One Way and Two Way Slabs

The Efficient Civil Engineer (by Dr. S. El-Gamal)

4 May 202004:16

Summary

TLDRDr. Sharif Gamal's video offers a concise overview of one-way and two-way solid slabs in construction. One-way slabs deflect in one direction and are designed with main reinforcement in one direction and minimum steel in the other. Two-way slabs, however, deflect in both directions, necessitating reinforcement in both. The video also explains how slabs supported on one, two, or four edges can be classified as one-way or two-way based on the ratio of their spans. Dr. Gamal emphasizes the importance of understanding these distinctions for accurate structural design.

Takeaways

🏗️ One-way solid slabs deflect in one direction, while two-way solid slabs deflect in both directions.
🔄 For one-way slabs, loads are directed to one direction, whereas two-way slabs distribute loads in both directions.
🛠️ In one-way slabs, reinforcement is primarily designed in one direction, with minimal reinforcement in the other to meet code requirements.
🔩 Two-way slabs require reinforcement design in both directions, with main steel (main steel) at the bottom and additional steel (secondary steel) above.
📏 All two-way slabs should have support on all four edges, unlike one-way slabs which can be supported on one, two, or four edges.
🌉 One-way slabs can be a special case like a cantilever, where loads are directed to one edge and supported by one beam.
📐 The classification of slabs as one-way or two-way depends on the ratio of the longest span to the shortest span; if greater than 2, it's one-way, otherwise, it's two-way.
📉 The maximum bending moment for a simply supported one-way slab under load is calculated as \( \frac{w l^2}{8} \), while for a cantilever, it's \( -\frac{w l^2}{2} \).
🏛️ The design of slabs involves careful consideration of support conditions and load distribution to ensure structural integrity and safety.
👋 Dr. Sharif Gamal's presentation provides a clear distinction between one-way and two-way solid slabs, highlighting the importance of understanding their structural behavior for proper design.

Q & A

What is the primary difference between one-way and two-way solid slabs?
-One-way solid slabs deflect in one direction, while two-way solid slabs deflect in both directions.
How do loads distribute in one-way solid slabs compared to two-way solid slabs?
-In one-way solid slabs, loads are directed to one direction only, whereas in two-way solid slabs, loads are distributed in both directions.
What is the reinforcement design strategy for one-way solid slabs?
-One-way solid slabs are designed for minimum reinforcement in one direction, with the other direction using only the minimum steel reinforcement required by code.
What is the term for the reinforcement used in the short direction of a two-way slab?
-In two-way slabs, the reinforcement in the short direction is referred to as 'main steel' or 'main reinforcement'.
What is the minimum requirement for the support of two-way slabs?
-Two-way slabs should be supported on all four edges.
What is a cantilever slab and how does it relate to one-way slabs?
-A cantilever slab is a special case of a one-way slab that is supported on one edge and deflects loads in one direction only.
How does the support configuration affect the classification of a slab as one-way or two-way?
-A slab supported on four edges can be classified as one-way or two-way based on the ratio of the longest span to the shortest span. If this ratio is greater than two, it is classified as a one-way slab; if less than or equal to two, it is a two-way slab.
What is the maximum bending moment for a cantilever slab under uniform load?
-For a cantilever slab under uniform load, the maximum bending moment is a negative moment of wl^2/2.
What is the significance of the ratio of the longest span to the shortest span in classifying slabs?
-The ratio of the longest span to the shortest span is used to determine if a slab supported on four edges is a one-way or two-way slab. A ratio greater than two indicates a one-way slab, while a ratio less than or equal to two indicates a two-way slab.
How does the design of reinforcement differ between one-way and two-way slabs?
-In one-way slabs, reinforcement is designed for one direction only, focusing on minimum reinforcement in the perpendicular direction. In contrast, two-way slabs require reinforcement design for both directions, with main steel at the bottom layer and additional reinforcement in the top layer.
What are the different types of one-way slabs based on support?
-One-way slabs can be supported on one edge (cantilever), two edges (two beams or walls), or four edges. The type of support affects how the slab deflects and how it is classified.

Outlines

00:00

🏗️ One-Way vs. Two-Way Solid Slabs

Dr. Sharif Gamal introduces the concepts of one-way and two-way solid slabs in civil engineering. One-way slabs deflect in a single direction and are designed with main reinforcement in one direction and minimum steel reinforcement in the other, which is only to satisfy code requirements. In contrast, two-way slabs deflect in both directions and require reinforcement in both directions. The main reinforcement, known as 'many steel', is placed at the bottom layer, while 'mary steel', or secondary reinforcement, is placed above it. The video also explains the support conditions for one-way slabs, which can be supported on one, two, or four edges, with a special mention of cantilever slabs that are a unique case due to their single-direction load and support. The classification of slabs as one-way or two-way based on the ratio of the longer to shorter span is also discussed, with a ratio greater than two indicating a one-way slab and a ratio less than or equal to two indicating a two-way slab.

Mindmap

Keywords

💡One-way solid slab

A one-way solid slab is a type of structural element that deflects primarily in one direction. It is designed to support loads that are applied along a single axis. In the video, Dr. Sharif explains that for one-way solid slabs, loads go to one direction only, and the reinforcement is designed for minimum steel in the direction perpendicular to the load application.

💡Two-way solid slab

Contrasting with one-way slabs, two-way solid slabs are designed to deflect in both directions, accommodating loads that are distributed across two axes. Dr. Sharif mentions that for two-way solid slabs, loads go to both directions, and reinforcement is designed for both directions, with main steel (many steel) at the bottom and mini steel (mary steel) above it.

💡Deflection

Deflection refers to the bending or displacement of a structural element under load. In the context of the video, deflection is used to describe how one-way and two-way solid slabs respond to applied loads, with one-way slabs deflecting in one direction and two-way slabs deflecting in both.

💡Reinforcement

Reinforcement in the context of slabs refers to the steel bars or mesh embedded within the concrete to enhance its strength and resistance to tensile forces. Dr. Sharif discusses the difference in reinforcement between one-way and two-way slabs, highlighting the need for minimum steel reinforcement in one direction for one-way slabs and designed reinforcement in both directions for two-way slabs.

💡Cantilever

A cantilever is a special case of a one-way slab that is supported on one edge and extends outward without additional support. The video script mentions that a cantilever slab is a special case where loads are applied in one direction, and it is important to calculate the moment carefully, especially noting the negative moment at the fixed support.

💡Supports

Supports are the structural elements that bear the weight of a slab. Dr. Sharif explains that the classification of slabs as one-way or two-way can depend on the number of supports and their configuration. One-way slabs can be supported on one, two, or four edges, while two-way slabs should be supported on all four edges.

💡Moment

In structural engineering, moment refers to a force that causes a rotation or bending and is calculated as the product of the force and the perpendicular distance from the axis of rotation to the line of action of the force. The video discusses calculating moments for cantilever slabs and simply supported slabs, with specific formulas for maximum bending moments.

💡Simply supported

A simply supported slab is one that is supported at both ends and is free to expand and contract along its length without restraint. Dr. Sharif uses the term to describe a condition where the slab is supported on four edges and can be classified as one-way or two-way based on the ratio of its longest to shortest span.

💡Span

The span of a slab refers to the distance between two supports. In the video, the concept of span is crucial for determining the classification of slabs. Dr. Sharif explains that if the longest span divided by the shortest span is greater than two, the slab is classified as one-way; otherwise, it is two-way.

💡Minimum steel reinforcement

Minimum steel reinforcement refers to the smallest amount of steel required by building codes to ensure the structural integrity of a slab. Dr. Sharif mentions that for one-way slabs, the design focuses on reinforcement in one direction, while the other direction only needs to satisfy the code requirements for minimum steel reinforcement.

💡Main steel (many steel)

Main steel, or many steel, is the primary reinforcement in a two-way slab, typically placed at the bottom layer to resist the tensile forces in the short direction. The video script explains that this layer of steel is crucial for the slab's strength and is part of the designed reinforcement in both directions for two-way slabs.

Highlights

One-way solid slabs deflect in one direction, while two-way solid slabs deflect in both directions.

For one-way solid slabs, loads are directed to one direction only.

Two-way solid slabs distribute loads to both directions, requiring different design considerations.

One-way solid slabs are designed for minimum reinforcement in one direction and satisfy code requirements for the other.

Two-way slabs necessitate reinforcement design in both directions, with main steel at the bottom and minimum steel above.

All two-way slabs should be supported on four edges to distribute loads effectively.

One-way slabs can be supported on one, two, or four edges depending on the structural design.

Cantilever slabs are a special case of one-way slabs, supported on one edge and subjected to one-directional loads.

Calculating moments for cantilever slabs requires special attention due to the potential for negative bending moments.

One-way slabs supported on two edges are still classified as one-way, regardless of the dimensions of the slab.

A slab supported on four edges can be classified as one-way or two-way based on the ratio of its longest to shortest span.

If the longest span is more than twice the shortest, the slab is classified as one-way.

For simply supported slabs, the maximum moment is calculated as wl square over eight.

Cantilever slabs under uniform load exhibit a maximum bending moment of negative wl square over two.

If the long span is less than or equal to twice the short span, the slab is classified as two-way.

Two-way slabs require reinforcement design for both directions to handle loads effectively.

The video concludes with a summary and an invitation to the next presentation.