Post tensioning is a method of prestressing concrete which employs use of high strength steel tendons, that are stressed after concrete placement. Post tensioning allows for the reduction in quantity of reinforcement, longer spans with fewer supports and thinner structural members, thereby reducing material quantities.
Holy Family Basilica project is a 4-level basement parking silo building in Nairobi, it provides much needed parking space within the Central Business District.
This parking silo architectural design is characterised by large spans of up to 8m between columns to accommodate a flexible parking layout. The slab conventional design sized the structural members as follows, 600mm deep beams and 175 mm thick slab. As regards the site earthworks, this project required excavation into the rock sub strata to accommodate the 4 stories below ground level.
Use of post tensioning technology was explored as a value engineering alternative, therefore the structure was re-designed as a post tensioned (PT) building and analysed for cost. Upon evaluation, a cost saving of Ksh.40 million was realised with the PT design hence it was adopted as the better value engineering option. Somers Engineering was then appointed to design, detail and install the post tensioning system.
The post tensioning design resulted in a 225mm flat slab of the slab with column support grids of 8m by 8m, flat soffit no down stand beams.
Cost comparison between the conventional and the post tensioned design highlighted the following;
- With elimination of the down stand beams, the rock excavation was reduced by 2.0m saving in volumes of rock excavation.
- With use of post tensioned tendons, quantity of steel used in the structural members was reduced by 60%
- Overall time savings from faster construction cycles: Striking off formworks 5 days after casting compared to 21 days for the conventional design.
- Simpler formwork construction as there are no down stand beams, saving on time and cost of formwork erection
- Cost of all services running vertically reduced as the building height reduced
- Overall savings of 20% on structural cost of the project.