1、PREFABRICATED STRUCTURESUNIT I INTRODUCTION Need for prefabrication Principles Materials Modular coordination Standardization Systems Production Transportation Erection. Prefabrication is the practice of assembling components of a structure in a factory or other manufacturing site, and transporting
2、complete assemblies or sub-assemblies to the construction site where the structure is to be located. The term is used to distinguish this process from the more conventional construction practice of transporting the basic materials to the construction site where all assembly is carried out.Prefabrica
3、tionis one of the architectural constructions. Large units of a building are produced in factories to be assembled, ready-made, on the building site. This technique permits the speedy erection of very large structures. Units may include doors, stairs, windows, wall panels, floor panels, roof trusses
4、 and even entire buildings. Prefabricated building:Prefabricated building is a type of building that consists of several factory-built components or units that are assembled on-site to complete the unit.The term prefabricated is buildings built in components (e.g. panels), modules (modular homes), t
5、ransportable sections (manufactured homes),It may also be used to refer to mobile homes. Different Between Prefabricated Constructions and Conventional Type:The conventional method of building a house is to transport bricks, timber, cement, sand, and construction aggregate, etc to the site, and to c
6、onstruct the house on site from these materials. In prefabricated construction, only the foundations and floor slabs are constructed in this way, while sections of walls and roof are prefabricated (assembled) in a factory (possibly with window and door frames included), transported to the site, lift
7、ed into place by a crane and bolted together. Need for Prefabrication: Cost of construction shorter construction time easy of expansion utilisation of material attractive finishes highly efficient for weather resistance single source assurance insurance advantageMaterial Properties In of Prefabricat
8、ed Structures Quicktoassemble Cost-effective Portable/movable Strong Waterproof, Moistureproof Fire ResistantPrefabrication Types:1. Conventional prefabrication construction is the most traditional construction method where all the construction activities are in-situ practices on site:2. Semi-prefab
9、rication divides as two sub-categories: system formwork and non-structural semi-prefabrication, involving a part of in-situ construction activities and a part of prefabrication. Normally, the non-structural semi-prefabrication is applied on faade, curtain walls, lost form systems and dry wall system
10、s;3. Comprehensive prefabrication involves a structural part and pre-finished construction. Examples of applications of structural comprehensive prefabrication include staircases, slabs, columns and beams: and4. Volumetric off-site fabrication encloses usable space but does not constitute the whole
11、building. Volumetric off-site fabrication is mainly used for facilities and includes solutions on office washrooms, plant rooms, building services risers and lifts.ADVANTAGES OF PREFABRICATION1. Self-supporting ready-made components are used, so the need for formwork, shuttering and scaffolding is g
12、reatly reduced. 2. Construction time is reduced and buildings are completed sooner, allowing an earlier return of the capital invested. 3. Quality control can be easier in a factory assembly line setting than a construction site setting. 4. Prefabrication can be located where skilled labour is more
13、readily available and costs of labour, power, materials, space and overheads are lower. 5. Time spent in bad weather or hazardous environments at the construction site is minimized. 6. Less waste may be generated and in a factory setting it may be easier to recycle it back into the manufacturing pro
14、cess, for instance it is less costly to recycle scrap metal generated in a metal fabrication shop than on the construction site. 7. On-site construction and congestion is minimized.DISADVANTAGES OF PREFABRICATION1. Careful handling of prefabricated components such as concrete panels or steel and gla
15、ss panels is required. 2. Attention has to be paid to the strength and corrosion-resistance of the joining of prefabricated sections to avoid failure of the joint. 3. Similarly, leaks can form at joints in prefabricated components. 4. Transportation costs may be higher for voluminous prefabricated s
16、ections than for the materials of which they are made, which can often be packed more compactly. 5. Large prefabricated sections require heavy-duty cranes and precision measurement and handling to place in position. Modular coordination:Modular coordination or MC is a dimensional system. It is a dim
17、ension and space coordination concept in which building and components are placed at their designations based on the unit or basic module known as “1M“ that equals to 100 mm. The use of MC is an important factor in IBS effective application as it completes the industry through quality control and in
18、crease of productivity.MODULAR HOMESModular homes are houses divided into multiple modules / sections which are manufactured in a remote facility and then delivered to their intended site of use. The modules are assembled into a single residential building using either a crane or trucks.Steel and/or
19、 wood framing are common options for building a modular homes.Modular components are typically constructed within a large indoor facility on assembly lines.Modular homesIndoor Construction of modular homesSuch dwellings are often priced lower than their site-built counterparts and are typically more
20、 cost-effective to builders and consumers.These new homes can be constructed in less time than it takes to build a home “on-site“. Importance of modular homes1. Indoor construction. Assembly is independent of weather which increases work efficiency and avoids damaged building material. 2. Favourable
21、 pricing from suppliers. Large-scale manufacturers can effectively bargain with suppliers for discounts on materials. 3. Ability to service remote locations. Particularly in countries such as Australia there can be much higher costs to build a site-built house in a remote area or an area experiencin
22、g a construction boom such as mining towns. Modular homes can be built in major towns and sold to regional areas.4. Low waste. With the same plans being constantly built, the manufacturer has records of exactly what quantity of materials is needed for a given job. While waste from a site-built dwell
23、ing may typically fill several large dumpsters, waste from a modular dwelling generates much less waste.5. Building Strength. According to manufacturers, modular homes are generally designed to be initially stronger than traditional homes.For example, replacing nails with screws and adding glue to j
24、oints. This is supposed to help the modules maintain their structural integrity as they are transported on trucks to the construction site. 6. Assembly Time. The modules that will combine to form the house are assembled off-site in a factory. These modules can take one to three months to be construc
25、ted but often take as little as 10 days to actually construct when they are first started. Then they are transported to the building site where a crane is brought in to assemble the modules together.The placement of the modules together generally takes several hours or days. Mobile HomesMobile homes
26、 or static caravans are prefabricated homes built in factories, rather than on site, and then taken to the place where they will be occupied. They are usually transported by tractor-trailers over public roads to sites which are often in rural areas or high-density developments. In some countries the
27、y are used for temporary accommodation on campsites. While these houses are usually placed in one location and left there permanently or they have the ability to be move.Behind the work fitted at installation to hide the base, there are strong trailer frames, axles, wheels and tow-hitches. The two m
28、ajor sizes are single-wides and double-wides. 1. Single-wides are eighteen feet or less in width and 90 feet (27 m) or less in length and can be towed to their site as a single unit. 2. Double-wides are twenty feet or more wide and are 90 feet (27 m) in length or less and are towed to their site in
29、two separate units, which are then joined together. Mobile homesProduction, Transportation and Erection:Considerations in production The general rule is that the connection device must be as foolproof as possible. It should be possible to place it in the mould correctly oriented and within the neces
30、sary tolerances, with a minimum of effort. 1. Avoid congestion At the location of connections additional reinforcing steel, embedded plates, inserts, block-outs etc. are frequently required. It is not unusual that so many items are concentrated in a small location that very little room is left for t
31、he concrete. It must be kept in mind that reinforcing bars and prestressing strands, which appear as lines on the drawing, take up real space in the elements. Reinforcing bars require more space than their nominal diameter, and there must be room for the curvature of bent bars. If congestion is susp
32、ected, it is helpful to draw large-scale details of the region in question. 2. Avoid penetration of the forms Units requiring holes in the forms should be avoided if possible, especially in steel moulds. Exceptions to this rule can be made if there is a substantial amount of repetition in the produc
33、tion. Holes in the forms may be necessary not only because the units are protruding from the elements, but also for the arrangement used to keep the units in place during casting. The units must also be designed so that they do not make the dismantling of the form impossible without damage to the fo
34、rm. Most forms are supposed to be used more than once. Connection units to be placed in the top surface during casting should be secured against the edges of the mould using purpose made holding devices. These devices are mostly costly, make it more difficult to obtain a smooth surface, or the holdi
35、ng device may hamper the placement of concrete or other surface material. The various disadvantages have to be evaluated before selecting the method. However, if the same steel plate is placed in the bottom of the form, it can be located with great accuracy, as it can be fixed to the bottom directly
36、. 3. Reduce post-stripping work A plant casting operation is most efficient when the product can be taken directly to the storage area immediately after removal from the form. Any operations required after stripping and before erection, such as special cleaning or finishing, welding on projecting ha
37、rdware etc. should be avoided. These operations require additional handling (increased possibility of damage to the elements), extra workspace and added labour, often with skilled trades. Sometimes a trade-off is necessary between penetration of the forms and post-stripping work. 4. Use repetitious
38、details It is very desirable to repeat details as much as possible. Similar details should be identical, even ifit results in a slight over-design. 5. Use standard items Hardware items such as inserts, studs, steel elements, etc., should be readily available standard items that are preferably from m
39、ore than one supplier. It also simplifies fabrication if similar product items are standardised as to size and shape. There is also less chance of error. The same principle applies to reinforcing bars, embedded plates, etc. 6. Be aware of material limitations Examples of this are the radius requirem
40、ents for bending reinforcing bars, standard lengths for certain sizes of inserts, etc. 7. Avoid non-standard tolerances Dimensional tolerances, which are specified to be more rigid than industry standards, are difficult to achieve. Connections, which require close-fitting parts without provision for
41、 adjustment, should be avoided as much as possible. 8. Allow alternatives Very often precasters will prefer certain details. The producer should be allowed to use alternative methods or materials, provided the design requirements are met. Allowing alternative solutions willoften result in the most e
42、conomical and best performing connections.Considerations for transportation During transportation any units protruding from the concrete element must be shielded in order not to create a hazard to people. Protruding units must be able to withstand any shocks they can be subjected to during handling.
43、 Protruding units, like reinforcing bars, can in many cases be difficult to handle during transportation. For example, a wall panel shall be transported standing at the edge, but has reinforcing bars protruding at the bottom. This will make it necessary to build up the support on the trucks, which i
44、s costly, takes time, and makes the load less stable. This problem can be solved by letting the bars protrude from the top of the element, but then the total height may make it difficult to negotiate the underpasses en route. The solution may then be to have the protruding bars replaced by insert an
45、d threaded bar, to screw in after the element has been transported to the site. If protruding units do not create the kind of problems described above because they do not stick out that much, there still may be some difficulties. For example: corbels pointing down during transport may necessitate a
46、lot of additional support provisions for the columns on the trucks. The consequence can be a less stable load, or decreased loading capacity of the truck. This kind of problem can be solved by making columns with the corbels in one plane only, and then place every second column top-to-bottom on the
47、trucks. Otherwise it is also possible to look for a corbel-free solution. Considerations for Erection To fully realise this benefit of fast erection of a precast structure, and to keep the costs within reasonable limits, field connections should be kept simple. In order to fulfil the design requirem
48、ents, it is sometimes necessary to compromise fabrication and erection simplicity. 1. Use connections that are not weather sensitive Materials such as grout, dry pack, cast-in-place concrete and epoxies need special provisions to be placed in cold weather. Welding is slower when the ambient temperat
49、ure is low. If the connections are designed so that these processes must be completed before erection can continue, costly delays may result. 2. Stability of the elements Some elements may require propping, shoring, bracing or fastening before the hoist can be unhooked. Planning for the fewest, quickest and safest possible operations to be executed before releasing the hoist will greatly facilitate the erection. 3. Stability of the structure In every stage of the erection process the stability of the structure as a whole must be planned
