Composite Construction Introduction to composite construction of buildings

Composite Construction Introduction to composite construction of buildings 1

General Steel and concrete These two materials complete one another: Concrete is efficient in compression and steel in tension Concrete encasement restrain steel against buckling Concrete provides Protection against corrosion and fire Steel bring ductility into the structure 2

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 3

Steel Concrete Composite Beam Composite beams are normally hot rolled or fabricated steel sections that act compositely with the slab. The composite 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. 4

The composite action increases the load carrying capacity and stiffness of the beam by 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. 5

size of the steel section is governed by serviceability considerations because composite beams tend to be used for long span applications Check that beam deflections during construction will not lead to significant additional concrete loads (due to ponding) that have not been allowed for in the design The bending resistance of the section is normally evaluated using ‘plastic’ principles 6

The plastic moment resistance is calculated using idealized rectangular stress Blocks. It is assumed that stresses of fyd and 0. 85 fcd can be achieved in the steel and concrete respectively 7

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. 8

The Various types of composite Beams 9

Shear Connectors These connectors are designed to Transmit longitudinal shear along the interface prevent separation of steel beam and concrete slab at the interface 10

most common type of shear connector used in composite beams for buildings is a 19 mm diameter by either 100 mm or 125 mm long welded stud. 11

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. 12

Composite slabs • consist 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 provides sufficient 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 13

If the slab is unpropped during construction, the decking alone resists the selfweight of the wet concrete and construction loads. Subsequent loads are applied to the composite section. If the slab is propped, all of the loads have to be resisted by the composite section. are usually designed as simply supported members in the normal condition 14

Profiled steel sheeting • depths ranging from 45 mm to over 200 mm • yield strengths ranging from 235 N/mm 2 to at least 460 N/mm 2 • . 8 mm and 1. 5 mm thick The various shapes provide Interlock between steel and concrete frictional mechanical 15

• decking may also be used to stabilise the beams against lateral torsional buckling during construction. • stabilise the building as a whole by acting as a diaphragm to transfer wind loads to the walls and columns • temporary construction load usually governs the choice of decking profile 16

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COMPOSITE COLUMNS A steel-concrete composite column is a compression member, comprising either a concrete encased hot-rolled steel section or a concrete filled tubular section of hotrolled steel. 18

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 Isection is much greater in the plane of the web (‘majoraxis bending’) than in a plane parallel to the flanges (‘minor-axis bending’). 19

The ductility performance of circular type of columns is significantly better than rectangular types. There is no requirement to provide additional reinforcing steel for composite concrete filled tubular sections. corrosion protection is provided by concrete to steel sections in encased columns 20

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. The plastic compression resistance of a composite crosssection represents the maximum load that can be applied to a short composite column. 21

Joints u Example of vertical shear transfer between beam and column 22

Aspects for using composite structures: u Architectural u Economical u Functionality u Service and Flexibility u Assembly 23

Aspects for using composite structures Architectural: u Longer spans u Thinner slabs u More slender column u More generous opportunities for design 24

Aspects for using composite structures Economical: u Reduction of height reduces the total of the building --> saving area of cladding u Longer spans with the same height --> column free rooms u Additional storeys with the same total height of building u Quicker time of erection: Ù Saving costs, earlier completion of the building Ù Lower financing costs Ù Ready for use earlier thus increasing rental income 25

Aspects for using composite structures Functionality: u Fire protection by using principles of reinforced concrete in which the concrete protects the steel 26

Aspects for using composite structures Service and building flexibility: u Adaptable structures Ù Modification during the life of the building Ù Modify services without violating the privacy of other occupants Ù Accommodation of service facilities in the ceiling within a false floor in a coffer box running along the walls 27

Aspects for using composite structures Assembly: u Working platforms of steel decking u Permanent shuttering u u Reinforcement of profiled steel sheetings Speed and simplicity of construction u Quality controlled products ensure greater accuracy 28

Construction methods Traditionally two counteracting methods of construction could be observed both connected with special advantages but also disadvantages worth mentioning. u Conventional concrete u Construction in steel construction method + freedom of form and shapes + easy to handle + high ratio between bearing capacity and weight + prefabrication + thermal resistance + high accuracy - time-consuming shuttering - low fire resistance - sensitive on tensile forces - need of higher educated 29 personal

Construction methods u Composite Construction comparing these two methods a combination of both presents the most economic way + higher bearing capacity + higher stiffness + plastic redistribution 30

Examples Millennium Tower (Vienna - Austria) u u u 55 storeys Total height 202 m Total ground floor 38000 m 2 Capital expenditure about 145 million Euro Time of erection: 8 months 31

Examples Millennium Tower (Vienna - Austria) 33, 05 m 42, 3 m m Concrete slab Concrete core Composite Slim floor beams Composite columns Composite frame 32

Examples Millennium Tower (Vienna - Austria) Total time of erection: 8 month max. speed 2 to 2. 5 storeys per week! 33

Examples Parking deck “DEZ” (Innsbruck - Austria) Erection of composite columns over 2 storeys Assembly of prefabricated concrete slabs 34

Examples Parking deck “DEZ” (Innsbruck - Austria) u u u 4 storeys Ground dimensions 60 x 30 m Max. span length 10. 58 m with 26 cm slim floor slab (= l/40) 35