Model prestressing tendons with couplers in STATIK.

In some prestressed structures it may be necessary to introduce the prestressing tendons in sections due to their dimensions or the construction process. To connect one tendon section to the next, it is necessary to place elements called couplers.

These elements are generally of 3 types:

- Couplers for the extension of previously installed and tensioned tendons. These couplers are usually installed in post-tensioned building slabs.

- Couplers that allow the lengthening of tendons that have not yet been tensioned or that have been partially tensioned. These couplers are commonly used in segmental bridges.

- Couplers that allow the tendon strands to overlap by applying the belt buckle principle. These couplers are suitable for rotationally symmetrical structures such as water tanks, digester tanks, etc., which require the arrangement of a circumferential post-tensioning.

The purpose of this article is to explain how we can model a structure whose construction process involves the arrangement of prestressing couplers in STATIK. In the program there is no element to model them, but what we will do is to simulate their effect.

Before presenting an example that we are going to solve with the program, we would like to make some important considerations to take into account when introducing tendons in the model that in reality are going to have couplers.

We must keep in mind that a coupler is a physical part that will have certain characteristics depending on the company that manufactures it. However, the same thing always happens in all of them: we have a tendon that arrives and another that leaves and there is a discontinuity. This means that it is necessary to know very well the characteristics of the element in the design phase in order to control the force coming from one tendon and the force coming from the other. A significant difference in forces (usually the manufacturer determines the maximum allowable difference) can cause debonding or uncoupling and render the system inoperative.

When we have a tendon already tensioned and we install a coupler that does not transmit the forces from the second tendon to the first tendon, we must ensure that the force that arrives from the second tendon to the coupler is equal (or very close) to the force with which the first tendon has arrived.

When the first tendon is not tensioned or is partially tensioned and we want the force of the second tendon to be transmitted to the first tendon, a coupler will be arranged to allow it and the manufacturer's prescriptions will be followed to ensure correct operation.

Here we are going to present an example in STATIK in which we are going to assume that we have a beam with two spans built in two phases in which there is a tendon also installed in two spans. There will be a partial tensioning of the first tendon section and we want that after the tensioning of the second section the force is transmitted to the first section.

We would like to make a brief remark related to the resemblance between the processes carried out in STATIK and the construction processes carried out in real life. Thus, it seems logical that different construction stages, different tendon groups can be defined and that it is possible to stress the tendons in different phases. Just like in real life. What we will not be able to do is to say that a tendon, which is physically defined within a certain structure line, is knitted in a phase in which that structure line is not active. With that in mind, the following development becomes even more logical.

The construction process will be as follows:

- The first span of the girder is executed, the first tendon span is installed and tensioned to 40% fpk from the initial end. A coupler is installed at the end to allow the transmission of force from the second tendon span to the first one.

- The second span of the beam is executed, the second tendon span is installed and tensioned to 75% fpk from the end.

The trick to getting the first tendon span to take the force coming from the second span is to set up the stressing process properly. Speaking in terms of STATIK there will be 2 construction stages:

- “Span 1” stage: this will consist of the first span of the beam to which the first tendon span will be assigned, which will be in STATIK the “Tendon Group 1”.

- Stage “Span 2”: it will be constituted by the second span of the beam to which the second tendon section will be assigned, which will be in STATIK the “Tendon Group 2”.

The stressing process of the first tendon section shall have two phases: one that occurs in the first construction stage (Span 1) and one that occurs in the second construction stage (Span 2). The second tendon span shall have only one stressing phase.

We do this as follows:

NOTE: in this example we do not consider wedge penetration to simplify the results.

1.- Stressing of the first span at 40% fpk from the initial end:

2.- Welding of the second section at 75% of fpk from the final end:

3.- We look at the force reaching the other end of the second tendon section:

4.- This will be the stressing force of the second phase of the first tendon section:

With this simple procedure we will make the force of the second tendon section to be transmitted to the first one, thus being able to consider the effects of the second section stressing on the elements that already existed previously.

By checking the values at the ends of the tendons we could also control the force in a coupler in case it is the type that does not transmit forces from one tendon section to another. With this data we would see if the limit given by the manufacturer is exceeded so that no debonding occurs.

As we have seen, STATIK has a large number of results outputs that will allow us to control at all times the forces at all points of our tendons and in each construction phase that we define.