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Transfer Slabs The Unbonded Post Tension Way!!

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Satish Jain Consulting Engineers
Satish Jain, PE, SE
Managing Director
Satish Jain Consulting Engineers Pvt. Ltd.

The office of Satish Jain Consulting Engineers Private Limited (SJC), Mumbai, India, has been designing unbonded post-tensioned slabs, beams and transfer slabs and executing it through their sister concern since the year 2012.

The simplicity of unbonded post-tensioning requires not bonding the strand to the concrete on purpose. This implies that the anchors are solely responsible for load transfer through the service life of the structure and to develop the tensile strength of the strand at the limit state. This requires stringent quality control on the anchorage system, the debonding mechanism for strands, skilled installation of anchors on site and cutting mechanism of stressing end tails after stressing. Advantages of the unbonded system over other systems include flexibility of strand layout, elimination of grouting post-stressing, elimination of ducts, lower friction losses, better control on design eccentricities and hence better load balancing.

The use of unbonded post-tensioning in India is reaching a good 10 years. In absence of specifications and codes for material quality and design of unbonded post tensioning of slabs and beams in India, the specifications of the Post Tensioning Institute of North America (PTI) and the American Concrete Institute’s building design code ACI 318 serve as excellent guides for the structural engineer of record to follow.

Transfer slabs are thick, rigid elements designed to carry gravity and seismic loads from one type of structural system with multiple floors above and transfer them to a different structural system below. Transfer slabs are designed to essentially act elastically under seismic loading. A very common example is a residential tower with parking floors at lower levels. No doubt, transfer floors can be an expensive addition to low-rise buildings of 10 to 15 stories but sky-high real estate prices in the city of Mumbai has pushed the developers to develop small plots of land or redevelop existing low rises into new low-rise residential buildings with parking levels with mismatched structural systems making transfer slabs necessary.

 

 

Two such transfer slabs projects are detailed below:

Project: Woodstock Juhu by Woodstock Realties Pvt. Ltd., Mumbai. Completion Year: 2014

This low-rise consists of 1 Parking Pit (Basement) + Ground + 1 Podium level + 10Residential levels. The transfer slab is located immediately above the podium level with a size of 4000 sq. ft. and a thickness of 800 mm. Conventional RC transfer slab would have resulted in a 1100 mm thick slab.

Materials used:

  • Strands:12.7mm diameter PE sheathed and grease filled strands conforming to ASTM A416 Grade 1860 were imported from The SIAM Industrial Wire Company, Thailand (A Tata Steel Global Wire Business).
  • Anchorage System: Zero-void encapsulated anchorage system consisting of anchors, wedges, a zero-void seal tube and zero-void grease caps were imported from General Technologies Inc. (GTI), USA.
  • Concrete Grade: M80 grade and Reinforcing: Fe 500.

Stressing of strands was completed in two stages of 50% each, stage one after the concrete had attained 70% of its strength and stage two after four residential slabs were casted above the transfer slab. Cutting of stressing end tails was done using a pocket shear machine, enabling the strand to be cut at least 25 mm inside from the edge of the slab. The cut end was then locked with a grease filled cap and then grouted with GP2 grout. It is important to note that abrasive cutting of strand tail ends using a blade is a strictly not permitted to safeguard the anchorage system against corrosion.

Project: Project DevAangan by SDC Properties Pvt. Ltd, Mumbai. Transfer Slab Completed Year: July 2018. Tower in progress.

This low-rise tower consists of Ground + 3 podium levels + 12 Residential levels. The transfer slab is located at the 3rd podium immediately. The floor plate size is 14000sq. ft. out of which about 8000 sq. ft is transfer slab of thickness 600 mm and the rest is garden level with thickness 300 mm. The residential floor plates above are 5000 sq. ft. each.

Materials used:

  • Strands:12.7mm diameter PE sheathed and grease filled strands conforming to IS14268 Grade 1860 from Tata Steel – Wire Division, India.
  • Anchorage System: GTI, USA make as mentioned above.
  • Concrete Grade: M50 grade and Reinforcing: Fe 500.

Stressing of strands was completed in a single stage upon 100% strength gain of concrete.

It is to be understood that with proper care in design and execution, unbonded post-tensioning can provide remarkable advan-tages in conceptualization of structural elements compared to con-ventional RC transfer slabs. Use of internationally accepted specifications in unbonded post-tensioning is a strong tool for the structural designer in adopting this cutting edge technology.

Satish Jain is a registered structural engineer in India and the USA with over 18 years of experience in the design of building structures both in India and the USA.

Email: satishj@sjcse.com, Web: www.sjcse.com

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