Slabs are a basic element of the building that support load from various structures above and effectively transfer loads to the beams or columns. There are numerous slab options in construction, but cantilever slabs stand out due to the unique structural advantages they offer, with limited support requirements. This blog will take a deeper look at cantilever slabs.
What is a Cantilever Slab?
A cantilever slab is a type of reinforced concrete slab that is supported at one end and freely extends at the other end, without any support. This slab resists loads and bending caused due to the overlying loads at the unsupported end. This slab is typically supported by a beam or a column and extends horizontally outward, making it ideal for structures that require minimal vertical support.
Materials Used in Cantilever Slab
- Reinforced concrete is the common material that uses steel reinforcement within the concrete structure to provide adequate support. The combination of steel and concrete helps in withstanding both compression and tensile forces.
- Steel in the form of beams or decks can be used in cantilever slabs to withstand heavier loads for a long span.
- Pre-stressed concrete– Pre-tensioned or post-tensioned steel rods are used along with the cantilever slab to increase the strength and span capabilities.
Design Considerations for Cantilever Slab
Load Calculations
- Dead Load– It measured the self-weight of the slab and the permanent fixtures like walls and other structural elements related to the slab.
- Live Load– It is the temporary load that measures the amount of furniture, people, and equipment loading it can withstand without failure. This load varies from building to building, based on the type of use.
- Concentrated Load- It is the pointed load that is transferred at a particular point.
Reinforcements
The top portion of the reinforcement should be placed along the length of the slab at the fixed end to withstand flexural forces. The bottom reinforcements should be placed along the width of the slab in the middle to withstand shear forces.
Span-to-Depth Ratio
The length of the cantilever slab should be determined based on the thickness. A longer cantilever requires a thicker slab to maintain stability and counter excessive deflection. As per IS 456: 2000, the span-to-depth ratio of the cantilever slab is 2:1, which can be extended up to 7:1.
Bending Moment
The fixed end of the cantilever slab experiences the maximum bending moment. Hence, the design must ensure that adequate reinforcement is provided to withstand stress and bending forces.
Shear Forces
Shear forces at the fixed end are higher compared to the free end of the slab. Hence, adequate reinforcements should be provided to support the shear forces.
Installation Process of Cantilever Slabs
Design and Planning
Ensure that the cantilever slab is properly designed considering all the factors to determine the design layouts, slab dimensions and materials specifications. Inspect the surface where the slab needs to be installed to ensure it is secure and can withstand loads.
Formwork
Install formwork based on the shape of the cantilever slab, ensuring that it supports the freshly poured concrete in place. In cantilever slabs, the formworks are typically placed at the fixed end. Ensure that the free end has the necessary support to avoid unwanted movement, sagging or twisting.
Reinforcement
The reinforcement should be carefully positioned according to the design. Ensure that the rebars at the fixed end are robust enough to handle high bending stresses, and the free end requires necessary reinforcement to prevent tension. Connect the rebars securely using wire to ensure that they are properly spaced and help withstand forces as a unit.
Pouring Concrete
The concrete that is mixed based on the design mix ratio should ensure that it is uniform in consistency and free from impurities. The mixed concrete is then poured evenly across the formwork and compacted using a vibrator to avoid air pockets and uneven surfaces.
Curing
After pouring, leave the slab to cure for an adequate period under moisture conditions to avoid cracking and other damages, and to attain maximum strength. After the concrete attains necessary strength to withstand on its own, the formwork can be removed.
Advantages and Disadvantages of Using Cantilever Slabs
Advantages
- It provides open spaces underneath the slab, offering an unobstructed view and functional space below.
- It creates striking visual effects, offering a more modern and dynamic look for both residential and commercial buildings.
- It helps save material and labour costs spent on additional structural supports.
- It offers flexibility in creating overhangs, terraces, or canopies without structural alterations.
Disadvantages
- It needs careful and precise design to prevent excessive deflection or bending.
- The thickness or reinforcement of the slab is comparatively high, which leads to high material costs.
- It requires more advanced calculations for load distribution and stress analysis, making it complex for design and engineering.
- Long-spanned cantilever slabs have a limited load-bearing capacity, which could lead to structural failure or over-sagging.
- The unsupported end of the cantilever slab is prone to cracking due to bending moments and stress, thus requiring frequent maintenance.
Applications
- It is used to build balconies that extend outwards from the building without the need for vertical supports underneath for an unobstructed view.
- It is ideal for overhangs above doors, windows, or entryways, providing enhanced protection against sun, rain, and other elements.
- It allows terraces for outdoor living spaces without columns, making them a popular choice for modern residential buildings.
- It can be used as canopies to shelter entrances, walkways, or waiting areas without obstructing pedestrian traffic.
- Used in the construction of bridges with overhanging decks, extending beyond the support.
- It is used to build flooding pool decks or infinity edges with no visual support.
Conclusion
In summary, cantilever slabs are highly effective and versatile structural solution that offers numerous architectural and design advantages. They provide uninterrupted spacing options that yield pleasing aesthetics. While they do have certain limitations in terms of design and maintenance, their numerous advantages outweigh these drawbacks. All these factors make cantilever slabs one of the most functional and aesthetically appealing solutions for residential and commercial structures.
Mithali Konety 
Hemali Patel