Buildings as urban climate infrastructure: A framework for designing building forms and facades that mitigate urban heat A Master of Architecture thesis by Nolan Loh with Professor Christoph Klemmt and Professor Pravin Bhiwapurkar
Abstract A novel architectural design framework built around a massing tool and evolutionary search algorithm is proposed to balance indoor and outdoor environmental conditions and demonstrate the potential for buildings to act as urban climate infrastructure by mitigating heat in dense urban environments. Within these environments, urban heat islands (UHI) are created when building forms, facades and other urban surfaces collectively lower urban cooling rates by altering radiation balances, airflow and moisture levels while increasing anthropogenic heat production. UHIs are associated with human health issues and other negative effects and are exacerbated by warming climates and extreme weather events. Existing UHI research and mitigation strategies address facade design (improved thermal properties, implementation of vegetation), but formal studies are conducted on larger scales than architects can implement into individual building designs.
The proposed framework expands upon existing early-stage architectural design methods to balance considerations of indoor and outdoor environments by generating voxelized building form and facades designs, analyzing each design using environmental simulations and evaluating the population of designs using an evolutionary search algorithm. Indoor environments are evaluated based on energy use intensity (EUI) and unusable daylight levels (uD). Microclimate conditions are evaluated using the Universal Thermal Climate Index model for outdoor thermal comfort (OTC). The massing tool is composed of scripts that generate sets of control points to alter building massing and distribute façade materials. This approach is flexible enough to pursue a wide array of massing and façade configurations but sufficiently constrained to provide architecturally feasible designs within specific site conditions. Environmental
performance simulations analyze microclimate conditions outside and environmental conditions within each iteration. Performance data is passed to an evolutionary search algorithm that determines which parameters to pass to the next population of designs. This process repeats until the user-defined stopping criteria is met. Quantitative information regarding the environmental criteria and building characteristics is used along with qualitative assessments of the design iterations to select a single iteration to develop. Prior to developing the building form and façade, a conceptual building core and column placement can be established to incorporate design considerations relaxed at the start of the framework. Application of the framework was demonstrated in the development of an alternative design for a 50+ story core and shell office tower in downtown Chicago, IL. The framework successfully ran 171 times
over three populations. During the hottest week of the year, the generated designs improved outdoor thermal comfort by up to 1.26oC, energy use intensity by up to 0.79 kWh/m2 and unusable daylight by up to 50.90% compared to the existing tower design. The framework established site-specific massing strategies that can mitigate urban heat and improve indoor environment conditions. Architecture firms could use the framework as a massing tool during early stages of design or government bodies could use the framework to inform urban developments. Keywords: urban microclimate, heat mitigation, massing studies, evolutionary search algorithms, core and shell office design
Introduction Problem: urban heat islands Thesis overview: framework proposal
Urban heat islands: Problem Urban areas experience higher temperatures than surrounding rural areas
Very strong heat stress in downtown Chicago
Rural areas with no thermal stress
Thermal image of 1995 Chicago heat wave <20 22 24 26 28 30 32 34 36 38 40< C
o
Urban heat islands: Impact Increased urban temperatures result in negative impacts on urban environments and human health
Urban temperature
Energy consumption
Greenhouse gas emissions
Pollutant concentrations
Mortality rate
Urban heat islands: Cause Building forms and facades collectively reduce urban cooling rates
Blocking airflow
Overall wind speed Wind speed below stagnation point
Disrupting the USL
Overall wind speed in surface layer above urban canopy
Solar exposure
Surface area exposed to solar radiation
Building form contributions to UHIs
Trapping radiation
Low roof albedo
Amount of trapped long-wave radiation
Amount of absorbed solar radiation
High facade reflectivity
Impervious surfaces
Temperatures at ground level
Moisture levels
Pedestrian thermal comfort
Air temperature
Building facade contributions to UHIs
Urban heat islands: Mitigation Existing UHI mitigation methods focus on surface treatments
Existing UHI mitigation methods
facades: Green facades, cool materials, retro-reflective facades
Roofs: Green roofs and increased albedo
Ground surfaces: Parks and paving materials
Urban heat islands: Need Shortcomings in existing UHI mitigation methods
Shortcomings in existing mitigation methods
Inaccessibility of UHI research for architects
Shaping thermal environments
Existing mitigation methods are most effective on roofs and do not address important characteristics such as building form which can alter airflow and trap radiation.
UHI studies investigate built environments on larger scales than are accessible for architects to implement into designs of individual buildings.
There is a need for early-stage design methods that mitigate urban heat by integrating feedback and decision making tools with iterative approaches to design generation.
Thesis overview Balancing outdoor and indoor environments using novel massing tool
Thesis statement
Scope of study
A novel architectural design framework built around a massing tool and evolutionary search algorithm is proposed to balance indoor and outdoor environmental conditions and demonstrate the potential for buildings to act as urban climate infrastructure by mitigating heat in dense urban environments.
Within the framework, building designs focus on meeting floor area goals and balancing outdoor thermal comfort, energy use intensity and unusable daylight levels during the hottest week of the year.
Evaluation criteria Assessing building impact on outdoor and indoor environments
Energy use intensity
Unusable daylight
kWh/m2
%
Energy simulation run using envelope characteristics from ASHRAE 90.1-2010
Daylight simulation calculating % of area that does not receive between 250 - 1500 lux
Indoor environment
Outdoor thermal comfort
C
o
Equivalent temperature relating thermal conditions to human physiological responses
Outdoor environment
Air temperature
C
o
UHI model using building form, typology and vegetation to adjust meteorological air temp
Mean radiant temperature
Wind speed
Relative humidity
C
m/s
%
Energy simulation determines surface temperatures relative to points in space
CFD simulation run using two most prominent wind velocities 10m off the ground
Meteorological data used from closest weather station
o
Environmental factors encompassed by outdoor thermal comfort
Case study Alternative design for 50+ story core and shell office building in downtown Chicago
Selected site
Expansive views to the north
The asymmetric site bordering the Chicago River gives script unique conditions to respond to and presents dense urban environment with an intense UHI.
Evaluation area Program
Site
Office: 1,370,000 SF Mech./BOH: 110,000 SF
Mix of industrial and office to the west
Retail/F&B: 7,000 SF Amenity: 14,000 SF Lobby: 15,000 SF
Taller buildings to the southeast
Total: 1,516,000 SF
0’
125’
250’
375’
500’
Case study Existing core and shell office design by Goettsch Partners
Mechanical floors
Offices 5â&#x20AC;&#x2122; setbacks along river create abundance of corner offices and signature formal element Tenant amenities Lobby, retail and F&B Trident columns open up riverwalk, celebrating relationship with river
Design and render by Goettsch Partners
Methods Framework: Stages of the massing tool Interpreting the results: using quantitative and qualitative analysis
Framework Massing tool driven by environmental performance
Preparation
Generation
Simulation
evaluation
termination
selection
conceptualization
Define voxel characteristics which will influence generated designs.
Voxelized form and facades shaped by parameter-generated control points.
Indoor and outdoor environmental simulations analyze performance.
Genetic algorithm evaluates population to determine which traits to pass to the next gen.
Stopping criteria determines whether to move to the next population or stop.
Forms and facades characterized to develop site-specific strategies and select design.
Selected design developed emphasizing design traits specific to the methodology.
Next population
Proposed massing tool
Preparation and Generation Massing tool balances design flexibility with constraints to create voxelized forms and facades
Voxel characteristics
Search space
starting form
Control points sculpt starting form
Control points distribute facade types
Voxel characteristics influence appearance and spatial qualities of generated forms.
Region that voxels can occupy account for site conditions and building form restrictions.
Initial form could be first pass at design or be used to influence the direction of the search.
Parametric control points sculpt starting form to meet floor area target and facilitate formal search flexible enough to produce a variety of massing strategies.
Parametric control points distribute three facade types (high glazing %, low glazing %, low with vegetation) to voxels to explore facade strategies for mitigating UHIs and improving indoor environments.
User input prior to running the scripts
Iterative design generation script
Simulation Indoor and outdoor environmental simulations inform decisions
Ladybug
Butterfly / openfoam
Adjusted air temperature
load weather file
Dragonfly / urban weather generator
Grasshopper plugins analyze the generated building forms and facades within validated environmental simulation software. The performance data is connected to a genetic algorithm run within the plugin Octopus.
Adjusted air speed
Mean radiant temperature
Honeybee / energyplus & Radiance
Ladybug
Outdoor thermal comfort (UTCI)
Energy use intensity
Unusable daylight
Framework results Sorting through qualitative and quantitative information
171 iterations completed successfully. Each iteration has accompanying data describing environmental performance, data describing the form and facade characteristics and rhino models with the facade types applied.
Mapping the results Looking for trends and design inspiration OTC vs. uD
116
75
OTC vs. eui
98
44
134
168
otc
EUI
uD
76, 98, 116
75, 132, 134
30, 150, 168
28.2oC - 28.8oC Base case: 29.5oC -1.3oC
3.86 - 4.82 kWh/m2 Base case: 4.65 kWh/m2 -0.79kWh/m2
46% - 65% Base case: 97% -51%
Additions to north side above podium
Stepping above podium and additions to south
Taller podium and wider tower
Extreme cases
Positive OTC massing trends
Selection Using qualitative and quantitative assessments
Assessing OTC of all iterations
Balancing outdoor & indoor environments
Qualitative assessments through drawing
Selected design
Top 30 OTC designs
Worst balance
Iterations sorted by outdoor thermal comfort to reduce number of iterations from 171 to 30.
Ideal balance
Designs evaluated by ability to balance indoor and outdoor environments and preliminary qualitative assessments to further reduce the number of iterations under consideration.
Section, plan and perspective drawings used to assess spatial and aesthetics qualities of the final four designs.
OTC: -4.3% EUI: -15.6% uD: -41.9%
Interpreting the form Understanding voxels and what they represent
Original voxel
Triangulated
Smoothed
increased resolution
The voxels do not represent a final form or a particular style. They instead represent a distribution of mass and can be expressed in different sizes, shapes and levels of refinement.
decreased resolution
Processing the voxels Increasing voxel resolution to address issues and define design direction
Structure
Fitting the core
Stepping on the southern side of the tower reduces the cantilevers and expands the urban street canyon to improve airflow.
The top tier of the tower must be expanded to the north and south half a voxel to fit the building core.
Observation areas
Connected spaces
Refined protrusions are strategically placed to take advantage of views north and south along the Chicago River.
The wrapping green facade creates opportunities for vertically connected spaces to create unique experiences and utilize the stack effect to improve natural ventilation.
Riverwalk & Podium
Facade aesthetic
Change in facade materials at corners of podium indicate where the form can be pulled back to reduce the presence of the podium and create a connection with the riverwalk.
The refined facade material distribution wraps around the building form, suggesting visually distinct facade types that can maximize this effect.
conceptual design Facade: incorporating passive strategies to define aesthetic tower: using environmentally-driven design to enhance human experience
Form and facade strategies Summarizing and interpreting framework results
Form strategies
Shift tower to the north to block radiation from reaching area north of site, lowering mean radiant temperatures
Focus facade types on top tier to reduce UHI effect
Facade strategies
Divide building into two primary vertical tiers
Use wrapping form to create atria that use the stack effect to reduce EUI
Expand southern urban street canyon to allow western winds through
Expand lower half of tower on north side to shade self and lower EUI
Focus facade on lower tier to reduce EUI and improve daylighting
Interpreting the facade Reflection: using angled panels to direct radiation back into the atmosphere
Reflection Primarily used in zones that receive high levels of solar radiation.
west elevation
east elevation
Panel angles correspond to sun angles and proximity of neighboring buildings. Radiation is reflected back into the atmosphere and to minimize energy absorbed by surrounding buildings and urban surfaces.
Interpreting the facade Absorption: using climbing vegetation to absort radiation before it strikes the facade
Absorption Primarily used in zones that receive high levels of solar radiation and are associated with multi-level tenants.
west elevation
east elevation
Climbing vegetation absorbs radiation in the summer without completely blocking daylight from entering the building. In winter months, the vegetation dies back to allow radiation to enter. This facade type is associated with slab openings to create atria that improve natural ventilation.
Interpreting the facade Reduction: carefully controlling the daylight levels within the office spaces
Reduction Primarily used in zones that receive low levels of solar radiation.
west elevation
east elevation
Glazing rotated and sheltered to carefully control daylight levels within the office spaces behind this facade type and minimize cooling and heating energy use.
Interpreting the facade Connection: emphasizing visual connection to the outdoors in select spaces
Connection Selectively used in sheltered zones with optimal views of the surroundings.
west elevation
east elevation
Zones sheltered from high levels of direct solar radiation with advantageous views receive minimal, transparent facades to emphasize visual connections between indoor and outdoor environments. This facade type is associated with slab openings to create atria filled with vegetation.
Creating comfortable environments
Section key Size of circle = wind speed Arrow indicates angle relative to coming out of the page Low cooling EUI
High cooling EUI
Vegetation and multi-height spaces are used to relieve cooling burdens and enhance experiences within. The sheltered atria are green interjections that leverage the stack effect to improve ventilation and create unique sectional experiences in towers that generally have limited vertical experiences within.
Buildings as urban climate infrastructure:
The framework is currently developed to look at a specific area around one building at a time. However, it is conceivable to expand upon framework to learn about size of the impact in both distance and magnitude of temperature changes. Relationships between environmental feedback technology and iterative design process presents benefits for both designers and planning officials.
With flexible iterative massing tools and greater levels of information describing environmental impacts of design decisions, government officials and designers can work toward purposefully shaping the urban thermal landscapes to mitigate urban heat and establish buildings as urban climate infrastructure.