What is a One-Way Slab?

A one-way slab is one of the major classifications of slabs in building construction based on flexural behavior. These are slabs that transfer a major portion of the incoming loads in one way and have an aspect ratio greater than 2.

This article discusses one-way slabs, their structural behavior, design reinforcement, and their advantages.

One-Way Slabs

The one-way slab is defined as a slab that transfers the loads in one direction perpendicular to the supports. According to the structural code, any slab with an aspect ratio (length/width) greater than 2 can be treated as a one-way slab. 

In a one-way concrete slab, as shown in the figure, each suspended span is effectively supported on two opposing sides by means of beams or walls.

A simple example of a one-way concrete floor system is commonly seen in residential construction, where we can see a slab supported on each end by bearing walls.

Fig One-Way Slab Supported by Walls in (a) Two Opposite Sides (b) Four Sides

 

  
Another, but slightly complicated example of a one-way slab floor would be slabs supported on beams.

Fig.One-Way Slab Supported on Two Sides by Beams

In a beam-girder system, the slab transfers loads to the beams, which in turn transfers the load to the girder that is perpendicular to the beam.

Fig. Slab-Beam-Girder System

In certain cases, there are also concrete slabs that are supported on four sides, but their aspect ratio is found to be less than 2, which would be categorized and designed as a one-way slab.

Structural Behaviour of One-Way Slabs

The structural behavior of a one-way slab can be understood by considering a rectangular slab supported along the sides. 

Load Action in One-Way Slab
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If the ratio of the long dimension to the short dimension of a four-side-supported slab panel is greater than or equal to 2.0, most of the load on the slab is transferred to the long pair of beams, that is, the load path is along the short dimension of the slab panel.

As shown, the load imposed on the slabs is transferred to the support, and then to the below structure. Under this action of load, the bending occurs along the shorter direction i.e. perpendicular to the supports. The structural action of a one-way slab under the imposed load can be visualized in terms of a deformed shape as shown below.

In a one-way slab design, the maximum bending moment is calculated corresponding to the design load, and the cross-section of the slab is calculated to resist this expected bending moment. 

Reinforcement for One-Way Slabs

Based on the structural behavior of one-way slabs, the main reinforcement to resist the flexural bending will be provided along the direction of the span. This forms the primary reinforcement for the slab.

One-Way Slabs Bending and Reinforcement details 

In addition, secondary reinforcement is also provided perpendicular to the main reinforcement to help control cracks from drying shrinkage or thermal contraction. Hence, this particular reinforcement is called temperature reinforcement or temperature steel.

For the design of one-way slabs in a slab-beam-girder system, structural reinforcement in the secondary direction is provided to resist bending in that direction, but less the main reinforcement along the span.

While arranging the reinforcement the main reinforcement comes below distribution reinforcement. This is to ensure that the main reinforcement occupies greater effective depth.

Fig. A typical one-way slab reinforcement details. The above reinforcement is laid based on Indian Standard codes and specifications.

Features of One-Way Slab

• One-way slabs can be designed efficiently for a span of up to 6m i.e. 20 ft between the beams.
• In the case of one-way slabs; their length perpendicular to the span is unlimited, providing flexibility in designing open and spacious interiors without the need for additional support. This gives more architectural freedom.
• It is suitable for residential and commercial light construction where floors normally rest on walls instead of columns. Hence reducing the need for fewer load-bearing walls.
• One-way slabs are easier to construct, which can lead to quicker project completion and potentially lower labor costs.

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