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Chapter: Civil : Prefabricated Structures : Introduction

Principles of Prefabrication

The Main reasons to choose Precast Construction method over conventional in method., The main reasons to choose. Precast Construction method over conventional in situ method,



v The Main reasons to choose Precast Construction method over conventional in method.

1.    Economy in large scale project with high degree of repetition in work construction.

2.    Special  requirement in finishing.

3.    Consistency in structural quality control.

4.    Fast speed of construction.

5.    Constraints in availability of site resources(e.g.  materials  & Laborites )

6.    Other space & environmental constraints.

7.    Overall  assessment  of  some  or  all  of  the  above  factors  which  points  to  the

superiority of adopting precast construction over convention method.


The following details gives. The cost implications of precast construction & conventional in situ method.


8. Large groups of buildings from the same type of prefabricated elements tend to v look drab and monotonous.


1.  Local Jobs are last.


v The  main  reasons  to  choose.  Precast  Construction  method  over  conventional  in  situ method.        

1.       Economy in large scale project with high degree of repetition in work execution.

2.       Special architectural requirement in finishing.

3.       Colysistonay in structural quality control.

4.       Fast speed of construction.

5.       Constraints in availability of site resources ce.g.materials & labour etc..

6.       Other space & environmental constraints.

7.  Overall assessment of some or all of the above factors which points to the superiority of adopting precast construction over conventional method

The following details gives the cost implications of precast construction & conventional in situ method.


v Prefabrication Elements :

1.     Flooring / Roofing system.

2.     Priciest Beams

3.     Precast Columns

4.     Precast  walk panels.

5.     recast Stabs.


v Classification :


The Prefabrication is classified as follow from the view of degree of Precast construction.


1.     Small prefabrication

2.     Medium Prefabrication

3.     Large Prefabrication

4.     Cast in Site Prefabrication

5.     Off-Site (or) factory Prefabrication

6.     Open system of prefabrication

7.     Closed system of prefabrication

8.     Partial prefabrication

9.     Total prefabrication


v Small Prefabrication :

The first 3 types are mainly classified according to their degree of precast


Elements using in that construction for eg.:brick is a small unit precast and used in building.


This is called as small prefabrication. That the degree of precast element is very low.


1.     Medium Prefabrication :


Suppose the roofing systems and horizontal members are provided with pretested elements those construction are known as medium prefabricated construction here th degree of precast elements are moderate.


        Large Prefabrication :


In large prefabrication most of the members like wall panels, roofing / flooring Systems, beams and columns are prefabricated. Here degree of precast elements are high.


          Cast - in - site prefabrication : OFF - site (factory)  prefabrication :


One of the main factor which affect the factory prefabrication is transport. The width of mad walls, mode of transport, vehicles are the factors which prefabrication is to be done on site on factory.

Suppose the factory situated at a long distance from the construction site and the vehicle have to cross a congested traffic with heavy weighed elements the cost in side prefabrication is preferred even though the same condition are the cast in site prefabrication is preferred only when number of houses and more for small elements the conveyance is easier with normal type of lorry and trailers. Therefore we can adopt factory (or) OFF site prefabrication for this type of  construction.

            Open system of prefabrication :

In the total prefabrication systems, the space framers are casted as a single unit and erected at the site. The wall fitting and other fixing are done on site. This type of construction is known as open system of prefabrication.


        Closed system of prefabrication :


In this system the whole things are casted with fixings and erected on their position.


        Partial prefabrication :


In this method of construction the building element (mostly horizontal) required are precast and then erected. Since the costing of horizontal elements (roof / floor) often take there time due to erection of from work the completion of the building is delayed and hence this method is restored. In most of the building sites this method is popular more. Son in industrial buildings where the elements have longer spans. Use of double tees, channel units, cored stabs, slabs, hyperboloid shall etc., are some of the horizontal elements.


This method is efficient when the elements are readily available when the building reached the roof level. The delay caused due to erection of formwork, delay due to removal eliminated completely in this method of construction Suitable for any type of building provided lifting and erection equipments are available.


        Total Prefabrication :


Very high speed can be achieved by using this method of construction. The method can be employed for frame type of construction or for panel type of or the total prefabrication can be on site or off-site. The choice of these two methods depend on the situations when the factory produced elements are transported and erected site we call if off-site prefabrication. If this method is to be adopted then we have a very good transportation of the products to site. If the elements are cast near the building site and erected, the transportation of elements can be eliminated, but we have consider the space availability for establish such facilities though it is temporary. The choice of the method of construction also depends on the following;


1.     Type of equipment available for erection and transport.

2.     Type of structural scheme ( linear elements or panel)

3.     Type of connections between elements.

4.     Special equipment devised for special method construction.


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