Post tensioning has gained significant popularity in recent years due to the numerous advantages it offers in modern construction and rehabilitation projects. Even relatively small structures, such as those with just a few floors (G+2), are increasingly adopting post tensioning techniques. This method is now widely used not only in new constructions but also in the restoration, rehabilitation, and strengthening of existing structures.
Case Study Overview
Consider a building originally constructed for residential purposes, which was later planned to be remodeled for commercial use. A major challenge arose due to the structural layout: the span between the eight interior columns was only 4 meters, which is inadequate for commercial requirements that typically demand larger open spaces.
To address this, the structural engineers aimed to remove alternate interior columns to effectively double the span to 8 meters. However, this presented significant structural challenges that required innovative solutions.
Evaluation of Alternatives
Before deciding on the method, the team considered multiple options:
Option 1: Installation of Structural Steel Beams
One option was to install 8-meter long structural steel beams beneath the existing beams to support the longer spans after column removal. This solution was rejected because the steel beams would reduce the headroom between floors, negatively affecting the building’s usability.
Option 2: Concrete Beam Encasement
Another option was to encase the existing beams in additional concrete, reinforced heavily with steel. Although structurally viable, this method made the beams considerably heavier and more expensive. It also reduced the headroom due to the added thickness, making it unsuitable.
Final Decision: External Post Tensioning
Given the drawbacks of the other options, the engineers chose an external post tensioning technique. This approach was cost-effective, preserved headroom, and provided the required structural strength to remove alternate columns safely.
Implementation of Post Tensioning
The project involved removing three alternate columns out of a row of eight. The beams connecting these columns were strengthened by encasing them with a 75 mm thick layer of micro concrete. This post tensioning system ensured continuous support across the spans where columns were removed.
Step-by-Step Procedure
The following process was adopted to achieve the column removal safely:
- Surface Preparation
The existing plaster on the beams was removed, and the concrete surface was roughened by hacking. Holes were drilled at specified intervals to install shear connectors, which were fixed in place using polyester resin grout. - Core Drilling
Core holes were drilled into the slab along the beams to allow the pouring of free-flow micro concrete, ensuring full encasement of the beams. - Reinforcement and Strand Installation
Reinforcement steel was fabricated and fixed alongside post tensioned mono strands encased in HDPE pipes. The ends of the post tensioned strands were anchored securely. - Shuttering and Micro Concrete Encasement
Shuttering was erected around the beams, and micro concrete was poured throughout the core holes to complete the encasement in one continuous operation covering all seven spans. - Initial Stressing
After seven days, once the micro concrete had reached a strength of 45 MPa, the post tensioned strands were stressed to 60% of their ultimate tensile strength (UTS). - Column Removal
The selected alternate columns were carefully chipped away using electric chipping machines while the beam was temporarily supported by screw jacks. - Final Stressing and Grouting
Once the columns were removed and the beam supported, the jacks were gradually released. Deflection of the beam was closely monitored before the post tensioned strands were stressed to 90% UTS and then grouted to secure the system permanently.
Illustrations
The system and its stages were documented through detailed diagrams, including cross-sections showing beam encasement and the final column removal. These visual aids helped illustrate the practical application and success of the external post tensioning method.
Conclusion
This case study highlights the effectiveness of external post tensioning in structural rehabilitation projects. The technique allowed the safe removal of alternate columns, creating larger spans without compromising headroom or significantly increasing costs. Post tensioning proved to be a practical and reliable solution for modifying existing buildings to meet new functional requirements.
As demonstrated, post tensioning is an excellent technique for strengthening, restoring, and modifying structures, and it is poised to become even more prevalent in the construction industry in the years ahead.