Precast Structural Systems
Precast concrete is a structural material that can be used as the primary structural system of a building transferring roof, floor, and lateral loads. Precast enables designers to integrate structural and envelope (architectural) systems reducing the total materials, detailing, costs, construction complexity, and more. The design versatility of precast concrete allows for almost any shape element, such as curved and radial sections, with flexibility in connection points and load paths. High strength, prestressed concrete structural members also deliver exceptional load-carrying capacity, which can result in smaller sections, longer spans, or both when compared to other structural systems. The design of precast, prestressed concrete structures depends on the integration of the structural system as a whole, the connections, and the individual components. Each aspect must consider the others as well as the functional requirements imposed by the building use. It is essential that design loads follow a load path from their point of origin to the final support or foundation. Although not always required by code, it is desirable to design the members and their connections to achieve a ductile, rather than a brittle, failure mode. In addition to resisting gravity loads, a principal consideration in building design is the lateral force-resisting system. There are a variety of precast concrete designs that can be used to achieve these goals economically and effectively. Precast structural systems can be designed as shear wall, moment resisting, or hybrid systems.
Benefits of Precast Structures
The considerable advantages of precast construction are combined with the inherent benefits of concrete to provide a superior construction product. These include:
- Inherent Fire Properties - Concrete has its own inbuilt fire resistance which is present during all construction phases. Fire resistance is typically achieved without the application of additional sprays or linings. This is an important inherent advantage over steel and timber solutions. Precast frames are generally designed for one hour inbuilt fire rating. This either totally eliminates or greatly reduces the need for additional fire protection and the associated costs.
- Economies are generated through reduced requirements for formwork, access scaffolding and less reliance on wet trades. Reduced on-site supervision by the main contractor is also a saving. Compared to cast in situ concrete, the following savings can be expected: Formwork 75 less Scaffolding 75 to 905 less Wet Concrete 905 less.
- Health & Safety - Once precast floor slabs are installed, they provide a safe working platform for site operatives. Simultaneously installing precast stairs offers safe and easy access between floors.
- Reduced Construction Programme, due to speed of construction, gives earlier return on investment, freeing up the project critical path and allowing earlier completion. It is estimated that a precast structure takes up to 205 less time to construct than a similar cast in situ structure.
- Greater Project Control from a completion/project management perspective and from a costing perspective.
- Factory Production ensures increased accuracy and quality of finish and decreases weather dependency. Compared with cast in situ structures, site labour is reduced by between 705 and 805 using precast. Work for following trades is reduced by between 305 and 705 depending on finishes.
- Build ability - Precast frames can greatly improve build ability because sensitive parts of the operation can be moved from the site to the factory.
- Larger Clear Spans - Reducing the number of columns is critically important in developments such as sports stadia and car parks. Longer spans and shallower construction depths can be obtained by using prestressed concrete beams and floors.
- Proven Designs and Methodologies - Precast frame design incorporates proven designs and methodologies which have been developed over many years.
- Sound Resistance - Precast structures meet the highest standards for resistance to sound transmission. Test figures show the airborne sound insulation of a 150mm concrete floor is 50 db.
- Composite Action - Prestressed precast elements act compositely with an in situ structural screed (topping), combining the benefits of precast and in situ construction.
- Less Wastage - Precast systems significantly reduce the amount of waste materials produced on site.
- Loose Reinforcement - The amount of loose reinforcement on site is reduced by between 805 and 905.
- Air tightness - Air infiltration in precast buildings is minimal because of the relatively small number of joints in the construction. This factor combined with the thermal mass of concrete gives excellent thermal performance.
- A Complete Service from design to manufacture to installation is available.