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Chapter: Civil - Prefabricated Structures - Introduction

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Systems of prefabrication

Systems of prefabrication
System is referred to a particular method of construction of buildings using the prefabricated components which are inter related in functions and are produced to a set of instructions.


Systems of prefabrication:

 

System is referred to a particular method of construction of buildings using the prefabricated components which are inter related in functions and are produced to a set of instructions. With certain constraints, several plans are possible, using the same set of components, the degree of flexibility varies from system to system. However in all the systems there is a certin order and discipline. The system of prefabricated construction depends oon the extend of the use of prefab components, their characteristics to be considered in devising a system:

 

i.                   Intensified usage of spaces

 

ii.                 Straight and simple walling scheme

 

iii.              Limited sizes and numbers of components

 

iv.              Limited opening in bearing walls

 

v.                 Regulated locations of partitions

 

vi.              Standardized service and stair units

 

vii.            Limited sizes of doors and windows with regulated positions

 

viii.         Stuctural clarity and efficiency

 

ix.              Suitability for adoption in low rise and high rise blocks

 

x.                 Ease of manufacturing storing and transporting

 

xi.              Speed and ease of errection

 

xii.            Simple jointing system

 


a)     Based on Disuniting of member

b)    b) Based on the construction

Based on Disuniting of member:

 

1.     System consisting of linear member disunited at joints

 

2.     System for prefabricates of entire rigid frame

 

3.     System consisting of I,T,U of straight members disunited at points of minimum moment.

 

4.     Two hinged and three hinged arches

 

System consisting of linear member disunited at joints:

 

Advantage:

 

Disuniting at joints gives linear member. This means that a great advantage and facilitates from the view point manufacture and assembly.

 

Disadvantage:

 

1.     Joints are at corners i.e. at points of maximum moment values, so forming the joint is difficult.

 

2.     Joints must be over dimensioned to cope with insitu concreting. And one alternate solution to replace moment resistant joints by hinged connection.

 

System for prefabricates of entire rigid frame:

In this system, to reduce the no of joints and to precast larger numbers I one piece leads to the prefabrication of entire frame. Production of the frames does not cause any particular trouble but the hoisting is more difficult and requires careful preparation.

 

The stress distribution of staright members during hoisting is in general statistically determinate.

 

Advantage:

 

1.     It is ideal for site prefabrication.

 

2.     Small number of joints so rapid prefabrication work is possible.

 

3.     Suitable for long walls consisting of great number of uniform frames.

 

 

 

 

 

 

 

System consisting of I,T,U of straight members disunited at points of minimum moment:




Another method of disuniting of structures is by division into different membranes at points where the moments are thin or smallest. This method is called as lambda method. Using this method hinge joints are made.

 

 

 

Advantage:

 

1.     Functions are made at points of minimum moments or at points of contra flexure.

 

2.     Disuniting the main girder in this manner makes the application of different skylights possible.

 

Disadvantage:

 

1.     Hosting and temperature bracing of L joined asymmetric frame members is particularly complicated.

 

2.     Temperature resting of frame member on each other necessaitates the use of canilevers having half depth and proper forming of this cause difficulty.

 

Two hinged and three hinged arches

 

Arched structures are normally two hinged and three hinged arches. Arched structures are normally used for bridging span more than 20-25m. Their production and placing is more difficult than straight members. Arch can be two hinged and three hinged but they can also be fixed at footings nd can be constructed with or without tie.

 

These members are generally precast and assembled in statistically determinant three hinged variance and middle hinge is only eliminated after placing is finished. The reinforcing bars protruding both sides are welded together and the joint between the members is filled in with insitu concrete.

 

Arch structure can be precast in either vertical or horizontal positions. In the first case, shuttering made of timber or concrete is required having the same curvature s the arch itself. The prefabrication of larger arches in theh horizontal position is found to be more economical The construction of arch trusses can be properly carried out in the horizontal position only.

 

 

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