What you must know about Post Tensioned Concrete Design

Summary

The video explores the benefits and design considerations of post-tensioned concrete structures compared to traditional reinforced concrete. It highlights how post-tensioning allows for more efficient designs with longer spans by pre-compressing the concrete, making it more resistant to cracking. The speaker emphasizes key design aspects, such as load balancing, tendon placement, and handling construction joints, while also addressing challenges like secondary forces and restraints. Careful attention to these factors ensures a durable and effective post-tensioned structure, offering insights for engineers aiming to optimize concrete design.

Takeaways

• 🔧 Post-tension concrete structures allow for greater span efficiencies and reduce cracking compared to traditional reinforced concrete structures.
• 🔩 In post-tensioning, tendons are cast into the concrete and then stressed with hydraulic jacks after the concrete hardens, creating pre-compression forces.
• ⚙️ The process of 'load balancing' helps distribute forces, but overloading can lead to permanent deflections. Balancing should only be around 80% of the dead load to avoid overstressing.
• 📐 Secondary actions caused by the draped tendons need careful design, as improper balance can lead to negative structural effects, such as permanent hogging.
• 🔗 Post-tensioning minimizes cracking by pre-compressing the concrete, but certain areas (e.g., construction joints) require additional reinforcement to avoid stress concentration.
• 🚧 Restraint locations, such as walls or stiff joints, can cause issues in post-tension designs by preventing proper compression, so careful detailing is crucial.
• 🔨 Pre-compression levels, or 'PNA,' vary depending on the use of the structure. Typical pre-compression is around 5.4 MPa, but it can go higher in specialized applications like watertight structures.
• 🔍 Tendon placement is critical, often determined by bending moment diagrams and slab deflection patterns. Tendons should be placed to resist tension forces effectively.
• 📏 Tendon lengths are most efficient between 12 to 30 meters; shorter tendons won't provide enough compression, while longer ones may experience excessive force losses.
• 🏗️ Fold areas or slab step-downs need careful handling to avoid cracking due to secondary forces, and adequate reinforcement may be necessary to secure these regions.

Q & A

What is the primary difference between traditional reinforced concrete structures and post-tension designs?

Traditional reinforced concrete structures use steel reinforcement in specific locations to resist tension actions, whereas post-tension designs cast in straight tendons within the structure and then apply hydraulic jacks to pre-compress the structure after the concrete has hardened.

How does post-tensioning help in resisting cracking?

Post-tensioning helps resist cracking by pre-compressing the structure, which means that tension forces need to overcome the pre-compressive forces before the concrete can crack.

What is the purpose of the secondary actions applied by the tendons in a post-tension structure?

The secondary actions applied by the tendons help balance loads from one location to another, allowing for more efficient load distribution and reducing the likelihood of cracking.

What is load balancing in post-tension design, and why is it important?

Load balancing refers to the practice of lifting loads in one part of the structure and transferring them to another to prevent overstressing. It is typically done for about 80% of the dead load to ensure the structure isn't over-stressed.

How can the balancing forces in a post-tension design be calculated?

The balancing forces can be calculated using a formula that considers the pre-stress force, the depth of elongation, and the length of the tendon. This helps determine the curve and the amount of load applied to the pre-compressive force.

Why is it important to limit the amount of pre-compression force applied to a structure?

Limiting the pre-compression force is important to prevent overstressing the structure and to account for losses that occur from the drape of the tendon and friction forces.

What are some ways to ensure that balance forces don't become too high in a post-tension design?

To ensure balance forces don't become too high, one can reduce the amount of post-tensioning, adjust the tendon length, or modify the tendon height to change the direct force from peak locations.

What is the significance of the minimum pre-compression force, also known as pre-stress, in post-tension structures?

The minimum pre-compression force, or pre-stress, is significant because it governs the level of cracking resistance. It is typically determined by building codes and the specific requirements of the structure.

How can detailing the location of temporary movement joints affect the performance of a post-tension structure?

Detailing the location of temporary movement joints is critical to prevent restraint, which can cause high forces and potential failure. Proper detailing ensures that the joints allow for the necessary movement without causing stress concentrations.

Why is it important to consider the curvature and drape of tendons in post-tension design?

Curvature and drape of tendons can cause secondary actions that affect the structure's performance. Proper consideration ensures that these effects are managed to prevent issues such as bowing or local failures.

What is the recommended range for tendon length in a post-tension structure for optimal efficiency?

The most efficient location for tendons is typically between 12 to 30 meters. Lengths outside this range can lead to inefficiencies due to either insufficient pre-compression force or excessive losses.