How to Calculate Quantity of Steel In Two Way Slab?

When constructing a slab supported by four adjacent beams, it’s crucial to understand the concept of a two-way slab. This type of slab handles loads through flexure in both perpendicular directions. In simpler terms, if the length-to-width ratio (L/B) is less than two, a two-way slab is implemented. These slabs, when uniformly loaded, take on a dish or saucer-like shape, resulting in bending moments in both directions.

Key Features of Two-Way Slabs

In two-way slabs, main bars are strategically placed in both the shorter and longer spans, incorporating a crank to prevent bending. Now, let’s delve into the details of designing a two-way slab with a real-world example.

Design Data

Consider a two-way slab with the following specifications:

• Length: 5 meters
• Width: 4 meters
• Main Bar: 12 mm, Spacing – 150 mm c/c, Alternate bent up
• Distribution Bar: 10 mm, Spacing – 150 mm c/c, Alternate bent up
• Slab Thickness: 150 mm
• Clear Cover: 20 mm
• Development Length (Ld): 40d

Step-by-Step Design Process

Step #1 – Bar Numbers

To determine the number of bars needed:

• Main Bar: (5000/150) + 1 = 34 bars
• Distribution Bar: (4000/150) + 1 = 27 bars

Step #2 – Cutting Length

Calculate the cutting length for both main and distribution bars using the provided formulas.

For Main Bar:
Cutting Length=4977 mm (or 4.977 m)Cutting Length=4977mm (or4.977m)

For Distribution Bar:
Cutting Length=5822 mm (or 5.822 m)Cutting Length=5822mm (or5.822m)

Step #3 – Top Distribution Bar

Determine the number and length of top distribution bars for both shorter and longer spans.

For Shorter Span:

• Number: 14
• Length: 4.96 meters

For Longer Span:

• Number: 12
• Length: 5.8 meters

Step #4 – Extra Top Bar

Calculate the number and length of extra top bars to counter bending moments.

For Shorter Span:

• Number: 20
• Length: 0.848 meters

For Longer Span:

• Number: 16
• Length: 1.04 meters

Step #5 – Weight of Steel

Calculate the weight of main bars, distribution bars, top distribution bars, and extra top bars using appropriate formulas.

Main Bar Weight=150.26kg

Distribution Bar Weight=96.98 kgDistribution Bar Weight=96.98kg

Top Distribution Bar Weight=42.84 kg (Shorter Span)Top Distribution Bar Weight=42.84kg(Shorter Span)

Top Distribution Bar Weight=42.94 kg (Longer Span)Top Distribution Bar Weight=42.94kg(Longer Span)

Extra Top Bar Weight=15.06 kg (Shorter Span)Extra Top Bar Weight=15.06kg(Shorter Span)

Extra Top Bar Weight=10.26 kg (Longer Span)Extra Top Bar Weight=10.26kg(Longer Span)

Summary – Bar Bending Schedule

Here’s the bar bending schedule for the two-way slab:

This comprehensive guide provides a detailed process for designing and calculating the quantity of steel needed for a two-way slab, ensuring structural integrity and stability.