Building construction is generally categorized into two types. The first one is called cast in-situ, where all the resources and materials are brought to the site where the construction is to be made. Then they are put together and fabricated into forming the building.
The other type is pre-engineered building or PEB. Usually, a pre-engineered building is totally composed of steel structures. A pre-engineered building is always built according to the suitable and available inventory and by employing manufacturing methods that conform to the structural and aesthetic requirements. The built-up sections of a pre-engineered building are actually fabricated at the factory to the exact size and then transported to the site. There at the site, the building is finally assembled out of the components using bolted connections.
Components of pre-engineered buildings :
- The primary framing structure of pre-engineered buildings used to consist of an assembly of I-shaped members called I beam. In pre-engineered buildings the I beams are formed by typically welding steel plates together to form the I sections. After they are formed, they are then taken to the field and assembled there to give structure to the pre-engineered building.
- Other structural components of a pre-engineered building include hot, and cold rolled steel sections, trusses, mill sections, etc. mill sections are preferred more than three-plate welded, castellated beams.
- Cold-formed Z and C shaped members are generally used as secondary structural elements to fasten and support the external cladding, and these are known as purlin members.
- Other than roll-formed profiled steel sheet; wood, tensioned fabric, precast concrete, masonry block, glass curtain wall or other materials may be used for the purpose of external cladding of the building.
- A sandwich panel is also an essential component of a pre-engineered building in which there are three layers, and a non-aluminum core is inserted in between two aluminum sheets.
- Mezzanine floors, bolts, and insulation form the other accessories of pre-engineered buildings.
Design methodology :
- The design of a pre-engineered building is usually based upon the stiffness matrix method.
- AISC, ASCE and IS:800 are the standard codes used for the construction of a pre-engineered building.
- The load calculations of a pre-engineered building are diverse. They usually include :
- Dead Load: it generally implies the self-weight of the structural components
- Live load: it generally implies the capacity of the structure to bear moving loads contributed due to humans
- Wind load: the thrust provided by the wind should also be taken into consideration
- Seismic load: the place where the pre-engineered building is constructed should be identified as to which seismic zone it belongs in. Earthquake load or seismic load should be considered for the place as a safety measure.
- All these load calculations are done taking different combinations, and a factor of safety is always added to the final load count so as to provide a cushion.
Advantages of pre-engineered buildings :
- Quality control: The primary advantage of a pre-engineered building comes in the form of quality control. Since all the structural components of a pre-engineered building are engineered beforehand, standards of different relevant codes are taken into consideration, and finally, these components are made in a factory; therefore, the quality is maintained. Generally, the quality of all these members is supervised by a quality control engineer or supervisor.
- Cost reduction: Due to monetary savings in design, the manufacturing cost and costs due to erection on site is reduced by a huge margin.
- Construction time is minimized: Due to the use of software for designing various components, the construction time is lessened.
- Low maintenance costs: Steel members are generally more durable than normal building components. The paints applied over steel members of pre-engineered buildings makes them able to withstand external forces and hence the maintenance costs that include refurbishing of paints, are cut down. Therefore the maintenance costs are much lower compared to conventional steel buildings.
- Erection time is lowest: Since the members are pre-manufactured, and skilled workforce is employed to assemble the prefabricated members, the time of the erection of the building is very less.
- Warranty period: Every building comes with a warranty period; a period for which it will stay erect and strong without the damage of any of its major components. The warranty period of pre-engineered buildings is generally on the higher side, with manufacturers usually claiming a warranty period of 20 years to be associated with a pre-engineered building.
Disadvantages of pre-engineered buildings :
Although pre-engineered buildings come with a bunch of advantages in the field of structural engineering, it is not fully rid of disadvantages. These can be enumerated as :
- Rusting or corrosion: The basic design of the pre-engineered building should focus on steel consumption. There is no point in using shortcuts while fabricating the steel components as it will affect the customer directly and also the manufacturer. If the steel used in the fabrication of the components or members of the pre-engineered building is sub-standard or of low grade, then the structure might become weak and will be exposed to heavy damage. The installation can also be stalled as the manufacturer would not be able to complete the construction within the committed cost. This is because losses will be incurred in repairing the damages. Moreover, if the paint used to coat the steel members is of low quality, then the underlying steel components will be exposed, and corrosion or rusting may creep in, which will result in the reduction of the steel strength and bearing capacity.
- Insulation costs: Insulation costs should be taken into account, and they generally tend to increase the overall cost of the building.
- External appearance: The framing and steel structures appear very unattractive when left exposed and diminish the aesthetic and architectural aspects of the building.