ADVANTAGES & APPLICATIONS
FOR LONG SPAN & HEAVY LOADING STRUCTURES:
P.T. has been proven to be economical for normal commercial buildings and to a certain extent, residential buildings, with spans greater than 7.0m.
For a typical structure with floor loading of 5.0 kN/m2 and with spans of approximately 10.0m, anticipated savings as compared to conventional RCC design:
Due to the more efficient design concept of post-tensioning, PT structures are generally thinner as compared its RCC counterpart.
E.g. for a typical residential project, RCC flat slab may be of 250mm thick, whereas PT slab of 200mm thick is sufficient.
NEAT & SIMPLIFIED LAYOUT:
For large spans of more than 10m, RCC design would generally adopt a ribbed beams framing system. This could be substituted with PT flat slab or banded slab system, which are very friendly for construction and fixing of services
Due to simplified structural layout of floor and reduced materials used, construction can be carried out faster with less material wastages.
Time saving is derived mainly from the following
• Reduced Steel Quantity
Steel laying is a very labour intensive activity, and by reducing the quantity of steel in PT design, the slab can be ready for concreting earlier.
Generally PT flat slab requires approximately 20 to 35kg/m3 of steel compared to that of RCC slab’s 80 to 120 kg/m3.
• Earlier Stripping of Shuttering
Post-tensioned slab would be ready for stressing operation when concrete strength achieves 28 N/mm2.
For a concrete grade of 35 N/mm2, this normally happens on the 5th day after concreting.
Upon completion of stressing, all the slab shuttering can be removed and recycled for other slab’s concreting.
This earlier stripping of shuttering would mean that the builder would need less shuttering materials and the finishing work at the lower floors can be carried out earlier.
IMPROVED DEFLECTION CRITERIA:
The presence of prestressing helps a PT floor slab to “arch” slightly after stressing of tendons.
This will help to reduce the overall downward deflection of floor slab and in turn help to alleviate the long term deflection problems, especially for long span structures.
Long term deflection, if left unchecked, would result in the cracking up of stiff non-structural members, e.g. brick walls, floor tiles, etc., in the long term (i.e. 5 to 10 years after construction).
ECONOMICAL & SUPERIOR DESIGN:
Post-tensioning offers savings to clients due to its superior structural behavior and other advantages.
Proposals have been made and accepted by local builders and developers in India as time and again the post-tensioning solution out performed the RCC scheme both in terms of costs, structural depths and other benefits to the future performance of the building.
* Earlier Stripping of Shuttering.
* Flexibility in providing service ducts and false ceilings.
* Larger column free space.
* Reduction in Dead load due to Thinner slab which will make cost savings
* Less no of work force engaged for Post tensioning works than RCC slabs
* Reduction in reinforcement quantity.
* Post-tensioning can thus allow a significant reduction in building weight versus a conventional concrete building with the same number of floors reducing the foundation load and can be a major advantage in seismic areas.
* Reduces occurrence of cracks.
* Freezing & thawing durability is higher than non prestressed concrete.
* Post-tensioning is the system of choice for parking structures since it allows a high degree of flexibility in the column layout, span lengths and ramp configurations
* Greater Flexibility for penetrations and openings in floors, both pre and Post Construction.