Flashcards in week 8 prefab Deck (23)
what is prefab?
Prefab is short for prefabrication.
It refers to any part of a building that has been fabricated at a place other than its final location.
For this reason, it can be referred to by other names such as off-site fabrication, off-site construction or off-site manufacture.
Prefabrication is an ‘umbrella’ term and it covers a range of different systems and processes. These will include structural, architectural and services elements.
types of prefab
There are two main families of prefab systems;
2D prefab and
These systems can be used on their own, used as hybrids with each other, or used in conjunction with traditional construction approaches.
explain 2d panel building
2D prefab is made up of pre-cut, pre-sized, pre-moulded or pre-shaped components that are assembled or installed on site.
They often arrive as flat-packed panels or non-volumetric systems, ready for assembly.
They might form the building envelope, stair cores, internal load bearing walls or lighter partitions.
They might be open or closed panel systems, precast concrete panels or other panel types.
2D prefab is easier to transport, lends itself to mass customisation and has infinite construction options, combined with speed of assembly
explain 3d modular building
3D prefab systems are three-dimensional structural units which are combined at site with other units or systems, or might comprise an entire small building.
They include pods, which are generally not structural modules, such as bathroom or kitchen pods.
They are a fast way to build, as they can be manufactured concurrent with site preparation, and can arrive on site almost complete.
3D prefab systems can be joined together to create larger spaces and they are increasingly demonstrating their ability to go multi-level.
The elements of 3D prefab may be structural elements, architectural elements or services elements, or they may be a hybrid of these
Stage 1: Planning
Stage 2: Proposal
Stage 3: Construction
Stage 4: Installation
Stage 5: Completion
After having secured the land for your project, consideration must be given to the type of prefabricated modular building and the construction method that will be suitable for the site and your budget.
Comply with the local building zoning requirements and that is within your budget.
If the project requires special planning and design to seek local Council's development approval, most firms will have a in house architects that can assist.
Detailed proposal is prepared which will include pricing on the modular building, recommended options, detailed specification and scope of work.
The proposal will include a scope of work and will list items that will be required to be undertaken by the client.
Should the proposal be acceptable, a deposit will be required to prepare drawings and for a contract to be drafted.
Once the drawings have been completed and a contract signed, the plans will be submitted for building approval before the construction can commence
explai construction phase
After the execution of the contract, shop drawings, material and colour samples will be presented for approval and selection.
Two parallel activities will take place during this stage, namely,
(1) the building will go into a production slot at the factory and
(2) the preparation of the site including clearing and grading if necessary, and foundation work. Timing will vary according to the size and scale of the building.
When the modules arrive on site, an installation crews will be there ready to begin the installation under the guidance of the appointed building contractor.
Depending on the size and complexity of the modular building, this can take between 5 and 15 working days for an average house.
However, if there is a great deal of work to be performed on site, such as tape, skim, prime and paint dry walls, or if it is a large building over 300 square meters, the installation will take longer.
This is the "handover" stage of the project where the work contracted for has been completed and the building can be occupied.
At this point a walk-through inspection will be conducted with the client or the appointed representative and a representative of the modular construction firm.
A list of any item that has not been completed to the client's is recorded that will be addressed in a specified time frame. Once this list is completed the building is then turned over to you.
After 3 months of occupation, the client will prepare a defects list which records items that require additional attention
explain offsite precasting concrete
Off-site precasting is carried out in a controlled environment to avoid weather related variables.
This minimises waste and delivers the highest level of quality assurance in the range of finishes and dimensional accuracy. It also reduces on-site materials storage and waste.
Controlled pouring environments in the factory also reduce concrete curing times through the use of temperature control and advanced mix design, including low water to cement ratios.
They can reduce the amount of cement required (the highest source of embodied energy) to achieve adequate strength to withstand transport and erection loads.
explain tilt up / onsite concrete
Tilt-up wall panels are cast on a horizontal surface and require tilting to vertical in their final location.
This is often a more practical solution on small sites or where transport access or site constraints preclude the use of off-site precast methods.
Precast panels are usually formed up on existing concrete floor slabs either as ‘stack casting’ (one on top of the other) or as individual slabs near their final point of erection.
Bond breaker compounds are used to ensure that stack cast slabs are separable after curing. Bond breaker failure (often due to poor application) can lead to considerable waste, cost and delay.
While tilt-up overcomes transport and other logistical problems, it is slower than precast because walls can’t be poured before or during floor construction.
why use precast panels
speed of construction
reliable supply — made in purpose-built factories and not weather affected
high level performance in thermal comfort, durability, acoustic separation, and resistance to fire and flood
highly flexible in form, shape and available finishes
ability to incorporate services such as electrical and plumbing in precast elements
minimal waste, as most waste in the factory is recycled
safer sites from less clutter
high thermal mass, providing energy cost saving benefits
problems of precast panels
Each panel variation (especially openings, bracing inserts and lifting inserts) calls for complex, specialised engineering design.
It is often more expensive than alternatives (can be offset by reduced construction times, earlier access by following trades, and simplified finishing and services installation).
Building services (power, water and gas outlets; conduits and pipes) must be accurately cast in and are difficult to add or alter later. This requires detailed planning and layout at design stage when plumbing and electrical trades are not usually involved.
Erection requires specialised equipment and trades.
High level site access and manoeuvring room for large floats and cranes free of overhead cables and trees is essential.
Temporary bracing requires floor and wall inserts that have to be repaired later.
It has high embodied energy.
Performance of Precast Panels can be judged by
explain peforamance indicator of apperarance
An almost endless variety of shapes, colours, textures and finishes is available for precast concrete. It can be moulded and shaped to suit almost any design or form.
Complex or detailed methods and finishes require a high degree of quality control and are often best handled at a off-site precast factory
Simple surface treatments are:
rebating and grooving
explain strcutual capacity
Structural precast concrete panels are a strong, durable and versatile building material, particularly suited to Australia’s harsh climatic extremes and requiring minimal maintenance.
Concrete is flood and fire resistant and doesn’t shrink, rot or distort.
It gains strength as it ages and in structural terms is ideally suited to the unpredictable conditions associated with climate change.
The inherent structural properties of precast walls mean that they do not require additional bracing to resist racking loads and that simple design of cross walls and junctions can provide adequate lateral bracing.
Precast finishes are highly impact resistant, withstand wear and tear, and require minimal repairs and maintenance.
explain sound insulation
Precast concrete provides one of the highest levels of acoustic separation of any common housing construction system for both internal and external walls.
Joints and openings must be detailed properly to maintain sound ratings. Precast concrete has fewer joints, and dimensional accuracy allows for the snug fitting of acoustically sound windows and doors.
Because services outlets are cast into wall panels, sound transmission weakness around switches and power outlets is avoided and transmission through cavities or air spaces is prevented
explain embodied energy
As with other high mass products such as brick, the embodied energy of precast concrete is arguably its most significant environmental impact.
The life expectancy of concrete structures is more than 100 years which requires it top be designed to be reused, extended or retrofitted to ensure that their initial embodied energy is amortised over their life span.
Careful design of precast panels for disassembly can facilitate reuse or recycling when the building reaches the end of its useful life.
Precast concrete elements can be crushed and reused as aggregate for new concrete or for road bases or construction fill, creating economic and environmental savings.
The erection sequence determines the order of transport. Careful planning of the erection sequence to minimise crane positions reduces transport and cranage costs.
Site access for trailers and cranes is a critical consideration at the design stage and may preclude the use of precast construction.
Check the delivery route to site before designing panels, and limit wall heights (including truck or float deck height) to fit under bridges, overhead cables and other height restrictions.
Site manoeuvring requirements for trailers and cranes can reduce these allowances.
describe different elements involved in prefabricated construction
Substructure - Foundations and works below ground
Frame - the structure of the building
Envelope - the external walls and roof that form the building
Services - The mechanical and electrical, plumbing services
Internal works - the internal walls, raised floor, suspended ceilings and applied finishes, fittings
Facilities - the major parts of the building. For use of the end-user, such as the toilet/washrooms, kitchens, lifts, plant rooms and building management system rooms etc.