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Intended Audience and Subject Matter
The National Institute of Building Sciences (NIBS)–under contract from the Army Corps of Engineers, the Naval Facilities Engineering Command, the US Air Force, the General Services Administration, the Department of Energy, and the Federal Emergency Management Agency—has developed this comprehensive federal guide for exterior envelope design and construction for institutional/office buildings.
The scope covers buildings constructed of steel, reinforced concrete, reinforced masonry and reinforced concrete masonry units and includes low-rise, mid-rise and high rise buildings. Typical buildings include administration (office) buildings of all sizes, from a small one-story base administration building to a twenty-story inner city agency office facility. Other building types include firehouses and police facilities, courthouses, military residences, many types of laboratories, various types of education buildings, hospitals, extended care facilities, clinics and many types of recreational buildings. Special use buildings such as airplane hangers, testing facilities, and stadiums, single family residences and wood frame structures are not included.
Though specifically intended for Federal Government agency projects, the information in the guidelines will also be applicable to many privately developed projects—whether of a commercial or institutional nature—which are essentially similar in use and construction to equivalent governmental structures. Because the guidelines are intended for use in the design of governmental structures, the intent is to provide a long-lived structure based on lifecycle costing since governmental ownership is typical in perpetuity. Thus a high standard of construction and maintenance is advised to achieve the aims of the agencies involved.
This is the first time a group of Federal agencies has developed a set of guidelines to be used for the design and construction of their buildings. Its publication and use is meant to assist in the development of uniform design and construction criteria for the Federal government. Instead of taking form as a printed document, which would be revised at long intervals, the Guide is made freely available as a "virtual" information source on the World Wide Web within the Whole Building Design Guide. It is anticipated that government agencies will devise methods of using the Guide to create their own "customized" documents to suit their building types, locations and administrative needs and to further their individual design and construction goals.
Private owners and their designers are free to use the Guide as a resource and can develop their own customized documents or simply refer their designers to useful sections of the Guide. The Guide is not a building code and does not attempt to specify mandatory criteria. Instead it provides design oriented information meant to assist designers in making informed choices of materials and systems to achieve performance goals in their buildings.
The Guide will be a "living" information source that will continually expand and change. It will be interactive, allowing users to enhance the content by adding resource papers, reporting on experiences, and helping to maintain a dialog with Guide management.
The details associated with this section of the BEDG on the WBDG were developed by committee and are intended solely as a means to illustrate general design and construction concepts only. Appropriate use and application of the concepts illustrated in these details will vary based on performance considerations and environmental conditions unique to each project and, therefore, do not represent the final opinion or recommendation of the author of each section or the committee members responsible for the development of the WBDG.
Richard Rush, in his book The Building Systems Integration Handbook, defines a building in terms of only four systems:
In this categorization, "The envelope has to respond both to natural forces and human values. The natural forces include rain, snow, wind and sun. Human concerns include safety, security, and task success. The envelope provides protection by enclosure and by balancing internal and external environmental forces. To achieve protection it allows for careful control of penetrations. A symbol of the envelope might be a large bubble that would keep the weather out and the interior climate in."
Dr. Eric Burnett and Dr. John Straube, in a number of writings, have also described the envelope in terms of performance and function. According to them, the envelope "experiences a variety of loads, including, but not limited to, structural loads, both static and dynamic, and air, heat and moisture loads." The enclosure must then support structural loads and control environmental loads, which include both long-term and short-term loads. The enclosure is also often used to carry and distribute services within the building. In addition, the envelope (primarily the wall) has several aesthetic attributes that can be summarized as finishes. (This description of the envelope is expanded in the wall section of this Guide.) Thus the systems and assemblies of the envelope are one of the four main building systems both in terms of their physical existence and in their contribution to overall building performance.
For this guide the building envelope also includes the below grade basement walls, foundation and floor slab (although these are generally considered part of the building's structural system) so that the envelope includes everything that separates the interior of a building from the outdoor environment. The connection of all the nonstructural elements to the building structure is also included. Finally, it is recognized that the exterior envelope plays a major role in determining the aesthetic quality of the building exterior, in its form color, texture and cultural associations.
The following Building Envelope Systems are covered in separate chapters:
- Below Grade construction
- Exterior Walls, both structural (providing support for the building) and nonstructural (supported by the building structure)
- Fenestration, both windows and metal/glass curtain walls
- Roofs, both low- and steep-slope
This guide demonstrates that the design of the envelope is very complex and many factors have to be evaluated and balanced to ensure the desired levels of thermal, acoustic and visual comfort together with safety, accessibility and aesthetic excellence. As can be seen from the list of functions of the building envelope detailed in the Function and Performance section, the envelope plays a role in almost every building function, either directly or indirectly in its relationship to other building systems.
Figure 1 illustrates the envelope systems of a typical building.
The guidelines for each of these systems are authored by an expert in the subject matter and are presented in the following uniform format:
- Introduction—An overall description of the system
- Description—The physical elements and properties of the system
- Fundamentals—A discussion of the design objectives to achieve ranges of performance
- Applications—The range of applications for building functions and external conditions
- Details—Generic construction details in CAD format with commentary
- Emerging Issues—A survey of significant current research and development
- Codes and Standards—Code philosophy and limitations and a summary of currently applicable codes and standards
- Resources—References, major publications, industry and professional associations, and Web sites
In addition, the following performance issues are examined for each of the envelope systems:
- Thermal performance
- Moisture protection
- Fire safety
- Daylighting and perimeter visual environment
- System maintainability
- Material durability
Beyond these major performance issues the following more specialized building performance topics are covered by separate authors in concert with the principal system authors and, where appropriate, integrated into the main text for each system:
- Seismic safety
- Safety against blast and chemical, biological and radiological (CBR) attack
- Safety against extreme wind
- Safety against flood
- Indoor air quality and mold prevention
- Sustainability and HVAC integration
The Whole Building Design Guide
The Building Envelope Design Guide is one of a series of guides in the Whole Building Design Guide (WBDG) that are intended to assist designers in the integrated design of assemblies and systems. As such, the Building Envelope Design Guide follows the general format of other guides in the WBDG and are internally linked to Resource Pages and other levels and sections of the WBDG.
The Whole Building Design Guide (www.wbdg.org) is an evolving Web-based resource intended to provide architects, engineers and project managers with design guidance, criteria and technology for "whole buildings". Accordingly, the WBDG covers the whole range of today's issues in building design, such as sustainability, accessibility, productivity, and safety—both from human and natural hazards. The WBDG is constantly augmented with updated and new information and is structured as a "vertical portal", enabling users to access increasingly specific information as they navigate deeper into the site.
The concept of the Whole Building Design Guide has been formalized to achieve four main goals:
- To simplify access to government and non-government criteria and standards information using a web-based approach so that valuable time is not wasted searching for this documentation.
- To replace outdated, redundant paper-based criteria documents.
- To guide managers and A&E firms through a "whole building" approach to building design so that high performance, longer-lasting buildings are produced.
- To provide a brief, up-to-date and informative resource that covers general and specific topics in an encyclopedic form. However, unlike a traditional encyclopedia, the WBDG enables the user to build up a private store of relevant information by direct links to other resources available on the Internet with a few mouse clicks.
The WBDG has become a primary gateway to up-to-date-information on a whole range of Federal and private sector building-related guidance, criteria and technology from a "whole building" perspective. Users are able to access information through a series of "levels" by way of three major categories: Building Types, Design Objectives, and Products and Systems. At a lower level are Resource Pages, which are succinct summaries on particular topics written by experts. Pages within the WBDG are cross-linked to each other, and hyperlinked to external Web sites, publications and points of contact, allowing easy access to related information. Agency-specific information is accommodated in a Participating Agency section. Other features include relevant Federal mandates, news headlines and a robust search engine.
The WBDG Web site is provided as an assistance to building professionals by the National Institute of Building Sciences (NIBS) through the funds and support of the NAVFAC Criteria Office, the U.S. General Services Administration (GSA), and the Department of Energy through the National Renewable Energy Laboratory (NREL) and the Sustainable Buildings Industry Council (SBIC). A Board of Direction and Advisory Committee, consisting of representatives from Federal agencies, private sector companies and non-profit organizations guide the development of the WBDG.
The Evolution of the Building Envelope
The Building Envelope Design Guide will constantly evolve, with its users participating in this evolution rather than simply using a set of fixed, definitive guidelines. They will thus be advancing the evolution of the building envelope itself. The first building envelope that protected humans from the elements was probably a cave that provided a degree of privacy and security. The earliest building envelopes were dome-shaped structures that combined wall and roof (Figure 2A). At an early stage, however, the two dominant forms of envelope evolved, depending on climate and available materials: the timber frame and the masonry wall (Figure 2B and 2C). Early shelters in the warm climates of Africa and Asia used timber or bamboo frames clad with leaves or woven textiles. In other regions and climates heavier indigenous materials such as stone, rock and clay baked by the sun were used to provide more permanent shelter and protection from the heat and cold (Figure 2D).
To this day rural regions in lesser developed countries still construct these forms of shelter. In the developed world we still use envelopes of timber frame and masonry walls, although both have evolved into a wide range of materials—some natural, others synthetic. Roofs evolved independently as waterproof elements with their own set of materials.
Thus, eventually the roof, wall and floor became distinct elements of the building envelope that have continued to this day with very little change in concept, use and even material. A medieval dwelling might have walls of wood, brick or stone, a wood roof structure, a slate tile or thatch roof and a floor of stone or hardened dirt. Such a dwelling can still be found today in many regions of the United States and the world.
To take one element of the envelope, the wall, its basic performance requirements have remained the same from medieval times to this day: protection of the interior from the elements and security for its occupants. However, our expectations have vastly increased, both in terms of absolute performance and the ability to control the impact of exterior water, sunlight and the ambient outside temperature on our interior environment. Depending on a society's structure and economy, such needs as degree of permanence of our exterior system, its scale and adornment and our desire to have a wide variety of exterior envelope choices may also vary considerably.
Compared to most of today's walls the medieval or renaissance masonry wall was simple. Initially the wall was a single homogeneous material—stone or brick-exposed on the exterior and interior. Such walls are still constructed today, although the wall is more likely to be reinforced concrete or concrete block. Before long, the historic wall would become adorned: a rough structural stone would be faced on the exterior with a precisely cut and fitted facing of fine stone or marble, and the interior would be faced with smooth plaster (Figure 3).
As soon as the structural wall became faced with different finish materials the beginnings of variable performance capability emerged. The separation of the structure and facing presaged the layered exterior wall of today. The structure of rough cut stone could rise independently from its facing, which could be prefabricated in the craftsman's shop. The high-quality facing and its detailing also provided protection from the weather for the structural masonry within (Figure 4).
Historic buildings and even historic revival buildings accomplished many of the building envelope functions by default: thick, heavy masonry was fireproof and good for insulation in both summer and winter. It was excellent acoustically and, with sheltering roofs and good water protective details, was reasonably watertight and draft free by the standards of the time. However, by modern standards, the wall had fixed and limited performance capabilities.
The big change in the concept of the wall—and the real beginning of today's concept of the building envelope—occurred with the invention of the steel, (and later, the reinforced concrete) frame in the nineteenth century. The exterior wall could become a screen against the elements and no longer be needed to support the floors and roof. However, for several decades steel frames were buried in masonry walls, and buildings continued to be designed in gothic or renaissance styles. The modern architectural revolution beginning in the early 20th century changed this and by mid-century the steel or concrete framed office building with its lightweight metal and glass curtain wall had become the new world-wide vernacular for larger commercial and institutional buildings.
When the wall became a nonstructural screen-in and no longer supported the upper floors and roof, it lost the beneficial attributes of mass but gained in providing performance options. Whole new industries arose that would develop insulation and fireproofing materials, air and moisture barriers and interior and exterior facings. More recently the exterior wall has become a major subject of building science studies, largely because of the wall's key role in managing heat gain, heat loss and moisture penetration. As a result, the modern wall now consists of a series of performance "layers" (Figure 5).
A cross section of a typical layered exterior wall of today illustrates the complexity that this approach leads to in practice (Figure 6).
A different material may achieve each performance requirement, with each performing a separate function, or some materials may perform multiple functions. For example, the air barrier may simply be a coating on a support layer.
Function and Performance
The complexities of today's wall can also be exemplified by listing its functional requirements, or needs that must be met. There are at least 13 distinct needs, as listed below. Most of these functions also apply to fenestration and the roof and a few also apply to below grade construction (see Table 1 in Section 7). Each function (with a few exceptions) has its own performance standards and methods of measurement, methods of testing for compliance, and acceptability criteria.
- Structural: If the wall is not part of the main building structure, support own weight and transfer lateral loads to building frame.
- Water: Resist water penetration.
- Air: Resist excessive air infiltration.
- Condensation: Resist condensation on interior surfaces under service conditions.
- Movement: Accommodate differential movement (caused by moisture, seasonal or diurnal temperature variations, and structural movement).
- Energy conservation: Resist thermal transfer through radiation, convection and conduction.
- Sound: Attenuate sound transmission.
- Fire safety: Provide rated resistance to heat and smoke.
- Security: Protect occupants from outside threats.
- Maintainability: Allow access to components for maintenance, restoration and replacement.
- Constructability: Provide adequate clearances, alignments and sequencing to allow integration of many components during construction using available components and attainable workmanship.
- Durability: Provide functional and aesthetic characteristics for a long time.
- Aesthetics: Do all of the above and look attractive.
- Economy: Do all of the above inexpensively.
Performance refers to the desired level (or standard) to which the system must be designed for each of the above functional requirements. For example, what dimensions of movement must be accommodated, and what is the expected useful life, or durability, of the system.
The building envelope represents a substantial percentage of a building's cost. (Some typical costs presented as a percentage of the whole building cost are shown in Table 1, below.) For a multi-story building, the envelope costs (dominated by the cost of walls and fenestration) may even exceed 20% of the total building construction cost.
The envelope is also a critical system in the determination of the overall performance of the building, with an emphasis on the thermal environment, lighting and acoustical characteristics. It is the prime determinant of the exterior aesthetic quality of the building. Clearly, the balance between the cost of the building envelope and its levels of performance will be of great importance in achieving the most cost-effective design of a building.
|Table 1. Typical building envelope costs as a percentage of whole building cost|
|Building Type||Floor Slab||Exterior Wall||Roofing||Total|
|JR. High School,
(Source: Means Square Foot Costs)
The Building Envelope Design Guide does not attempt to provide cost information for estimating purposes. There are a number of reasons for this:
- Construction costs fluctuate and rapidly become out of date. Published indices attempt to ameliorate this problem but they tend to be very broad in scope and are of limited use in application to a specific project. Also the state of the local market at the time of bidding and construction often has a major effect on cost.
- Construction costs vary greatly according to the project location. In broad terms, in the U.S. there is approximately a 200% spread between the least and most costly states in terms of construction cost.
- Very rapid change in cost—generally upward—in such key materials such as steel and lumber are common and cannot be predicted in advance.
- Government agencies and other institutions and businesses that manage large construction programs generate detailed cost information that refers particularly to the type and quality of the buildings they construct. This forms a valuable database that is specific to the owner's location and needs. Architectural and engineering firms and contractors also develop cost data that applies to their projects.
- Specialty construction cost estimating and management firms develop very large and detailed databases on cost because they are focused entirely on cost management issues. On any significant project they should be employed from the outset as part of the design team with responsibilities for cost management.
For these reasons, the developers of the Building Envelope Design Guide believe that cost management should be based on local information procured before the design process for budgeting purpose and during the design process for cost management purposes. The use of life-cycle costing, economic analysis and value engineering can be used to the extent that they suit the owner's economic goals. Clearly an agency or institution that expects to own a building for its entire useful life is well advised to budget on a life-cycle, and many government agencies now require that this be done.
Private owners may have other aims, but the ultimate building operators will all benefit from a building in which life-cycle costs have been considered. A future in which it is clear that energy costs must be expected continually to rise—with the possibility of energy itself becoming scarce within today's building lifetime—makes it the more necessary to design for minimum energy usage consistent with meeting functional and environmental needs. Building envelopes designed with opportunities for the use of day lighting and natural ventilation obviously can play a major role in such considerations.
Function, Performance, Design and Construction Relationships
The systems and elements that comprise the building envelope are each discrete assemblies that, in many instances, are designed by specialist consultants and installed by specialist sub-contractors. The mechanical system, for example, will be designed by a specialist engineering consultant and installed by several different mechanical sub-contractors. Some assemblies have a variety of methods of design and procurement. A metal and glass curtain wall may alternatively be a proprietary catalog assembly, a custom assembly designed by the architect with input from manufacturers or a consultant, or designed by a consultant to meet the architect's requirements. Each system, however, also has functional, performance, design and construction relationship to others.
Functional performance for the thermal environment, moisture protection, sustainability and durability are shared by all 4 of our major subsystems: walls, fenestration, roofing and below grade. Thus each has to be designed to contribute its appropriate share of the overall functional effectiveness in meeting the performance requirements for the whole building. Acoustic performance for the exterior is the responsibility of the wall system and, to a lesser extent, the fenestration, while daylight transmission and control is the responsibility of the fenestration and the roof (if there are skylights, although these will be designed by the fenestration designers). Natural ventilation, if provided, will be a fenestration design problem but will also have major repercussions on the HVAC design. If the HVAC system employs perimeter heating or cooling this must be integrated with the envelope performance requirements. Interior air quality is primarily an HVAC issue, mainly concerned with outside air supply and filtering. The exterior wall will also have some performance requirements relating to materials and permeability.
These relationships and some others are "flagged" in the matrix shown in Tables 2 and 3. Table 2 shows the list of basic performance requirements that are covered in this guide. Table 3 shows the list of secondary performance requirements that are covered. In addition, aesthetics is shown as an "influence". Unlike the other performance requirements, aesthetics is not subject to scientific testing and measurement. Nonetheless—particularly for the wall and fenestration systems of the building envelope—it is a powerful influence in the system and material selection process. The attributes of color, texture and pattern are familiar. The attribute of "association" refers to issues such as the use of stone to represent solidity and permanence (besides its possible measurable attributes of durability), or the desire of some colleges to require red-tile roofs on new campus buildings.
The group of practice considerations refers to issues that appear in all systems and are critical to the successful implementation of the whole project, from concept to commissioning. Innovation refers to emerging methods, materials and processes that may improve the performance, cost or appearance of a system as a whole or any element of it.
X Major determinant or influence
(X) Minor determinant or influence
|Table 2. Function and Performance Relationships of the Building Envelope: Basic Performance Requirements|
|BASIC PERFORMANCE REQUIREMENTS||Thermal||X||X||X||Wall and glazing insulation and relative quantities largely determine thermal performance in medium and high rise buildings (small footprint), roof more influential in low rise (large footprint).|
|Moisture Protection||X||X||X||X||All systems important, particularly glazing interface with walls, roof interface with skylights.|
|Acoustics||X||(X)||Protection against outside acoustic environment. Walls major determinant, with glazing a secondary influence.|
|Light Transmission||X||(X)||Glazing major determinant, both quantity and location: roof may be important if skylights are provided.|
|IAQ||(X)||(X)||HVAC system is major determinant, together with natural ventilation (operable windows) if provided. Wall openings (grilles) must provide outside air supply to HVAC system.|
|Mold Protection||(X)||(X)||HVAC system is major determinant, together with natural ventilation (operable windows) if provided.|
|HVAC Integration||X||X||(X)||Wall and glazing insulation are critical in determining HVAC loads, together with roof if large footprint building.|
|Natural Ventilation||X||Operable glazing is traditional way of providing natural ventilation, but coordination with HVAC system is essential to ensure energy efficient system.|
|Durability||X||X||X||X||All systems contribute to overall durability of building.|
|Sustainability||X||X||X||(X)||Materials and performance of walls, glazing and roof have major influence: below grade construction may contribute but few alternative are available for design and construction.|
X Major determinant or influence
(X) Minor determinant or influence
|Table 3. Function and Performance Relationships of the Building Envelope: Safety, Aesthetics, Practice and Innovation|
|SAFETY REQUIREMENTS||Fire Protection||X||X||X||Wall, glazing and roof materials are critical and strongly regulated by code. HVAC and fire protection systems are key systems for smoke and fire reduction and elimination.|
|Floods||X||(X)||X||Below grade and first floor wall construction are critical but building location is the key determinant. Protection of low level glazing is important.|
|High Winds||X||X||X||Building envelope is particularly vulnerable to high winds because wind action attacks the building exterior surfaces.|
|Seismic||X||X||(X)||X||Building structure is main defense against earthquakes. Heavy nonstructural precast wall panels and glazed curtain walls may need special attachment detailing to structural frames subject to large racking deformations in high seismic zones. Heavy roof materials (tile) are vulnerable and need positive attachment.|
|Blast, CBR||X||X||X||X||Building structure is main defense against blast: non-structural wall panels and glazing need special design and attachments to structure, roof may contribute debris due to suction effect. HVAC main protection against CBR.|
|AESTHETIC INFLUENCES||Aesthetic||Aesthetic issues primarily relate to building exterior (public view) and interior of exterior wall ( occupant view).|
|Color||X||X||X||Related to materials: natural (stone etc) metals (metallic or painted finishes) and availability of a range of alternatives.|
|Texture||X||X||X||Smooth (metals), smooth to rough (natural materials, such as marbles, granites etc) and cast textures on concrete.|
|Pattern||X||X||X||Primarily related to joints between panels and glazing, and the size and shape of panels.|
|Associations||X||X||X||Local context and cultural associations, e.g. natural stone versus stucco.|
|PRACTICE CONSIDERATIONS||Cost||Pervades the choice of all materials and systems.|
|Material||X||X||X||X||Cost and availability of materials.|
|Installation||X||X||X||X||Cost (and time) of installation.|
|Life Cycle||X||X||X||X||Evaluation of cost and performance in relationship to owner's (and society's) needs and resources.|
|Codes & Standards||X||X||X||X||Pervades the use of almost all materials and systems.|
|INNOVATION||Innovation||Includes the need for innovation and trends in innovation, through research and market forces initiated by codes and regulations e.g. developments in glazing technology to respond to energy conservation codes.|
|Performance||X||X||X||X||Performance enhancement is a continuing goal, often related to marketing.|
|Cost||X||X||X||X||Strongly driven by competition induced by bidding process.|
|Aesthetic||X||X||X||A strong driver for exterior materials and finishes, glazing and steep slope roofing.|
The functional and performance relationships shown are also accompanied by physical connectivity. Many of the building envelope assemblies are connected to and supported by the building structure. Since the envelope receives certain loads—such as wind and seismic, and, in some instance—blast, its members must be capable of distributing these loads to the building structure besides resisting them within their own subsystems. Obviously, if the HVAC system employs perimeter heating and cooling, the distribution system will be part of the overall building HVAC distribution. The physical relationship of the roof assemblies to the structure is critical to their performance.
These relationships mean that the building envelope cannot be designed in isolation. It is a function of design management to ensure that the performance attributes and the physical connections between the envelope and the rest of the building are integrated in concept and execution from the commencement of the design process.
Safety, Security and Building Codes
Safety has long been the traditional focus of building codes, starting with the earliest codes related to fire safety. Protection against earthquakes has been the subject of codes in seismic regions for the last three quarters of the twentieth century. These codes have become very highly developed and have a strong engineering and scientific base assisted by governmental, institutional or industrial programs of research and development. Some codes also mandate requirements for floods and high winds, but these are much less developed than those for earthquakes.
In general, the intent of codes has been to mandate minimum standards that provide acceptable safety at an affordable cost. While codes also provide some property protection, this has not been a major target and has not been a stated intent of the codes. Many features of building codes are understood to represent a minimum standard and in practice are almost always exceeded for reasons of comfort and amenity. For example for many decades all U.S. building codes have agreed that the minimum floor area of a bedroom is 70 square feet with a minimum dimension of 7 feet in any horizontal direction; however, few bedrooms in even the most economical tract house are of this size.
Codes that do not appear to offer an accepted everyday benefit—and that are seen as adding cost to the project—are all too often treated as a maximum. This has created problems when buildings correctly designed and constructed to a seismic code have suffered significant damage—which the code permits, provided that life safety is not compromised. In part owing to the success of codes in protecting public safety, attention has begun to shift toward property protection because of the large economic losses incurred by earthquakes, high winds and floods. This loss is only partly due to the cost of repair and reconstruction; deeper economic losses are incurred by business and service interruption and losses such as market share and tourism.
As a result, designers are being encouraged to create designs for performance against natural hazards that are appropriate for a building's occupancy, function and importance. This means designing for an acceptable level and type of damage. This also generally means designing for performance above that anticipated by normal code-conforming design.
Another development in design against natural hazards has been the encouragement of multi-hazard design. The intent here—as in many other developments in the design process—is to implement a higher level of design integration. This involves ensuring that buildings subject to more than one natural hazard—for example both floods and earthquakes—take advantage of design methods that assist in countering all the hazards that apply and identify and find solutions for those instances where measures may conflict. Thus while elevating a building above grade is an excellent solution for design in a flood plain, great care must be taken to avoid "soft first stories" that have proven disastrous in earthquakes.
While building safety has traditionally focused on natural hazards, building security is focused on societal and political hazards—those created by criminal or disaffected members of our own society or foreign elements that see the U.S. as an enemy. Some degree of concern has always been present in design, of which keys and locks are the most familiar. Remote surveillance systems have been around for some time in retail stores and other sensitive environments. The tragedy of September 11, 2001 and the condition of war that has existed since that time have thrown a much more intense light on the need for building security.
The major difference between attack hazards and natural hazards—beyond a deliberate attempt to damage the building and cause casualties—is that of probability. Although natural hazards vary greatly in probability there is considerable statistical information on the issues of where, how big and how often a given hazard will occur, and considerable scientific study is devoted to this topic. Human-caused hazards have very little history, and the relatively few instances provide a very poor statistical database. As a result, it is at present very hard to know the extent to which it is prudent to take steps to mitigate attack by blast and/or chemical, biological and radiological (CBR) weapons. Meanwhile, much research and development is underway, ranging from testing vehicle barriers to the development of building code provisions that reduces the risk of progressive collapse from damage as a result of bombing.
This is an area that may be expected to evolve rapidly. Some of this evolution will be technical. Some will be societal and political as it begins to become apparent how large and how long-term the threats remain. The questions of where, how big and how often need much more convincing answers than are available at present before the full impact on building design can be gauged. Meanwhile, this subject is superimposed on the traditional range of issues with which the designer must grapple. Again, the need is for design integration with of security concerns considered from the outset.
Although many historic materials are still in use—roofs are still constructed of copper and slate, and walls employ natural stone—the building envelope has been a prime target of innovation since the first quarter of the twentieth century. Innovation has been most significant in the wall and fenestration systems of the envelope and has been driven by four main influences:
- Cost reduction for a competitive market
- Enhanced performance
- Material innovation and industrial research & development
These influences are all related to one another. Much industrial research and development has been aimed at obtaining a competitive edge through performance improvement or cost reduction. For example, pre-cast concrete fabrication enjoyed about two decades of great success because the material and shapes appealed to architects. Innovation in pre-casting techniques, form design and fabrication and surface finishes resulted from collaboration with architects and the effort to be competitive as a supplier. Ultimately, however the pre-eminence of the pre-cast panel fa¸ade was seriously threatened by the rise of glass fiber reinforced concrete, a synthetic material innovation that was lighter and more economical than concrete and more easily formed into sculptured shapes.
The construction industry is intensely competitive. Much of this stems from the traditional approach to contractor and supplier selection—the competitive bid. This places a premium on lowest cost, resulting in innovation by contractors and sub-contractors trying to get an edge on their competitors. Another aspect of cost is that of reduction in construction time, which translates into cost reduction for a building project's entire stakeholder. Thus, for example, the separation of the building envelope wall and fenestration from the structure enabled the structure to be erected faster, while prefabricated components such as curtain wall assemblies and pre-cast panels were fabricated off-site.
The effort to reduce on-site labor through componentization also originated in the effort to reduce costs and construction time. It is noteworthy that the building industry was able to achieve extraordinary feats of construction time using traditional materials when labor costs were low. For example, the Empire State Building in New York City was constructed in just over 12 months—at the height of the great depression—by laborers working 24 hours a day for a contractor who used the first fast-track scheduling process. A booming construction industry and post-war labor costs soon made such an approach prohibitive.
Aesthetics has also been a powerful influence on the envelope. The most significant application—that of the development of the curtain wall—depended initially on the creation of a market by architects. Since the first all-glass skyscrapers were sketched by Mies van de Rohe in 1919 and 1921 (Figure 7), architects strove to achieve ever simpler and purer glass forms.
The metal and glass curtain wall became a feature of some of the seminal works of the International Style that dominated world-wide design after World War II. Le Corbusier's Pavilion Suisse, Paris (Figure 8A) designed in 1929 was one of the most influential. The most significant development of the curtain wall, however, occurred in the United States. First designed as an expensive, refined and elegant custom artifact, it gradually became a standard commodity, and today is the least costly way to enclose a structure. Perhaps more important, for several decades the glass box perfectly symbolized, in its image of contemporary elegance and modernity, the aspirations of American corporate architecture from Wall Street to Main Street. The United Nations building of 1947 (Figure 8B) and the Lever Brothers building of 1952 (Figure 8C) had enormous impact on architects and owners alike. Within a few years every city in the U.S. had its blue, glass-curtain walled cubes, from corporate headquarters to modest savings and loan branches (Figure 8D). Today, very refined custom walls are still being conceived that can demonstrate an extraordinary scale and delicacy (Figure 9).
Many problems, such as leakage and obtaining pleasing glass colors, had to be solved in early curtain walls. Manufacturers and influential architects working together managed to do so. The first curtain walls were expensive, custom-made assemblies. The proprietary curtain wall has now evolved into the least expensive way of cladding a building. However, very refined custom walls are still being conceived that can demonstrate an extraordinary scale and delicacy. (Figure 9)
Future innovations still in their infancy are the double-skin curtain wall that aims to provide controlled natural ventilation and hybrid systems that aim to achieve substantial energy savings as a hedge against an uncertain energy future. (Figure 10)
A number of categorizations of the systems or subsystems that comprise a building are in existence. Richard Rush, in his book The Building Systems Integration Handbook, defined only four systems:
Thus the systems and assemblies of the envelope are one of the four main parts of the building both in terms of their physical existence and in their contribution to overall building performance. The envelope protects the other systems from harsh aspects of the outside. It also works in conjunction with the other systems to ensure a safe and benign environment for the building occupants. Thus the envelope is a gatekeeper, allowing certain aspects of the exterior into the building, rejecting some and changing the nature of others. The design of the envelope is very complex and many factors have to be evaluated and balanced to ensure the desired levels of thermal, acoustic and visual comfort together with safety, accessibility and aesthetic excellence.
This guide provides information and recommendations to assist the designer in achieving their goals. While levels of performance are discussed and many quantitative criteria are explained, no mandatory requirements are stated. Those are the purpose of building codes and regulations. This guide assumes that any design work will conform to code requirements but deal also with many functions that, while not subject to code, are essential for building excellence.