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The Pearl River Tower, in Guangzhou, China, has been designed to be the most energy efficient of all the world's supertall structures. Although the design team's original goal of constructing a "net zero-energy" building that would sell its excess power to the local electrical grid is unlikely to be achieved, the structure is expected to consume nearly 60 percent less energy than a traditional building of similar size and could serve as a model for future "carbon-neutral" towers.

Seeking Zero energy

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B y R og eR e. F R eche t t e III, p. e ., l eed a p, a n d R ussel l g I lch R I st

he industrialization of the world has led to great innovation, great technological advances, and powerful national economies. It has also resulted in an incredible appetite for energy, most notably energy generated through the use of fossil fuels. This massive consumption of fossil fuels has sharply increased the levels of carbon dioxide (CO2 ) in our atmosphere, resulting in a steady but rapid warming of the planet. The ramifications of this man-made environmental shift are not yet fully understood, but many scientists believe the results could be catastrophic. Many factors have contributed to this crisis. But while

The 310 m tall Pearl River Tower, in Guangzhou, China, is expected to be the most energy efficient of all the world's supertall structures, opposite. As designed, the project should come close to being a "net zero-energy" building. Such buildings do not require the city or region in which they are located to generate any additional energy on their behalf. Photovoltaic cells will be integrated into the tower both as the building's skin--in the form of spandrel panels-- and as a source of power. To achieve the greatest productivity, the cells will be located in an asymmetrical arrangement at the roof level, where the system will not only provide electricity but also function as a solar shade for the section of the building that will be most susceptible to the negative effects of direct solar radiation.

© 2008 American Society of Civil Engineers

Civil Engineering January 2009

© 2008 American Society of Civil Engineers

It was important to som that this holistic approach produce an array of solutions that would be compelling at a conceptual level and would survive the rigors of design development and future value engineering exercises.

the city or the region to generate any additional energy on its behalf, modifications to the original design arising from economic considerations and regulatory challenges made this goal unachievable. But thanks to an all-inclusive design philosophy that wove together a variety of measures designed to reduce the building's dependency on the city's electrical grid, the Pearl River Tower is expected to come as close as possible to that goal. Such a high level of performance requires a design team to consider a host of issues, among them the site of the structure, the passive and active energy sources available, the types of building materials to be used, and the desired indoor air quality. The team must also determine ways of integrating these issues into the building design in a substantive manner rather than including them in a way that is purely for show. It is thus necessary to determine such basic elements as site conditions, the building's orientation, the local wind speed and direction, and the path of the sun in the region and to draw on such sophisticated approaches and technologies as radiant ceilings, doublewall systems, photovoltaic devices, and wind turbines. It was important to som that this holistic approach produce an array of solutions that would be compelling at a conceptual level and would survive the rigors of design development and future value engineering exercises. This demanded a design approach that looked not to form but to performance. In this way, superfluous architectural detailing was avoided by ensuring that all of the systems possessed a degree of interdependency. The structure of this wide but narrow tower is based on a composite system that utilizes both structural steel and reinforcedconcrete elements to resist gravity and lateral loads. The primary lateral-load-resisting system features an interior reinforced-concrete core and a series of composite megacolumns that are linked by a large, multistory system of structural steel X braces on the narrow edge facades of the building. The perimeter columns are linked to the reinforced-concrete core wall and the corner megacolumns by a system of two-story outrigger and belt trusses at the major mechanical levels. Engaging the perimeter columns with the outrigger trusses increases the effective moment mechanism of the lateral system while the belt trusses work to equalize the loads in the perimeter columns. Structural steel moment frames also are provided on the broad faces of the building for additional resistance. Inherent redundancy and robustness are achieved with the addition of the belt trusses and perimeter moment frames. The thicknesses of the core walls range from 700 to 1,500 mm over the height of the building. The megacolumns consist of large built-up structural steel I sections that are up to 900 mm deep by 700 mm wide; these I sections feature 100 mm thick plates surrounded by reinforced-concrete encasements that are 3,000 by 2,700 mm for the bottom half of the tower and 2,500 mm square for the top half. The structural steel X braces located between the megacolumns also are formed of built-up I shapes that typically are 600 mm deep by 600 mm wide and have plates that are 50 to 100 mm thick. Each system of X braces is roughly six stories tall. The perimeter columns generally are built-up shapes below the uppermost outrigger and are belt truss systems and rolled sections above that point. The perimeter columns for the lowest third of the tower consist of built-up I shapes 600 mm deep by 600 mm wide with 100 mm thick plates; there are also 50 to 100 mm thick cover plates on and between the flanges because of the loads from the lowest outrigger and belt truss system. The middle third of the

transportation and industrial activity have long Four large openings approximately the city of Guangzhou experiences some of the 6 by 6.8 m located at the mechaniworst air pollution on the planet, the city and been recognized as major sources of CO2 emiscal floors will function as a type its province, Guangdong, are a major focus of sions, the emissions associated with the built of pressure relief valve for the this environmental initiative. The Pearl River environment may actually be the single greatest building, and vertical-axis wind turbines installed in each openTower project, which features both active and contributor to global warming. Therefore, when ing will harvest wind energy. The passive approaches to limiting carbon emissions architects and engineers design buildings, it will building's design will capitalize on through new technologies as well as reduction become increasingly imperative that they work the pressure difference between to reduce the amount of energy consumed during the windward and leeward sides of strategies, could play an important role in develthe structure, facilitating airflow the construction phase as well as limit the CO2 oping a new model that will both provide higher through the openings. emissions that are generated by their buildings living standards and achieve important environonce they are in use. mental goals. Designed by Skidmore, Owings & Merrill llp (som), of Chicago, som became involved in the Pearl River Tower project in 2005 the 310 m tall Pearl River Tower, in Guangzhou, China, is scheduled when it was hired by the China National Tobacco Company to for completion in 2010 and is expected to be the most energy effi- design a headquarters building for the Guangdong Tobacco Comcient of all the world's supertall structures. As designed, the project pany, one of the largest companies in Guangzhou. The design brief should come as close as possible to a net zero-energy building. Such envisioned a "high-performance" tower that would consume siga building does not require the city or region in which it is located nificantly less energy than is typically needed by a building of this to generate any additional energy on its behalf and thus is environ- size and type. The design team interpreted "high-performance" mentally benign. With such buildings, the city or region can keep its to mean a structure whose energy-saving systems and strategies power generation stable or possibly even decrease it. In this way the would work together in an integrated fashion to consume nearly city or region can expand, increase its density, and prosper without 60 percent less energy than does a more traditional building. The result was som's design for the 71-story Pearl River Tower, the need to consume additional fossil fuels, thus avoiding the potential which includes associated conference facilities that increase the total increases in harmful greenhouse gases. The Chinese government recently set the goal of reducing the footprint of the project to approximately 204,000 m². Although nation's carbon emissions by 10 percent by the year 2010. Because the initial goal was to construct a building that would not require

Cross Section of Air Temperatures at Perimeter Zones

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© 2008 American Society of Civil Engineers

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Reduction meant finding as many opportunities as possible to reduce the amount of energy consumed.

tower consists primarily of built-up I shapes that are 600 mm deep by 600 mm wide and have 50 to 75 mm thick plates. Below grade, the perimeter columns are encased in concrete to simplify the construction interface with the basement levels, which are generally concrete slabs reinforced in two directions. The outrigger and belt truss elements also are built-up I shapes ranging in depth from 600 to 1,000 mm with widths up to 600 mm and plate thicknesses of 50 to 100 mm. The typical floor framing takes the form of rolled wide-flange beams supporting a deck slab of concrete on metal with a total thickness of 160 mm. The shear studs are welded to the beams to provide composite action with the slabs. The maximum floor beam spans are approximately 13 m. The foundations for the tower take the form of a 3,500 mm thick reinforced-concrete mat extending under the core and megaframes as well as 4,000 mm thick reinforced-concrete spread footings under each of the perimeter columns. All foundations bear on rock and extend approximately 28 m below grade level. The initial goal--designing an edifice that would not require the city or region to generate any energy on its behalf--was based on four concepts: reduction, absorption, reclamation, and generation. Reduction meant finding as many opportunities as possible to reduce the amount of energy consumed. These reduction strategies focused on the expected largest consumers of energy within the building--namely, the heating, ventilation, and air-conditioning (hvac) system and the lighting system. The reduction strategies incorporated into the design of the Pearl River Tower included the following: · An internally ventilated, high-performance active double wall with mechanized blinds on the northern and southern facades; · High-performance, triply glazed facades on the eastern and western sides of the structure; · A so-called chilled radiant ceiling with a perimeter chilledbeam system, both using chilled water (approximately 14.5ºC) delivered through a serpentine arrangement of copper piping that is fixed to the back of a curved metal shape for the ceiling system and a series of metal fins for the perimeter beams; · A "decoupled" ventilation system providing only fresh air that is cooled by the chilled-water system described above and delivered via a raised access floor; · A dehumidification system using heat collected from the double-wall facade as an energy source;

Pearl River Tower Incident Solar Stress Model

· A low-energy, high-efficiency lighting system using radiant panel geometry to assist in the distribution of light. Absorption, the second design concept, made it necessary to focus on strategies designed to take advantage of the natural and passive energy sources--namely, wind and solar--that will pass around, over, and under the building's envelope. The absorption strategies used on the Pearl River Tower included the following: · A wide-scale photovoltaic system integrated into the building's external solar shading system and glass outer skin, which is located on the southern facade; · The use of fixed external shades and integrated photovoltaic devices on the eastern and western facades, as well as integrated photovoltaic devices within the western facade shades; · Maximizing the use of natural lighting via controls that respond to light and are integrated into a system of automated blinds; · Vertical-axis wind turbines designed to take full advantage of the building's geometry. The third concept--reclamation--relied on strategies to harvest the energy that would already be resident within the building. Once energy has been added to the building, it can be reused repeatedly. For example, the Pearl River Tower is designed to use recirculated air to help heat or cool the fresh air from the outdoors before it is delivered to the occupied areas of the building. Naturally, this strategy is dependent on the outside air conditions and requires absorption chillers. Generation, the final concept, envisioned the use of microturbine technology to generate clean power in an efficient and environmentally responsible manner. Indeed, the original plan was based on the projected ability to generate enough electricity within the structure to sell the excess to the local electrical grid. Having the ability to generate power more efficiently than can be achieved by the city's grid would result in a net reduction in greenhouse gases associated with the building's normal operation. For example, a typical electric power utility grid is less than 30 to 35 percent efficient by the time the energy has reached the building from the power plant source, according to "Energy Efficiency in the Power Grid," a white paper produced by abb, Inc., of Zürich, Switzerland, in 2007. By contrast, the on-site generation plant that was designed for the Pearl River Tower was expected to generate power with an efficiency exceeding 80 percent. The original concept for the building involved linking as many as 50 microturbines--each approximately the size of a large kitchen refrigerator and powered by such fuels as kerosene, biogas, diesel, methane, propane, or natural gas--to create a generating capacity of 3 MW. Unfortunately, these plans were placed on indefinite hold when the

January 2009 Civil Engineering

Wind Velocity Vectors at Mechanical Floors

Guangzhou utility decided that it would not connect the microturbines to the local electrical grid, which is often unreliable. Because the tower would not be able to sell its excess power to the utility, the cost of the microturbine system could no longer be justified. Moreover, the elimination of this technology meant that the goal of achieving a net zero-energy building was unattainable. But the potential benefits of the microturbines were so compelling that the building's basement has been designed so that it can be retrofitted to accommodate the devices should the local utility ever change its stance. The facade of the Pearl River Tower will feature an internally ventilated double-wall system made up of doubly glazed, insulated units integrated into 3.0 by 3.9 m unitized panels, as depicted in the figure on page 47. Two hinged 1.5 by 2.8 m singly glazed leaves will be fixed to the back face of the mullion to create an approximately 200 mm deep cavity with a small air gap at the base. Within the cavity is a motorized silver venetian blind system in which the perforated blinds measure 50 mm wide. The position of these blinds--fully open, open at a 45 degree angle, or fully closed--will be controlled by a photocell that tracks the movement of the sun and is connected to the building management system. The exterior glazing will take the form of insulated, tempered glass with a low-emissivity coating; the inner layer will be an operable clear glass panel that can be opened for maintenance. The units will be suspended from the top at each level and laterally supported at the bottom. This integrated facade assembly provides exceptional thermal performance as well as good visibility through the glass, and it should allow for the enhanced use of natural lighting. In turn, this should make it possible to reduce the amount of artificial lighting required

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High-Efficiency Equipment

Daylight Responsive Controls

High-Performance Glazing

High-Efficiency Lighting

in the space while preserving the excellent views--via the perforaComparison of Pearl River Tower tions--even when the blinds are fully closed. Of greater importance, with Hypothetical Baseline Building: the double-wall arrangement will be a vital component in maintainProjected Annual Energy Consumption ing the balance between maximizing transparency and achieving a high standard of comfort for the building's occupants. As sunlight strikes the exterior doubly glazed skin, some of the resulting solar heat gain will enter the cavity between the outer and inner glazed layers. Fortunately, the cavity will act as a natural chimney. The cooler air from the occupied office areas will enter the cavity via a gap at floor level and act as a pressure relief valve to allow more fresh air to enter the occupied areas. The trapped air in the cavity will then be extracted through the ceiling void to, depending on the outside temperature, either preheat or precool the interior air. By maintaining a low temperature on the interior layer of glass-- the layer closest to the occupants--the mean radiant temperature in the office space will be decreased. This will then reduce the operative temperature of the space from, say, 27ºC at the perimeter to 23ºC farther inside the office, as depicted in the figure on page 41. This lower operative temperature will create an environment of improved thermal comfort at the perimeter zones and should directly improve the flexibility and usability of the areas closest to the exterior glazing. account for convective and radiated heat that must be cooled by the A similar system is used on both the southern and northern facades, hvac system. Heat gains at the perimeter of the building are much in part for controlling glare but also because the northern facade is more variable and usually difficult to control because of solar energy exposed to solar gains from the west in the late afternoon. transmission through the glazing. A water-based radiant cooling and The Pearl River Tower, as mentioned above, will also feature a displacement ventilation system addresses each heat transfer mode decoupled radiant cooling ceiling that works in conjunction with with the appropriate cooling or ventilation system: radiant loads are an under-floor ventilation air delivery system. This combination controlled by the radiant system, and convective loads are controlled should provide improved comfort in all respects while simultane- by a combination of the radiant system and the displacement ventilaously reducing the building's energy demand and maintenance tion system. This approach works better than a conventional forced costs. Furthermore, by requiring less material, it will also reduce overhead mixing system, which is entirely convective, because water the structure's capital costs. is a far more efficient transfer medium than air. Projected Energy The office space within the tower will encounter The system proposed for the Pearl River Tower heat gain daily from a variety of sources. The people Savings from Large- will also mean that significantly less energy will be in the offices, the ambient and work space lightScale Sustainable required to power fans than in a standard variable ing, the computers, and other office equipment all Design Strategies air volume system. With a conventional air volume system, the warmer air, typically the return air, can increase temperatures within the building interior as it migrates to return grilles located throughout the f loors. But the 40,000,000 decoupled system proposed for the Pearl River Tower uses the exterior 35,000,000 double-wall enclosure as the return 30,000,000 air plenum. As the return air within 25,000,000 the perimeter zone is drawn to the building's exterior, the effect on the 20,000,000 interior zones is minimized. 15,000,000 The decoupled ventilation system 10,000,000 also enabled the design team to reduce 5,000,000 the building's floor-to-floor height from 4.2 m to 3.9 m--the equiva0 lent of reducing construction by five stories. It also reduced the costs associated with the exterior envelope and, through the projected energy savings, provided what may be the most environmentally beneficial aspect of the Pearl River Tower design.

Pearl Design Baseline High-Efficiency Plant Demand Base Vent Radiant Cooling "No Flow" Features Heat Recovery Integrated PV Wind Turbines

The Pearl River Tower will implement vertical-axis wind turbines that are capable of harnessing winds from both prevailing wind directions with only a minor loss in efficiency.

The system also enabled the design team to optimize the plan layout turbines that are capable of harnessing winds from both prevailby eliminating fan rooms and reducing the size of air shafts. This, in ing wind directions with only a minor loss in efficiency. The turn, resulted in a reduced core area that increased the usable space on building's design capitalizes on the pressure difference between each floor and thus increased the building's revenue potenthe windward and leeward sides of the building and Wind Tunnel should facilitate airflow through the four openings. On tial. Moreover, the decrease in fan equipment provided the Test Data at the windward side, a stagnation condition causes the space on the mechanical floors for the wind portals that made the building's wind turbine system possible, space Wind Portals locally increased pressure to be higher than the undisthat would not have been available if a conventional turbed pressure approaching the building. On the ventilation system had been used. leeward side, a low-pressure area is induced Wind looms large in the design of tall by the high-velocity flow at the sides and buildings. It builds up positive presroof of the building. sure on the windward side of the The effect of the wind traveling structure and--through vortex through the four openings was shedding around the sides and carefully studied in a wind tunover the top of the buildnel testing rig that featured a ing--creates large pockets scale model of the Pearl River of negative pressure on the Tower. This testing took airleeward side. But if the air flow measurements of the is allowed to pass through wind speeds as the winds the building, the differapproached the building ential pressure from front and also measured the corto back is reduced, as are responding air velocities the forces on the building. within the building's four Moreover, such an approach openings. The model was confers environmental benethen rotated within the tunnel fits structurally in that it reduces to simulate what would happen the quantity of steel and concrete when the wind approached from that is required to maintain the all possible directions. building's stability. The results indicated that as the air The Pearl River Tower incorporates passes through the openings, the wind accelerfour large openings approximately 6 by 6.8 m, ates and the velocity increases. If the wind strikes one on either side of the mechanical floors at the building at a perpendicular angle to an openlevels 24 and 48. These openings function as a ing, the velocity will drop. But from almost every type of pressure relief valve for the building and other angle, the increase in wind velocity will also as a source of wind energy. Indeed, the facades of the structure exceed the ambient wind speeds. In most cases, the velocity increases have been designed to decrease the drag forces and optimize the wind should be more than twice the ambient wind speeds. velocity passing through these four openings. In particular, the broad Thus, placing one vertical-axis wind turbine within each of the sides of the structure will be aligned perpendicular to the prevailing four openings of the building will take advantage of the increased winds, which for most of the year are from the south, to create a power potential of the airstream. These wind turbines are low-vibrapositive pressure on the windward side and a negative pressure on the tion, low-noise units that operate within a wide range of wind direcleeward side. By contrast, most buildings are typically aligned so that tions and should provide power year-round. Therefore, the Pearl the narrower facades point toward the prevailing winds. River Tower not only should realize structure-related cost savings Given that the power potential from wind speed is a cube func- as a result of adding the four openings but also should be able to avail tion of wind velocity, the wind power potential at these four loca- itself of relatively free energy by harvesting the accelerated winds that tions should be maximized. Thus, a small increase in velocity can will pass through these openings. translate to a larger increase in potential power. Even a relatively Like an increasing number of buildings worldwide, the Pearl benign wind speed of 2 m/s should generate an energy output River Tower will integrate photovoltaic modules into the building approaching 8 m/s. envelope rather than include such devices simply as an extra feaThe Pearl River Tower will implement vertical-axis wind ture. The modules on the Pearl River Tower will serve both as the

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© 2008 American Society of Civil Engineers

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The cumulative benefit of all the environmentally beneficial strategies included in the design of the Pearl River Tower will significantly reduce the amount of energy needed to operate the building.

building's skin--in the form of spandrel panels--and as a source of the system not only provides electricity for the building but also power. By avoiding conventional spandrel panels, the incremental functions as a solar shade for the section of the building that is most cost incurred in incorporating the photovoltaic modules should be susceptible to the negative effects of direct solar radiation. reduced and the life-cycle cost improved. The cumulative benefit of all the environmentally beneficial Photovoltaic systems that are integrated into buildings often strategies included in the design of the Pearl River Tower will have lower overall costs than when they require separate, dedicated significantly reduce the amount of energy needed to operate the mounting systems, according to Building Integrated Photovoltaics: A Case building. The most notable reductions anticipated are associated Study, a report prepared in 1995 by Gregory Kiss, with the mechanical systems of the building, but The design of the Pearl River Jennifer Mahadev, and Raman Mahadev for the measurable savings should also be realized in the Tower initially envisioned the use of microturbine technology U.S. Department of Energy's National Renewcooling and lighting systems, as well as in the air to generate clean power in an able Energy Laboratory, in Golden, Colorado. and water delivery systems. efficient and environmentally The energy consumption of the building has Careful study of the expected solar radiaresponsible manner. Although tion on the Pearl River Tower revealed that the these plans were placed on indefi- been modeled and compared with that of a hyponite hold when the Guangzhou thetical baseline building that features the same photovoltaic cells would be most productive utility decided that it would not geometry but rather than utilizing similar enviif they were installed in certain locations on connect the microturbines to the the building's envelope. As a result, the cells local electrical grid, the building's ronmentally beneficial measures relies instead on basement can be retrofitted to the more established strategy of using air in place have been located in an asymmetrical arrangeaccommodate the devices should of a water-based radiant ceiling system to cool the ment at the roof level of the structure, where the utility change its stance. building. As the figure on page 44 indicates, the Pearl River Tower is expected to consume approximately 58 percent less energy on an annual basis than the hypothetical baseline building. Ground was broken for the project in August 2006, and construction of the Pearl River Tower proceeded through 2007 and 2008. The highest portion of the structure should be erected during the fourth quarter of 2009, and the building should be completed by October 2010. In developing the design for the Pearl River Tower, the scope of the services that som provided included architecture, structural engineering, and mechanical, electrical, and plumbing engineering up through the detailed design phase of the project. As required in China, som worked closely with a local design institute--in this case the Guangzhou Design Institute--to ensure that all the necessary local and statutory approvals were obtained with respect to planning, zoning, and building codes. Although many of the energy-saving strategies and technologies in the design of the Pearl River Tower were not new per se, their use in China has been limited. This fact, combined with the reluctance of the Chinese authorities to import existing technologies or manufactured goods from other parts of the world, meant that the performance criteria from projects in the United States or Europe were not easily transferred to this project. Furthermore, som's work was subject to peer reviews by leading experts in China. While the requirement

for these reviews is understandable, such assessments tend to be based more on theory than on practical experience. As a result, it was sometimes difficult to convince other project team members of the viability of the proposed solutions. Moreover, efforts to address the concerns of the Chinese clients by using examples from projects in the West often were not successful. The design team has learned numerous lessons from this project, including the critical fact that attempting to design and build a tower that does not require the city or region to generate any additional energy on its behalf is a formidable undertaking, especially when the tower is supertall and is located in a city with a notoriously unreliable electrical grid. It is clear that the Pearl River Tower would have been a challenge to design and construct even if it had been in London or Chicago. But this is especially true in Guangzhou, where the humid climate will rigorously test the high-performance facade as well as the radiant cooling and fresh air systems and the associated control systems. On the other hand, it is likely that most, if not all, of the building's components will be obtained from within China, substantially reducing the embodied energy consumption that has become prevalent in construction projects in the Western Hemisphere. Although ultimately the design for this project resulted in a structure that will not achieve a net zeroThe facade will feature an internally energy status, the process has ventilated double-wall system that provided strong analytical eviincorporates a motorized venetian dence that the original goal is blind system controlled by a photocell that tracks the movement indeed possible. When comof the sun. The exterior glazplete, the Pearl River Tower ing will take the form of will help China lead the way in insulated, tempered glass with a low-emissivity coatdeveloping supertall buildings ing; the inner layer will that use energy with the utmost be an operable clear glass n efficiency. panel that can be opened

for maintenance.

Roger E. Frechette III, P.E., LEED AP, is the director of sustainable engineering and Russell Gilchrist the director of technical architecture for Skidmore, Owings & Merrill LLP, of Chicago. This article is based on a paper the authors presented at the Council on Tall Buildings and Urban Habitat's 8th World Congress ("Tall and Green: Typology for a Sustainable Urban Future"), which was held in Dubayy (Dubai) in March 2008.

P roject c redits Owner: Guangdong Tobacco Company, Guangzhou, China Architect and engineer: Skidmore, Owings & Merrill llp, Chicago Wind tunnel testing and computational fluid analysis: Rowan Williams Davies & Irwin, Inc., Guelph, Ontario

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© 2008 American Society of Civil Engineers

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