Steel Concrete Composite Structures
The term “steel-concrete composite structure” implies the use of steel and concrete parts in combination to construct one structure. The main benefit of composite elements is the ability to integrate the characteristics of each material to create a single component that functions more effectively overall than its individual constituent parts.
Steel-concrete composites are the most common types of composite material used in construction, although there are other composites made of steel and other materials, such as timber and concrete, plastic and concrete, etc. Concrete and steel blend well together despite having extremely distinct characteristics. For instance, concrete provides protection against corrosion and thermal insulation, whereas steel gives a structure ductility, while steel components are prone to buckling. Concrete can also prevent or delay buckling.
Concrete is a material that performs well in compression but less so in tension. While steel us an excellent material under tension. Steel-concrete composite components combine the tension-resistance of steel with the compressive strength of concrete to create a highly effective and lightweight unit. These elements are frequently employed in structures like bridges and multi-story buildings.
The goal of composite construction is to produce strong, lightweight structural components for the construction industry and other associated sectors.
Design Standards
In the United States, the design and construction of steel and concrete structures are primarily governed by the following codes and standards:
For steel:
AISC (American Institute of Steel Construction): The AISC is responsible for creating standards related to the design, fabrication, and erection of steel structures. The main design code is the "AISC 360 Specification for Structural Steel Buildings," which provides requirements for the design of steel structures.
For Concrete:
ACI (American Concrete Institute): The ACI is responsible for developing codes and standards related to the design, construction, and maintenance of concrete structures. The primary design code is "ACI 318 Building Code Requirements for Structural Concrete," which provides the requirements for designing reinforced concrete structures.
In addition to these primary codes, there are other codes and standards that may apply depending on the specific project requirements, such as:
ASCE/SEI 7 (American Society of Civil Engineers/Structural Engineering Institute): Minimum Design Loads for Buildings and Other Structures
IBC (International Building Code): A model building code developed by the International Code Council (ICC) which has been adopted by many jurisdictions in the United States.
It is important to note that local and state building codes may have additional requirements or amendments to these national codes, so it's essential to consult with local authorities for the most up-to-date requirements in a specific location.
Applications of Steel Concrete Composites
In warehouses, multistory buildings, bridges, marine constructions, and other structures, composite construction is frequently employed. Beams and girders, floor systems, and column systems are categories that broadly describe several uses in the aforementioned buildings.
Composite Slabs
Typically, reinforced concrete is cast on top of profiled steel decking (trapezoidal or re-entrant) to create composite slabs.
During the construction phase, the decking can serve as formwork, a work platform, or external reinforcement when building composites. A decking is manually spread out over the floor space after being hauled into place in bundles.
The depths of the slabs usually start at 5”. Concrete is most frequently used to make slabs because of its bulk and stiffness, which may be used to lower floor deflections and vibrations and accomplish the necessary fire protection and thermal storage. Due to its better strength-weight and stiffness-weight ratios as well as its simplicity of handling, steel is frequently employed as the foundation beneath the slab.
Galvanized steel is used for the decking, which is normally 3/64” thick. Stiffeners may be used to stiffen the top flange and support hangers for relatively lightweight items to be suspended from the soffit in order to prevent local buckling. Roll-on embossments, also known as dimples, are placed on the decking profile to trap concrete around the re-entrant portions of the profile and enable interlocking.
Re-entrant or trapezoidal decking typically measures about 2” deep and has an unsupported span of around 10’. Trapezoidal profiles that are 3” deep have an unsupported span of about 15’. Deep decking is trapezoidal decking that is deeper than 8”, and the decking troughs may be reinforced further if needed. Deep decking has an unsupported span of about 20 ft
Composite Beams
A composite beam consists of an I- or W-shaped steel segment that is connected to a concrete slab on top by shear connectors. They have been acknowledged as one of the most cost-effective structural solutions for both bridges and multistory buildings.
Downstand Beams
A downstand beam and a composite slab are joined together using shear studs that are through-deck welded. Another option is to cover the top flange of the steel beam with a slab of precast concrete. The useful span has a range of around 20 to 40ft. Other layouts allow downstand beams to span 65 ft or more.
Composite Floors
A composite floor system is made up of steel beams, profiled metal decking, and a reinforced concrete slab. These materials come together to form a profile that is primarily meant to sustain traffic loads as well as gravity or dead loads.
For a range of construction types, notably elevated parking garages and multistory commercial buildings, composite floor systems are frequently used as floor slabs and bridge decks.
Evidently, the flooring of buildings and bridges should be rigid and substantial enough to minimize deflection and vibration. Reinforced concrete is unquestionably the best material in this situation. However, the supporting beam or girder should possess a greater strength-to-weight ratio, which only steel can provide.
Shallow Floor
Shallow floors, which can be used for spans between 13’ and 30’, are those where the majority of the steel section is within the depth of the concrete slab. When compared to downstand beams, the slab rests on the top surface of the bottom flange rather than the top flange, with the amount of torsion given to the beam being a crucial factor. Precast concrete or in situ concrete on deep steel decking with a typical thickness of 9” may be used for the slab.
The advantages of shallow floors include the lack of interruptions caused by downstand beams and the fact that additional fire protection is frequently not required due to the placement of the slabs and beams within the same zone.
Composite Columns
Composite columns can be constructed from steel tubes filled with concrete or from steel components encased in concrete. Composite columns offer the following advantages:
Steel pipe or tube has a higher bending resistance when filled with concrete.
Steel casing prevents spalling and keeps concrete contained.
Concrete infill enhances compression resistance and stops the steel casing from buckling in specific areas.
Steel casements are used in place of the supporting steel and formwork.
Composite columns can have significant strength for a relatively small cross-sectional area, allowing for the maximization of the usable floor space. There are several distinct forms of composite columns; the two most popular are an open steel section encased in concrete or a hollow steel tube filled with concrete. The steel section’s compression resistance is increased by the concrete infill, preventing the steel from buckling. Due to its fire resistance, the column may be left uncovered or merely minimally shielded.
Advantages of Steel Concrete Composites
Performance, value, and speed of construction are all advantages of composite construction.
Compared to concrete construction, composite systems are over 25% lighter. As a result, site construction and installation are simpler, and labor expenses can be reduced. Construction timeframes are significantly shortened by the weight reduction of composite materials relative to reinforced concrete and the avoidance of a large amount of false work.
Concrete is strong in compression, whereas steel is strong in tension, hence combining the two has been shown to significantly improve the structural performance of the resulting composite unit. According to the findings of an ASCE study, using steel-concrete composite can boost a floor slab's maximum shear strength by 85%.
When compared to reinforced concrete and structural steel, steel-concrete composite can result in overall cost reductions of up to 10%. In addition to enhancing the composite members’ strength, steel enclosed in concrete shields the entire building from the damaging impacts of fire, disasters, and corrosion.
These strengths can be used to join the two materials into a structure to create a highly effective, lightweight design that can successfully withstand both axial and flexural forces. The following are additional perks and benefits:
A steel-concrete composite can have a high level of strength from a cross-sectional area that is relatively small.
The forces in those elements supporting the composite themselves are lessened due to its lighter weight. Supporting members’ expenses, including foundation expenditures, might be decreased in this way.
Composite materials’ superior strength-to-weight ratio enables compact designs, which are anticipated to be aesthetically pleasing, affordable, safe, and environmentally friendly.
Composite solutions do away with the pricey temporary operations like propping, stripping, and others involved in conventional concrete formation.
In accordance with calculated requirements, steel and concrete can be arranged to achieve the appropriate combination of strength.
Steel allows composite beams to span greater distances without the use of intermediary columns.
Composite columns make connecting to steel beams of a steel-framed construction easier and eliminate the need for lateral strengthening and time-consuming lateral tie fixing.
Construction in bodies of water is made easier by composite columns made of steel tube or pipe casing.
Applying composites in marine building makes it possible to lay concrete underwater. For concrete infill, driven steel pipes and sheet piles act as essential and long-lasting formwork.
There is no need to wait for the previously cast floors to get stronger before casting the subsequent floors. Only modest quantities of temperature bars are necessary to control cracking thanks to the steel decking system’s positive moment reinforcement for the composite floor.
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