Design principles in prefabricated structures unit iii ce6016 pfs

DESIGN PRINCIPLES
UNIT – III
CE6016 PREFABRICATED STRUCTURES

Contents
 Disuniting of structures
 Design of cross section based on efficiency of material used
 Problems in design because of joint flexibility
 Allowance for joint deformation.
Prakash Kumar Sekar A.P Civil Engineering

Disuniting of structures
 The solution of problems connected with the transportation and placing of
structures demands as a rule their disunity into smaller members
 In general there is a trend towards the use of large members because of reasons
like lower cost in hoisting, savings in joint cost and speed construction
 Advances in the construction technology also lead to the use of larger members
 Disuniting is performed in places having minimum moment

System of Disuniting
Systems consisting of linear members disunite at joints
Systems for prefabrication of entire rigid frames
Systems consisting of L, T and U shaped or straight
members disuniting at points of minimum moments

Design of cross section based
on efficiency of material used

Cross Section for precast structures – Beams & Columns

One of the great advantages of precast reinforced concrete
structures over monolithic ones lies in the possibility of
forming cross section which from the viewpoint of the
theory of strength of materials
The economy of cross section is measured by the form
factor Ф value of which is
Ф = F / F’
F = area of the investial cross sections
F’ = area of a rectangular cross section which the depth h
width b
For Rectangular cross section Ф = 1

For I , T , U & V shaped section, Ф <1.
For reinforced concrete structures their cross section being
not homogenous the relation is not so simple and the
value of suppliesonly an approximate measure of savings
For I profile steel section Ф = 0.31 to 0.33
For prestressed concrete section Ф = 0.45 to .50

Fretted structures, Trusses and Virendeel
There is generally no difference between construction between
fretted and solid beams
The reason for different openings in the body of the fretted
beams is merely to obtain savings in the materials and to lessen
the dead load

Virendeel
The use of reinforced concrete structures having divided cross section
as have virendeel columns and truss is becoming increasingly common
due to economic and aesthetic reasons
The manufacturing of these structures in a horizontal position requires
less material for their shuttering, their reinforcement and concreteing is
lso fairly simple.
Other structures like tubes, pipes can be easily fitted to these strucures

Joints and problems in Joints
due to Joints flexibility

Dimensioning of joints
 While lengthening of steel bars for joint the reinforcement in the precast and
cast insitu joint will be different hence reinforced concrete code book is
necessary for design
 Then lengthening of tensioned steel bars should be avoided, but if such a
lengthening should be inevitable proper welded joints should be provided
 Best welded lengthening is an arc-welded joint consisting of four welds and
two laps

Problems in design because of joints flexibility
General design
Based on architecture
Based on structural analysis

General design Based on Architecture
 Modern layout in dimensions and gridlines setting out as this will enable
easy standardization in precast design
 Suitable size of panels position of joints and edge details
 Standardization of sections will ensure the minimizing of panel types and
major design variation as far as possible

General design Based on Structural system
 Overall structure framing design and stability at various stages of construction
 Selection of structural elements to be precast
 Types of connection design to ensure structural adequacy and practically
insitu execution
 Connection design to allow for panel tolerences, adequate space within joints
to avoid rebar elasticity and congestion

Major consideration in design of precast
concrete systems
 Load assessments and load paths
 Establishment of suitable structural form or system
 Precast components selection and panelization with stardization

Joints and connections
 In precast connection design apart from strength requirement, other
consideration such as ease of manufacturing, election and tolerance for
work efficiency
 In theory all joint connection can be designed with structural adequacy for
its performance needs with precast elements manufacture as per design
 All joints shall also cater for water tightness, durability, fire and aesthetic
conditions

Type of Precast joint
Compressive joint
Tensile joint
Shear joint
Flexural and torsion joint

Compressive joint
Using direct bearing as intermediate medium such as mortar
Tensile joint
Use of steel splice connections, welding of cast in steel plates,
lapping, grouting or starter / site laid rebars
Flexural and torsional joint
Moment joint connection can be achieved with force coupling
using splice, bolting, welding, composite joint casting with rebars etc..,

Allowance and joining of prefabricated
members
 Precast reinforced structure are of many types
Load carrying structural members
Space bordering members
Surface forming structural members
 Load carrying structures can be divited into main groups according
to their disuniting into members or by dividing prefabrication
having identical dimensions

Design principles in prefabricated structures unit iii ce6016 pfs

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