Slab, its types, design example and slab design excel sheet download.

slab

Slabs are one of the most essential structural elements in construction, providing flat surfaces such as floors and roofs.

They distribute loads to beams and columns, ensuring the stability and functionality of buildings.

Two common types of slabs are one-way slabs and two-way slabs.

What is a One-Way Slab?

A one-way slab is a reinforced concrete slab that primarily bends in one direction.

This happens when the slab is supported on two opposite sides only, with the other two sides free or simply spanning between beams.

The length-to-breadth ratio (L/B) of a one-way slab is greater than or equal to 2.

Characteristics of One-Way Slabs:

• Bending occurs only in one direction.
• Reinforcement is provided along the shorter span (primary reinforcement) and shrinkage/temperature bars are placed perpendicular to it.
• Commonly used in rectangular rooms, corridors, and other spaces with significant length compared to breadth.

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What is a Two-Way Slab?

A two-way slab bends in both directions and is supported on all four sides.

This type of slab is used when the length-to-breadth ratio (L/B) is less than 2, enabling the load to be distributed in two perpendicular directions.

Characteristics of Two-Way Slabs:

• Bending occurs in both directions.
• Reinforcement is provided in both directions to handle bending moments.
• Suitable for square or nearly square spaces, such as auditoriums, warehouses, and industrial buildings.

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Differences Between One-Way and Two-Way Slabs:

Feature	One-Way Slab	Two-Way Slab
Load Distribution	Along one direction only	Along both directions
Support Conditions	Two parallel sides	All four sides
L/B Ratio	≥ 2	< 2
Reinforcement	Along shorter span primarily	In both directions
Application Areas	Corridors, narrow rooms	Square or near-square rooms

 

Design Example of a One-Way Slab

Problem: Design a one-way slab for a residential building floor spanning 4 meters (width) and 10 meters (length). The live load is 3 kN/m², and the floor finish load is 1 kN/m². Use M20 grade concrete and Fe415 steel.

Step 1: Determine Effective Span

The effective span for the one-way slab is the shorter span, i.e., 4 meters.

Step 2: Calculate Total Load

Dead Load = Self-weight of the slab = 25 × Thickness (assume 150 mm thickness initially)

Dead Load = 25 × 0.15 = 3.75 kN/m² Total Load = Dead Load + Live Load + Floor Finish Load = 3.75 + 3 + 1 = 7.75 kN/m²

Step 3: Moment Calculation

Factored Load = 1.5 × 7.75 = 11.625 kN/m² Factored Bending Moment (M) = wL²/8 = (11.625 × 4²) / 8 = 23.25 kN.m

Step 4: Reinforcement Design

Using M20 concrete and Fe415 steel, calculate the area of steel (Ast) using the bending moment formula:

Ast = (M × 10⁶) / (0.87 × fy × d)

Assume an effective depth (d) of 125 mm. Ast = (23.25 × 10⁶) / (0.87 × 415 × 125) = 419.77 mm² Use 12 mm diameter bars spaced at 150 mm center-to-center.

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Design Example of a Two-Way Slab

Problem: Design a two-way slab for a hall measuring 6 m × 6 m with a live load of 4 kN/m² and floor finish of 1.5 kN/m². Use M25 grade concrete and Fe500 steel.

Step 1: Check Slab Type

L/B = 6/6 = 1 (less than 2). Therefore, it is a two-way slab.

Step 2: Calculate Total Load

Dead Load = 25 × Thickness (assume 150 mm initially) Dead Load = 25 × 0.15 = 3.75 kN/m² Total Load = Dead Load + Live Load + Floor Finish Load = 3.75 + 4 + 1.5 = 9.25 kN/m²

Step 3: Moment Calculation

Factored Load = 1.5 × 9.25 = 13.875 kN/m² Moments are calculated using coefficients from IS 456:2000: Mx = αx × w × L², My = αy × w × L² Assume αx = 0.052 and αy = 0.045 for simply supported edges.

Mx = 0.052 × 13.875 × 6² = 25.93 kN.m My = 0.045 × 13.875 × 6² = 22.41 kN.m

Step 4: Reinforcement Design

For Mx and My, calculate the reinforcement using: Ast = (M × 10⁶) / (0.87 × fy × d) Assume an effective depth (d) of 125 mm for initial design. Astx = (25.93 × 10⁶) / (0.87 × 500 × 125) = 477.25 mm² Asty = (22.41 × 10⁶) / (0.87 × 500 × 125) = 412.07 mm²

Use 10 mm bars at 150 mm c/c for Astx and 12 mm bars at 200 mm c/c for Asty.

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