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Diving Deeper into Post-Tensioning Theory
Post Tension - Fundamental Theory
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Diving Deeper into Post-Tensioning Theory
(Structural)
(OP)
24 Jan 21 10:16Hi All,
I'm relatively new to post-tensioning (PT) design and have been exploring various theories. While I grasp the basics like P/A and P applied at a distance from the centroid, there are still some gaps. Specifically, without grout, I wonder how force transfer occurs between the tendon and concrete, aside from the anchorage zones. I've thought about catenary action when a draped tendon straightens under tension, but this aspect isn't often discussed. Without bonding tendons and neglecting catenary action, it seems only the anchorage zones matter for the force interaction between tendon and concrete.
RE: Post Tension - Fundamental Theory
(Aerospace)
24 Jan 21 15:27If a moment causes a differential in tension/compression within a beam, the reverse is also true. Think of an archery bow: the string doesn't contact the bow except at the ends, yet it still creates a bending moment.
RE: Post Tension - Fundamental Theory
(Structural)
24 Jan 21 15:57Here's a thread where I encountered a similar challenge:
Thread One and Thread Two where experts like Rapt and KootK discuss similar topics.
My Personal Open Source Structural Applications:
https://github.com/buddyd16/Structural-EngineeringOpen Source Structural GitHub Group:
https://github.com/open-struct-engineerRE: Post Tension - Fundamental Theory
(Structural)
24 Jan 21 16:31
In a recent sketch, I showed that [P*e] is equivalent to load balancing, stemming from the load balancing effect of tendon curvature which moves the pre-compression force to the tendons' level. While some may view it as obvious, this realization came late in my career.
You're correct about your understanding. The main forces involved are the anchorage and load balancing forces. For further clarity, consider whether to focus on:
a) The concrete alone, with superimposed tendon actions, or;
b) The combined concrete and tendons, despite their non-composite nature in PT.
RE: Post Tension - Fundamental Theory
(Structural)
25 Jan 21 02:26Fevola,
Your concern is valid. Unbonded tendons indeed result in catenary action. This is why ACI318 requires a minimum bonded reinforcement, ensuring the member acts in flexure rather than catenary.
RE: Post Tension - Fundamental Theory
(Structural)
26 Jan 21 00:10Quote (rapt)
There is only catenary action, if there is no bond.
Concerning flexural resistance, not just catenary action, but member pre-compression also plays a role.
The bonded reinforcement enhances capacity and promotes conventional flexural behavior, yet does not replace the resistance generated by tendon drape. Catenary action remains a primary resistance source.
RE: Post Tension - Fundamental Theory
(Structural)
27 Jan 21 18:06While I don't endorse unbonded reinforcement use in North America, it's for different reasons. Flexural action is still present, as evidenced by T.Y. Lin's book "Prestressed Concrete Structures".
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Frequently Asked Questions
- What is post-tensioning?
Post-tensioning is a method of reinforcing concrete or other materials with high-strength steel strands or bars, commonly known as tendons. These tendons are tensioned to apply compression to the concrete, enhancing its performance and load-carrying capacity. - Why is post-tensioning used?
It is used to provide additional strength to structures, enabling longer spans, reduced material use, thinner slabs, and greater structural efficiency. It's ubiquitous in large-scale projects such as bridges, buildings, parking structures, and more. - How does post-tensioning work?
Tendons are placed within the concrete form before the concrete is poured. Once the concrete has gained sufficient strength, these tendons are tensioned with hydraulic jacks, transferring compressive force to the concrete. This prestresses the concrete, enhancing its performance. - Are there different types of post-tensioning systems?
Yes, there are two primary types: bonded and unbonded systems. In bonded systems, tendons are encapsulated in a grout-filled sheath, creating bond along the tendon length. In unbonded systems, tendons are coated with grease and encased in a plastic sheath, providing free movement relative to the concrete.
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