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Prefabricated buildings, often referred to as prefabs, are structures with components—such as walls, roofs, and floors—that are manufactured in a factory or plant. These components may be fully or partially assembled before being transported to the construction site. This approach is favored for its cost efficiency, rapid construction, and potential for reuse. Prefabricated buildings are commonly used for temporary construction sites, office spaces, medical facilities, evacuation centers, schools, apartment complexes, and single-family homes.
Prefabrication offers greater efficiency compared to traditional on-site construction due to its controlled manufacturing environment. Since most buildings feature repetitive sections like walls, roofs, and floors, a streamlined production process can be developed. This approach allows for continuous improvement and optimization of the manufacturing operations.
The concept of prefabricated buildings can be traced back to nomadic times when people needed transportable shelters while exploring new lands. During the colonization era, there was a demand for portable houses and camps. However, once colonies were established, the need for such prefabricated structures diminished.
The extensive development of prefabricated construction began in the 20th century, particularly during and after the World Wars. With a shortage of construction labor as men were engaged in war production, there was a significant housing shortage that persisted into the postwar period. This situation prompted the exploration of alternative construction methods to meet housing demands.
Despite being built from modules, prefabricated structures must adhere to the same building codes as traditional buildings. These codes, which can vary widely from state to state or county to county, present a challenge for both permanent and prefabricated construction. In the United States, modular buildings must comply with the International Building Code, which is regularly updated to reflect new developments but may differ between states.
Before a prefabricated building is installed, the construction site undergoes an inspection to assess the soil type needed for the foundation. A third-party inspection ensures compliance with local, state, and international building codes. Following these inspections, modular companies must secure building permits, utility permits, and occupancy permits.
With advancements in construction standards and codes, alongside the increasing demand for housing and office spaces, prefabricated buildings continue to evolve as an alternative to conventional construction methods. Technologies like Building Information Modeling (BIM) provide digital representations of structures, facilitating better management of on-site assembly and reducing risks associated with prefabricated construction.
Prefabricated buildings offer numerous advantages to manufacturers, contractors, and end-users. By dividing activities between on-site and off-site locations, project schedules and costs can be managed more flexibly, assuming effective project management is in place. Off-site fabrication benefits from the efficiencies of optimized assembly lines. Furthermore, the trend towards eco-friendly and sustainable construction presents significant market opportunities for prefabricated buildings. Both the residential and non-residential sectors are expected to see an increase in market share in the coming years.
One of the primary benefits of manufacturing through a production line is the faster turnaround time. Workers perform their specific tasks repetitively with defined operational sequences, making their actions more efficient than those on a conventional construction site. Additionally, some operations can be automated. In terms of project planning, prefabricated construction is quicker because certain activities can be conducted simultaneously. For instance, while the site is being cleared and the foundation constructed, the actual building components such as walls, roof, and floors can be fabricated off-site. With prefabrication methods, a building can be delivered to the site up to 90% complete, allowing a facility to be occupied within days or weeks of delivery.
Weather plays a significant role in the construction industry as it is an uncontrollable factor. It can slow down projects, delay completion dates, and generally disrupt workflow. All companies must account for weather conditions when planning a project.
In contrast to permanent construction projects, weather has minimal impact on the assembly of prefabricated buildings since 90% of the process occurs in controlled environments. Prefabricated buildings are constructed "off-site" in specialized plants, allowing for the completion of new facilities regardless of weather conditions. This integration of site and "off-site" construction methods enables projects to proceed even in adverse weather or emergency replacement scenarios where traditional construction methods may be impractical.
Prefabrication helps to mitigate the effects of external factors. Structural components are produced under more controlled conditions, minimizing the impact of the external environment. Techniques such as shop welding and precast concrete manufacturing are less affected by uncontrolled factors.
Quality control is a significant advantage of prefabricated building construction. In conventional site construction, local inspectors visit the work site at various stages. However, in prefabricated building manufacturing, rigorous inspections and scrutiny are applied throughout the entire manufacturing process.
Prefabricated building manufacturing facilities typically employ quality control inspectors, as well as state and third-party inspectors, who approve prefabricated components at every stage of production to ensure strict adherence to drawings and quality standards. In modular plants, key employees and crew foremen often have many years of experience and have worked on diverse projects with various designs.
In contrast, traditional construction workforces often consist of unskilled laborers provided by local employment agencies. These laborers are assigned to projects to meet labor demands without consideration of their skill level. Unlike the experienced employees in modular construction, traditional construction workers are often employed on a temporary or part-time basis, working for different companies on a project-by-project basis.
Implementing quality control is more straightforward in prefabricated construction compared to on-site construction because structural components are designed with repeating features. Standardizing component dimensions and tolerances is easier, as molds, formworks, and temporary fasteners are consistent for typical building components, resulting in uniform dimensions. Additionally, the quality of prefabricated buildings is more likely to be consistent across different locations, provided they use the same components, due to the reduced impact of local site conditions.
The cost of prefabricated buildings may be similar to or even higher than that of traditional construction. However, the key benefits of prefabrication lie in its time and efficiency. Traditional construction projects often take six to nine months to complete, whereas prefabrication can achieve the same results in about half the time. This efficiency leads to cost savings and faster implementation of the structure. Additionally, prefabricated buildings offer the advantage of precision engineering and design for customized and unique components in each project.
Certain prefabricated buildings are designed for temporary use, making them ideal for applications that need temporary workspaces, such as construction projects, remote healthcare services, or research activities. These buildings can be easily disassembled and relocated to different sites, allowing for minimal disruption and preservation of the original job site.
This advantage arises from the efficient use of raw materials and reusability inherent in the prefabrication process. In conventional construction, there tends to be more material waste and temporary components like formworks, fasteners, jigs, and fixtures that are discarded after use. On-site constructed buildings are typically permanent; once their intended purpose is served, they often remain unoccupied until repurposed or demolished. In contrast, prefabricated modular buildings are easier to repurpose due to their mobility.
Fabrication shops offer a more controlled environment compared to on-site conditions. By conducting most of the work in these shops, workers are exposed to fewer safety hazards and threats, such as working at heights, adverse weather, confined spaces, and nearby construction activities. In fabrication shops, construction operations can be efficiently separated and ergonomically designed, minimizing these risks.
The lifespan of any building is influenced by several factors, including installation, maintenance, and environmental conditions. Generally, prefabricated buildings have a lifespan of over 30 years and often exceed their original purpose. Prefabricated buildings are governed by the same construction standards and regulations that apply to traditional construction methods.
Like traditional buildings, prefabricated structures undergo rigorous scrutiny and inspections. What sets them apart is the continuous inspection of components and materials during the assembly and manufacturing processes. This ongoing oversight makes prefabricated buildings more adept at addressing potential defects, errors, and subpar materials.
The concept of prefabricated buildings dates back several centuries, with roots tracing to the 15th century when William the Conqueror invaded England. Over time, prefabricated buildings have evolved to meet societal needs, including their use as prefabricated farm structures during the Gold Rush of 1848 and the appearance of portable cottages in Great Britain during the same era.
In the early 20th century, prefabricated home kits were sold through catalogs, with precast concrete materials available for foundations. The expansion of the prefabricated building industry accelerated after World War II, particularly with the introduction of Quonset huts as temporary barracks for soldiers. The simple design of these huts laid the groundwork for a growing industry that now offers a variety of prefabricated buildings to meet the needs of manufacturers, farmers, and homeowners.
Prefabrication primarily falls into two categories: volumetric (or modular) and panelized. Both types use materials such as timber, steel, concrete, and fiberglass, with some incorporating a combination of these materials.
The various components of prefabricated buildings are engineered at a factory and delivered to a location where they are assembled. Prefabricated buildings are a viable and reliable alternative for creating work space, storage area, and flexible manufacturing facilities. The components of prefabricated buildings include framing, secondary supports, wall and roof panels, door and window frames, fasteners, and sheeting and insulation.
While larger components might seem like the core elements of prefabricated building assembly, the most crucial items are the screws, bolts, nails, and fasteners that secure the various parts together. These fasteners are meticulously engineered for durability and ease of installation.
Prefabricated components include construction elements such as windows, doors, and trusses, which are not complete panels on their own. Although these materials involve minimal off-site assembly, they offer greater flexibility in use, as they can be adapted and installed based on specific site conditions.
Prefabricated panels are two-dimensional components assembled on-site to construct a building. They typically require more on-site work compared to modular prefabricated buildings. These panels are often available as sub-assemblies with complete finishes and installed features like windows, doors, and insulation. Alternatively, panels can be supplied as bare structural frameworks, with additional components added on-site. The types of prefabricated panels vary based on material and form.
Structural Insulated Panels (SIPs): This type of panel consists of two structural facings with a layer of insulating material sandwiched in-between. The two structural facings or boards can be metal sheets, plywood, and cement. The insulating material can be polymer foams such as expanded polyurethane and polystyrene foams. These are manufactured by gluing the three pieces with strong adhesives or by letting the foam expand and cure whie forming between the facings. In both processes, the facings are clamped together. Pressure and temperature are applied until the adhesive or foam has cured.
Insulated Precast Concrete Panels: The construction of insulated precast concrete panels is similar to SIPs, where two structural facings bound an insulating material. In this type, the facings are concrete layers, called wythes. These wythes are usually pre-stressed to achieve higher structural performance. The insulation is a rigid material with proprietary designs. Since all three components are rigid and can act as load-bearing members, they can be combined and separated depending on the intended function. The connection between the wythes can be stiff, sliding, or deflecting. Fully composite panels are rigidly connected, which makes them able to resist higher loads. Non-composite connectors can slide or deflect and are strong in tension but weak in shear. They enable the wythes to act independently with each other. Non-composites are mostly used for applications such as refrigeration and cooling where high insulation is required.
Insulated Concrete Forms (ICFs): This type of panel uses rigid insulating materials as permanent formworks for creating reinforced concrete walls. The formworks, ties, and other supporting elements are prefabricated and installed on-site. They can be made as modular units that can interlock together to form a building. Ready-mix concrete is poured on-site, creating a permanent wall. Afterward, finishes and cladding systems can be directly applied to the insulating material. Though the structure is not completely prefabricated, the time and labor spent are mostly off-site. Most ICFs offer better performance than other panels since the main load-bearing structure is steel-reinforced concrete. Due to their monolithic construction, they are stronger and more resistant to moisture penetration.
Timber Frame Panels: These types of prefabricated panels are timber stud walls with plywood or proprietary facings attached to either side of the walls. Insulating materials are then fitted with insulation. Access for utilities such as cable conduits and piping is easier to install compared to other panels. Timber frame panels are cheaper, but the downsides are their weaker load-bearing strength, poor sound resistance, and susceptibility to biological attacks such as mold and termites. Chemical preservatives, fungicides, and insecticides are added to prevent such biological attacks.
Lightweight Steel Frame Panels: In this type, the main load-bearing members' studs are made of cold-formed steel, usually C-sections. They are assembled by welding, bolting, or other fastening methods. Facings and insulation materials such as gypsum board, stone wool, oriented strand brands (OSB), and expanded polystyrene foams (EPS) are added. The insulation can be placed within the thickness of the steel (cold frame) or outside of the steel framing (warm frame). Lightweight steel frames have a higher strength-to-weight ratio than other panels, but their capacity is limited to resist mostly static loads and some lateral loads such as wind and earthquakes. Another drawback is the high thermal conductivity of steel and the risk of interstitial condensation. Thus, a thicker insulation material is required.
Prefabricated modules are three-dimensional components, typically composed of four shop-assembled panels. These modules are arranged adjacent to or stacked on top of each other to form the complete building. They are connected on-site using inter-module connections that are bolted together. Modular buildings are a popular type of prefabricated construction, offering all the benefits associated with prefabrication. A single module can serve as a complete building, requiring minimal site work. Modular buildings come in various types depending on their construction methods.
Four-sided Modules: This type of module is manufactured with four closed sides creating a cellular space. The panel frames are load-bearing can transfer both vertical and lateral loads. The maximum height for this form is typically 6 to 10 stories, depending on site conditions. Applications for four-sided modules are hotels, small residential buildings, housing compounds, and dormitories.
Partially Open-sided Modules: This module consists of one or more walls made up of an assembly of panels that do not completely span the entirety of the wall. The partitions are open, serving as accessways or corridors connecting adjacent modules. Edges of the partially open sides have corners or intermediate columns or posts that transfer the vertical load as a replacement for the load-bearing panel. The typical height and applications for this form are the same as that of the four-sided modules.
Open-sided Modules: This module consists of one or two sides that are designed to be fully open. The long sides are usually removed so that a larger space can be created by attaching to other adjacent open-sided modules. The loads are transferred to the corner posts, which are connected to the edge beams by gusset plates and bracings. Since open-sided modules have lesser load-bearing members, they are not applicable for creating tall buildings. The typical height of this type of form is about two to three stories. Applications for these modules are hospitals and schools.
Hybrid prefab systems combine both three-dimensional modular components and two-dimensional panel elements to create either a whole building or specific parts of it. Also known as mixed modular and panel systems, this approach leverages the strengths of both methods. Modular units offer superior construction quality and detail but may face limitations due to assembly and transportation constraints. Three-dimensional modules are typically used for high-value, highly serviced areas like kitchens and bathrooms. In contrast, panelized components, which are flat-packed or ready-to-assemble, are utilized for floors and walls in more open spaces.
These types involve stand-alone modules or complete buildings that are delivered and installed on-site with pre-prepared foundations. They require minimal site work but are constrained by hauling capacity limits and road width and height clearances.
Most prefabricated buildings adhere to established design, engineering, and fabrication processes. While these methods are reliable, certain design variations enhance the marketability and appeal of prefabricated structures. One notable example is the Quonset hut, a distinctive design that has been in use for many years and remains popular as a temporary prefabricated building.
Quonset huts can be considered among the earliest examples of prefabricated buildings. Their design has been utilized by various civilizations throughout history and continues to be used today. However, in many municipalities, the appearance of Quonset huts has been restricted or banned.
Quonset huts are constructed from steel alloys and are characterized by their dome-shaped arch steel design. Among various prefabricated building types, Quonset huts are known for their affordability, low maintenance, and resilience against diverse weather conditions and earthquakes. Their design, which dates back centuries, has been used by multiple civilizations for shelter.
Modern Quonset huts build on the advantages of their ancient predecessors, incorporating contemporary engineering techniques and technologies. They are manufactured in two-foot-wide sections that can be easily reshaped and customized to meet specific needs. The fundamental structure of Quonset huts relies on the arch shape, providing structural support and an unobstructed interior design without intrusive supports.
The arches of Quonset huts are made of galvanized steel. The various panels are corrugated to enhance and strengthen the arches. This aspect of Quonset huts offers the unique features of surface and structure being the same. The segments and arches are bolted together with the arches anchored to the foundation slab. A base plate connected to the arches, lines up the arches without the need to twist them in place.
Fiberglass is used as a building material for prefabricated buildings due to its lightweight and durability. It is a mold-free and rust-free material that can be used for outdoor structures without being damaged by water or the build-up of moisture due to its tightly sealed structure. In the construction of prefabricated buildings, fiberglass is used for wall cladding material because of its resistance to wind, heat, and cold effects.
Fiberglass is available in a variety of shapes and designs, with numerous color options. Its flexibility makes it resistant to tensile stress due to its plastic composite base material, which enhances the strength and durability of fiberglass prefabricated buildings. Additionally, fiberglass provides excellent structural integrity and insulation, comparable to R-18 and R-24 ratings.
Prefabricated buildings can be used as temporary locatable buildings that are used indoors or outdoors. They have a panel-type construction with a frame made of steel or aluminum with a roof. The walls of shelters can be made of several types of materials, including fiberglass, glass, and polyvinyl chloride (PVC). Prefabricated temporary shelters can be installed and set up quickly. They are durable and maintenance-free and are capable of withstanding harsh weather, including radical temperature changes and humidity.
While prefabricated shelters can serve as huts for personnel protection, they are also commonly used as storage units for rescue equipment, sound dampening for pumps, and protection for sound room equipment. Available in various shapes and sizes, these shelters can be adapted to a wide range of applications. Their quick assembly and implementation, typically taking half the time of traditional methods, make them particularly suitable for emergency situations.
Temporary, relocatable modular buildings can be reused or repurposed many times and be moved to different sites. They are intended to serve as short-term buildings and are designed for easy teardown and relocation. The designation “temporary” refers to how the structures are installed. With proper care, temporary prefabricated buildings can last for 25 to 30 years, after which the components are typically repurposed or recycled.
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