The basics of post tensioned concrete design | how to design post-tensioning

Summary

This video, presented by Brendan, a structural engineer from Australia, explores the growing popularity of post-tension design in residential and commercial structures. Brendan explains why post-tensioning can offer stronger, more efficient designs with longer spans compared to traditional reinforced concrete (RC) methods. Key concepts such as pre-compression forces, tendon placement, and load balancing are discussed in detail, highlighting how they help manage tensile forces and enhance structural performance. The video also covers practical considerations for detailing post-tension systems, including tendon placement, high/low points, cantilever design, and managing restraints and penetrations.

Takeaways

• 🌟 Post-tensioned designs are becoming popular for their ability to provide stronger, thinner, and more efficient structures compared to standard reinforced concrete (RC) designs.
• 🔩 The primary advantage of post-tensioning is the introduction of pre-compression forces which counteract concrete's weakness in tension, delaying tensile cracking.
• 📏 Post-tensioning allows for the redistribution of forces within a structure, moving load from areas of high stress to areas of lower stress.
• 📈 The position of tendons relative to the neutral axis significantly affects the structural behavior, with implications for both upward and downward forces.
• 🏗️ Analyzing the deflected shape of a structure is crucial for determining the optimal positioning of high and low points in post-tensioning.
• 📍 High points in tendons should be located at areas of maximum hogging moment, and low points where deflection is highest.
• 🚫 Care must be taken with tendon diversions in slab folds to avoid thrust forces that could compromise the slab's integrity.
• 🔗 The distinction between live and dead ends in post-tensioning is important, with live ends being where tendons are stressed and dead ends where they are anchored.
• 🏢 The amount of post-tensioning (P on A) required depends on the desired degree of crack control, ranging from low to high depending on the application.
• 🔄 Load balancing in post-tensioned structures should not exceed 80% to prevent detrimental hogging forces.
• 🚧 Restraint considerations are critical in post-tensioning design, as excessive restraint can lead to cracking or other structural issues.

Q & A

What are the key benefits of post-tensioned design over standard reinforced concrete (RC) design?

Post-tensioned designs offer stronger structures that can span further, have thinner slabs, and are more efficient. The two key actions of post-tensioning—pre-compression and the tendon’s force to flatten—help overcome concrete’s weakness in tension, improving the overall performance of the structure.

How does pre-compression in post-tensioned concrete improve structural strength?

Pre-compression helps concrete by adding compression before any tensile forces act on it. Since concrete is weak in tension, this pre-compression means higher tensile forces are required to cause cracks, thus improving the structural strength and reducing cracking.

What are the primary and secondary actions involved in post-tensioning?

The primary actions include the pre-compression force that resists tensile forces, and the tendon’s force to flatten, which helps redistribute load. The secondary action depends on the tendon’s position in relation to the neutral axis—when above or below the neutral axis, the tendon causes the slab to deflect either upwards (hogging) or downwards (sagging).

Why is understanding the deflected shape of the structure important in post-tensioned design?

The deflected shape helps identify where high and low points for tendons should be placed. Low points should be where deflection is greatest (sagging), and high points where hogging occurs. This ensures the tendons provide the most efficient load distribution across the structure.

How should tendons be positioned in cantilevers for effective post-tensioning?

In cantilevers, tendons should generally be kept high and positioned above the neutral axis to effectively counteract the deflection forces and make use of secondary effects like pre-compression to reduce deflection.

What considerations should be made when positioning tendons around folds or steps in the slab?

Tendons should be diverted gradually through slab folds or steps to prevent large net forces that could thrust tendons out of the slab. The diversion should occur over 8 to 10 times the step height, and care should be taken to ensure the tendon’s path transitions smoothly.

What are 'live ends' and 'dead ends' in post-tensioned design, and where should they be located?

A 'live end' is where the tendons are stressed, usually with a jack, and a 'dead end' is where they are anchored. Both ends impart bursting forces into the structure, so they should be placed in areas where there’s sufficient concrete cover, ideally not too close to surfaces. Live ends should have additional reinforcement, especially in pan-stressed locations.

How much post-tensioning is generally needed for different structural requirements?

The amount of post-tensioning is determined by the crack control needed. Low crack control requires about 0.7 MPa of stress, moderate control (typical for residential/commercial spaces) requires 1.4 MPa, and high crack control for critical areas like wet decks requires 2.5-3 MPa.

What is the purpose of load balancing in post-tensioned design, and how is it achieved?

Load balancing ensures that the upward or downward forces created by the tendons do not overly counteract the dead load. Typically, no more than 80% of the load should be balanced to avoid excessive hogging, ensuring the structure remains stable and efficient.

What are common mistakes to avoid in post-tensioned designs regarding tendon lengths?

Tendons should not be too short (less than 6 meters) as they cannot be stressed effectively, nor too long (over 40 meters) due to excessive friction losses. The ideal length for efficient post-tensioning is between 12 to 24 meters.