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7/26/2019 Chandresh-Choudhary1

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Composite Construction

Introduction to compositeconstruction of buildings


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General

These two materials complete one

another:

Steeland concrete

Concrete is efficient in compression and steel intension

Concrete encasement restrain steel against buckling

Concrete provides Protection against corrosion and

fire

Steel bring ductility into the structure


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Composite construction refers to any

members composed of more than one

material. The parts of these composite

members are rigidly connected such that

no relative movement can occur.

The main composite elements in buildings

are

1.Steel Concrete Composite Beam

2.Composite Slab

3.Composite Columnn


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Steel Concrete Composite Beam

Composite beams are normally hot rolled or fabricated

steel sections that act compositely with the slab. Thecomposite interaction is achieved by the attachment of

shear connectors to the top flange of the beam. These

connectors generally take the form of headed studs.


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The composite action increases the load

carrying capacity and stiffness of the beamby factors of up to 2 and 3.5 respectively.

It is normally designed to be unpropped

during construction, and must be sized to

support the self-weight of the slab, and other

construction loads, in their non-composite

state.


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size of the steel section is governed by serviceability

considerations because composite beams tend to be usedfor long span applications

Check that beam deflections during construction will not

lead to significant additional concrete loads (due toponding) that have not been allowed for in the design

The bending resistance of the section is normally evaluatedusing plasticprinciples


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The plastic moment resistance is calculated using

idealized rectangular stress Blocks.

It is assumed that stresses offyd and 0.85 fcd can be

achieved in the steel and concrete respectively


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Composite beams are generally shallower (for any

given span and loading) than non-composite beams,and they are used commonly in long span applications.

Consequently, deflections are often critical.


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The Various types of composite Beams


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Shear Connectors

These connectors are designed toTransmit longitudinal shear along the interface

prevent separation of steel beam and concrete slab

at the interface


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most common type of shear connector used in composite

beams for buildings is a 19 mm diameter by either 100mm or 125 mm long welded stud.


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The property of shear connector most relevant to design is the

relation-ship between the shear force transmitted, P, and the slip at

the interface, s This load-slip curve should ideally be found from

tests on composite beams.


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Composite slabsconsist of profiled steel decking with an in-situ

reinforced concrete topping.

The decking(profiled steel sheeting) not only acts as

permanent formwork to the concrete, but also providessufficient shear bond with the concrete so that, when the

concrete has gained strength, the two materials act

together compositely

span between 3 m and 4.5 m onto supporting beams

or walls


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If the slab is unpropped during construction, the

decking alone resists the selfweight of the wet

concrete and construction loads. Subsequent loads areapplied to the composite section.

If the slab is propped, all of the loads have to beresisted by the composite section.

are usually designed as simply supported members in thenormal condition


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Profiled steel sheeting

yield strengths ranging from 235 N/mm2to at least 460

N/mm2

depths ranging from 45 mm to over 200 mm

.8 mm and 1.5 mm thick

The various shapes provide Interlock between steel and concrete

frictional mechanical


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decking may also be used to stabilise the

beams against lateral torsional buckling duringconstruction.

stabilise the building as a whole by acting as adiaphragm to transfer wind loads to the walls and

columns

temporary construction load usually governs

the choice of decking profile


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COMPOSITE COLUMNS

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A steel-concrete composite column is a compression

member, comprising either a concrete encased hot-rolledsteel section or a concrete filled tubular section of hot-

rolled steel.


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The presence of the concrete is allowed for in two

ways.

protection from fire

It is assumed to Resist a small axial load

to reduce the effective slenderness of the steel

member, which increases its resistance to axial load.

The bending stiffness of steel columns of H-or I-section is much greater in the plane of the web (major-

axis bending) than in a plane parallel to the flanges

(minor-axis bending).


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There is no requirement to provide additional reinforcing

steel for composite concrete filled tubular sections.

The ductility performance of circular type of columns

is significantly better than rectangular types.

corrosion protection is provided by concrete to steel

sections in encased columns


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The plastic compression resistance of a composite cross-

section represents the maximum load that can be applied to

a short composite column.

While local buckling of the steel sections may be

eliminated, the reduction in the compression resistance of

the composite column due to overall buckling should

definitely be allowed for.


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JointsExample of vertical shear transfer

between beam and column

removed after concreting

bracket with shear connectors

contact pieceweld seam

reinforcement

bracket for the lower flange

shot-fired nails


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Aspects for using composite

structures:

Architectural

Economical

Functionality

Service and Flexibility

Assembly


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structures

Architectural:

Longer spans

Thinner slabs

More slender column

More generous opportunities fordesign


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structures

Economical: Reduction of height reduces the total of

the building --> saving area of cladding

Longer spans with the same height

--> column free rooms

Additional storeys with the same

total height of building

Quicker time of erection:

Saving costs, earlier completion of the building

Lower financing costs

Ready for use earlier thus increasing

rental income


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structures

Functionality:

Fire protection by using principles of reinforced

concrete in which the concrete protects the steel


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structures

Service and building flexibility:

Adaptable structures

Modification during the life of the building Modify services without violating the privacy of

other occupants

Accommodation of service facilities

in the ceilingwithin a false floor

in a coffer box running along the walls


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structures

Assembly: Working platforms of steel decking

Permanent shuttering

Reinforcement of profiled steel sheetings

Speed and simplicity of construction

Quality controlled products ensure greater accuracy


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Construction methods

Traditionally two counteracting methods ofconstruction could be observed both connected with

special advantages but also disadvantages worth

mentioning. Conventional concrete

construction method

Construction in steel

+freedom of form and

shapes

+easy to handle+thermal resistance

- time-consuming shuttering

- sensitive on tensile forces

+high ratio between bearing

capacity and weight

+prefabrication

+high accuracy

- low fire resistance

- need of higher educated

personal


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Construction methods

Composite Construction

comparing these two methods a combination of both

presents the most economic way

+higher bearing capacity

+higher stiffness

+plastic redistribution


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Examples

Millennium Tower (Vienna - Austria)

55 storeys

Total height 202 m Total ground floor 38000 m2

Capital expenditure about 145 million Euro

Time of erection: 8 months


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Examples


Composite columns

Concrete core

Composite Slim floor beams

Concrete slab

42,3 m

Composite frame

42,3 m


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Examples


Total time of erection: 8 month

max. speed 2 to 2.5 storeys per week!


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Examples

Parking deck DEZ (Innsbruck - Austria)

Erection of composite columns over 2 storeysAssembly of prefabricated concrete slabs


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Examples

Parking deck DEZ (Innsbruck - Austria) 4 storeys

Ground dimensions 60 x 30 m

Max. span length 10.58 m with

26 cm slim floor slab (= l/40)

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