Principle
Construction materials of different properties are combined to interact and respond against loads in synchronization rather than individually. These composite materials are physically connected to utilize their distinct strengths and features to form a single unit stronger than any separate parts.
Composite construction is meant to achieve efficient and lightweight structural solutions for construction and other related industries.
Background
This engineering and construction method has been widely practiced over a century ago, more dominantly involving the steel-concrete connection. Concrete-encased steel sections were initially developed as composites with the purpose of overcoming the effects of fire while ensuring stability against axial and bending forces.
Composite columns with reinforced concrete core and steel pipe or steel tubular shell were introduced to provide an integral and permanent formwork. Profiled galvanized metal sheets were also introduced to eliminate traditional formworks and at the same time, increasing the strength of reinforced concrete slabs.
Standards
Design of composite beams and composite slabs for buildings are covered by BS EN 1994-1-1
Design of Composite Steel and Concrete Structures are covered by BS EN 1990-1999 Eurocode 4
Australian Bridge Design Code, Section 6: Steel and composite construction
BS 5400 British Standard code of practice for the design and construction of steel, concrete and composite bridges
ASHTO: Standard Specifications for Highway Bridges; 24 CFR 200 Sub-part S
Concrete Slab Stresses in Partial Composite Beams and Girders, American Institute of Steel Construction, Vol. 21
Applications
Composite construction is extensively used in bridges, multistorey buildings, warehouses, marine structures, and more. Many applications in the mentioned structures are categorized as beams & girders, floor systems, and column systems.
Composite Beams and Girders. Composite beam includes a steel section in I or W shape attached to a concrete slab by shear connectors atop of it. They have been recognized as one of the most economical structural systems for both multistorey buildings and bridges.
Obviously, building and bridge floors should be stiff and massive enough to reduce deflection and vibrations. In this case, reinforced concrete is undoubtedly the material of choice. The supporting beam or girder, however should have a superior strength-weight ratio, a quality that only steel can offer.
Composite Floor Systems. Composite floor system consists of steel beams, profiled metal decking, and reinforced concrete slab. These materials are combined in a compact and very efficient way to form a profile that is basically designed to hold gravity or dead loads as well as traffic loads.
Composite floor systems are mostly used as bridge decks and floor slab for wide range of building classifications largely for elevated car parks and multistorey commercial buildings.
Composite Column Systems. Composite columns can either be concrete-filled steel tube or concrete-encased steel element. Either way, composite columns are advantageous as follows.
Flexural resistance of steel pipe or tube is maximized when provided with concrete infill
Steel casing prevents spalling and confines the concrete.
Concrete infill delays local buckling of the steel casing and enhances compression resistance
Steel casement replaces formwork and reinforcing steel.
Advantages
The success and versatility of composite construction can be stated in a simple and straight forward explanation - concrete responds excellently in compression and steel behaves the same in tension.
Joining the two materials together as a structure, these strengths can be used to achieve a highly efficient and lightweight design that can effectively resist both axial and flexural forces. Other benefits and advantages are as follows:
Composite systems are over 25% lighter than concrete construction. As a result, site erection and installation are easier, and labor costs can be minimized.
Steel-concrete composite can have high strength from a relatively small cross-sectional area.
The reduced weight of composite itself reduces the forces in those elements supporting them. In this way, supporting members including foundation costs can also be reduced.
Superior strength-to-weight ratio of composite materials allows compact designs which are expected to be aesthetical, economical, safe, and green.
Composite systems eliminate the costly activities of traditional concrete forming like propping, stripping, and other temporary works.
Steel and concrete can be arranged to produce an ideal combination of strength according to calculated requirements.
Concrete-encased steel elements has good resistance to buckling, fire, and corrosion.
Composite beams can cover longer spans without the need of intermediate columns, thanks to steel.
Composite columns reduce the requirement of lateral reinforcement and time-consuming fixing of lateral ties, as well as providing easier connection to steel beams of a steel-framed structure.
Composite columns involving steel tube or pipe casing simplifies foundation works and construction in bodies of water.
In marine construction, pouring of concrete under water is made possible by applying composites. Driven steel pipes and sheet piles serves as integral and permanent formworks for concrete infill.
Concreting of succeeding floors may proceed without having to wait for the previously cast floors to gain strength. The steel decking system provides positive moment reinforcement for the composite floor, requiring only small amounts of temperature bars to control cracking.
Summary
The benefits of combined steel-concrete construction are characterized by three major aspects: speed, performance, and value.
Steel-concrete composite is about 30% lighter than reinforced concrete and slightly heavier than structural steel by 2%. This significant reduction in weight of composite materials compared to reinforced concrete along with the elimination of huge amount of false works contributes to a huge factor in reducing construction timelines.
With concrete being strong in compression and steel in tension, the combination of the two materials has proven and excellent results in enhancing the structural performance of the product composite unit. Application of steel-concrete composite can increase the maximum shear strength of a floor slab by 85% according to the results of a study by ASCE.
Overall savings using steel-concrete composite can be as high as 10% compared to reinforced concrete and 7% when compared to structural steel. Steel encased with concrete does not only improve the strength of the composite members but protect the entire structure from adverse effects of fire, calamities, and corrosion.
Why Consult ESC?
ESC Steel Structures is one of the top providers of structural steel fabrications around the world.
ESC’s capabilities include Steel Bridge Structures, Bailey Bridge, Pre-Engineered Steel Buildings / Pre-Fabricated Steel Buildings, Offshore Steel Structures, Mining and Excavation Structures, Pressure Vessel Fabrication, Steel Towers, Rail Infrastructures, Material Handling Structures, and more.
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