WRITTEN BY KLINE ASSOCIATE ENGINEER, KATHERINE FARLEY, P.E.

Post-tensioning (PT) is widely used in large supporting elements such as transfer girders, transfer slabs, and mat foundations. While the advantages of PT are widely known for typical building construction, PT is particularly well suited for the design of transfer elements in buildings. PT transfer girders, for example, will typically be small in comparison to steel or non-prestressed concrete girders. Moreover, the active PT force will counteract gravity loads to reduce long-term deflections. The design of these members can be challenging due to the large PT force concentration required to pick up several levels above. The effects of applying this magnitude of PT force at once can cause several issues, including massive cracks and blow-outs. For this reason, stage stressing becomes a crucial design step. The engineer of record (EOR) should determine if stage stressing is required, and if required, the EOR should be the one to specify all stage stressing requirements.

Below is a list of common FAQ’s contractors may have while dealing with stage stressing:

Q1. How do I know if stage stressing is required?

Stage stressing requirements should be provided in the structural drawings. However, it is not uncommon for the EOR to delegate this responsibility to the PT supplier, or in some cases to ignore them altogether. If the project has PT transfer girders, transfer slabs, podium slabs, or a mat foundation, stage stressing may be required. If the drawings are silent regarding stage stressing, it is appropriate to contact the EOR through an RFI.

To dive into the nitty-gritties, the first indicator that stage stressing is required are initial concrete compressive stresses larger than 0.60f’ci. When an engineer recognizes initial compressive stresses are an issue, there are only a couple ways to reduce these stresses. Either reduce the PT force or increase the initial compressive strength of the member. The second option is often not feasible because the required initial concrete strength would be unachievable given a tight construction schedule. Therefore, reducing the amount of PT initially applied to the member through stage stressing is the preferred option.

Q2. Do I need shoring during any part of the stage stressing operation?

If stage stressing is designed properly, no shoring of the transfer element is required after the initial stage stressing sequence is complete and stressing records are approved by the EOR. Once the initial PT force is applied, the member gains enough structural capacity to support its own self-weight, plus additional loading from the levels above. The engineer should design the stage stressing sequence in a way that eliminates post-initial stage shoring.

Q3. Is camber required during stage stressing?

No. However, deflection at early stages of stressing can be an issue in many cases. The engineer may request cambering to compensate for any large short-term deflection occurring during stage stressing. When requesting camber, the engineer should pay special attention to the interaction of adding PT at different stages and the long-term effects. Understanding this will eliminate deflection issues that can impact third party work, such as façade and window installation.

Q4. How is the number of stages determined?

The number of stages is determined by the engineer based on the required PT force together with the number of levels supported by the transfer element. Based on practical experience, a good indicator of the number of stressing stages for a structural element is the ratio between the actual initial tensile stress and 6√f’ci.

For instance, if a transfer slab supporting 6 levels above has an initial stress of 12√f’ci, then:

Q5. For bonded PT transfer girders, does it matter which tendons are stressed first?

Yes. The arrangement of bonded PT tendons within a girder is driven by the center of gravity of the strands (CGS) at the anchorage relative to the center of gravity of the concrete (CGC). In cases where multiple tendons are required, the anchorages will most likely be arranged in rows. As PT force is applied at each stage, the engineer should verify that the CGS of every stage is within the kern (middle third) of the section in both the vertical and horizontal directions.

Q6. Can I partially stress a tendon in any given stage?

No. It is recommended that tendons (bonded and unbonded) be stressed to their full jacking force in one stage. For this reason, the size and arrangement of stressing anchorages must be coordinated with the stage stressing requirements in order to ensure that the PT force and CGS at each stage are acceptable to the EOR. Stressing a tendon multiple times to achieve the jacking force can result in damaged or broken strands. Such a procedure can also lead to unsafe practices.

Post-tensioning is a powerful method for the design of transfer elements, not only because of the advantages mentioned above but also because the engineer is able to fine-tune forces and stresses within the members. Though the answers to the above FAQs apply to most situations for typical projects, every job is unique. Therefore, it is vital that an experienced PT designer who understands the behavior of transfer elements with the applied loading be engaged to design the members and specify the required stage stressing.

If you need a post-tensioning expert or have any questions as you prepare for your next building project, consult an expert!