How Pre-stressing Enables Thinner Concrete Structures?

Refer to IS 1343: 2012 while working with pre-stress concrete structures

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Think of long-spanning bridges, roofs, and huge stadiums without central supporting pillars. How do engineers erect these structures when concrete is such a brittle material?

Well, that's because of pre-stressed concrete.

What is Pre-stressed Concrete?

Pre-stressed concrete is a type of concrete subjected to compressive stresses before it is brought to external service loads.
The pre-stressing is achieved by using high-strength steel cables or tendons, which are stretched and anchored to the concrete elements. When the concrete is subjected to external loads, the pre-stressing forces in the steel cables or tendons help to balance the applied loads and increase the structural capacity of the element. Where this technology is used?

Application of Pre-stressed concrete in the construction industry

  • Bridges: Pre-stressed concrete is commonly used in the construction of bridges, as it provides superior structural integrity and stability. The pre-stressing allows for greater spans between supports, reducing the number of supports required for a given length of the bridge.
  • Buildings: Pre-stressed concrete is often used in high-rise buildings and other structures where weight must be kept to a minimum. By pre-stressing the concrete, it can provide superior strength-to-weight ratios than traditional reinforced concrete.
  • Slabs: Pre-stressed concrete slabs are used in floors, roofs, and other flat surfaces. The pre-stressing increases the load-bearing capacity of the slab, allowing for larger spans without the need for additional support columns.
  • Piles: Pre-stressed concrete piles are used in foundation systems to provide extra support and increase the stability of the structure. Pre-stressed piles can bear higher loads than traditional piles and are more resistant to movement.
  • Tanks: Pre-stressed concrete is used in the construction of water tanks, as it helps to reduce the amount of leakage due to cracks or other imperfections in the concrete. The pre-stressing provides greater stability and integrity to the structure, reducing the risk of failure.

Apart from these, Pre-stressed concrete has a wide range of applications and is commonly used for the construction of railway sleepers, runways, hollow-core planks, and concrete pipes.  

Want to use pre-stress on your construction site? Refer to the IS code!

The Bureau of Indian Standards publishes IS codes like these for the harmonious development of the activities of standardization, marking, and quality certification in construction. These codes are prepared after extensive research done across the country by reputed construction companies and organizations. The codes are freely available in public resources and can be downloaded from the internet.

But what about pre-stress concrete? Does any code exist to know better about pre-stress technology? Yes! Refer to IS 1343: 2012.

IS code for Pre-stressed Concrete

IS 1343: 2012 covers various types of pre-stressing systems that can be used, such as pre-tensioned and post-tensioned systems, and the materials and methods to be used for pre-stressing.

Here are the two key features of the code:

  • The code is composed according to the concepts of the Limit State Method
  • The code is aligned with the revised version of IS 456

Now, let's dig deeper into what this code contains.

Overview of IS 1343: 2012  

The code is divided into four sections for ease of use. Here is the brief about each section:

1. Section 1 - General

This section covers the fundamentals like the scope of this IS code, references, terminologies, and symbols.

2. Section 2 - Materials, workmanship, inspection, and testing

This section deals with the material and material handling aspects. For example, the concrete grade for pre-stressed structures should be within the range of M 30 to M 80.

3. Section3 - General design requirements

The pre-stressing can be done using pre-tensioning or post-tensioning, according to which losses are calculated using the formulas mentioned in this section.

4. Section4 - Structural design: Limit State Method

The pre-stress elements are designed using the limit state method. In this method, the structure should satisfy the requirement of the limit state of collapse (flexure, compression, tension, shear, torsion), the limit state of serviceability (deflection. cracking, maximum compression), and the limit state of vibration.

Now that you are well aware of the requirements, let's look at the steps to design a pre-stressed element:

Steps to design an element using IS 1343:2012

Here is a sample design for a pre-stressed steel element using the Indian Standard IS 1343:2012:

1. Determine the design load and the required cross-sectional area of the element.

2. Select the type and size of the pre-stressing steel based on the design load and the allowable stresses in the steel. The pre-stressing steel should have a high strength-to-weight ratio and should be corrosion-resistant.

3. Determine the spacing of the pre-stressing steel based on the size and type of the steel, the cross-sectional area of the element, and the amount of pre-stressing required.

4. Design the concrete cross-section to resist the design loads and to provide an adequate bond with the pre-stressing steel. The concrete should have a high strength-to-weight ratio and should be properly cured to ensure maximum strength.

5. Determine the pre-stressing force and the pre-stressing losses based on the type and size of the pre-stressing steel, the spacing of the steel, and the concrete properties.

6. Design the anchorage system to transfer the pre-stressing force to the concrete and to provide a sufficient bond between the steel and the concrete.7. Design the element for the serviceability limit states, including deflection, cracking, and fatigue.

8. Design the element for the ultimate limit state, including the strength and stiffness of the element under the design loads.

9. Fabricate and install the pre-stressed steel element according to the design and the relevant construction stand

That's how you can use IS 1343 while working with pre-stress.

At the end of the code, you will find annexures that provide detailed information about specific relevant topics.

Annex A includes the list of IS codes referred to, Annex B gives details about sheathing ducts for pre-stressing., Annex C elaborates on the testing of systems with mechanical anchorages, Annex D for Moment of resistance for rectangular and T-sections, and Annex E for the names of committee members who helped design the code.

You now got the complete idea of pre-stress technology. As you might be knowing, pre-stress can be beneficial in multiple ways. Here are a few of the benefits of using pre-stress:

Advantages of Pre-stressed Concrete in construction

1. Increased Strength: Pre-stressed concrete has a higher strength-to-weight ratio than other types of concrete, which makes it more suitable for larger construction projects. The pre-stressing process increases the strength of the concrete by up to 20%.

2. Improved Durability: Pre-stressed concrete is highly resistant to cracking and breakage, making it an excellent choice for structures that must withstand harsh conditions or frequent use. As a result, it can last longer than other types of concrete.

3. Reduced Construction Time and Cost: Due to its increased strength and decreased need for reinforcement, pre-stressed concrete can be used in large constructions with fewer materials and less labor. This reduces the overall cost of construction and shortens the amount of time needed to complete the project.

4. Improved Load Bearing Capacity: Pre-stressed concrete can support greater loads than other types of concrete due to its increased strength. This makes it ideal for large buildings and bridges.

5. Aesthetic Appeal: Pre-stressed concrete can be designed to have an attractive appearance. This makes it suitable for use in a variety of architectural projects.

In this article, you got a complete idea of IS code, its use, and the benefit of pre-stress technology.

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