Green Pittsburgh facility showcases unconventional innovations
With features that include a cable-supported, stainless steel-topped roof and exhibit halls naturally illuminated by daylight, Pittsburgh's new David L Lawrence Convention Center breaks the stereotype for U.S. convention facilities.
The 1.5 million-sq.-ft. center, with its official opening set for September 20, will be the largest U.S. building to receive a Leadership in Energy and Environmental Design (LEED) designation from the U.S. Green Building Council. It will, appropriately, host the second international Green Building Conference and Exposition November 12-14.
New York City-based architect Rafael Viñoly says his design was inspired by Pittsburgh's imposing suspension bridges and extraordinary topography, which is often likened to that of San Francisco. The convention center is located on the south bank of the Allegheny River, a few blocks east of the merging of the Allegheny and the Monongahela to form the Ohio River.
Viñoly was selected in a two-part process that reportedly included the first design competition conducted for a civic building in Pittsburgh since the H.H. Richardson-designed Allegheny County Courthouse was built in 1884. The initial request for proposals drew 25 responses. Four firms were subsequently invited to participate in a design competition, with each receiving a stipend of $100,000.
The roof is the convention center's most prominent exterior feature. Its shape approximates that of a catenary curve — a pure curve that results from uniform loading. Stainless steel was selected as the roof covering both for its durability and for its "glimmering" quality.
Fifteen circular masts, which are concrete as they rise through the building, have pin-topped steel sections to allow roof movement in one direction. Cables draped over the masts are anchored to a "bow" truss on the river-facing side of the building and a "stern" truss on the opposite side. This structural plan produced more than 250,000 sq. ft. of column-free exhibition space, reportedly the largest such area in the U.S.
"The most important architectural feature of the convention center is its ability to solve both an urban design problem and a programmatic challenge," Viñoly has stated. "A single structural solution addresses both the need for a large, uninterrupted covered space and at the same time defines the architecture of the building as an integral part of Pittsburgh's tradition of engineering excellence and remarkable urban profile."
The building incorporates a main exhibition hall, a secondary exhibition hall, meeting rooms, two lecture halls, and a 33,000-sq.-ft. ballroom. It has 330,000 sq. ft. of total exhibition space.
Out of the 'black box'
Daylighting was incorporated early in the design process, according to David Rolland, project director with Rafael Viñoly Architects. He noted that convention centers are frequently described as "black boxes," although many European centers incorporate daylighting. At the David Lawrence Convention Center, the exhibit hall has 67,776 sq. ft. of glazing, including 35,740 sq. ft. of glass and 26,900 sq. ft. of skylights. Seventy-five percent of the exhibition space will receive natural light. While Rolland adds that trade show exhibitors are accustomed to working in an environment which gives them complete control of lighting, he believes they will find the addition of daylight a positive experience.
Convention officials have nevertheless not overlooked exhibitors who may want to shut out daylight — for example, in order to make a visual presentation. An internal shading system will permit the main exhibit hall to be darkened to a level of about 5 fc. The system is noteworthy for both its scale and configuration. For example, to cover a non-rectangular glass wall area, an articulated arm descends and a shade then unrolls from it.
Another novel feature of the convention center is its ability to rely only on natural ventilation during typical spring and fall periods, making it unnecessary to operate its mechanical system. David Linamen, principal-in-charge with Butler, Pa.-based Burt Hill Kosar Rittelmann Associates (BHKR), the project's mechanical/electrical engineer, says this capability should pertain when outside temperatures are in the 48-62-degree range.
Computational fluid analysis was used to study natural ventilation patterns. Air rises almost continuously from the river, where wind is channeled by typography and other buildings, according to Linamen. It is introduced into the exhibit hall about 12 ft. above the floor. Cooler air descends, while warmer air is pushed upward by the stack effect, exiting at roof level through a series of dampers. At its maximum height, the ceiling is 118 ft. above the exhibit hall floor.
"Both the stack effect and the amount of incoming air can be controlled by adjusting the dampers," says Ronald Stabert, BHKR lead mechanical engineer. The volume of incoming air is also regulated by sensors that are designed to keep CO2 levels inside the building below 1,500 ppm.
Fabric ducts let building breathe
Polyester-based fabric air ducts suspended from the roof cables are an important component of the HVAC system. The 32-in.-diameter ducts, which are flexible enough to move with the roof structure, enter the exhibition hall 24 ft. above the floor and rise to 46 ft., following the sloped contour of the roof cables. Instead of a single large duct, two parallel ducts were used. They deliver air at a velocity of 6,000-9,000 cfm. Air is fed from two sides of the building to provide uniform distribution throughout an exhibit hall that is more than 300 ft. wide.
The exhibit hall's lights are mounted between the translucent ducts, turning the ducts from a functional necessity into a nighttime aesthetic feature. The fabric ducts were "almost an ideal solution to all the duct issues we had to resolve in the exhibit hall," Linamen says.
The original plan was to fabricate the ducts from sheet metal, which would have been too heavy and would have necessitated the use of aesthetically compromising flex joints and linear diffusers. By contrast, the fabric duct has perforations ranging in diameter from 1/2 in to 1 1/4 in. over its entire length. At the 46-ft. height, the holes are largest and are located mostly at the bottom to maximize air distribution.
A Teflon-coated fiberglass fabric is suspended from the underside of the roof trusses, providing better light distribution and helping to define the building's 60-ft.-wide bays.
Delivering chilled air at a temperature of 41 degrees rather than a more typical 54 degrees permitted a 60% reduction in the volume of air necessary to maintain the desired floor-level temperature, Linamen says. "Cooling capacity is a direct function of temperature difference between the air coming out of the system and the temperature you're trying to maintain," he adds.
The roof system, which has been described as "a living, breathing assembly," has a life of its own, Rolland observes. The cable tension system will change shape depending on where a load is applied. Snow loads will slightly displace it. "That's not a problem," he insists. "All external cladding systems just must be designed to be able to accommodate the fact that the roof moves."
Down the drain — twice
The Lawrence Convention Center conserves water through recycling. Water that has been used in lavatories and sinks for hand washing is routed through a combination of biodegradation and aeration processes. Linamen says submicron filters remove all undesirable components except viruses. The water is also exposed to ultraviolet light to improve its clarity. This "gray water" is then used for flushing toilets and urinals before being routed to the city sewer system. Recycled water can provide half of the 200,000 gallons required daily for these functions when the convention center is at its peak occupancy of 25,000.
Rolland says the building's design reflects a true integration of structural, mechanical, electrical, and architectural disciplines, without treating each as a separate element. For example, weldments that anchor the roof cables are aesthetic features of the south building terrace.
The design of such elements is often not within the architect's domain, says Rolland, but not in this case: "We work with a variety of materials and the expression of an architectural aesthetic. This project allowed us to work with the structural engineer in using their materials as part of the architectural expression.
"Rafael was always very clear about the fact that the architecture of the building is the structure," Rolland says. "No need to hide anything — just reveal it and make it part of the design."
A major objective of the design was to create a truly attractive venue that would be the antithesis of the starkness of many convention facilities, according to Rolland. Thus, outdoor terraces on the river and city sides of the building will be accessible to the public.
The parallelogram-shaped convention center footprint is largely the result of the river's geometry. Viñoly intends the design to help merge the grids of the city and the river, to forge a stronger tie between Pittsburgh and its riverfront.
Viñoly and Linamen were among attendees at an orientation meeting at which the convention center's program requirements were outlined. When they met again two weeks later, they were pleased to discover that their respective design concepts were remarkably compatible, according to Linamen, which was "almost too good to be true." Since the roof was to be the center's key design element, its compatibility with the HVAC distribution concept was a significant project milestone.
The suspension bridges that inspired Viñoly's design also provided a starting point for the structural engineering, says James O'Callaghan, senior associate with London-based structural engineer Dewhurst Macfarlane & Partners. His firm had previously worked with Viñoly on a number of other projects, including the Kimmel Center for the Performing Arts in Philadelphia.
O'Callaghan noted the similarity of the convention center's roof design to a typical suspension bridge, with its tall masts that serve as cable anchors. The masts support 152 cables that are 9 in. in diameter.
When viewed in section, the "bow" frame on the river side of the building, the "stern" frame on the opposite side and the masts — which rise progressively to a height 192 ft. above the main floor level, in order to give the eastern end of the convention center up upward tilt — resemble the profile of a ship.
"The whole building becomes an enclosed load path, or closed system, that resolves all its forces within itself, rather than into the ground," O'Callaghan says. Prestressed lower damping cables restrict vertical movement of the roof to 2 ft.
Fire protection was more challenging than expected, Linamen notes. Because the exhibit hall is too tall for the effective use of sprinklers, the ultimate resolution was to specify water cannons. The 25 units, mounted 15 ft. above the floor, are capable of shooting up to 600 gpm of water over a distance of 300 ft. The water cannons are used in conjunction with an aspirating smoke detection system that employs a continuous punctured rubber tube, which is attached to the bottom chord of roof cables, to monitor the opacity of the air for the presence of fire.
The HVAC system plan initially envisioned the use of water from Pittsburgh's underground aquifer (often called the city's "fourth river") to reduce the need for cooling towers. A test well yielded promising results. But underground contamination from a nearby abandoned service station was subsequently encountered — a situation Linamen says would have been impossible to remediate satisfactorily, as it would have produced a gradual warming of water that was being returned to the aquifer. Another option — seeking approval to discharge this cooling water into the river for a period of five years — was rejected because it was considered too problematical. "If we could have gotten all the water we wanted from the aquifer, we wouldn't have needed any cooling towers," Stabert says. The center now has four cooling towers. Linamen called the inability to proceed with the initial plan "a big disappointment."
No dice on ice
Another disappointment for energy conservation advocates was owner Pittsburgh Sports & Exhibition Authority's decision not to incorporate an ice storage system in the building's chiller plant, which Linamen says had a projected one-year payback. With the design for such a system in an advanced stage, the authority decided to hire a third-party provider, Westboro, Mass.-based Noresco, to build and operate the plant, thereby eliminating a $5 million item from the project's capital budget.
Yet another potential energy-saving feature not implemented was to direct a flow of condenser water over the roof to help cool the water and provide a 5-10% increase in the overall efficiency of the system. Linamen was not surprised that this idea was dropped. "It was an easy value-engineering target" due to its lengthy life-cycle payback, he says. Nevertheless, it garnered more attention than any of the building's other energy-saving concepts, and rooftop piping was installed to allow for possible future installation of such a system.
Despite the disappointments, Linamen says the features that were incorporated will reduce the convention center's operating costs about 33% compared to a comparable building designed in accordance with ASHRAE Standard 90.1 (1999).
From a contractor perspective, the challenges of the convention facility centered essentially on four factors, according to Arthur Hunkele, project director with Turner Construction Co.: 1) The project was designed by a signature architect; 2) it was publicly bid; 3) it was fast-tracked; and 4) it had an objective of attaining a high LEED rating.
Turner headed a joint venture that included Pittsburgh-based contractors P.J. Dick and ATS. The joint venture provided construction management on an agency basis. All 57 prime contracts were publicly bid and held by the owner.
The initial plan was to complete two-thirds of the new facility by building up to the former convention center, then utilizing elements of the old building for the new facility. This also would have allowed the old center to continue operating as long as possible.
When the old building was demolished down to its deck, however, it became clear that a retrofit of the remaining structure could not satisfactorily resolve forces generated by the convention center's unique roof. Consequently, it became more cost effective to completely demolish the old building, which was done in the summer of 2001. The first phase of the new building opened in February 2002.
The project plan required the contractor to stage the work in the exact opposite of an ideal sequencing procedure. "We like to build to daylight — start in a corner and work our way out, so you have the greatest chance of an easy demobilization," Hunkele says.
Because of the goal of attaining LEED accreditation, particular attention was directed to the recycling of construction materials and debris. Usable materials were donated to Construction Junction, a Pittsburgh organization that sells them at nominal cost to low-income buyers.
|Structural steel/misc. metals||89,952,800|
|Furniture, fixtures, equipment||10,278,580|