BIMETAL HI-PERFORMANCE FACADE Michael Helmer
Samuel Xia
William Searcy
Façade Design We laid out our façade area with a simple grid to detail a smaller area and allow that single sector to repeat across the entire façade bay. The vertical grid was laid out in five foot increments because it ended squarely on the 20 foot bay and provided an adequate sized space for occupants to sit within the façade’s spatial definition. The horizontal grid was defined by what we wanted that area to do in terms of performance. We came to three sectors; the spandrel portal, the viewport and the clerestory. The spandrel portal is the bottom portion and rises to 30 inches above the floor slab. The height was determined from our design goal to make the façade occupiable and 30 inches is a standard desk height. With this height, occupants are able to move desks next to the facade when lighting is good or views outward are present. The central zone is the viewport and rises eight feet above the spandrel zone. This zone’s function is to allow for people standing next to the façade to have views outward and have the best solar heat gain while not allowing glare to happen inside the space. This area is covered with a bimetal screen that acts as a moveable mesh that reacts to solar radiation to increase and decrease opacity. The third zone is the
clerestory that takes up the remaining three and a half feet of the facade. This area is completely transparent and, with the help of the light shelf, directs light towards the interior ceiling to create diffused lighting. Two Part System Our façade design is a two part system that mitigates glare while maximizing solar heat gain. To mitigate glare, there is a system of horizontal and vertical shading devices. The horizontal shelf runs through the façade and shades the viewport while also directing light to the ceiling for diffused lighting. The vertical shade provides support for the horizontal shelf and protects from direct western light. The vertical shade tapers toward the façade as it gets lower to allow some late day light in when the light shelf is no longer efficient. In the viewport sector, we used bimetals to increase and decrease opacity with sunlight to allow light in during the times of day that solar heat can be gained.
Midterm Progress At the midterm, there were some big issues we needed to overcome to make this design plausible. Our solar shades were successful for what we wanted to achieve and those remained the same. The issues mostly came in the viewport zone and the bimetal shading device. The first issue was the bimetal screen was too dense. It didn’t allow for adequate views out and didn’t meet the daylighting minimum we set for ourselves. The second major issue was we didn’t accurately predict the screen movement. To fix that, we designed the metal fins for the movement we wanted and tested different configurations of fins to achieve the opaque versus transparent relationship we desired.
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BiMetal Screen We used a bimetal assembly for our viewport shading device to allow for views and solar heat gain but deflect direct sunlight to reduce glare. A bimetal product, in its most simplistic form, is two different metals attached to each other such as aluminum and copper. The different metals have different expansion and contraction rates, making them curl or flatten when exposed to different temperatures. For ours, we designed the strips to curl when cold and flatten when heated to block the direct sun light. The spacing of the strips was determined from wanting to achieve a 50 percent opaque surface when flat and a 10-20 percent opaque surface when cooled.
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Materials For materials, we already determined a large portion of our façade with the bimetal screen. As for the rest of the materials we wanted to use the most sustainable products to add to the performance goals our façade achieved. With this in mind, we chose wood as our primary framing material. Wood works well because it is readily available, renewable and recyclable. The issue with wood, especially in a climate like Oregon’s, is that it will rot when continuously exposed to water. That is why we chose a finish called alkaline copper quaternary (ACQ) to be applied to our wood framing. We chose ACQ because it is a low VOC treatment that is water based unlike treatments that use harmful chemicals as a base. It is an organic compound created with carbon and hydrogen instead of chromeated copper arsenate which is more toxic than ACQ. We also looked at borate which is another treatment that would work but current development of the product leaches from wet wood overtime. The fixed shades are designed to be folded aluminum. We chose metal here to contrast the wood aesthetically and to increase reflectivity for the light shelf.
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Simulation Analysis - Daylight Factor The facade’s fixed horizontal shade and bimetal screen successfully reduce the amount of direct sunlight entering the space. The open bimetal condition reduces the amount of direct sunlight by 37% compared to an all glazed facade with no shading. The closed bimetal condition reduces direct sunlight by 51% compared to the same un-shaded facade. Even after reducing the overall bimetal screen density to 50% opaque when closed, the facade system still does not allow for the appropriate daylighting factor of 2.00% for 50% of the room area. Both open and closed bimetal conditions fall short of the required 50% area threshold. This may be a combined result of the bimetal screen and the large fixed horizontal shade above. Therefore a control simulation was run with only the horizontal shade and no bimetal screen which narrowly achieved the required 2.00% daylight factor over 50% of the space. Further investigation into the bimetal pattern and opaqueness will be required to solve this issue. - Spatial Daylight Autonomy Although the Daylight Factor metric was not satisfied by the facade, the Spatial Daylight Autonomy metric of 300 lux for 50% of the area is met by both open and closed bimetal conditions.
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- Shadow Studies Annual solar analysis reveals that the facade’s fixed horizontal and vertical shades provide substantial shadows from summer and equinox sun keeping the interior space cool and protected from solar heat gain. The bimetal screen creates variegated shadow patterns along the interior space during winter months reducing glare and providing shaded pockets for occupants. - Solar Heat Gain Coefficient Annual solar exposure analysis shows that the open bimetal condition and horizontal shade provide adequate protection from direct sunlight which results in an estimated SHGC of 77% or a 23% reduction in solar heat gain compared to an unshaded facade. Additionally, the closed bimetal condition and horizontal shade provide substantial shading from direct sunlight which results in an estimated SHGC of 64% or a 36% reduction in solar heat gain compared to an un-shaded facade. - Solar Exposure Annual solar exposure analysis shows that amount of direct sunlight hitting the southern exterior is drastically reduced with the horizontal shade and bimetal screen. An un-shaded facade is shown to receive direct sunlight along 95% of its southern exterior during midday summer hours while both the open and closed bimetal conditions reduce this 20% (a 78% reduction in area). The reason for both conditions performing identically may be that the horizontal shade is the driving factor in direct sunlight reduction while the bimetal has little effect during summer months. The amount of direct sunlight allowed to enter the space during winter months however is affected by the bimetal screen. An un-shaded control facade allows for 75% of the interior floor to be struck by direct sunlight while the open bimetal condition reduces this area to 45% (a 40% reduction in area) and the closed bimetal condition reduces the area to 25% (a 66% reduction in area).
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- Radiance The main purpose of the bimetal screen is to control glare for interior occupants. Radiance analysis of winter solstice, equinox and summer solstice conditions shows that both the open and closedbimetal conditions are effective in reducing overall glare while providing multiple shaded areas for occupants within the space. The bimetal screen provides protection from low angle winter glare while the horizontal shade provides substantial protection from high angle summer glare. However, neither the bimetal screen or horizontal shade significantly reduce mid angle equinox glare resulting in several spots of high glare along the interior floor and glazing. - Passive Air Flow Solar heat gain creates a low pressure warm zone along the interior glazing while cooler air from and adjacent interior space forms a high pressure zone to the north. Providing horizontal louvers at floor level along the north wall and operable clerestory glazing at the south facade allows cooler air to enter the space low and rise as it heats up. The heated air can then escape to the exterior to clerestory vents. This process resembles a passive displacement ventilation system and keeps warm air along the glazing from traveling deeper into the space. Thermal simulations support the predicted displacement ventilation strategy during winter and summer months but shows a reversal of the airflow direction during equinox conditions. In the reversed scenario cooler air enters through the operable clerestory vents while warm air escapes via louvers along the north. It is unclear why this reversal occurs and may be investigated further with additional simulation.
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Fit Facility Results We built a prototype of our façade and used the FIT facility to see if our design performs as well as the digital results showed. We looked at two different sensor readings over a two week span to see if our façade performed to our standards. The first readings were taken from four photo-chromatic sensors that measured the amount of light hitting and passing through our façade. The second readings came from thermal ribbons placed on the exterior and interior face of the glazing. This gave us readings on how much direct light is passing through the glass and passively heating the interior space. We compared our findings with the readings from sensors placed on a blank portal to see how well our prototype performed. For the photo-chromatic readings, our façade performed quite well but there were some odd findings. The interior corridor findings averaged out at 2461 lux which is great daylighting for our space. The blank averaged out to be 3352 lux at 12pm which means that our prototype was successful in reducing the amount of direct light to a level that is suitable for a working environment. However, the interior sill readings showed a different result. The prototype portal averaged out to be 5512 lux at 12pm which is still a decrease from the 10074 lux the exterior façade sensor read at that same time. The different
result came from the blank portal which averaged out to be 3763 lux which is almost 2000 lux less than the prototype portal. After revisiting the FIT facility, it began to make sense why we got these readings. In front of the of the blank portal there is a cluster of trees that would block the high noon direct light which would lower these readings. Looking at the graph of our findings for this parametric, the trees shading is clearly seen with the prototype portal increasing exponentially compared to the blank portal. The thermal readings also boded well for our prototype. Across the board the prototype portal read two degrees cooler than the blank portal. The interior sensor also read slightly warmer than the exterior meaning the amount of direct sunlight our façade prototype was letting in was warming the interior space. In a climate like Oregon’s, this is desired because it is primarily a warming climate where buildings need to be heated more than they need to be cooled. The odd findings came with the blank portal that the interior read colder than the exterior at 12pm. The 9am and 3pm readings showed what we would expect where they read the same temperature but the 12pm temperature is unexplained.
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Future Development There are some issues still unresolved with our faรงade. Currently, our faรงade does not meet the two percent daylight factor for 50 percent of our space. This is due to our bimetal fin configuration not letting in enough natural light. If we were to move forward with this design we would look at different configurations and the possibility of opening up the faรงade more than our current 10-20 percent transparent design.
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