This article gives you comprehensive insights into prefabricated buildings. Read further to learn more about:
- An overview of prefabricated buildings
- Benefits of prefabricated construction
- Risks and challenges
- Types of prefabricated constructions
- And much more…
Chapter One – Overview of Prefabricated Buildings
Prefabricated buildings, or prefabs, are buildings with components (walls, roof, and floor) that are manufactured in a factory or manufacturing plant. These components can be fully or partially assembled in a factory which is then transferred at the construction site. This method of building construction is preferred due to its cost efficiency, fast turnaround, and reusability. Common applications of prefabricated buildings are temporary construction facilities, office spaces, medical camps, evacuation centers, schools, apartment blocks, and single-detached houses.
Prefabrication is more efficient than conventional on-site construction since manufacturing through a production line is more controlled. Because most buildings have repeating sections of walls, roof, and floors, a manufacturing process can be developed by putting together a sequence of operations. These operations can be studied and improved making the manufacturing process more efficient.
There are several recorded prefabricated buildings constructed throughout history which can be traced back to nomadic times when people were migrating to discover new lands. Colonization brought the need for transportable houses and camps. However, the need for prefabricated buildings was less urgent back then since once the colony was settled, there was no need for further development.
The method was only extensively developed starting in the 20th century. During the first and second world war, available labor for construction was reduced since men are mostly needed for the manufacture of war goods. This led to housing shortages that endured until the postwar era. Alternative methods of housing construction were considered to meet the demand. However, due to high initial costs and specialized labor required, the business has a higher risk than conventional construction. On top of that, housings produced through prefabrication have several defects and problems which ended up in governments compensating owners who unknowingly purchased housings with defects. A notable event that further depressed the acceptance of prefabrication was the collapse of a 21-story prefabricated tower block, known as the Ronan Point, in 1968.
Due to developments and standardization of construction and manufacturing methods, coupled with the growing need for housing and office spaces, prefabricated buildings are being innovated continuously in place of conventional construction. The emergence of modeling tools and processes such as BIM (Building Information Modeling) helps architects, engineers, and contractors by digitally representing the characteristics of the structure. BIM enables effective management of the assembly on-site, which reduces business risks associated with prefabricated construction.
Chapter Two – Benefits of Using Prefabricated Buildings
Prefabricated buildings provide plenty of benefits for manufacturers, contractors, and end-users. The idea of dividing activities on- and off-site enables greater flexibility on the project schedule and costing, provided that the planners have effective project management capabilities. Off-site fabrication also brings the advantages seen in optimized assembly lines. With regards to market opportunities, prefabricated buildings will ride the trend of eco-friendly or green sustainability. Market share of the industry, both residential and non-residential, is expected to rise in the following years.
Faster On-site Construction
One of the main benefits of manufacturing through a production line is a faster turn-around. Workers perform their respective tasks repetitively with defined operational sequences making their actions more efficient than workers in a conventional construction site. Some of the operations can be automated as well. With regards to project planning, prefabricated construction is faster since some of the activities can be done simultaneously. Take site clearing and foundation construction, for example. Actual construction of walls, roof, floors, and even finishing does not start until the site is cleared and the foundation fully cured. With prefabrication, some of the construction can be manufactured off-site while on-site operations are taking place. Moreover, structural members may be available already due to reuse or transfer. These are particularly useful for constructing temporary facilities where site activities are desired to be reduced to a minimum.
Reduced Effects of Uncontrolled Factors
These factors are weather, pollution, and other site restrictions. Weather is one of the major causes of construction delays since it affects worker productivity and safety. Harsh weather also accelerates wear and tear of equipment which causes their untimely failure. Restricted work activities and equipment breakdown can halt the entire construction. Also, ambient site conditions, along with pollution, can decrease the quality of work. Concrete curing, painting, and welding are some of the processes that can be affected.
With prefabrication, these external factors can be minimized. Structural components can be fabricated under more controlled conditions without being affected by the external environment. Shop welding and precast concrete manufacturing are some of the methods that are minimally affected by uncontrolled factors.
Higher Quality and Consistency
Since prefabricated components are manufactured in a controlled environment, it is easier to produce consistent output due to work familiarization. Repeated operations cause fabricators to have a natural flow of actions with minimal interruptions. This condition then contributes to enhanced craftsmanship. Also, operations such as concrete curing and painting can be controlled with consistency. Mix ratio, curing time, temperature, and moisture can be controlled which is difficult to achieve on-site.
Quality control is less complicated to implement than on-site construction since the structural components are designed to have repeating features. It is easier to standardize component dimensions and tolerances. Molds, formworks, and temporary fasteners are the same for a typical building component that produces constant dimensions. Also, the quality of a prefabricated building is more likely to be consistent with other prefabricated buildings constructed at different locations, provided that they have the same components. This is because the construction is less affected by local site conditions.
Faster on-site construction, reduced risks, and consistent quality ultimately result in savings. But aside from these, prefabricated buildings are cheaper because of the efficient utilization of raw materials and bulk production. Prefabricated building components can be produced in a batch process or a production line. Both of these processes have a predetermined amount of raw material needed to produce a particular number of building components. This effectively minimizes raw material wastage. Bulk production, on the other hand, contributes to cost reduction since it is cheaper to produce in larger volumes. Lower supplier prices and reduced manpower and transportation costs by bulk ordering and production all turn into savings.
Some prefabricated buildings are designed to be temporary. These are preferred in applications requiring temporary working spaces for project-based works such as construction jobs, remote healthcare services, research, and so on. Prefabricated buildings can be easily disassembled and transferred to different sites. This functionality also results in minimal alteration and preservation of the job site.
This benefit stems from the process’s efficient raw material utilization and reusability. Conventional construction not only has more wasted material, but has temporary components such as formworks, temporary fasteners, jigs, and fixtures which are discarded after construction. Buildings from on-site construction are mostly permanent. After its intended use, the building will be unoccupied until repurposed or demolished. Prefabricated, modular buildings are easier to be repurposed because of their mobility.
Fabrication shops have a more controlled environment than the conditions on-site. Exposure of workers to safety hazards and threats such as working at heights, weather, constrained spaces, and adjacent construction operations are greatly minimized since most of the work is done in fabrication shops. In the fabrication shop, different construction operations can easily be separated and ergonomically designed.
Chapter Three – Risks and Challenges in Prefabricated Construction
In a way, prefabricated construction challenges the existing methods of conventional construction. Prefabricated construction is not always applicable and may cause more drawbacks instead of gaining the expected benefits. Prefabricated construction can be considered as high-risk, high-reward that threatens trust from all stakeholders. Below are some of the risks and challenges associated with prefabricated construction.
High Capital Cost
Establishing a manufacturing line for a prefabricated building requires very high capital in comparison with conventional construction. Though the benefits, in the long run, can outweigh the initial costs, this creates a perception of the industry being high-risk, high-reward. This makes it difficult for small investors to enter the market while larger, more established players continue to widen the gap in competition.
Issues in Variety and Volume
Some advantages of prefabrication are derived from the repetitive nature of production. These are only true if there is a demand. The industry is very broad, with large amounts of special requests and unique customer requirements. This provides a challenge to the manufacturer, who must be able to adapt and innovate to create solutions that fit the customer’s needs. In addition, the volume of orders can be less than the breakeven point when stocking a particular style of prefab. This defeats all economic and time advantages supposedly offered by prefabrication.
More Complex Design Process
Aligning, connecting, and interlocking the components require additional engineering and planning. The initiation and design phases of the building require more support from specialized crafts that have experience in modular fabrication and assembly. This will lead to higher engineering costs.
Questions Regarding Performance and Lifespan
Despite advancements in structural modeling and simulation are improving, it is still difficult to predict the actual structural response of a prefabricated building as a whole. There is a lot of uncertainty on their performance and lifespan. Most of the time, it is difficult to prove claims of prefabricated component manufacturers without support from published research. Only actual site performance can determine the reliability and lifespan of the building. Thus, more research and case studies are needed to prove the realistic benefits of the method.
Chapter Four – Different Prefabricated Constructions
Prefabricated construction can be divided according to the degree of fabrication. It starts from individual elements which progress into assemblies and stand-alone structures. Below are the different types of prefabricated constructions.
Components include construction elements such as windows, doors, and trusses, which in itself are not a complete panel. Among the prefabricated materials, these have the least amount of off-site assembly, but usage is more flexible since they can be placed and installed according to actual site conditions.
Prefabricated panels are two-dimensional components that are put together on-site to form a building. They require more on-site work than modular prefabricated buildings. These components are commonly available as sub-assemblies with complete finishing and installed features such as windows, doors, and insulation. Panels can also be supplied as bare structural frameworks in which the additional components can be later added on-site. There are different types of prefabricated panels which vary according to 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 either by gluing the three pieces with strong adhesives or by letting the foam expand and cure while being formed in 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 an insulating material is bounded by two structural facings. 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, along with the 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 its monolithic construction, they are stronger and more resistant to moisture penetration.
Timber Frame Panels: This type 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 its 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 are added such as gypsum board, stone wool, oriented strand boards (OSB), and expanded polystyrene foams (EPS). 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 its capacity is only limited to resist mostly static loads and some lateral loads such as wind and earthquake. 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 in construction usually made up of four shop-assembled panels. Several modules are placed adjacent or on top of each other which forms the whole building. Modules are connected by inter-module connections which are bolted on-site. The term modular buildings are synonymous with prefabricated buildings because it is the most popular among all types. All advantages of a prefabricated building are seen in a modular building. A single module can be the complete building itself with minimal site works required. There are different types of modular buildings according to their form of construction.
Four-sided Modules: This type of module is manufactured with four closed sides creating a cellular space. The panel frames are load-bearing which 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 which can serve as accessways or corridors that connect 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. 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 by attaching to other adjacent open-sided modules a larger space is created. 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.
- Modules Supported by Primary Structure: In this type of module, an external steel structural frame is added to support and transfer loads. The external structure can provide open spaces at or below ground levels while the modules are stacked above. It can also act as a full support for the modules while enabling the walls and partitions to be non-load bearing. Typical applications for this type are residential and mixed-retail buildings.
Hybrid prefab systems utilize both three- and two- dimensional components from modular and panel systems to create a whole or a part of a building. This type can also be referred to as mixed modular and panel systems. Modular units have the advantage when it comes to construction quality and detail but are sometimes limited by assembly and transportation constraints. The three-dimensional modules are used for highly serviced and higher value parts such as kitchens and bathrooms. Panelized components are added to the assembly due to their flat pack or ready-to-assemble construction. Two-dimensional panels are used for floors and walls of more open areas.
These types are stand-alone modules or buildings. Complete buildings are delivered and installed at the site with prepared foundations. Complete buildings require the least amount of site works but are limited by hauling capacity limits and road width and height clearances.
- Prefabricated buildings, or prefabs, are buildings with components (walls, roof, and floor) that are manufactured in a factory or manufacturing plant. These components can be fully or partially assembled in a factory which is then transferred at the site.
- Prefabrication is more efficient than conventional on-site construction since manufacturing through a production line is more controlled.
- Some of the benefits of utilizing prefabricated buildings are faster on-site construction, reduced effects of uncontrolled factors, higher quality and consistency, cost efficiency, reusability, less raw material wastage, and reduced safety hazards.
- The risks and challenges, however, are difficulty in transportation and handling, low variety, more complex design process, and uncertainty in performance and lifespan.
- Prefabricated buildings can be classified according to the degree of construction. The different types of constructions are component, panel, module, hybrid, and complete buildings.