9 dec 09 | Engineering News-Record
Design Team Held to Competition Promises
by Nadine M. Post
Masdar Initiative reconvened competition jury halfway through design of its sustainable headquarters in Abu Dhabi to keep on positive-energy track.
It's a tired, old story and the bane of many a design team: Win an international design competition and then watch helplessly as all the brilliant concepts get whittled away by the developer and its "value engineers" until the constructed building bears little resemblance to the original submission.
It's a brand new story, and both the delight and challenge of the Chicago-based design team that beat out 158 others to design the 116,372-sq-meter headquarters for the Masdar Initiative in Abu Dhabi--billed as the world's largest carbon- and waste-neutral office building. In a giant exception to the rule, the client is holding Adrian Smith + Gordon Gill Architecture (AS+GG) and its primary engineers, Thornton Tomasetti (TT) and Environmental Systems Design Inc. (ESD), to their competition promise of delivering not just a sustainable building but a 3% "net-positive-energy" building, as if the promise were a sacred vow.
To keep its own team and the design team hyped on its positively energetic design, the Masdar Initiative reconvened its 12-person competition jury for a formal review about halfway through design. "I've never had that experience," says Gordon Gill, a partner of AS+GG. "It was extremely helpful and a part of probably one of the most mutually collaborative processes" he has ever been involved with, he adds.
As many as 40 people--including Masdar's team--attended the review, held Nov. 19-21, 2008, at AS+GG. The sessions provided assurances that neither the client nor the design team was straying from the goal of sustainability, says Gill. Robert Sinn, a principal in the Chicago office of structural designer TT, calls the review an "eye-opener," describing it as a couple of days of "not-so-easy conversations, with Gordon always on the hot seat."
Juror Dennis A. Andrejko, a professor of architecture at the University at Buffalo, Buffalo, N.Y., calls the design review "really exciting" because of the intention to make the project a showcase for sustainability. "I think this post mortem to recap how the design process was evolving was a great thing," he says, adding that he had never before heard of such a process. There were "good, solid and comprehensive discussions" about putting back some elements that were a bit problematical, rather than taking a more expedient approach and leaving them out, he adds.
Discussions got down to the nitty-gritty. The group even questioned whether escalators or stairs alone would take people down to the transit tunnels. The decision: provide both. The group also debated whether to plant indigenous desert landscaping or fruit trees in the office building's shaded roof gardens. The decision: orange trees, because the micro climate being created for the building's passive solar system would support their growth and the gardens could then easily offer something rare and special in the desert, says Gill.
Piling and grade beam work is finished, and the slab on grade is half complete for the headquarters, which is part of a 6-sq-kilometer development called Masdar City (http://masdarcity.com/en/index.aspx). The estimated $22-billion mini city is an initiative of the emirate-owned Abu Dhabi Future Energy Co. The Masdar City master plan by Foster & Partners, expected to be built out by 2016, describes a sustainable development laid out as a high-density, auto-free city with mixed uses. Masdar Headquarters, which will house about 2,500 people, including about 200 from the International Renewable Energy Agency, is scheduled to be completed in 2012. "We have a budget for the headquarters but while the negotiation for the contractors is still pending, we do not want to publicize the cost as it has commercial implications," says a Masdar spokesman.
When asked whether the current financial crisis in neighboring emirate Dubai will have any effect on the Masdar project, the spokesman says, "We are not prepared to talk about Dubai and the impact but it doesn't have any impact for Masdar Headquarters so far." The general construction contract is expected to be let early next year.
As designed, the headquarters would yield zero carbon emissions and zero liquid and solid waste. The strategy is to reduce energy consumption through passive solar and efficient systems to a level that can be sustained through on-site energy production. A rooftop photovoltaic (PV) system is expected to produce 3.5 MW of electricity each year by harvesting the desert site's abundant sunlight. It will supply 100% of the building's total energy, and building-integrated PV panels will provide a 2% surplus. In principle, "The building is either protecting itself from the sun's heat or harvesting solar energy," says Gill.
Though sources say sustainability adds an initial first cost, Masdar officials say the premium will not be known until the building's final cost is known. "We hope to make green building competitive with conventional building eventually," says the spokesman.
Headquarters plans call for a rather mundane seven-story, concrete-framed office building that by itself would not raise a single eyebrow. But the architect has designed a cover for the office building that is a real eye-opener. When built, it will likely rank as the world's largest trellis. The seven-acre steel mesh is designed as a surface for the PV panel array. The trellis, a trussed trapezoid in plan, will be held up by 11 "wind cones," each resembling a cooling tower but framed in steel pipe and clad in glass. The trellis will double as a giant canopy, though there is no solid surface, shading the 23,286-sq-m building site from the sun's heat.
The wind cones, each enclosing a courtyard or some other public space and open at the top, do double duty as part of the development's natural ventilation system. "The temperature in the cones, without air-conditioning, will be 15 to 20 degrees cooler than the outside air," says Gill.
The sustainable systems planned for the building are all based on proven technology but they combine to produce something new, says Mehdi Jalayerian, managing principal of ESD. "That's the success of this project," he adds. No "extremely exotic" systems are being used, he says, because the team wanted to make the project buildable and the systems functional.
Jalayerian lists three fundamental planning steps: reduce building energy use through shape, configuration, siting and with a high-performance envelope; apply efficiency to building systems through very efficient lighting, narrow lease span from the building perimeter to the courtyards for daylight penetration, efficient layout of ambient and task lighting with user controls and efficient ventilation-system motors; use renewable energy, in this case through a PV system that takes advantage of the region's abundant sunlight and through collecting the cooling-system condensate for water features.
"In comparison to an efficient typical building, this building has 35% of the energy requirement and uses half the energy typically used for cooling," says Jalayerian. Water use is designed to be reduced by 70% over the code's baseline through collection and recycling. For the overall energy balance, embodied energy in potable water due to the desalination process was considered part of the overall city infrastructure requirement, not the building's, according to AS+GG.
There are many bells and whistles to sustainability in this project: the shaded micro climate created using the canopy and glazed insulated outer walls, and shaded green roofs and solar thermal tubes to supply the building with air-conditioning, a plan that uses the sun to actually cool the building. There also will be earth ducts to reduce the temperature of outside air and serve double duty as underground corridors that will connect the public garden space to the proposed transit system. Finally, the wind cones act not only as modern wind towers, supplying ventilation and exhausting warm air through the space, they also maximize natural daylight throughout the building. The cones have operable windows to give occupants the option of naturally ventilating interior spaces, and they create courtyards in the public spaces.
The building's form is sculpted in response to an extensive environmental analysis to optimize total system performance. RWDI performed wind-tunnel tests to determine airflow movement, including how the wind cones were going to behave. A scale model of the building and the future city around it was instrumented using small pressure sensors. Results were then correlated back to an airflow analysis, which takes into account meteorological data.
"We've never done anything with this kind of [building] shape" and the 11 wind towers, says Duncan Phillips, an RWDI principal in the firm's Dunstable, U.K., office. RWDI also studied the wind's effect on the trellis.
Sustainable materials, selected based on life-cycle analysis, are expected to be used throughout the complex. The headquarters' specifications were developed in synchronization with the Masdar City master specification, a document that is continuously updated. Rigorous reporting and monitoring are included for both the design and construction teams, says Omar Mustafa Waqfi, the Masdar Initiative's specifications manager.
The aim is to reach the highest level of sustainability, says Waqfi. For example, the headquarters' concrete has a low-carbon footprint achieved by using a cement replacement and an on-site batch plant to eliminate truck deliveries. Reinforcing steel is specified with over 95% recycled content and structural steel with 60% minimum recycled content.
The headquarters design team contributed directly to the master spec for various products as well as systems, including wind-cone performance, underfloor air distribution and optimization of the photovoltaic products and positioning.
The complex has two major integrated but structurally independent, assemblies that share a common podium: the concrete office blocks and the steel PV-support trellis and its wind cones. The independence of the two systems will give the base-building contractor the option of starting either assembly first, says TT's Sinn.
There are two, seven-story office blocks. Where the floors are punctured by the 51-m-tall cones, made of painted, hollow pipe sections, openings are framed by concrete columns sloping to match the profile of the cones. Curved ring beams are used at each floor opening to stabilize the sloping columns.
One of the project's big challenges was to prove to local authorities that the 7,500-ton steel frame did not need fireproofing, says Sinn. This was demonstrated with the help of fire protection engineer RJA, which created different fire scenarios to determine a fire's impact on steel strength even if the sprinklers did not work, says the engineer. That, combined with high-temperature structural analysis, showed that "we could lose practically half a cone" and all the others would keep the trellis from falling in, says Sinn.
The geometrically identical cones have a belled form, shaped by a diagrid frame of hollow pipe sections. Diagrid members are straight pieces; horizontal rings are curved. The 325-mm-dia pipes have various wall thicknesses depending on structural requirements. The base of each cone is 27 m in diameter, while the topmost ring is 7 m in diameter.
The trellis has an upper, flat mesh surface and a lower, wavy mesh surface. A section on the lower trellis has been dubbed "the peel" because it flares out from each cone 26 m above the base of the cones (see drawing). The peel section is then organized on a repeated 31.5-m square module in plan. The rest of the lower trellis consists of a diagonalized mesh of steel pipes attached to the perimeter of the peel, forming a checker-board pattern. Together the peel and the mesh surface at the lower trellis brace the bottom chords of the trellis trusses, which form the waved pattern.
Cantilevered trusses with bottom chords that follow the wavy profile of the lower trellis connect lower- and upper-trellis surfaces. Truss depths range from 1.3 m to 15 m at the root of the peel. With a maximum cantilever length of 57 m, the trusses are designed to limit the frequency at the tip to prevent adverse fluttering effects from wind action, says TT.
Each cone receives eight trellis trusses at equal spaces along its circumference. The top chords are composed of 450-mm-dia pipe sections and are braced at 11 m on center to create a horizontal platform for the PV-support grillage.
Sinn says there is a complicated joint at which the cone continues up and the wavy surface diverges. In this case, a steel casting will join the several members.
For constructability and economies of scale, all parts of the headquarters building, designed using building information modeling, were developed to maximize repetition and modularity. For example, because the trellis and the concrete building are independent of each other, the trellis can be built before the concrete building and act as a shade for the workers. "It's an option" for the contractor, says Sinn.
Jalayerian speaks for his project colleagues when he says he fully embraces the demands that the client Masdar has put on the design team. "I think it's wonderful," he says. "It's aggressive, and it's progressive."