This article explains the usage of post tensioning in one of the sky scrapers in India. This also gives a note on the challenges faced, structural analysis done and the cost effectiveness of the PT solution.
About The Structure
Lokhandwala Minerva (Fig.1) is an 84 floor1007 ft high (307 meters) supertall skyscraper structure under construction in Mumbai, Maharashtra, India. Construction started and estimated to be completed in 2018. It has two towers of 84 floors each which include floors for parking, podium garden, services, residential and penthouse. Structural System of this floor plate consists of PT slab and Beam having span of 39.36ft (12m) from Ground floor to 12th floor and 25th floor to 46th floor. In podium level PT slab is design for a loading of 0.42 Kip/ft2 (20KN/m2). There is transfer level PT slab at 53rdfloor. Grade of concrete used in typical floor level M50 and in Transfer level M70. For avoiding practical difficulties and tendon congestion we have used 15.2mm strands.
Why Post – Tensioning
The need to create long, column free floor plan with excellent architectural look that made the structural consultant to choose post tensioned concrete. The use of a highly effective post tensioned concrete system allowed for thinner slab. Shallow post tensioning beams reduced the overall building mass and increase floor height. For transfer plate post tensioning helped to minimizes Sizing the rebar congestion.
Analysis And Design
Entire Global modeling and Analysis was carried out by Structural consultant as per privilege codes of Indian Standards (Fig.2). Design of PT slab, Beams and Transfer plate carried out based on Analysis model received from consultants. Software’s used for designing RAPT and ADAPT.
How PT Helped to be Cost Effective
It Reduced slab thickness providing direct saving in concrete costs for typical slab as well as foundation. For the typical level slab implementation of the PT Beams and slab systems gives more floor heights with respect to conversional beam slab systems. And handling cost such as lifting of materials, human resources and time. Reduce concrete volume and the usage of PT leads direct savings in steel reinforcement quantities.
Transfer plate designed at 53rd floor. Columns from 53rd floor and above floors are floated from the transfer plate. Since the transfer plate depth is 78.7in / 106.3in (2000mm / 2700mm), casting has done in two stages. The First stage of concrete 27.55in (700mm) and 51.18in (1300mm) as second stage. After casting of first stage concrete, the shuttering was removed and first stage concrete acted as shuttering for second stage concrete. This helped to reduce the shuttering and back propping cost. Shear links has been detailed like shear stools/colums to facilitate and to avoid the congestion of laying difficulties. This arrangement also helps in the easy laying of tendons.
Post tensioned concrete system was the best choice for structural framing for this sky scrapers structure. Post tensioning system reduced the building mass and in terms reduces the vertical and lateral load. Construction challenges like shuttering, propping and concreting at higher level due to transfer plate / Girders can be handled by PT system by Designing and detailing in stage/layer wise in such way that first stage (40% thickness) will act like shuttering for rest of Transfer Plate.