Composite Slabs & Columns – Advantages and Basic Concepts

Composite Slabs & Columns – Advantages and Basic Concepts

Read Introduction on Composite Structures Here

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
• 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

Profiled steel sheeting

•  depths ranging from 45 mm to over 200 mm
• yield strengths ranging from 235 N/mm2 to at least 460 N/mm2
• 0.8 mm and 1.5 mm thick
• The various shapes provide Interlock between steel and concrete

• decking may also be used to stabilise the beams against lateral torsional buckling during construction.
• stabilize 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

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 hot-rolled steel. 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’). 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

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 cross-section represents the maximum load that can be applied to a short composite column.

Aspects for using composite structures:

• Architectural
• Economical
• Functionality
• Service and Flexibility
• Assembly

Architectural

• Longer spans
• Thinner slabs
• More slender column
• More generous opportunities for design

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

 Functionality

• Working platforms of steel decking
• Permanent shuttering
• Reinforcement of profiled steel sheetings
• Speed and simplicity of construction
• Quality controlled products ensure greater accuracy

 Construction methods

Traditionally two counteracting methods of construction could be observed both connected with special advantages but also disadvantages worth mentioning.

Composite Construction

comparing these two methods a combination of both presents the most economic way

• + higher bearing capacity
• + higher stiffness
• + plastic redistribution

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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