insulation. Taking these actions allow for the resolution of the explained contradiction among
the design objectives innovatively.
In the next step of the design procedure, the parametric model of the apartment complex is defined.
Then, the database is set, separate models for structural and thermal simulations are created, and the
GA-based breeding code is scripted [23, 24]. Details of inputs for structural and thermal analyses are
provided in Table 3. The displacement and utilization diagrams of the generated solutions and, the bar
charts that show their thermal energy demand from November to the end of March are to be stored in
the database.
Table 3: The environmental and structural properties of the mid-rise residential complex
Material and thermal properties
CLT plates density = 500 kg/m3 ,Conductivity = 0.13 W/mK ,Specific heat 1600 J/kgK
Thermal emittance = 0.9 ,Solar absorptance = 0.7 , Visible absorptance = 0.7
Embodied energy =10 MJ/kg , Embodied carbon =0.71 kgCO2/kg
Material cost estimation
CLT structural system cost: Design + material supply +installation = 435-490 CAD/m2 (40-45 CAD/ft2)
≈ 450 CAD/m2
(Exchange rate CAD to USC = 0.75) , Design + material supply +installation = 326 – 367 ≈ 350 USD/m2
Material and load properties
CLT plates: Spruce-Pine-Fir Density = 400 kg/m3 (25 lb/ft3) Thickness = 8-20 cm (3 1/8 – 7 7/8 in)
Load case 1 = self-weight + Roof load (promenade roof) of 2.8 kN/m2 (58.5 psf) [25]
Load case 2 = self-weight + residential live load of 1.92 kN/m2 (40 psf) [25]
Load: 0.01 people/m2 +11 equipment w/m2 + 7 lighting w/m2, 150 lux, continuous dimming, AllOn schedule [26].
Conditioning: heating setpoint: 21C cooling setpoint 26 C
Ventilation: Air change per hour = 0.5 ACH
Hot water: On | peak flow 0.03 m3/h/m2 | 65C supply temperature | All On
Construction
Roof = 95mm CLT + 80mm Rigid foam EPS 035 insulation + 95mm CLT + 20mm Gypsum Fiber Board
Partition = 20mm Gypsum Fiber Board + 95mm CLT + 20mm Gypsum Fiber Board
Ceiling = 95mm CLT + 30mm Impact sound insulation + 95mm CLT + 20mm Gypsum Fiber Board
External floor = 95mm CLT + 80mm Rigid foam EPS 035 insulation + 95mm CLT + 30mm Pine wood
Façade = 30mm Pine wood + 80mm rigid foam pur no coating Insulation + 150mm CLT + 20mm Gypsum Fiber Board
According to the described design objectives, a series of design alternatives is generated whose global
warming potential is less than 1,000,000 kgCO2eq and ideal total heating energy from November to the

Proceedings of the IASS Annual Symposium 2019 – Structural Membranes 2019
Form and Force
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end of March is less than 5000 MJ (Figure 6). Then, the generated solutions are explored to find a series
of suitable design alternative whose total floor area is greater than that of the Habitat 67. Figure 7
displays the distribution of solutions regarding their gross floor area and global warming potential.
Figure 8 displays the distribution of solutions based on their gross floor area and ideal total heating
energy from November to the end of March. In both graphs, solution number 228 has been chosen as a
suitable solution. The performative characteristics of solution number 228 are displayed in Table 4.
Figure 6: South-East view of the solutions whose global warming potential is less than 1,000,000 kgCO2 eq and
ideal total heating energy from November to the end of March is less than 5000 MJ. The solutions are sorted based
on their gross floor area in descending order.
Figure 7: The plot displays the distribution of solutions regarding their gross floor area and global warming
potential. Habitat 67 and some of the suitable solutions made of CLT panels are highlighted.

Proceedings of the IASS Annual Symposium 2019 – Structural Membranes 2019
Form and Force
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Figure 8: The plot displays the distribution of solutions regarding their gross floor area and ideal total heating
energy from November to the end of March. Habitat 67 and some of the suitable solutions made of CLT panels
are highlighted. Table 4: The performative characteristics of solution number 228
5. Conclusion
It is almost a decade that designers have been more concerned about the sustainable development of
human-built environment and reduction of building construction impact on our environment. This paper
represents a series of design alternatives for a mid-rise apartment complex, inspired by the configuration
of Habitat 67. In contrast to the prefabricated reinforced concrete construction of the Habitat 67, Cross-
Laminated Timber (CLT) panels are utilized as the structural components of the residential complex.
The application of the GA+TRIZ methodology helped to obtain relatively more suitable solutions by
directing the designer to choose better structural material and reduce the life cycle impact and heating
energy demand of all generated solutions.
Acknowledgments
My thanks to Professor Peter von Buelow for his support in using ParaGen interface [23] and Hydra Lab
at the University of Michigan.

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Form and Force
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