What are Composite Deck- Slabs?

Composite slab decks are concrete slab structures constructed over steel decks using support from steel beams. The steel deck act as permanent formwork for the concrete and once it sets, the whole deck and concrete act as a single unit and act monolithically to sustain the loads. 

Composite Deck-Slab
Image Credits: ASCE Library

Composite slabs are assisted by composite steel beams that perform a composite interaction with the slab.

Composite slabs are mainly used for steel buildings constructed for industrial, commercial, or even residential sectors, due to the faster construction and economy they offer. 

In this article, the general construction details, behavior, and benefits of composite slabs are explained in detail.

Components of Composite Deck-Slab Systems

Components of a Composite Deck-Slab

The composite action of the floor system combining the concrete and steel deck is achieved by embedding the studs in the concrete during the concrete pouring as shown in figure-1 and 2.

Components of a Composite Deck-Slab

The composite action of these slabs helps to reduce the loads coming over the frame and hence the foundation. 

The frame of the column supports the steel beams. The steel beams with shear connectors are connected to the profiled steel deck sheets as shown above. 

Usually, the structural engineers and architects design the slab thickness and mesh reinforcement for concrete slabs by relying on the design specifications of the metal deck manufacturers. 

Construction of Steel Decks for Composite Slabs.

The steel beams or girders support the steel decking. Shear connectors are then drilled or welded, to make the bond with the steel deck and concrete with the supporting beam. After which, the designed mesh reinforcement is laid over and concrete is poured and left for curing. Once the whole unit sets, the slab together sustains the loads. 

Shear Connectors to connect beams and steel deck and concrete slab

Principles of Composite Steel Deck-Slab Systems

In a composite steel deck-slab system, the concrete acts as the compression material, and the steel decks that are bonded to the bottom of the concrete act as the tension steel. Hence, together a composite deck slab behaves like a simple reinforced concrete beam, where the deck acts as the rebar.

⏩A composite deck slab is commonly designed as a single-span beam. This means only a positive bending moment or positive reinforcement is provided by the steel deck.

⏩The maximum load-carrying capacity of a composite deck-slab system should be limited to the bending capacity, vertical shear, and maximum acceptable deflection. ANSI/ SDI C-2011 provides the design methods for composite steel deck-slab systems

⏩Composite steel deck utilizes structural concrete fill poured over the top of the steel deck. The design of the concrete should be by ACI 318 with a minimum compressive strength of 3000psi. ASC Steel Deck's load tables are based on either 145 pcf normal weight concrete or 110 pcf lightweight concrete. Composite deck systems can be designed with lower or higher-density lightweight concrete, but the effect on fire rating must be considered if applicable to the project.

How Steel Deck Supports Concrete in Composite Slabs?

Steel deck acts as a permanent formwork for concrete in composite concrete slabs by providing several key functions and benefits:

1. Structural Support

The steel deck serves as the bottom reinforcement and structural support for the concrete slab during construction and throughout its service life. It distributes loads and provides stability, allowing for the placement of concrete and subsequent construction activities.

2. Formwork

The steel deck acts as a formwork system for the wet concrete during the pouring and curing stages. It defines the shape and thickness of the concrete slab and holds it in place until the concrete sets. This eliminates the need for traditional temporary wooden or plastic formwork, saving time and labor.

3. Composite Action

Once the concrete cures, the steel deck and concrete slab work together in a composite manner. The bond between the steel deck and concrete allows them to act as a single unit, enhancing the load-carrying capacity and stiffness of the slab. This composite action increases the slab's strength and performance.

4. Fire Resistance

Steel deck typically has fire-resistant properties, providing a level of fire protection to the concrete slab. This can be crucial in buildings where fire safety is a concern.

5. Economical and Efficient

Using steel decks as permanent formwork is often more cost-effective and efficient than traditional formwork methods. It reduces material waste, labor requirements, and construction time.

6. Smooth Finish

Steel deck often has a smooth surface, which results in a quality finish for the exposed ceiling in buildings with open ceilings. This eliminates the need for additional finishes or treatments.

7. Reduced Construction Height

The steel deck is relatively thin, allowing for reduced overall construction height compared to thicker conventional slabs. This can be beneficial in projects with height restrictions or where maximizing floor-to-ceiling height is essential.

8. Versatility

Steel decks can be used in various types of buildings, including commercial, industrial, and residential structures, making it a versatile choice for different construction projects.

In summary, steel deck acts as permanent formwork for concrete in composite concrete slabs by providing structural support during construction, serving as a formwork system, enhancing the slab's strength and fire resistance, and offering cost-effective and efficient construction solutions. This approach is commonly used in modern construction to optimize performance and reduce construction time and costs.

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