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What is the effective span of a cantilever slab?

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In structural engineering, the term “effective span” refers to the distance over which a slab or beam is designed to safely support loads. For cantilever slabs, which are beams or slabs supported on one end and free on the other, this concept is crucial for ensuring stability and structural integrity.

What is the Effective Span of a Cantilever Slab?. Image 1 of 4

Understanding Cantilever Slabs

A cantilever slab is a projecting concrete ledge that is fixed at one end and free at the other, resembling a diving board in its support structure. This design allows for overhanging structures without external bracing, making it a popular choice for balconies, canopies, and certain architectural features.

Determining the Effective Span

The effective span of a cantilever slab is typically defined as the length from the face of the support to the free end. However, when calculating the effective span, engineers must consider additional factors such as the width of the support and the back span – the portion of the slab that extends into the structure it protrudes from.

According to Eurocode 2, the effective span of a cantilever slab should be taken as the length from the face of the support plus half the width of the support1. This accounts for the distribution of forces and the stiffness provided by the support structure.

Design Considerations

When designing a cantilever slab, engineers must ensure that the slab can withstand not only the self-weight and live loads but also the moments and shear forces induced by its cantilevered design. The slab’s thickness, reinforcement, and connection to the support structure are all critical factors in its ability to maintain structural integrity.

For instance, the main reinforcements are typically provided at the top of the cantilever and extend into the back span by at least 1.5 times the length of the cantilever or 0.3 times the length of the back span, whichever is greater2. This reinforcement helps to resist the tensile forces generated by the bending moments.

Practical Limits

Practically, cantilevers are usually limited to a span of about 25 to 35% of the back span3. This limitation ensures that the cantilever does not experience excessive deflection or vibration, which could compromise the structure’s safety and serviceability.


The effective span of a cantilever slab is a fundamental concept in structural engineering, ensuring that cantilevered elements are designed with the appropriate balance of strength, stiffness, and stability. By carefully considering the effective span and related design parameters, engineers can create cantilever slabs that are both functional and safe.


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This article has provided an overview of the effective span of cantilever slabs, touching on key design principles and practical considerations. For those interested in the detailed structural design process of cantilever slabs, including load analysis and flexural design, resources such as Structville’s comprehensive guide can offer valuable insights

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