Architecture/Construction
SMITH
—From the Foreword by James Timberlake, FAIA
THE DEFINITIVE REFERENCE ON PREFAB ARCHITECTURE FOR ARCHITECTS AND CONSTRUCTION PROFESSIONALS
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ritten for architects and related design and construction professionals, Prefab Architecture is a guide to off-site construction, presenting the opportunities and challenges associated with designing and building with components, panels, and modules. It presents the drawbacks of building in situ (on-site) and demonstrates why prefabrication is the smarter choice for better integration of products and processes, more efficient delivery, and realizing more value in project life cycles. In addition, Prefab Architecture provides: ■
A selected history of prefabrication from the Industrial Revolution to current computer numerical control, and a theory of production from integrated processes to lean manufacturing
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Coverage on the tradeoffs of off-site fabrication including scope, schedule, and cost with the associated principles of labor, risk, and quality
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Up-to-date products featuring examples of prefabricated structure, enclosure, service, and interior building systems
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Documentation on the constraints and execution of manufacturing, factory production, transportation, and assembly
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Dozens of recent examples of prefab projects by contemporary architects and fabricators including KieranTimberlake, SHoP Architects, Office dA, Michelle Kaufmann, and many others
In Prefab Architecture, the fresh approaches toward creating buildings that accurately convey mature and expanded green building methodologies make this book an important voice for adopting change in a construction industry entrenched in traditions of the past.
A GUIDE TO MODULAR DESIGN AND CONSTRUCTION
RYAN E. SMITH is Director of the Integrated Technology in Architecture Center (I TAC), an interdisciplinary research consortium at the University of Utah College of Architecture + Planning in Salt Lake City, Utah (www.itac.utah.edu). Smith’s research and teaching focuses on promoting integration that leads to sustainable and lean design and construction practice.
PREFAB ARCHITECTURE
“Prefab Architecture . . . is beyond theory, and beyond most of what we think we know about pods, containers, mods, and joints. This book is more than ‘Prefabrication 101.’ It is the Joy of Cooking writ large for the architecture and construction industries.”
PREFAB ARCHITECTURE A GUIDE TO MODULAR DESIGN AND CONSTRUCTION
RYAN E. SMITH FOREWORD BY JAMES TIMBERLAKE, FAIA
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chapter
5
Fundamentals
This chapter will discuss the fundamental technical and constructional principles related to prefab architecture. These fundamentals include the following categories: • System: Structure, Skin, Services, Space • Material: Wood, Steel/Aluminum, Concrete, Polymer/ Composite • Method: Manufacturing and Fabrication • Product: Made to Stock, Assembled to Order, Made to Order, Engineered to Order • Class: Open versus Closed • Grid: Axial and Modular
5.1 Systems Building systems are generally thought of in five different categories: site, structure, skin, services, and space and stuff.1 Prefabrication can be used to deliver everything but the site. Most “stuff,” including furnishings and fixtures, are so easily changed and their lifespan varies from year to year that it will not 99
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F U N D A M E N TA L S
stressed skin panels in metal or wood. Mass structures are less common.
Space Services
SYSTEMS
Skin Structure Site Figure 5.1 The major building systems that have emerged in construction are identified graphically from most durable to least: site, structure, skin, services, and space.
be considered as a prefabricated system. Therefore, for the purposes of organizing the information herein, the focus will be on offsite fabricated structure and enclosure systems with a brief discussion of interior space and service systems of buildings in relation to architecture.
5.1.1 Structures Structures are load-bearing and lateral-resisting systems that transfer dead loads induced by gravity on the mass of the building and live loads induced by habitation, wind, rain/snow, and dynamic loading of thermal and movement stresses to the ground. Structures include foundations, frames, load-bearing walls, floors, and roofs. Buildings are made up of two general types of structures to resist vertical and horizontal loading: • Mass structures can be solid load-bearing to transfer load not through distinct elements, but through surfaces and solids. Mass structures are built of stacked wood, laminated wood, concrete, or
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• Frame structures act as skeletal systems in post and beam, space frame, and diagrid. These are primarily made of wood, steel/aluminum, and/ or reinforced concrete—materials that are strong enough to resist both tension and compression stresses and support multistory buildings. Frames are the most common structural system due to their flexibility for non-load-bearing infill and ease of erection. Frame systems are composed of vertical columns or posts and horizontal spanning elements such as beams or girders. Frames are inherently gravity loadbearing, but rake under later loads due to wind, seismic, or other dynamic loads such as disproportionate live loading. Therefore, frames require some type of lateral load-resisting system. Three major types of lateral systems exist: brace frames, shear wall, and rigid frame. • Brace frame: The junction of column to beam can be laterally braced with diagonal members of steel. There are various types of braces. In the United States, the most common are “X” bracing and chevron bracing. In seismic regions, sophisticated systems of braces have been introduced including eccentrically braced and unbonded braces. Brace frames provide a stiff structure and are more cost effective than a rigid frame or shear wall in many instances because they can be bolted together quickly onsite. The braces may be welded and bolted directly to the beam and column connection or use a gusset plate that transfers load between the elements. Brace frames, however, leave unsightly and spatial obstructions in bays at gusset plate connections that limit flexibility in future change or in routing utility services through the building.
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Figure 5.2 The three lateral load-resisting systems include left: brace frame; middle: rigid frame; and right: shear wall.
• Shear wall: Shear walls provide lateral resistance to horizontal loading. Shear walls infill bays between columns and beams. Traditional onsite construction may use site-cast concrete or reinforced block as a shear wall. Prefabrication would suggest that the shear panels be fabricated offsite and placed into structural bays. Panels can be fixed to the steel sections by welding or bolting connections along the edges. Steel connection plates are embedded into precast panel corners and anchored with steel shear studs. This connection is filled with mortar to ensure that the panel is secured to the corner of the column and beam. • Rigid frame: Most frame structures are separated from enclosure. Save in the case of exterior shear walls that act as exterior enclosure infill, frames must be enclosed in order to provide exterior protection from elements, thermal differences, and interior space separation/fire separation. Frame load-bearing structures can be framed in a variety of relationships with infill such as inline (integrated), online (aligned), or offset (separated). Frames require infill so the treatment of thermal insulation becomes critical. Exposing frames to the exterior
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and interior presents problems with thermal bridging. Best practice would suggest putting the frame on the interior with insulation on the exterior, or to place the frame on the exterior of the enclosure; however, this is difficult not to create thermal bridges with floor-spanning elements that must attach to the vertical frame structure and thus create a bridge for energy transfer. • Cores: Buildings can contain a core that provides a center area of services such as stairs and elevators. Since these vertical shafts need to be fireproofed, site-cast concrete is usually used to act as shear core as well. Steel frame structures can be attached to these cores in three ways: (1) steel embeds are placed into the concrete core with a flange that engages the structural steel beams to be bolted; (2) the steel connection plate is set flush in the concrete wall and is welded to the beam onsite; or (3) the core is cast with a recess to accept the steel beam on a steel embed–bearing plate. In all of these cases, care must be taken to minimize onsite welding as much as possible. In this case, using precast cores instead of site-cast cores and providing embeds, faceplates, or recesses in the
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precast walls that can accept the steel structure may increase the speed of construction and quality of precision.2 • Space frames: A space frame is a 3D truss formation that consists of lightweight interlocking members that create a latticework. Space frames are used for long-span roofs and can be formed to make hollow columns or girder elements. Their strength-to-weight ratio is high, making this an ideal solution for few points of support and prefabricated structures that have a high degree of repetition. Space frames derive their strength from the inherently rigid triangulation. They are rigid but also ductile, with movement and bending occurring across each of the individual elements or struts. Space frames are attributed to Alexander Graham Bell at the turn of the twentieth century, but Buckminster Fuller made them popular in architecture.
F U N D A M E N TA L S
Space frames have become less common in the latter part of the twentieth century due to their cost, but still present an opportunity for prefabrication when implemented in higher profile buildings where structure is left exposed. • Diagrid: Short for “diagonal grid,” this is a structure that uses triangulation as well. Members are placed on the diagonal, as opposed to horizontal and vertical standard frame structures. The diagrid is then able to act as a vertical gravity load-bearing structure and a lateral load-resisting structure simultaneously. As such, this requires less material, upward of 25 percent, than would be required in a conventional structural system that uses post and beam and a separate lateral resisting system. Diagrids can often be found in nature such as in plant formations and bone structures. In this context, they are referred to as “lamella structures” in which members are con Figure 5.3 Cores offer lateral resistance to buildings and may also contain vertical services such as stairs, elevators, and mechanical shafts. Figure 5.4 Space frames have existed since the turn of the twentieth century, but did not take hold in architecture until Buckminster Fuller made them popular. Architecture students at the University of Utah constructed this space frame in the 1960s on the Salt Lake City campus.
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nected along a pattern of intersecting diagonal lines to form a mesh surface. Contemporary architecture has many recent examples of diagrid structures, for example, in the Hearst Tower, designed by Norman Foster; the Seattle Library, designed by Rem Koolhaas; and the Tokyo Prada store, designed by Herzog and de Meuron. Diagrid frames may be prefabricated in large grid panels and connected onsite as a superstructure.
5.1.2 Skins Building skins or enclosures mediate between interior and exterior environments. The protection from exterior extreme temperatures and elements is the primary function of an enclosure system. Architecturally, enclosure systems provide the primary aesthetic communication of a built work. Structures and services are becoming ever more specialized, but building enclosures are still the responsibility of the architect. Therefore, how a community receives the building and how the building performs environmentally is a result of enclosure design. Envelopes constitute both exterior wall and roof systems. Exterior skins as both the separating and linking element between interior and exterior environments must perform a variety of tasks, including: • Function: Pragmatic purpose of the building skin, comfort, shelter, view • Construction: Elements of the building skin and how they are assembled • Form: Aesthetics of the building skin, cultural and contextual response • Environment: Performance of the building skin in lifecycle
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Figure 5.5 A diagrid structure uses 25 percent less frame material than with a traditional orthogonal grid. This grid is used in the Seattle Public Library, designed by OMA Rem Koolhaas.
Each of the four aspects must be fully considered to create architecture that responds to the needs of its inhabitants and the societies for which it is built. Architects and construction professionals are under increasing pressure to deliver on both construction and ecology of building skins. How a building skin is developed as a series of elements that can be fabricated and quickly assembled has a large impact on overall project budget. This sequence must be well integrated into the entirety of the other criteria. Likewise, the long-term performance in both initial and operational energy as well as durability and maintenance should be considered with respect to the other three criteria in order to justify the investment of the building skin. Arguably, the building skin is the most dominant system among structure, services, and space.3 This is true not only in terms of design aesthetics, but in the functions it must perform and the impact it has on ultimate energy performance throughout its lifecycle.
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The building skin in many respects determines the weight and ultimate sizing of structure, as well as the performance of services and interior systems of the building. From a performance perspective, skin must ventilate; protect from radiation, conduction, convection, and daylight; insulate and potentially integrate energy systems. Other functional criteria include flame spread and structural loading. All of these functional criteria have an impact on the aesthetic criteria. In addition, functional, aesthetic, and ecological considerations determine the constructional criteria that will be employed and the degree to which prefabrication is used in a given building. Construction and design are inseparably linked as structure and enclosure design determine the visual appearance of the building. Load-bearing components, such as beams, supports, and walls, and the spacing of them define the rhythm, division, and proportion of the building skin.4 Classifying building skins according to construction or assembly can be based on the following criteria: • Load transfer (bearing and non-load-bearing): Bearing skins include traditional structures such as stacked masonry, timber, or contemporary cast concrete barriers. Non-load-bearing structures are the most common today and separate building structure from the exterior enclosure skin. These are composed of wood, glass, metal, ceramic, or stone claddings. From the perspective of function, construction, aesthetics, and energy, the separation of skin and structure is a natural evolution of contemporary desire for flexibility within the lifecycle of a facility. • Shell arrangement (single-skin or multilayered): Solid wall construction can act as a single skin, relying on one material or layer to perform both structure
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and enclosure. Today, however, with increased expectation on building skins to perform a variety of functions, layers are assembled each having defined functions to serve. Air gaps may be provided for water condensation as well as placing insulation and vapor barriers in strategic relationship with one another in order to control dew points and condensation in the building skin system. Even the most rudimentary wall in residential construction is incredibly sophisticated in the functions it must perform. • Transmission (transparent, translucent, opaque): Across the board of load transfer and shell arrangement, a variety of levels of transparency, translucency, and opacity are possible. Contemporary glass facade systems offer the capacity to open up expanses of exterior wall for view; however, this also introduces concern of radiation transmission. With contemporary materials and arrangements of shells, enclosures are able to perform much better than just 10 years ago while maintaining desired transparency. • Structure–enclosure–space relationship: The integration of these three elements has an impact on one another. Spatially, building skins can be placed in front of, behind, or in line with the structural systems of a building. The placement of the building skin, in larger measure, determines the aesthetic communication as well as the energy performance by way of creating or mitigating thermal breaks. This also affects interior spatial arrangements, as structure interior can obstruct space definition and arrangement, but also present opportunities for expression when designed in an integrated fashion. Prefabricated facades consisting of panels of wood, glass, metal, stone, or precast/GFRC cladding are produced in factories and installed onsite. These systems are multilayered, multimaterial, with each layer
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performing specific functions of protection from water, air infiltration, visibility, thermal transmission, and so forth. These layers are assembled in the factory and erected onsite to the superstructure; or an armature is attached to the superstructure and cladding elements in glass, metal, concrete, or stone are placed on the frame in the field. Commonly used non-load-bearing enclosure systems are glass curtain wall, metal facade, precast cladding, and masonry (including stone and brick). Less common but becoming popular are wood and polymer (plastic) facades. Figure 5.6 The relationship of frame structure and enclosure determines the expression of the building as well as the thermal performance. Enclosures may be outside of the vertical structure, in line with it, or inside of the structure.
Figure 5.7 These large, glazed prefab units are being fabricated in China for the Highline 23 project in New York City, designed by Neal Denari Architects. Front, Inc. developed the glazing system.
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5.1.3 Services The services of a building include the heating, ventilation and air conditioning, plumbing, electrical, and any conveying equipment such as elevators and escalators. The air handlers, condenser units, air-to-air exchangers, and heat pumps are by default prefabricated mechanized units. Mechanical ducting for airflow has been automated in design to fabrication for many years. Prefabrication of services as it relates to architecture refers to a higher level of unitization. Services may be produced as modules that can be located in buildings. Bathrooms, kitchens, communication rooms, utility rooms, and service walls are outfitted in the factory and then placed efficiently inside building structures. Conceptually leaving so-
Figure 5.8 This bathroom service pod is fabricated with plumbing, fixtures, and finishes and shipped to be installed as an interior module within a building structure.
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phisticated equipment and higher-level finishes inside of the units to factory work before placing them onsite controls the quality and warranty. For example, restrooms and kitchens for utilitarian functions that have a high degree of repetition are ideal for service prefab. This includes service kitchens for restaurants; and kitchen and bathroom units for housing, dormitory, and hotel construction. More on these types of service units will be discussed in Chapter 6.
5.1.4 Space Materials used to define interior spaces are by definition not exposed to inclement weather. Therefore, polymers, finish wood panels, and newer materials not appropriate for exterior applications can be exploited on the interior. Interior systems provide the primary human dimension of architecture by which its inhabitants experience space. Architects therefore, have specified interior spaces in most cases to be the more expensive per unit volume portion of buildings. An exhaustive look at space-making elements of buildings is beyond the scope of this book. Materials for interiors can be subdivided into panels, tiles, coatings, and coverings.5 All of these systems may be easily applied in a factory environment, shipped, and erected onsite. This is rarely done, however. Interior space is the most temporary of all building systems, but it is also the most expensive over the lifecycle of a facility, considering the rate at which change occurs. Interiors can be changed every time a new tenant or owner moves in. To accommodate this need, manufacturers are beginning to develop prefabricated interior systems that allow for easy assembly and disassembly. A company called DIRTT (Do It Right This Time) has developed a prefabricated interior tempo-
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5 . 2 M AT E R I A L S
rary partition and floor system. The wall system is an open-source product that can accept many different types of materials from 3-Form resin panels to wood paneling to glass tiles as well as different electrical configurations.
5.2 Materials Prefabrication can be accomplished in virtually any material. Although most elements today are some type of composite made from one or more materials, the primary material in a compilation determines the flow of the material through its lifecycle from who harvests the material, manufactures, fabricates, and finally installs it. The building industry trades are set up to handle certain types of materials throughout this lifecycle due to tooling, manipulation, and install expertise. For example, in the last decade, structural insulated panels have fallen under the purview of framers, albeit not successfully in many cases, because they traditionally are used as exterior structure and enclosure walls for housing. For our purposes, materials will be organized by wood, steel/aluminum, concrete, polymer, and composite. The primary material can also determine in what system, element, and building type it is used. Today there are more choices of materials than ever before. With the advent of nano materials and composites, the traditions of concrete, wood, and steel may seem historic. However, these materials are still high performers for their cost and the reality is that alternative structural materials outside of these three seems highly unlikely in the near or long-term future of building. In prefabrication, alternative materials are having greater impact, as their potential to make way for innovative solutions is greater. This is because
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they can be carefully controlled and manipulated with a specialized labor force that understands these new materials and may be able to implement them with specific skill sets. Materials have properties and performance characteristics related to the parameters of how they are used in buildings and what job they perform. Outside of aesthetics, materials must perform a range of functions from structure, attachment, infiltration, and thermal resistance. For example, materials for structures are generally steel and concrete because they are readily affordable and available. Labor crews have been established to handle these materials and their associated systems. Tools, machines, and factories are well established to develop and manipulate steel and concrete. Design standards exist for both steel and concrete structures regardless if they are developed on- or offsite. Glass, polymer, and aluminum are found in enclosure systems that are non-load-bearing because they are lightweight and offer light transmission, but are less suitable for structures. In smaller buildings, wood can be used for structure and enclosure as well. Glass is manufactured as large sheets and polymer in recent years has been used as panel, shell, and pneumatic pillows on enclosures. Facade construction has implemented precious metals including copper, bronze, and durable alloys such as stainless and titanium. John Fernandez in Material Architecture classifies materials by families according to their extrinsic and intrinsic properties.6 Families include metals, polymers, ceramics, natural materials, and composites. A family, such as ceramics, has consistent material properties across its material types including brick, concrete, stone, glass, and the like. These materials are brittle, made from the earth, and are dense and hard. Metals, polymers, ceramics, and natural materi-
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