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Post-Tensioning in Structures – A Case Study

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POST-TENSIONING
Er. P Surya Prakash
Managing Director
SatyaVani Projects and Consultants Pvt. Ltd.

Introduction

The Post-tensioning method is used under a pre-stressing process in which it has marked advantages over the conventional non-stressed structures like greater span to depth ratio, higher moment and shear capacity. These methods are generally adopted in the making of PSC girders, sleepers, Bridges, Slabs in buildings, Concrete Pile, Repair and Rehabilitations, Nuclear Power Plant etc. The increasingly extensive use of this method is due to its inherent advantages and its nature of easy application to a wide variety of structures with different geometrical shapes and design solutions. The use of post-tensioned floor slabs and reinforced concrete core walls has become increasingly popular in high-rise construction.

Prestressed Concrete is being used as an alternative to reinforced concrete all over the world for decades. In the present times the post tensioning systems are very widely used for structures involving major constructions. Now, Prestressed technology, with the attainment of Design maturity gained over the years, provides efficient solutions for various structural members and situations viz., floor structure, vertical element, lateral load resisting system.

The underlying principle of this technology lies in using high tensile strength steel alloys, producing permanent pre-compression in areas subjected to Tension. A portion of tensile stress is counteracted which thereby reduces the cross-sectional area of the steel reinforcement.

Methods

  • PRETENSIONING: Placing of concrete around reinforcing tendons that have been stressed to the desired degree.
  • POST-TENSIONING: – Reinforcing tendons are stretched by jacks while keeping them inserted in voids left before-hand, during curing of concrete. These spaces are then grouted fully with pressure pump to ensure a strong bond of steel tightly to the concrete.

Advantages of PT Technology Applications

  • Post-tensioning allows longer clear spans, thinner slabs, fewer beams and more slender, dramatic elements.
  • Thinner slabs mean less requirement of concrete. It means a lower overall building height for the same number of floors and floor-to-floor height. In turn, it also translates to considerable savings in mechanical systems and façade costs.
  • It results in a significant reduction in building weight than that of a conventional concrete building with the same number of floors, leading to reduced foundation load and thus can be a preferable choice of Design system in seismic areas.
  • In this system beams and slabs can be continuous, i.e. a single beam can run continuously from one end of the building to the other.
  • Reduced occurrence of cracks improved seismic performance and faster cycle time of construction are some notable positives associated with this system.
  • Freezing & thawing durability is higher in structures using PT than non-prestressed concrete.

 

 

Materials / Equipment used in Post-Tensioning:

  • The basic element of a post-tensioning system is called a tendon. A post-tensioning tendon is made up of one or more pieces of prestressing steel, coated with a protective coating and housed inside of a duct or sheathing which provides one coat of corrosion protection.
  • A tendon will have anchors on each end to transmit the forces into the structure.
  • Long tendons may also have intermediate anchors along their length.
  • The prestressing steel can be a high strength steel strand or a high strength steel bar.
  • Prestressing steel is manufactured to applicable ASTM requirements.
  • Typical strand sizes are 0.50 in. and 0.60 in. diameters, and bar sizes can typically range from 1 in. to 2.5 in. To give an idea of the high strength of this type of steel— A typical steel strand used for post-tensioning yields about 243,000 psi. while in comparison, a typical piece of rebar yields about 60,000 psi.
  • Inside the duct or sheathing, the prestressing steel is covered in a protective coating that provides another level of corrosion protection. This coating can be a specially formulated type of grease, or it can be a specially designed type of grout.
  • When grease is used, the prestressing steel is permanently free to move relative to the sheathing and the tendon is referred to as an unbonded tendon. When grout is used, the steel is permanently bonded to the sheathing and is referred to as a bonded tendon.
  • The hydraulic jacks used to tension the post-tensioning tendons range from compact 60 lb jacks used to stress mono-strand tendons and small bar tendons, to very large jacks requiring special hoisting equipment, used to simultaneously tension all the strands in large multistrand tendons.

Process in Brief

Tension is applied to prestressing steel by using a hydraulic stressing jack. The jack bears against one of the anchors that is embedded in the concrete and pulls the steel to a predetermined force. As the tensioning is applied, the steel gets elongated, and the concrete element gets compressed. When the proper tensioning force is reached, the prestressing steel is anchored in place. The anchors are designed such that they provide a permanent mechanical connection, forever keeping the steel in tension, and the concrete in compression.

Case Study of a Commercial Building in A.P.

Description of Study Model:

Project Details

  1. Purpose of building: Commercial
  2. Shape of building: Regular (rectangular)
  3. No of stories: (2B+S+11)
  4. Type of wall: Masonry
  5. Height of stories: 4.5m (similar)

General Conditions of Area of Construction

  1. Area: Mangalagiri, Andhra Pradesh
  2. Zone: III
  3. Soil Type: Moderate
  4. Zone factor:0.16
  5. Response Reduction Factor R=3.0

 

 

Material Properties

To carry out the work in RAPT software, the properties of the material such as concrete and steel need to be defined. Similarly, the loads should be defined, under live load, dead load, seismic load and wind load types.

  1. Grade of concrete and steel: M40 and Fe500
  2. LL=5KN/m2
  3. FF=2KN/m2
  4. Grade of prestressing strand FP=1860 N/mm2
  5. Overall depth of drop panel = 400 mm
  6. Width of drop panel   = 2900 mm
  7. Depth of slab   = 220 mm
  8. Width of strip B = 8100 mm
  9. Dia. of strand used=12.7mm
  10. Number of strands in column strip =8nos.
  11. Number of strands in middle strip = 7nos
  12. Properties of Tendon used in designing
  • Type of tendon- cold drawn steel.
  • Modulus of elasticity- 210000 N/mm2.
  • of wires in one strand-7
  • Yield stress of tendon-1860 N/mm2.
  • Tendon jacking stress -1481 N/mm2.

Methodology

All the process which was carried out while designing the structure is represented in the form of a flowchart given below:

Modelling using Staad Pro And Rapt Software

  1. Centre line plan was prepared in AutoCAD to simplify the work in STAADPRO.
  2. Grids was laid in STAADPRO.
  3. Structure was modelled according to the plan selected in STAADPRO.
  4. PT slab is analyzed and designed by RAPT

Assigning of Various Loads

  1. Reinforcing bars of desirable diameter was allowed.
  2. Loads of various types were defined along with mass source for seismic loading.
  3. Loads like live load, seismic load and wind load were assigned to structure according to IS 875 and IS 1893.

Design of Structure

  1. Design of different members was carried out with the help of software.
  2. Detailing preferences were selected.
  3. Rebar selection rules were applied.
  4. Detailing was also done using the same software.
  5. Report generation was also done in software.

 

 

Conclusions

  • Among the various types of concrete prestressed concrete is the best concrete for obtaining highest strength in the constructions of major structures and for getting longer life span of a structure.
  • There is a considerable percentage saving in concrete and steel; Owing to the participation of the entire concrete cross-section, more slender designs are possible.
  • It allows for smaller deflections when compared to structural members with steel and reinforced concrete. Good crack behaviour and therefore better protection of the steel against corrosion.
  • The structure attains high fatigue strength, since the amplitude of the stress changes in the prestressing steel under alternating loads are quite small.
  • There is constant serviceability even under considerable overload, since temporary cracks close again after the removal of overload.

In future this post-tension technology will be one of the most competitive solutions adoptable for structures on a large scale. As the thickness of the PT slab is much lesser than the R.C.C flat slab, aesthetic look of the building gets largely enhanced leading to a clear height from a longer distance. Using a PT Slab rather than a RCC flat slab is preferred and advisable for a commercial building. Construction of a structure using PT Slab makes more sense from the viewpoint of reduced deadload too as it results in a lighter structure.

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