School Design in Sierra Leone
D-Lab: Building Technology
Project Description
Partnered with USACF (US-Africa Children’s Foundation), the goal was to help design a school that was going to be in Masiaka, Sierra Leone. The school needed to be designed specifically for the environment it was in, taking into account airflow, lighting, materials, cost, and available space.
Climate Analysis
To help better understand what the climate may be like in Masiaka, data was collected from the nearest weather stations, which included: Kindia, Guinea; Faranah, Guinea; and Freetown, Sierra Leone. The primary data used at all the locations was the UTCI, which is the Universal Thermal Climate Index, and it incorporates air temperature, wind speed, mean radiant temperature, and humidity ratio. All three locations showed that during the main school hours, 8:00 AM to 5:00 PM, the UTCI showed there was primarily moderate to strong heat stress. Data was also collected in these regions to show how shade and protection from the wind affected the UTCI. All the locations saw the UTCI lowered when shaded from the sun, however, Freetown saw a significantly larger drop off than the others. Conversely, all locations saw a rise in UTCI when protected from the wind but Freetown saw a more significant rise in UTCI, most likely due to it being located near the coast. . Only Freetown was proven to have any prevailing winds with winds coming mainly from the west while Kindia and Faranah saw no trends in wind direction. There are a couple things that can be taken away from this climate analysis in terms of the climate in Masiaka. First, there will likely be moderate to high heat stress when outdoors and unshaded; however, staying shaded and keeping proper airflow in the school will minimize the heat stress. Secondly, there will either be prevailing winds from the west or no prevailing wind at all so the school and its orientation should account for both.
Outdoor Thermal Comfort
In an environment where electrification is not readily available, thermal comfort is solely based upon passive features in the building. This means not relying on air conditioning or heating to create a tolerable environment that feels comfortable. “Feels like” is a subjective term that presents a range of temperatures or environmental conditions that are tolerable by the students who would be attending this school. To qualify this sensation, the team investigated the variables that had the largest impact on thermal comfort. This testing involved using a Kestrel 5400 portable heat stress tracker/weather meter. To highlight specific variables, environments were selected that would isolate them as best as possible. These variables included wind speed, relative humidity, and solar radiation (sunlight/cloud cover). The predicted main contributor to thermal comfort was solar radiance. This was based on the team's personal experience with transition to direct sunlight and the difference shade makes on hot or cold days.
Brick Construction and Testing
MIT’s D-Lab has always been mindful of sustainability and environmental impact when designing. In alignment with these values, the team performed validation testing on alternatives to traditional western concrete blocks. The experiment tested five different Compressed Earth Block (CEB) mixtures with varying cement content. Taking into consideration the locally available materials, the mixtures consisted of clay, sand, gravel, water, and cement.
Each mix was made by hand and then packed into wooden brick molds. This included crushing dried clay and hand mixing as seen in the pictures below. The mixes were then tested for compressive strength by crushing them in an INSTRON. From calculations, we knew that each brick had to be able to sustain compressive loads of 8 PSI. The bottom table on the right is the brick data from our team specifically, which had bricks made of 2 parts gravel, 1 part sand, and 1.5 parts clay (No cement).
When analyzing the performance of the different mixes, as seen in the first data table and graphic on the right, it was unexpected that the 0% cement with one part sand and 2 parts gravel, outperformed the 3% cement mix and that performed on par with the 5% cement mix. This was even surprising for the instructors overseeing the testing. This was confirmed to not be an anomaly, because each brick broke at about the same load (187.701 PSI,183.363 PSI,169.031 PSI). This could potentially be due to packing methods used by the teams. In future testing, the packing process should be standardized to provide more reliable results across the different cement content mixes. There’s a sudden jump in performance for the 7% cement mix, which is more in line with predictions that Cement Stabilized Earth Bricks (CSEB) would perform significantly higher in compression testing. but this confirms that cement stabilized bricks are not needed for wall construction. All of the mixes also performed beyond the needed 8 PSI, which also confirmed that a rammed earth wall made of CEBs could effectively accomplish the job needed to be performed by the building. Concrete, with a compressive strength of 3000-5000 PSI would be unnecessary for the construction of the walls for this project.
Comparing the results of each mix there is a strong correlation between the strength of bricks and the gravel content. Though clay is needed to hold the mix together and sand is needed to have uniform distributed loading through the bricks, gravel seems to be the determining variable for compressive strength. Considering the data and the embodied carbon of making and transporting cement compared to the other components (clay, sand, gravel), cement should not be used in the construction of these buildings. The bricks have been shown to perform well beyond standards without the significant increase (2-4 times the embodied carbon) involved in adding cement content.
Graph of the compressive strength of each brick mixture based on cement content
This graph is a collection of measurements done with the light meter.. The yellow section, which spans from 106 lux to 212 lux, is the range of light where work can be done but not preferred. Below is not workable (red), and everything above is workable (green).
Daylight Measurements
How much light is needed is a key metric to understand when designing a building, especially when it will likely only use natural light. In order to better understand how much lighting is needed, a light meter was used to measure light intensity within everyday lives. Measurements were taken solely based on sunlight as well as using overhead classroom lights so that intuition could be built on how much light the students will need in school. Each measurement taken was accompanied with a location, a relative feel about the intensity of the light, and if it was suitable to work in. The light intensity for these instances was measured in lux which is lumens per meters squared or essentially how much light is in the room. Based on 18 measurements, the conclusion was made that 106 lux was the bare minimum light needed to work and then anything above 212 lux was easily able to be worked in. Future tests and simulations will need to be done to determine if the school design will be able to get enough light but if it can at least 200 lux then it will be suitable for the students.
School Design
The next step involved creating a design for the school and using the information learned as well as certain goals that we wanted to achieve. This design needed to allow for effective learning spaces but also allow for space where the students could work and play outside of the classroom. This led to a design, as seen in picture, that consisted of two classrooms oriented in an L-shape with a curved raised roof spanning the entire two classes. This allows for a shaded outdoor space for students to learn, play, and hangout. The design keeps the square footage of the classrooms the same as previous designs but their orientation gives space for a new shaded space. There are about 12 windows in total, allowing for light to enter in the morning and the afternoon and the raised curved roof helps provide natural ventilation.
Construction and Testing of Physical Model
The team built a physical daylighting model from cardboard for use with a heliodon in order to simulate the interaction of the building with sunlight at different times of the year and different times of the day. This provided high level data on how much light would make it into the building and the courtyard throughout a given day or time of the year. This model doesn’t compensate for weather or cloud cover. It did demonstrate the aesthetic and visual aspect of lighting that could be recorded by a camera or seen with the human eye. This also allowed for quick alterations to roof height and window size/location that might change the amount of light entering the building. Cardboard was used to simulate the color and reflectivity of rammed earth/ CEB which is about 0.2. The cardboard was creased to create a sloped/rounded roof. The individual pieces were hot glued to seal out light between walls and the base. From testing it was determined that the inside corners were darker than visually preferred so windows between the buildings would be needed to transfer light from undesirable directions. Also the lack of clerestory windows reduced glare on the expected teaching surfaces/walls. This confirmed the Kéré roof design as a viable option and the L-shaped design a potential arrangement for the buildings with alterations to allow light capture from non-normal directions to the window orientations.
Daylight Simulations
Using climate studios, simulations were run to see if this design could get enough light to function as a school. This orientation, with the short side facing north, provided light levels ranging from 300-700 lux which was deemed suitable to work in.
To determine how much light our design would get, we used ClimateStudio and ran simulations on our design. This involved modeling the design and then testing to see how much light the classrooms would get during different orientations.
These simulations were very important because one of the main concerns with our design is how the expanded roof and courtyard will affect the amount of sunlight entering the buildings. Almost every orientation of the building was simulated and the data collected provided valuable insight on our design. All the orientations gave enough light for the students to do work, however, the orientation seen in the Figure, with the short side facing north, provided the most balanced daylight levels and least harsh glares.
The light levels within the building ranged from about 300-700 lux which as discussed before is more than suitable for a classroom. Shades and louvers can also be added to the windows to better direct the sunlight and to allow more light in during the early and late hours of the day. Overall, the daylight simulations demonstrated that our design will be able to get enough sunlight throughout the day to function as a school.
Natural Ventilation
Outdoor Airflow
When deciding how to place the three buildings in relation to each other and in which orientation, one key consideration is the outdoor airflow and how it is affected by the school’s orientation. In order to make these decisions, we used Eddy, which is a tool within ClimateStudio to help test how airflow will interact with certain models. The orientations tested were primarily with the short end facing north, however the spacing between the three buildings was varied. The two main considered spacings can be seen below in the figures with the first configuration, Figure A, having the buildings slightly overlap in the vertical direction while the other, Figure B, has more spacing between the buildings. Since there is a chance of prevailing winds from the west, that was the primary wind direction tested with the first configuration creating a wind tunnel between the buildings and creating more turbulent airflow. The second configuration allows for air to travel between the buildings making the airflow much calmer and not inducing a wind tunnel. When the winds were tested coming north and south, the configuration of the buildings had little effect so that was no longer regarded as a consideration. The final configuration can be seen in Figure above with the buildings placed vertically next to each other, but with space in between. Several other configurations were tested but none had better airflow than the final configuration and kept into account the other design considerations like daylights and indoor airflow.
Building/Indoor Airflow
The sloped roof design, as seen in the figure on the right, provides assisted buoyancy-driven ventilation by harnessing the building airflow dynamics. There are three different direction-lines of air that encounter the building. There is a stagnation line of air that is normal to the building and passes through from one side to the other, providing a cross-breeze through the building. There is an upwind vortex at the bottom of the building that causes eddies in the air. There is another streamline of air that, knowing the structure in front of it, travels up and over the roof. This air speeds up to meet the air that is traveling directly through the building similar to an airfoil design of a wing. This increase in velocity will, assumedly, create a negative pressure at the top of the open-concept ceiling see in the figure on the right. In response to this, the hot air will be pulled up and out of the building, assisting the natural buoyancy-driven flow already provided by the movement of hot air as it rises. This would work with strong prevailing wind as long as the building is oriented with the slope of the roof opening to the wind direction.
In order to address a wind pattern similar to Kindia, Guinea, where there is no prevailing wind, the team added windows to allow ventilation from at least two directions with the potential to add louvers for non-normal directions. As seen in figure on the right, this window pattern allows for ventilation for multiple directions of wind. This design includes french doors between the two classrooms for when wind is traveling in the direction shown as south facing in the diagram. All calculations on the airflow can be seen on the right.
Window Design
The window design opens upward allowing a controllable amount of airflow and sunlight in while also protecting against rainfall.
When designing the windows for the school and this specific design, three criteria were taken into account: the ability to let in sunlight, allowance for airflow, and protection from the rain. Using those restraints, the solution of a casement window appeared to work best. This type of window opens upwards allowing the teacher or whoever is using the window to control the exact amount of airflow coming in as well as the exact amount of daylight to come in. Since the classrooms are already sufficiently lit, these windows are perfect because they don’t allow for a large amount of direct light to come in, but do allow a little to enter in addition to light bouncing off the ground. This window design also affords that the windows can be opened even when it is raining since it will block all vertical rain and most horizontal rain from entering the school. This window design is simple, easy to use, and effective given the conditions as well as satisfying all the criteria needed.
Truss Design
One of the interesting aspects of the school design is the raised curved roof, but that leads to a more complicated and intricate truss design. To help test and design the trusses for the roof, Grasshopper and Karamba applications were used. The truss design used was a reinforced bowstring which allows the roof to be curved but also ensure that it will not collapse on itself. The final truss and roof design can be seen on the right, with the width spanning over 12 meters and the length of the roof being closer to 20 meters. The other aspect of the truss that was examined was the type of material used, wood or metal. Both are structurally sound when used as the truss material and both do not cause any high stress points within the structure either. The differences among the two materials lies in structural mass and embodied carbon. The steel trusses result in a total mass of 647 kg while the wood trusses only have a total mass of 347 kg. However, the wood trusses account for 427 kg of CO2 which is nearly 200 kg more than the steel trusses. Taking both into account, as well as pricing and the availability of the materials, the wood seems to be the best option. While the carbon released is not negligible, compared to the other considerations, wood is the best option for its weight and availability. Additionally, 200 kg of CO2 is offset by not driving about 500 miles which might be prevented by choosing wood over steel since more trips at longer distances will likely need to be made when using steel as the main material. Overall, the wood trusses will be more than sufficient and not have any stress points regardless of wind direction.
This image shows how the outdoor space would be occupied, including having picnic tables outside to promote students to eat and work together, while also giving the teacher the option to teach outdoors.
Social Aspect
One of the cornerstones of this design is the outdoor shaded space provided by having the classrooms oriented perpendicular to each other. The main reason for including this in the design is because of the importance of having a space for students to be together outside of the classroom. The outdoor shaded space is over 835 sq. ft. which is ample room for students to be able to run around and play without the sun beating down on them. The climate studies showed how important shade is to improving thermal comfort, especially in Masiaka, which was another reason this space was so important. Additionally, the plan is to have picnic tables located in this space because it will give a space for students to eat together, work together, and on especially hot days, it gives the teacher the ability to teach outside. Fostering this sense of community by having a space to hangout away from the sun is crucial in improving the students social skills but also just allowing a place for them to relax from school. This space promotes students to play outside which has proven to help improve behavioral and communication skills, inventiveness, and executive function in kids. The goal of this design was to cultivate a sense of community and allow a place to play for the students and this shaded space provided does just that.
Conclusion
Considering the simulations and data gathered from calculations and qualitative observations, the team determined that the L-Shaped building design with Francis Kéré’s raised roof design will provide a thermally comfortable environment with multiple modes of operation either outside or inside class instruction. The space will also provide a space shaded recreational space with the courtyard that we expect to foster community and sports which USACF and HOSL both want to encourage. Also, the Francis Kéré inspired roof provides increased buoyancy driven ventilation and shaded overhangs to reduce direct solar radiation while also creating a conductive barrier for heat transfer right into the building. These elements would be replicable in multiple L-shaped building designs were this approach to be expanded upon. The site placement decision to stagger the buildings is to reduce air pressure coefficient loss between buildings and better take advantage of wind patterns, similar to Kindia, Guinea, with no prevailing winds.
The team would highly encourage the use of fans to reduce heat stress caused by conductive heat transfer from the room to the shaded courtyard as well as in the classroom. Utilizing the provided solar power, this would enhance the thermal comfort of the students while also providing some sensation of cooling. They would also encourage the building project to use rammed earth walls or CEB for construction of the building due to the embodied carbon of cement and the readily available nature of clay, sand, and gravel on site. Going forward, onsite wind data would greatly increase the accuracy of airflow simulations. This could be accomplished by setting up a Kestrel weather station on the site to log data throughout the year. Despite the compromises in thermal comfort that will be experienced with passive cooling systems and natural ventilation, this project will provide local instruction and access to incredible educational resources that will change the lives of the students who have access to this site.