专业英语(中英)可持续发展及结构工程师.docx

Sustainability and the Structural EngineerThe desire for high-performance, green, sustainable, and Leadership in Energy and Environmental Design (LEED)-certified buildings has become a major force in todays construction market. As the hype surrounding green construction evolves, the role of the structural engineer in contributing to sustainable building seems to have been diminished to that of simply being a specifier of construction materials rather than a designer of sustainable structures. The real issue for the structural engineering profession is determining what role the structural engineer should play as green building evolves into a collective design and construction consciousness. The answer is simple: the structural engineer needs to emphasize the contributions that good design decisions can make to the sustainability of a structure.Historical Perspective To look at the present and future of the green building movement, it is necessary to look at how it got to where it is in the first place. There has been so much talk of sustainable this and green thatnot just in the construction world, but in every facet of lifethat it is sometimes difficult to recall a time when green was not the hot topic it is today. But it was not always this way and it did not happen overnight. So when did this concern for the environment begin? Throughout history, there have always been those who have appreciated and respected the natural world, but ironically, instead of being immersed in nature, it may actually have taken a step awayto the tune of around 240,000 miles (386,243 km)for people to gain the perspective of earth as something precious and vulnerable. British astrophysicist Fred Hoyle identifies that moment as the first photos of earth taken during the Apollo 11 mission in 1969, saying, “Something new has happened to create a worldwide awareness of our planet as a unique and precious place. It seems to me more than a coincidence that this awareness should have happened at exactly the moment man took his first step into space” (Nelson 2009). A picture is indeed worth a thousand words.Since the 1960s, there has really been some sort of buzzword or phrase or concern associated with the environments and humanityswell-being. The discussion has evolved from air quality touching on personal health (pollution), to water quality (community health), to resource management (responsibility for the environment), to the term “green” (protection of the environment), to the current landscape sustainability (preservation of the environment). Looking forward, the next phase is likely to be stewardship, or management of the environment. Again, the point is that the environmental movement is constantly maturing.Involvement of the Structural EngineerA structural engineer may very well ask, “Well, why should this be important to me?” or perhaps (hopefully). ” Where do I fit into this picture?” and even, “What can I do?”The first question can be answered without getting into the specifics on whether global warming is a real issue or predicting a doomsday for the environment. Simply, there is an ever-increasing demand for green buildings. The domestic market has seen a major downturn in construction startsnonresidential starts in 2010 were down 16% from 2009 and down 64% from the peak in 2006yet demand for green construction has gone from an estimated 2.5% of new construction in 2005 to 12% in 2008 and 32% in 2010. And the demand is expected to jump by more than 40% by 2015. As a staff member at the Environmental Protection Agency once put it, the green buildings movement “isnt so much about jumping on board or being left behind as it is jumping on board or being run over.” Simply put, it is a movement that is not going away. As far as the role of the structural engineer in sustainable design, it is instructive to start with buildings and the environmental burdens they carry. Buildings in the United States account for nearly 40% of all energy use, compared with approximately 30% each for transportation and industry (and buildings are tied to both of those areas as well). U.S. buildings are also responsible for 65% of total domestic electricity consumption, 30% of total domestic greenhouse gas emissions, 136 million short tons (123 million t) of construction waste on an annual basis, and 12% of potable water use. The buildings structural engineers are involved in designing have a major impact in the areas of water consumption, waste generation, energy use, and environmental emissions.Certainly, there are building systems that have a much greater impact on the environment than the structural frame. Lighting and HVAC systems are responsible for most of a buildings energy use over time, whereas the structural systems impact starts relatively low and does not increase over life of the building. However, as HVAC, lighting, and other building systems become more and more energy efficient, the environmental impact of the structural framing system will actually increase relative to those systems.Structures, Materials, and Leadership in Energy and Environmental Design This trend is not unrecognized by the green buildings community. Every rating system, standard, and life-cycle analysis (LCA) tool includes the consideration of structural framing systems and materials. One may start with a rating system, Leadership in Energy and Environmental Design (LEED), which is the most recognized brand of green building programs. A growing percentage of the general public, and certainly most of the design community, is familiar with what the LEED program is, and there are countless presentations, articles, and websites available describing the inner workings of the program. In a nutshell, it is a prescriptive rating system that assigns points or credits (100 in all, plus 10 “extra credit” points) to the use of various green materials and practices in a building project. If a project gets 40 points, it is LEED certified, 50 points for LEED silver, 60 points for gold, and 80 points for platinum.The LEED Materials and Resources section focuses on three main areas: recycled content and reuse of materials, regional materials, and biobased content. Designers of LEED structural steel-framed projects have accumulated a Recycled Content point or two as the result of steels high recycled content. Steel may have also contributed to gaining Regional Materials credits for the percentage that was recovered and manufactured within 500 miles (805 km) of the project site, along with the contribution of other materials, such as concrete. LEED projects with wood framing systems typically gain Certified Wood and/or Rapidly Renewable Materials credits. And if a building retains major portions of the framing system from its previous life or employs salvaged materials from elsewhere, Building Reuse or Materials Reuse credits can be earned. Finally, there is also a credit for Construction Waste Management. (Interestingly, this one gives points for diverting construction site waste from the landfill but not for minimizing site waste in the first place.)In essence, what LEED does in the Materials section is to put together a list of material considerations and assign thresholds to them (e.g., achieving 10% recycled content gets one point, 20% gets two). In doing so, it has created credits that can bring out the best in all major structural materials, as all materials have their own sustainable qualities. For example, with respect to structural steel, the LEED credit for Recycled Content encourages the use of a highly recycled product that has several very attractive sustainable characteristics:1.U.S.-produced structural steel has an average recycled content of 93.3%, the highest of any building framing material, and a recycling rate of 98%also the highest of any building framing material. It has a high strength-to-weight ratio compared with other framing materials. It has a significant potential for material reuse; whereas other products can only be recycled into a lower-quality product, steel can be recycled over and over again and remade into new members without any loss of quality (AISC 2010a).2. The steel production process practices superior water resource management: 95% water recycling rate with no external discharges, resulting in a net consumption of less than 70 gal.per short ton (290 L=t).3. The steel industry has increased productivity by a factor of 24 since 1980, reducing labor hours per ton from 12 to 0.5. It has increased material strength by 40% since 1990 36 to 50 ksi(248,211 to 344,737 kPa), decreased energy use by 67% since 1980, decreased the carbon footprint of steel (per ton) by 47% since 1990, decreased energy intensity (per ton) by 29% since 1990, and earned an EPA best industry performance designationAmerican Iron and Steel Institute (AISI 2008).4. Structural steel is fabricated regionally in off-site facilities and erected on-site, resulting in minimal waste generated onsite (and any waste that is generated is fully recyclable and resalable).5. Steel buildings are easily deconstructed, enabling reuse of the steel members. Steel framing allows easy integration of mechanical systems, resulting in low floor-to-floor heights, less building volume, and lower energy consumption. It also allows for large window areas, resulting in plentiful natural lighting, higher occupant comfort, and reduced electrical consumption. In addition, exposed steel removes the need for architectural finishes and their associated environmental impacts.6. The bulk of steels carbon footprint is tied to electrical use; there is a tremendous opportunity to reduce this footprint as the electric utility grid becomes more renewable.The LEED system does not specifically provide credit for all of these factors, and even the factors it does consider are not fully considered. The Regional Materials (MR-5) credit is a good example of this. This credit awards one point if 10% of the building materials are recovered/harvested and manufactured within 500 miles of a project site. But in attempting to simplify the application of such a credit, LEED fails to recognize several issues involving regional materials:1.The supply chain for cradle-to-cradle (or closed-loop manufactured) materials is different than that of a cradle-to-grave material. The credit, as written, only gives consideration to the source of material used in the manufacturing process; it does not consider the end of life of the product. If a cradle-to-cradle product can be recycled into the same or similar product within 500 miles of its “disposal,” then the same benefit is gained as if a new product were manufactured from material sourced within 500 miles of the new project site. Encouragement of the sustainable benefits of the utilization of cradle-to-cradle material would be gained by adding language recognizing the end-of-life use of the material and documented by current consumption patterns of similar recycled materials in the projects locale. For example, although there is not a structural steel mill within 500 miles (805 km) of every project site in the United States, there is an electric arc furnace mill (which uses recycled steel scrap to create new steel products) within 500 miles of virtually every point in the lower 48 states, and each one of these mills collects its scrap within a radius of roughly 400 miles (644 km). In other words, scrap flows to the point of its most efficient use.2.A significant amount of confusion has existed regarding the interpretation of the term “manufactured” in the current LEED MR-5 credit. The issue has been whether all manufacturing stages relative to the final product must occur within 500 miles (805 km) of the project site or whether only final manufacturing must occur within 500 miles (805 km) of the project site. This confusion has been heightened by allowing fractional credit calculated by weight. A simple solution to this confusion would be to allow the fractional calculation to be performed on either a weight or cost basis. In that way, if raw material sourcing and final manufacturing accounted for 70% of the cost and occurred within 500 miles (805 km) of the project, generating local employment and economic benefits, then 70% of the products value (reduced, if not all the raw material sourcing was from within 500 miles of the project site) would be included in the final calculation of regional content.3. The draft credit is intended to support local communities and arbitrarily defines local communities as being 500 miles (805 km) from the project site. But in many regions of the country, the raw materials required for product manufacturing are not available from within a 500-mile (805-km) radius of the project site. The extraction, harvest, or recovery of these raw materials may represent only a small percentage of the labor requirement to produce the final product. The unintended consequence of the draft credit is to discourage local manufacturing in favor of large central manufacturing facilities close to the sourcing of the raw materials, thereby negatively impacting local communities.4.The Commerce Clause of the U.S. Constitution guaranteesthe free flow of trade between states. The adoption of LEED requirements by a governmental jurisdiction for nongovernmental projects may trigger a legal challenge on the basis of the implied restriction on the sourcing of material from beyond a 500-mile (805-km) radius. Courts in numerous cases have struck down regional trade restrictions even if they involve only a portion of the overall product supplyDean Milk Company v. City of Madison, Natl. Solid Waste Mgmt Assoc. v. Charter Co of Wayne, and Bremmer v. Rebman, to name a few. In the Bremmer v. Rebman case, the court held that “any local regulation, which, in terms or by its necessary operation, denies the equality in the markets of a State is, when applied to the people and products or industries of other States, a direct burden on interstate commerce among the States, and, therefore, void.”5. Although the intent of the credit has been modified to focus on the support of local communities, the language still references the 500-mile (805-km) transportation distance in the original MR-5 credit. This 500-mile (805-km) radius does not take into account various modes of transportation such as rail or water, which are approximately four times more efficient than truck transportation. And on top of that, even the designated distance500 miles (805 km)is debatable.Of course, this is from the perspective of the steel industry. Other materials might have similar or different complaints or compliments regarding various credits. The point, again, is that the system is not perfect in what it does and does not include, nor is it perfect in terms of weighting the various credits and aspects against one another. That said, LEED has been integral to bringing the environmental impact of buildings to the forefront, encouraging more environmentally friendly design and construction practices, and generally pushing the green buildings movement forward.Material Selection or Frame Design?The result of the methodology employed by LEED is that structural engineers have focused on material selection to gain credit points to assist the overall project to gain a higher level of certification. Certainly, the proper selection of structural materials is not a bad thing, but it has resulted in a shortsightedness on the part of some structural engineers when it comes to sustainability. A sustainable structure is not a structure that has the most sustainable materials used in it, but rather it is a structure in which use of the selected materials is optimized in the design process for maximum sustainable benefit. For a structural engineer, sustainability should be more about design than material selection.The LEED program itself is recognizing this as it evolves. The next version of LEED guidelines is due out in 2012. In the first draft of that version, the Regional Materials section was altered to no longer apply to structural materials, and the Recycled Content credit for structural materials was changed to become a prerequisite. The second draft has been further modified to reintroduce these credit opportunities to an overall structural section as one credit path in a list of multiple options. It remains to be seen how the final draft will address structural materials. Regardless of what occurs in the next version of the LEED system, the role of the structural engineer has always been and should continue to be that of a designer that optimizes structural framing systems, not just a specifier of materials.Life-Cycle AnalysisOne way this is happening within LEED is through the inclusion of LCAs within the LEED credit system. Historically, LEED has taken a prescriptive approach to sustainability; LCAs take an analytical one. Traditionally, LCAs quantify the environmental impact of a product like, say, structural steel. They are especially useful when it comes to building materials because unlike ongoing building systems, once building materials are installed, their environmental footprint is established. Secondary impacts on building operations, such as thermal mass and thermal bridging, typically are treated as part of the building system.At the beginning of an LCA study for a product, a functional unit is determined; in terms of steel, one ton is an appropriate functional unit. Next, a life-cycle inventory (LCI) is performed on the basis of the functional unit. An LCI is basically an environmental inventory of the product; it provides the inputs and outputs of making the product between defined boundaries within the supply chain. For structural steel, the product LCI is typically considered from the collection of scrap to the outbound gate of the mill when the product ships to