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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increases exponentially. This study is important because it points to the reality that although production processes by using lean strategies are important, manufacturing and assembly efficiencies have been focused on while transport in general has been neglected. As movements toward larger subassemblies are providing schedule savings, transportation may be just as important in determining the feasibility of fabrication for onsite construction by virtue of project cost. A hybrid approach to using not only modules but also panels and components when needed can be a wise solution for achieving a cost-to-benefit strategy for a given project. The cost-effective distance of transport found by Seaker and Lee are consistent with numbers established from both ISBU engineer Buro Happold and research performed by the American housing company Pulte. Adrian Robinson from Buro Happold states that, for most projects, a 200 kilometer, or 124 miles, is the limit of cost-efficient transport. This distance was found in research in preparation for the Travelodge projects. The use of ISO containers expedited this benchmark by being able to send the modules through shipping. Had the modules been fabricated near the site, the labor costs would have been cost prohibitive. In developed countries like the United States, prefabrication makes sense when travel distance is closer. Likewise, Mark Hodges from Pulte Homes, who has invested in prefabrication and supply chain integration, and who ships modules for rapid assembly on market rate housing, states that their system is limited to 125 miles from the plant.29 This number continues to emerge as a standard in the building industry from factory to site. Logistically, it is not cost beneficial to ship from farther distances unless a large margin is made up in labor, time, or material costs.
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Although prefabricators will often advertise capacity to deliver upward of 500 miles from factory location, this is more a marketing effort to secure additional work. Tom Hardiman at the Modular Building Institute states that 125 miles, as a rule of thumb, has much to do with the locations of the various modular builders. If a manufacturer or supplier is within a 300 to 400 mile distance, the industry will naturally parse itself into 100 to 150 mile radius sections for shipping. In the modular industry a network of dealers or general contractors who do business, share information and work together to cover territories. For specialized fabricators on projects which demand their services and have accompanying budgets, shipments of great distances can be justified. However, on normative construction projects distance plays an increasing importance into overall project costs. Michelle Kaufmann and modular builder Kullman Buildings Corp., have documented in their operations that although prefabrication distance is decreased by 5 percent, the cost of transportation increases 5 percent for offsite construction. This number considers capital cost only and does not take into account the travel time of crews to and from the jobsite or factory. It is safe to suggest that the number of trips to and from the jobsite is more than to and from a factory location. However, these costs are absorbed by overall project budgets, rarely broken out as a separate line item. Therefore, it is difficult to make an accurate comparison.
7.6 Setting Prefabricated building elements arrive to the site ready to be placed. Setting and assembling elements is the final step in the process of construction including hoisting, positioning, adjusting, connecting, and
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stitching. Elements designed for prefabrication and onsite assembly will need to be designed to accommodate lifting points, sometimes called “pick points.� Pick points are designed by an engineer to ensure that the lifting points coincide with the distribution of weight of the element. This is critical so the element will stay stable during craning and will be able to be placed square, or on a level plane. Lifting points may be anywhere on the element, but careful consideration should be given to the final aesthetic of the pick point. Questions such as whether the pick points will be covered by finishes or hidden within an assembly makes this decision less critical. Pick points may also be part of the architectural aesthetic or coincide with ultimate attachment points of the element to another element or foundation, floor, or existing building once installed, as was employed in the St. Ignacius
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Church by Steven Holl. This can be difficult because forces for hoisting and placing are different than the ultimate loads being distributed in an element once installed. Pick points should be carefully determined if the panel or module is finished to a high degree so that straps, cables, and buckles do not damage the elements while in hoisting. A pick point on a module is calculated at thirds, but always considering uneven weight distributions in a particular unit. For wooden modules, often a wraparound belt strap is used. This requires the modules to be over-structured so when lifted they do not break at the midspan. Precast uses lift points, lift lugs, or anchors for transport and assembly. These are embedded into the panel during the precast process in the factory. In order to simplify the positioning of the panels, the ele-
Figure 7.12 This module, for a Marmol Radziner Prefab House in California, is being hoisted with three belt straps and one spreader bar to distribute load to the hydraulic crane.
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Figure 7.13 In order to maneuver this on-hook module, the setting crew uses guide ropes to locate the exact placement of the modules.
Figure 7.14 Locating a corner or two during the set is key to getting the exact placement of the module. This may need to be performed a few times in order to get its placement within a tolerable dimension.
ments have reference and fitting surfaces. Often these are scrupulously numbered so there is no confusion about how they are installed. Bar codes, numbering, lettering, and other methods of identification are used to organize the assembly sequence onsite. These can be placed directly on the elements themselves.
pick points and the crane should be carefully considered with the design and construction team during early project planning. This may affect the design of the elements for assembly from their size and configuration.
Various types of rigs or spreader bars can be used to lift elements. Although direct lifting is an option for smaller elements, spreader bars are used for most projects in order to keep forces perpendicular to the subassemblies, and reduce the possibility of introducing unwanted bending forces within the element. This is especially true for modules. Spreader bars are essentially beams or structures that distribute the loads of lifting over the spreader instead of onto the prefabricated element itself. This is especially critical in modular construction where point loads in conspicuous places may induce eccentric or blunt forces that can permanently damage the module or cause the module to fail structurally. Spreader bars are supplied by the entity performing the setting. However, the design of the interaction of the spreader with the
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7.6.1 Craning For most assembly, elements will be lifted directly from the flatbed trailer to their final location. Cranes lift the element and carefully locate its place onsite. Onsite crew guide elements into place and make connections. Ideally, the onsite work process does not impede the maximum workflow of the crane. Rental of large cranes is expensive, and therefore, the machines should be used as much as possible when procured. Once the riggings are in place, the maneuvering of elements “on-hook� is typically performed by one or two guide ropes. Weather conditions will prevent the setting of prefabricated elements when wind speeds exceed 10 mph. Any joints or openings, which remain exposed at the end of the day, are covered with a tarp to protect against possible rain damage.
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CRANE TYPES There are two main types of cranes: mobile cranes and fixed cranes. Mobile cranes can be truck-mounted, which have the crane integral to the truck such as in rough-terrain and all-terrain combinations; or they can be crawler cranes, which have a base similar to a front-loader with rotating tracks. The following is a description of the most common crane types used in setting prefabricated elements.
• Truck mounted hydraulic cranes Rough
terrain for unimproved worksites in which access is difficult
Simple
truck-mounted crane can run at highway speeds, but cannot do rough terrain
All-terrain Pick 40-
truck-mounted crane is combination of the two previous examples
and carry capability
to 75-ton capacity
• Crawler cranes Greater
flexibility onsite
Transported 40-
on trailer to site
to 3,500-ton capacity
Ships
on eight trucks
Self-assembly
The contractor on the job designs cranes. In a project in which the fabrication company is acting as general contractor as well, a decision regarding crane type must be made in tandem with the design of the prefabricated system. General principles of cranes are that their capacity is inverse to the reach or radius. The greater the radius, the lower the weight the crane can hold. However, in order to accommodate greater loads and increase reach, larger capacity cranes must be used. The selection of the type of crane is based on weight and reach. The craning of modules requires a crane of greater capacity than those commonly kept onsite during in situ construction projects. Site cranes often have a capacity of less than 5 tons, whereas the cranes used for lifting modules often have a capacity in the range of 40 to 75 tons.
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Selecting a crane depends on the load to be lifted, the height clearance needed, the mobility of the crane to perform multiple jobs, or the reach of the crane, the number of lifts, and the availability of the crane. Tower cranes are much more expensive and are only warranted when multiple levels of installation of prefab are going to be accomplished. For single sets of modules or a few modules, truck-mounted hydraulic telescoping cranes are desirable. This is conflicting, however, because tower cranes have a much larger capacity than truck-mounted cranes, but at the level of building prefab components this is rarely an issue.30 Boom size also determines load capacity. For example, a standard truck-mounted hydraulic crane with a smaller 25- to 70-ft boom can handle 22 tons. A 100-ft boom crane can handle 33 tons—larger and
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Fixed cranes are not mobile, but can carry greater loads and reach greater heights and distances. Although fixed cranes ultimately are moved, while onsite they move very little for economic reasons. The most common type of fixed crane is a tower crane.
• Tower cranes Used Up
when space is a premium
and over reach
Usually
fixed to foundations
Strategically
located for maximum reach
Figure 7.15 Left and Middle: Mobile lifting cranes are versatile, able to move throughout the site and reach distances manageable by small to medium-sized projects. These cranes have a 40- to 75-ton capacity, generally adequate for lifting prefab elements for building construction, and a reach of 180 ft high and 160 ft wide. Right: Tower cranes are stationary and costly but have great lifting capacity and reach.
stronger cranes are readily available, but access of the larger truck will quickly become an issue on residential streets and alleys. Maximum weight allowed for truck transit is 80,000 lbs in gross weight. To put this in perspective, this 40-ton gross weight maximum on highways has a 60 percent weight buffer over that needed for a typical timber-framed house which, at 2,000 S.F. weighs 120,000 lbs or 60 tons, with a code-prescribed dead load weight of 60 lbs/S.F. This means that even if the house were flat-packed as densely as possible, it would generally still weigh less than the maximum weight for truck transit and be able to be lifted by a 25-ft boom in three lifts, or a 100-ft boom in two lifts. In general, it is more economical to go with a small, accessible crane to lift in multiples than with a large crane that will lift once or twice.
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7.6.2 Foundations For modular construction, foundations can either be piers, linear footings, or continuous footings. Wood modules generally place distributed loads on foundations as they distribute loads similar to a bearing wall condition. Depending on how they are developed, steel framed modules, such as those that Kullman Buildings Corp. produces, a point load rather than a distributed load is placed on a foundation. Therefore, slab-on-grade is not a typical solution for this type of modular construction. Rather, perimeter and pier foundation systems are the best solution. Site-cast foundations are never entirely plum; certainly they are much less precise than elements that have been factory produced. Therefore, setting of elements on foundations often includes shims to achieve level.
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Figure 7.16 Three types of foundations that can be used for modular construction include: Left: piers; Middle: linear stem wall; and Right: full stem wall. Modular construction can be designed to distribute load to vertical structure at corners alleviating the need for full-engaged stem wall bearing at the perimeter of the module.
7.7 Tolerances Tolerances exist to accommodate the normal manufacturing and installation inaccuracies that occur in construction as a result of moisture, thermal differential movements, material discrepancies, and human error during assembly. During detailing, designers need to work with fabricators and contractors to determine the tolerances for a given project. Each detail has its own accommodation for forgiveness in dimension discrepancy and if two materials are coming together each must respect the other in its accuracies. Larger elements require greater tolerances, especially if they cannot be altered. Calling for increased tolerances does increase the cost of a project. This requires an integrated effort in order to coordinate elements on the jobsite for assembly. Because factory methods improve the craft of construction, tighter tolerance can typically be achieved in offsite construction relative to onsite construction. Today’s equipment and machinery allow for
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tolerances up to 20 millionths, given the right temperature requirements. This is used for highly precise work in medical and mechanical applications, but in building, these kinds of tolerances are not necessary. Given the inaccuracies of uneven sites, site-poured foundations without tight tolerance, the precisions of prefab may be high, but the tolerance between the elements must allow for dimensional discrepancy. Therefore, tolerance refers to the desired allowance of dimensional inaccuracy. For prefabricated construction this is between elements themselves, and the elements in relation to onsiteconstructed portions of the building. In prefabrication, tolerances fall into two categories: part or subassembly tolerances and assembly tolerances. Part tolerance refers to the tolerance of the parts that make up the component, panel, or module including the making of elements from MTS parts. Assembly tolerance refers to the tolerance of the element or subassembly itself and the process of placing the subassemblies onsite.
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The ultimate determination for tolerance is dependent on where and how it will be assembled onsite. Tolerances are therefore calculated accumulatively in sets of assemblies, such as a series of modules or panels. For example, in a set of six cladding panels set within a structural bay each having a tolerance of ±1/16 in., the overall dimensional tolerance of the assembly is as follows:
= or an overall dimensional tolerance of 3/16–1/4 in.
=
Tolerances reflect the dimensional error as a result of onsite construction inaccuracies of human assembly. For example, Office dA designed the Arco gas station to have ±1/64 in. accuracies in the stainless CNC panels. The dimensional discrepancies due to human error during assembly varied upward of ¼ in. Connections are therefore designed with tolerance within them to accommodate this error. Joints that are unforgiving inevitably must be manipulated again in order to fit. Often methods such as slotted holes, neoprene washers, elastic joints, loose fitting joints, and reveals are used to make up this dimensional difference.
± 0.15” or 3/16” - 1/4” tolerance
±1/16”
±1/16”
±1/16”
±1/16”
±1/16”
±1/16”
Figure 7.17 This image illustrates the principles of accumulated tolerances in a six-panel column bay. Each panel has a dimensional tolerance of ±1/16 in. The overall dimensional tolerance for this assembly is 3/16 to 1/4 in.
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DIMENSIONAL TOLERANCES FOR U.S. CONSTRUCTION Concrete Dimension of footing
–1/2 inch, +2 inches
Squareness of residential footing
1/2 inch in 20 feet
Plumbness of wall
±1/4 inch in 10 feet
Variation of wall from buidling line
±1 inch
Variation in wall thickness
–1/4 inch, +1/2 inch
Plumbness of column
1/4 inch in 10 feet, no more than 1 inch overall
Variation in level of beam
±1/4 inch in 10 feet; ±3/8 inch in any bay; ±3/4 inch for entire length
Variation in level of slab soffit
same as for beam
Structural Steel Plumbness of column
1 inch toward or 2 inches away from building line in first 20 stories; 2 inches toward and 3 inches away for above 20 stories
Beam length
±3/8 inch for depth of 24 inches and less; ±1/2 inch for greater depths
Wood Floor evenness
±1/4 inch in 32 inches
Wall plumbness
±1/4 inch in 32 inches
Exterior Cladding Aluminum and glass curtainwall
varies depending on manufacturer
Structural glass curtainwall
varies depending on manufacturer
Metal cladding (CNC)
±1/64 inch in 15 feet
Interior Finishes Plumbness of metal framing
±1/2 inch in 10 feet
Flatness of suspended ceiling
±1/8 inch in 10 feet
Modules Wood modules
±1/4 inch in 32 inches
Steel modules
±1/8 inch in any one direction of the individual modules
Figure 7.18 Dimensional tolerances for U.S. construction: These are general rules of thumb and not meant to be standards. Each project may also require a specific dimensional tolerance that deviates from this list for the intended purpose.
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MATE-LINE STITCHING Seams can be concealed or revealed as part of the tectonic of the building. In modular construction walls and ceilings, finishing or “stitching” is accomplished in the field using standard GWB finishing techniques. Flooring can be applied onsite, in the factory, or in a combination of the two. For floors finished entirely in the factory, standard flooring transitions can simply be applied onsite. A combination of factory and site finishing is the most common. Below are a few examples of stitching finishes in modular construction from Kullman Buildings Corp.: • Carpet: Typically the tack board is installed in the factory and the carpet is sent as ship-loose. • Ceramic tile: Tile can be set in the factory, allowing one tile to be set onsite over the mate line. It is generally best to perform grouting as a single process onsite. • VCT is set in the factory such that the tile, which will cover the seam, will be cut about ¼ in. narrower, allowing a precise fit to be made onsite. • Concrete: Grout or self-leveling compounds can be placed in the seam joint onsite. • GWB: One full sheet of GWB is left off of the factory finish and applied onsite.
Figure 7.19 The interiors of modular and panelized projects have “mate lines” that need to be stitched together onsite in order to seamlessly connect finishes.
continued
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Figure 7.20 This panelized house by Bensonwood has a seam from one floor to the next at the exterior wall. This area has been left clear in order to make a structural connection between the two floors. The exterior siding will be stitched together once the connection is made.
Figure 7.21 This is an example of a brick veneer stitch that occurs between a mate line between two stacked steel frame modules. The mate line, or seam, is left open in order to make a structural connection and then it is: Top: stitched with a flashing detail to cover the seam; and Bottom: in-filled with brick veneer performed onsite.
Tolerances are established by individual industry associations such as American Institute for Steel Construction for structural steel frames and Precast Concrete Institute for the precast industry. These standards determine the dimensional accuracy of the manufacturing process to ensure that construction assembly is more easily accomplished. Dimensional discrepancies, when unintended, can present problems and are therefore considered undesirable. However, tolerance is needed in every material part and subassembly so that onsite assembly is smooth and without
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labor and schedule increase. Tolerances also increase the quality of the building by providing a means of movement and system change out over time. It is recommended that each project establish its tolerances based on goals and expected outcomes as well as schedule, budget, and availability of labor skill. Prefabricated elements, when combined with onsite work, often determine the tolerance of construction. On the other hand, if the prefabricated element is small and insignificant to the overall cost of the
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project, it will be custom fabricated to meet the dimensional needs and tolerances established onsite. Prefabricated wall cladding panels will closely govern the story heights and the length of the building, or part of a building, where they are used. The structural frame is usually erected by site-work methods using site-cast reinforced concrete, and the prefabricated units, whether mass-produced to standard sizes or specially made for the particular building project, are fitted to it. An accurate tie-up between the respective dimensions of structure and cladding units is essential, and only a certain degree of tolerance may subsequently be allowed for either. Grids for building construction therefore must be established so that onsite and offsite work might be coordinated. This is usually performed with prefabrication based on modular grids, not axial. Modular grids allow for dimensional coordination across elements onsite and offsite. It is necessary, when intending to use extensive prefabrication of components, to design the building from the start on a reference grid related to the intended module.
7.7.1 Joints Where building subassemblies meet there is a joint. The appearance and performance of joints is important. Joint appearance and location is determined by the system being employed and the grid used. The joints are fixed by production, design, and transport. Joints make up the dimensional discrepancy by virtue of the actual dimension of the joint. Joints must be protected from the weather by virtue of constructional attachment such as lap joints, drip grooves, and other strategies for cladding detailing or are simply joined by sealant. Although sealant may be required or desired for moisture control, details should at all trials work toward quality detailing through geometry and attach-
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ment and, as a last resort, chemical sealants. Bolted connections or connections which allow for disassembly have an easier time being recycled. Joints perform moisture and thermal control and acoustical protection. Prefabrication allows for fewer joints in the construction system providing fewer places for a building to fail, less labor to attach or seal, and less labor onsite. Fewer products and subassemblies means less cost, therefore, fewer joints likewise require less onsite assembly time thus reducing overall project cost.31 To deal with tolerances in construction at joints, a number of fitting mechanisms can be employed. The following have been taken from Allen and Rand’s Architectural Detailing:32 • Sliding fit: One element overlaps another and is positioned by sliding. If there is a dimensional discrepancy, the gap is covered by one of the elements sliding over the other. If two adjacent elements are fitted against one another, sliding is simple; however, when a third or fourth plane is introduced fitting is more difficult. These sliding planes can be mitigated with three or more dimensions by allowing for generous openings and lapping to occur and adjustable fit joints to allow for the tolerance to occur. • Adjustable fit: Building elements must be positioned accurately and therefore are designed so alignment can be adjusted during or after assembly onsite. Oversized holes and horizontally or vertically slotted anchors allow dissimilar systems such as an enclosure panel and a structural floor to connect to one another. Once proper alignment is made, a method for securing this detail is needed. It may be a weld or simply friction created at the bolted connection. Disassembly favors bolted friction or slip critical connections over welded or glued connections.
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• Butt joint: This detail is an alternative for joining elements at a miter joint. The joint is a lapping of (A) element past (B) element placing the pieces perpendicular to one another in order to hide imperfection in the detail. The real benefit is removing knife joints common with miters. This can be used in connection with reveals and adjustable fit connections. A quirk miter is a corner detail in which elements are joined using a built-in reveal, no knife edges, and is forgiving to retain symmetry of a miter joint. • Edge: The edge of elements, when exposed, should be carefully considered. A sharp edge is susceptible to nicking, breaking, denting, or the like. On the other hand, chamfered edges allow for easy wearing and will not impale people. In prefabrication this is important to consider and may make end elements different in manufacture than others. Corners may need to be shaped and reinforced differently than other elements in the assembly.
7.8 Conclusion
Figure 7.22 The following are fitting mechanisms for negotiating dimensional tolerances from Top to Bottom: sliding, adjustable, reveal, butt joint, and edge.
• Reveal: Offsetting materials so they do not slide past one another but let the tolerance be taken up in the separation dimension is a good way to align elements in relation to one another. The reveal often creates a shadow line that conceals the lack of precision of the detail. Transition of one system or material to another or change in direction from one element to the other element makes a reveal which adds visual interest and tolerance accommodation.
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Architects dealing with offsite fabrication must think more like product designers. In speaking with a product designer about the connection between design and production, he stated that he would not think of designing a product without working to develop the method for production as an integral process. This is because the cost of a project and the time that it takes to manufacture it determines its viability in the marketplace. Product development therefore is the process of including all the activities that take place from market interpretation to finished product designs. Included in this equation are prototype production and test activities. Designers of products and prefab architecture must see their ideas from concept through to end use.
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