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One of the largest net-zero energy office buildings in the world has opened in Silver Spring, Maryland, as the corporate headquarters of United Therapeutics. Solar panels collect energy for the structure, and geo-exchange wells, an earth labyrinth, natural ventilation, and electrochromic glass—among other special design choices—minimize energy use. And building performance monitors ensure employees and operators know exactly how much energy is being generated and used.

The Unisphere, a 210,000 sq ft, six-story office building that is powered, heated, and cooled completely from sustainable energy technologies located on-site, is one of the largest net-zero energy office buildings in the world. Recently completed in Silver Spring, Maryland, along with its adjacent parking garage, the building is the corporate headquarters of United Therapeutics, a biotechnology company founded in 1996 to develop a treatment for pulmonary arterial hypertension (PAH). Founded by Martine Rothblatt, Ph.D., in an effort to develop a treatment for her daughter, the company is now worth $3 billion and continues its efforts to help those with PAH regardless of their income level.

This focused effort on helping people extended to the construction of the net-zero facility. From the start, Rothblatt wanted to create a welcoming space that used as little energy as possible while generating as much as possible so that the structure honored the environment and the people who live in it by eliminating the building's carbon footprint.

The company chose EwingCole, of Philadelphia, as the architect; interior designer; structural engineer; mechanical, electrical, and plumbing engineer; fire-protection consultant; and lighting designer.

A large central atrium is the center point of the office building and serves as an important element of the site's net-zero design, which utilizes solar panels, geo-exchange wells, an earth labyrinth, natural ventilation, and electrochromic glass, among other strategies, for conserving and producing energy. Solar panels attached to the building and its attached parking garage generate 1,175 MWH/year of energy to balance out the amount used by the facility.

The office building is elliptically shaped as a direct reference to the shape of the universe and is oriented east-to-west to maximize the performance of its north and south facades. This results in reduced energy use, better daylighting, and maximum solar power generation. The Unisphere also embraces the street level by including a public plaza for small gatherings or concerts, with seating, landscaping, and retail shops. A water feature that is part of a wall at the south end of the plaza runs continuously in warm weather, the water jets installed into the paving simply for the enjoyment of passersby.

While there are several ways to approach a net-zero energy design, either by generating energy on- or off-site or by purchasing renewable energy credits, the mandate in this case was to generate the energy on-site from renewable sources and to use that energy to power the building, methods which constitute a net-zero energy project in its strictest sense. So all the renewable energy sources were connected physically and electrically to the building's structure and to the local electrical distribution system, so that any excess energy could be sent to the grid. The restricted urban site of 67,583 sq ft, coupled with shading from adjacent buildings, made this the greatest challenge on the project. While multiple forms of renewable energy were investigated, Ewing-Cole ultimately decided to meet the goal through photovoltaic modules.

This was not as simple as calculating the number of modules that would be required to offset the anticipated energy use of a typical building of this size, however. To ensure that the company was a good steward of environmental resources, the design team first evaluated all design decisions and system selections to determine how to reduce energy usage as much as practicable. This was accomplished through extensive energy modeling, which began in the preschematic design phase and continued through construction so that the team could predict the energy impact of each decision.

United Therapeutics invested in physical and virtual mock-ups of the building to ensure that the energy-related design complexities could be fully evaluated before final construction. The physical mock-ups included a full section of two adjoining offices that include the central atrium's curtain wall and the offices' raised floors, partitions, furniture, lighting, and exterior curtain walls, which include operable glazing and photovoltaic modules. This allowed the building systems to be tested for functionality and user preference while highlighting potential construction issues that could then be resolved before the full building construction.

In addition to this physical mock-up, a mock-up of the integrated control networks that would be used to monitor the energy systems was constructed at the beginning of the project timeline. Although this put pressure on the construction manager and control contractor to conduct work that is typically saved for the end of the project, the importance of proving the communication capabilities of these systems early in the timeline was key to the project's overall success.

Several communication issues were identified and resolved during this virtual mock-up process, providing the foundation for a much smoother installation and integration process toward the end of the project. At the completion of this complex project, all parties involved agreed that the physical and virtual mock-ups were worth the additional cost and front-end preparation.

A county-owned parking garage at the site had to be demolished before construction could begin. The garage consisted of a precast-concrete-clad steel superstructure that supported three levels of elevated parking decks and covered the entirety of the proposed site. A deep foundation system of concrete drilled piers extended down to bedrock to support this superstructure. During the design phase, the project's columns were located to minimize interference with those piers, but in some locations, excavation depths for the new construction did require pier removal. The contract documents identified specific piers to be removed in their entirety and required that the remainder be cut off 2 ft below the new slab-on-grade elevation.

The Unisphere is built to maximize the available site within the surrounding urban context; its footprint abuts an existing structure to the west, extends to within 10 ft of the property line to the south, and in some areas extends to the property lines to the north and east. The site is bounded by county and state streets to the north and east, by existing United Therapeutics-owned buildings to the west, and by an unrelated existing 12-story hotel to the south.

The subsurface stratigraphy below the site consists of existing fill, residual soils, disintegrated rock, and gneiss bedrock. The Unisphere includes a partial basement, located 21 ft below street level, and various intermediate slab-on-grade elevations throughout the building. The residual soils were capable of supporting the slabs-on-grade but did not have sufficient capacity to support concentrated loads at the locations of the building columns.

The building columns are thus supported by reinforced-concrete spread footings bearing on disintegrated rock with an allowable bearing capacity of 20 ksf and reinforced-concrete drilled piers, varying from 3 to 5 ft in diameter, bearing on bedrock with an allowable bearing capacity of 80 ksf. The top of the disintegrated rock varies across the site from 9 to 21 ft below the existing street elevation to the north. The top of bedrock was located another 5 to 10 ft below the top of the disintegrated rock. Shallow spread footings were used where excavation depths for the slab-on-grade landed within the disintegrated rock layer, and deep drilled piers were used to reach the higher-capacity bedrock layer where excavation landed within the unsuitable residual soils.

The basement is located on the site's north side, directly adjacent to a street and to a seven-story concrete structure supported by drilled piers that is owned by United Therapeutics. Excavations for the Unisphere's basement foundations typically extended 25 to 30 ft below the street level with a local area of excavation extending 36 ft below. This excavation was supported with a system of soldier piles, rock pins, horizontal bracing, tiebacks, and lagging designed by Superior Foundation Inc., based in Vienna, Virginia.

The basement walls on the north side include a soil-side waterproofing system consisting of continuous sheets of waterproofing material and vertical, in-plane drainage board. A layer of permeable filter fabric placed between the wood lagging and the drainage board prevents infiltration of fine soil into the drainage system. The drainage board is positively connected to weep holes located at the base of the concrete basement walls, below the slab-on-grade, to tie in to the crushed stone subbase and the below-slab perforated pipe interior drainage system.

The concrete basement walls were poured directly against this waterproofing assembly, which required that cantilevered shoring towers be built on the interior side of the basement-wall formwork to resist the large lateral pressures created by the wet weight of concrete as the 21 ft high basement walls were placed.

Excavations for the basement foundations also extend approximately 10 ft below the foundation walls that support the existing United Therapeutics-owned buildings to the west. This condition was addressed with a system of underpinning, also designed by Superior Foundation.

While the geotechnical investigation did not identify a groundwater table elevation that was a concern for the project, seams within the rock do allow groundwater to flow into fractures, creating perched water at various elevations. An under-slab drainage system consisting of crushed stone and interconnected perforated pipes was installed below all basement slabs to address this. This system worked in conjunction with a perforated foundation drain, surrounded by crushed stone and geotextile fabric, located at the base of the below-grade exterior walls, to manage the groundwater.

Aboveground, the six-story building uses a reinforced-concrete frame with two-way, flat-plate slabs and drop panels spanning to concrete columns. While the parking garage and office building appear visually distinct, they are structurally one: The first four floors use floor plates measuring approximately 50,000 sq ft. These include 250 by 110 ft parking decks, and, within the elliptical shape, 200 by 100 ft of office space and 100 by 38 ft for the central atrium footprint.

Each floor plate includes multiple slab elevations to accommodate raised flooring for the office spaces, depressions for localized exterior roof decks, and sloping slabs within the parking decks. Columns within the elliptical portion of the floor plate are set back from the exterior wall so that most floor plates cantilever by approximately 7 ft and some by as much as 20 ft near the tip of the elliptical floor plate. The lateral system is reinforced-concrete shear walls, typically located at stair and elevator cores.

The elliptical portions of the floor plates are clad with a continuous sloping curtain wall system that is supported at each concrete floor slab. The curtain wall system includes aluminum outriggers from the vertical mullions that support sloping solar panels and a decorative metal mesh panel system.

Solar panels-3,000 in total-top the structure. The upper level of the parking deck is covered by solar panels supported by a 20 ft tall elevated tubular steel framing platform. To orient the solar panels for proper exposure, the elevated steel is arranged in a low-slope sawtooth configuration. The highest roof level is covered by a solar tray that measures approximately 315 by 108 ft and slopes approximately 10 ft in the short direction to properly orient the solar panels. The solar tray is supported by structural steel framing that rises 17 ft at its lowest point and 27 ft at its highest point above the concrete roof deck. The solar panels are attached to a standing-seam metal roof system that is overlaid on the steel frame. This framing cantilevers approximately 20 ft at the high and low points of the tray slope, and the roof diaphragm is stabilized with steel moment frames that transfer lateral forces to the concrete superstructure at the concrete roof plate below.

A rooftop stormwater catch basin collects nearly 780 gal. per minute of rainwater, draining the captured rainfall into several vegetated areas within the roof and along the sides of the building, rather than into the city's sewer system. The vegetated areas, which are 6 ft deep and contain soil and large plants, are visible from windows at several levels, offering employees stress-reducing views of nature.

To ensure that the Unisphere could renewably generate all the electrical and thermal energy that it would need on-site, EwingCole took advantage of multiple energy-saving systems. The elliptical shape of the office structure allowed EwingCole to maximize building performance by tapering the building in elevation, each of the six stories becoming slightly narrower than the one below it. This provides an optimal balance of daylighting, views, and space for solar panels.

In addition to the rooftop panels, solar panels are mounted on the south facade of the structure, as mentioned above. They also serve as sunshades and points of interest for people visiting the plaza. The solar modules mounted on the Uni-sphere convert, on average, 22 percent of the sun's contact energy into electricity.

On sunny days, the modules generate energy that exceeds the building's immediate usage requirements. This excess energy is sent to the electrical grid, powering nearby homes and businesses. On overcast days or during overnight hours, when the modules do not generate enough energy to cover the building's energy usage, the Unisphere draws energy from the power grid. At the end of the year, the Unisphere should draw no more energy than it sends to the grid. An energy report generated for the first quarter of this year shows that the building is performing better than the models expected.

The Unisphere also makes use of a ground-coupled geoexchange system as a key component of its reduced energy footprint. This system uses the mass of the earth below the building as a storage device. Excess heat is absorbed by the earth in the summer and extracted in the winter when needed. Highly efficient water-to-water heat pumps concentrate the heat from the geo-exchange system and convert it into usable chilled and hot water for distribution within the building. Ultimately, nearly 20 mi of geo-exchange tubing was installed below the building for this system.

The original design determined that 52 closed-loop wells drilled to depths of 500 ft were required to serve the building's heating and cooling profile. Because of the restrictive nature of the site, 80 percent of the wells were placed beneath the building and parking structure. This was made possible by United Therapeutics' work to change Maryland's well regulations, which were originally based on potable wells and were, until this point, uniformly applied to closed-loop geothermal wells. United Therapeutics was able to secure a change in the regulations in 2015 after negotiating with the Maryland Department of the Environment throughout 2014. It was a critical precedent for the company to set and served the client's goal of pushing for better, more sustainable design within the industry.

The current Code of Maryland Regulations requires specific clearances to foundations, underground sewers, and property lines in addition to maintaining a specific well spacing. All these requirements were coordinated through a composite site plan showing underground utilities and foundations. The regulations also required that wells within the building footprint include vaults to provide access to the wellheads if required for future maintenance.

While drilling the wells, two instances of transference occurred. Transference refers to breakthrough of water and drill cuttings from one bore to another during drilling and before the wells are grouted. Transference poses a risk in fractured crystalline rock, where water flow is restricted to narrow fracture zones, so the project site was susceptible to this issue. These fractures can connect or create pathways between bores, which can convey high-pressure water and cuttings from one open bore to an adjacent open bore. If an ungrouted loop accidentally receives water and cuttings from another bore that is being drilled, the drill cuttings and water pressure can potentially destroy the loop.

As a result of the transferences, two additional bore locations had to be identified on the already congested site and the two damaged bores were abandoned. Through coordination among the design team, the construction manager (Whiting-Turner Contracting Co., of Greenbelt, Maryland), and the well driller (Allied Well Drilling, of Laurel, Maryland), the issue was quickly resolved. In future projects with this type of geology, the design team plans to identify several alternate well locations during the design phase that can be used if necessary.

Another design element that minimized energy usage on-site is a 0.25 mi long concrete earth labyrinth that is located 12 ft below the lowest level of the parking garage. This natural ventilation system, a mix of air and earth, mitigates temperature differences between the atrium and the exterior. Outside air is pulled into the earth labyrinth's underground chambers, where the air comes into contact with the thermal mass of the system's concrete and the earth. It is then passively heated or cooled before it is delivered to the atrium through low-velocity floor diffusers at the perimeter of the atrium's central swimming pool.

The six-story-high central atrium reflects the elliptical shape of the office building with its own footprint; the shape is also repeated in the swimming pool, which features two 20 m lap lanes. The pool can be used by swimmers, but it is also used as a heat sink to help balance the overall heating and cooling system; the water is heated by reject heat from the building. Light-emitting diodes (LEDs) that change color with the temperature of the water illuminate the pool.

The atrium pool can also be covered with a structurally insulated panel that permits it to be used as a walking surface while simultaneously reducing heat loss and evaporation. The water is treated with a combination of sphagnum moss and carbon dioxide to minimize reliance on chlorine and other chemicals.

Nearly 200 active chilled beams provide localized cooling within the remainder of the building by taking advantage of the heat-conducting properties of water, which is about 30 times more effective at transporting energy than traditional, air-based distribution systems. As a result, each office in the Unisphere is efficiently cooled using roughly one-third the amount of supply air as a standard office. This approach results in smaller ductwork and reduced energy to operate fans.

The use of a natural ventilation is another energy-saving component. Panels beneath the office's floors and along the building perimeter can be opened to admit fresh air when conditions permit. At these times, the heating/air-conditioning system can be deactivated to save energy. Openings above the ceiling into the atrium provide a cross-flow of air. The 5,000 sq ft atrium acts as a thermal chimney, using the buoyancy of warm air to in-duce airflows throughout the building and to improve the natural ventilation effect.

When needed, variable-speed fans at the top of the atrium are modulated to induce additional airflow. Occupants can also control their window positions and ceiling fans to tailor conditions to their comfort preferences through a custom-programmed controller.

Energy recovery is included in all airstreams, which permits a transfer of energy between the exhaust and ventilation airstreams during mechanical ventilation modes that ultimately reduces the amount of energy that is used to condition the ventilation air. The recovery device contains a desiccant technology embedded in a wheel that permits both heat and moisture to transfer between the airstreams without mixing them, providing temperature and humidity benefits. A unique unit configuration with dual recovery devices improves humidity control within the office area, resulting in total energy transferred approaching 80 percent efficiency.

Daylight harvesting also plays a part in reducing the energy profile of the building. Electrochromic glass technology has been installed in the exterior walls throughout the office areas. This special glazing changes tint level based on a variety of factors, including season, location of the sun, cloud coverage, reflection from adjacent structures, and occupant preference. "Smart" tinting means that the glass properties can adjust to support the building's heating and cooling systems by tinting in the summer to reduce heat gain and going transparent in the winter to maximize heat gain. The Unisphere's electrochromic vision glazing is paired with daylighting panels made by OKALUX North America, of White Plains, New York. This permits diffused light to enter the space to support daylighting while providing the equivalent thermal performance of a wellinsulated wall by using a superinsulating material.

Daylight harvesting throughout the facility allows artificial lighting to be dimmed or turned off when adequate daylight is available to illuminate the space. High ceilings and efficient windows increase the admission of daylight into the core of the building, while photoelectric sensors automatically adjust the building's artificial lighting output in real time. But individual comfort is also prioritized, so workstation controls can override the sensors. Users can also adjust temperature, window position, ceiling fan speed, and window tint.

Nevertheless, the full integration and automation of the various systems is critical to optimizing the building's performance. From power monitoring and lighting controls to electrochromic glazing, the Unisphere relies on the interconnection and interoperability of each building system to operate at peak efficiency.

The building automation system serves as the Unisphere's main "cognitive" source. This system is the central means of monitoring and controlling the building's heating, ventilation, and air-conditioning (HVAC), responding to the daily interior and exterior environmental conditions to initiate the appropriate sequences of HVAC operation. In addition, the system can make operational adjustments based on building schedule and occupant behavior through occupancy sensing and carbon dioxide monitoring.

The central system automatically collects data from other building systems through a combination of a virtually partitioned building systems network and direct physical network connections. Through these virtual and physical connections, the system can receive and react to various inputs. For example, it can use input from the automated lighting-control system's occupancy sensors to reduce HVAC operations based on real-time occupancy. Additionally, the interconnection of the system with automated windows and electrochromic glazing can close windows and adjust the tint, depending upon the building's mode of operation.

The automation system has also been customized with a graphical user interface to provide real-time data to the facility operator, who can easily monitor and optimize the overall building performance. The interface was custom-developed to provide a snapshot of the overall building performance on a home screen while also allowing the operator to dive into each building system for more specific data if necessary. The graphical interface places the most critical building performance information at the fingertips of the operator and allows for more streamlined trending and troubleshooting of all building systems from a single workstation.

Because energy performance is essential to the Unisphere, the interface also displays data from the central power monitoring system. This monitoring system captures energy production and consumption across multiple categories, from the overall building load to individual office branch circuits. The system automatically conveys this data to the building automation system to enable trends to be accounted for and adjustments to be made.

Overlaying the entire integration platform is a cloud-based data analytics tool that reviews input and changes of status in real time to identify current or potential issues. Summary reports are provided to facilities staff to ensure that the building is operating at peak efficiency and recommend when to perform preventive maintenance. The data analytics tool is customizable and can adjust over time based on the building's performance history.

As a "living" building, it was important to United Therapeutics and the design team that the Unisphere communicate its story and real-time conditions to building occupants. This is done through a series of both static and dynamic installations and devices placed throughout the building. These installations communicate complex strategies in very simple terms that the casual observer can understand in a short period of time. If interested in learning more, they are guided to the Unisphere's website,, for more information.

The Unisphere's energy use is monitored and recorded down to the electrical circuit level, which provides an extremely high level of granularity for analysis and display. This provides a quick snapshot of the building's current operating mode, which can vary by floor. Real-time and historic energy use is communicated via a graphical interface on the fifth floor, which has the most foot traffic because it is the connecting floor between the new office building and the existing United Therapeutics building to the west. A screen engages occupants by displaying real-time energy use, providing information on the building's sustainable aspects, and supporting a series of what-if scenarios that allow occupants to change hypothetical variables to understand the impact that their decisions would have on the building's performance.

The most impressive display feature is the atrium's massive "Energy Dial." The building automation system feeds data to the sundial-like sculpture, whose 40 ft long light beams lengthen and contract depending on energy consumption. The sculpture pulsates during the warmest part of the day. A nearby touch screen explains it further.

The Unisphere celebrated its grand opening in late September 2018. The commissioning effort was completed in December after all systems were determined to be operating and reporting as specified. The EwingCole design team is currently at work on another net-zero energy project for United Therapeutics that will incorporate many of the features of the Unisphere, in addition to a few new ones.

Jason Fierko, P.E., LEED AP, a principal of EwingCole, of Philadelphia, was the lead mechanical engineer for the Unisphere project; Jared Loos, P.E., AIA, the chief executive officer of the firm, served as the principal in charge; Jennifer Wampler, AIA, LEED AP, a principal of the firm, was the project architect; and Peter Welsh, P.E., S.E., a principal and the director of structural engineering at the firm, served as the structural engineer of record for the project.

Architecture; interior design; structural engineering; mechanical, electrical, and plumbing; fire protection; and lighting design EwingCole, Philadelphia
Project management Stranix Associates, Dunn Loring, Virginia
Contractor Whiting-Turner Contracting Co., Greenbelt, Maryland
Commissioning authority Cornerstone Commissioning Inc., Boxford, Massachusetts
Green building consultant Atelier Ten, New York City Excavation Superior Foundation Inc., Vienna, Virginia
Well drilling Allied Well Drilling, Laurel, Maryland
Environmental graphics Hush Studios Inc., Brooklyn, New York

© ASCE, Civil Engineering, June, 2019


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