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The use of post-tensioning in buildings provides efficient, economic and elegant structural solutions while controlling deformations and cracking under service conditions.

  • Keangnam Landmark Tower
    Post-tensioning for a symbol of Vietnam’s economic growth – the 350m tower that is now the country’s tallest building.
    Vietnam - 2009 read more

    Keangnam Landmark Tower

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  • One Pacific Centre redevelopment
    Diaphragm wall and basement construction works for the redevelopment of a commercial building in Kwun Tong.
    Hong Kong - 2007 read more

    One Pacific Centre redevelopment

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  • Skylight Tower apartments
    Design and installation of post-tensioning for the 72,500m² of slabs in an apartment complex.
    Vietnam - 2012 read more

    Skylight Tower apartments

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  • Ex-Yuen Long Estate redevelopment
    Construction of shaft-grouted barrettes and carrying out static load testing in a complex geological setting.
    Hong Kong - 2013 read more

    Ex-Yuen Long Estate redevelopment

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Use of post-tensioning in buildings achieves substantial benefits for all parties.

Owners benefit from savings in materials in the structures and their foundations, reduced financing costs due to shorter construction periods, less maintenance, more usable space for a given building height and reduced structural deflection.

Architects benefit from more aesthetic freedom and larger column-free spaces.

Contractors gain through shorter construction periods as formwork is often simpler and cycle times are reduced as the use of post-tensioning allows the formwork to be stripped earlier. The approach also reduces energy consumption and allows for fast and easy installation of electrical, air-conditioning and other services.

VSL’s products and services

VSL provides building construction services from design assistance before tendering to full execution of the post-tensioning. Services include:

- Preliminary design assistance at the conceptual stage to select the best option for the floor system and provide preliminary sizing and quantities;

- Assistance throughout all detailed design stages with a constant aim of optimising the savings in materials, achieving sustainability for the structure and easing construction to reduce the cycle times and the resources required;

- The complete supply and installation of the post-tensioning materials, including a turnkey service package provided by VSL’s teams on site.

Associated products and services:

  • Blast protection
  • Heavy lifting
  • Precasting and frame erection
  • Bearings and movement joints
  • Repair and maintenance
  • Strengthening
  • Dampers

Advantages of VSL’s services:

  • Larger spans and cantilevering structures, allowing larger column-free spaces to provide more flexibility for the framing or the partitioning of the completed building as well as greater architectural freedoms
  • Thinner slabs, with a reduction in structural depth and weight that saves materials and reduces foundation loadings
  • Better structural performance, with improved deflection and cracking behaviour.
  • Improved constructability, with less material to be handled and placed. A smaller quantity and simpler layout of passive reinforcement mitigates congestion, allowing easier installation of curtain walls
  • Faster construction cycles, with simpler formwork, less propping, earlier stripping and quicker turn-around
  • Flexibility in services layout as post-tensioning permits the design of a flat soffit (underneath surface of the floor) whereas a non-prestressed floor might require beams and thickening at the support areas. This simplifies the design and installation of mechanical and electrical networks.

Contributing to sustainable solutions

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The savings in concrete and rebar materials mean that post-tensioned structures allow for substantial reduction in CO2 emissions during the construction stage, of up to 27%.

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How it works

Post-tensioning tendons are installed at suitable locations to maintain compression and ensure the slab’s strength.

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A post-tensioned raft foundation under construction

Main components

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Each tendon is made from the following components:

  • Strand - the tensile member of the tendon
  • A passive ‘dead-end’ anchorage
  • A stressing anchorage or ‘live-end’ to allow both the dead end and the live end to transfer the post-tensioning forces from the tendon to the concrete
  • Couplers are sometimes located between the two ends of tendons, serving temporarily as a live end for a section of the tendon that has already been installed

The stressing anchorages can also be used as fixed ("dead-end") anchorages, which may be preferred in some situations for practical or financial reasons.

Main systems

Bonded versus unbonded

The unbonded monostrand system offers the following advantages:

  • Thin, light and flexible tendons that allow maximum tendon eccentricity in relatively thin members and are easy to handle and place
  • Only small friction losses during stressing
  • Factory provision of corrosion protection for the prestressing steel
  • No grouting necessary

The advantages of the bonded systems include:

  • Full exploitation of the yield strength of the prestressing steel
  • Improved cracking behaviour by activation of bond forces, reducing the amount of additional non-prestressed steel required for crack control compared with an unbonded system
  • For the flat duct system: thin tendons allow maximum tendon eccentricity in relatively thin members
  • For the multi-strand system: the ability to transfer large forces using large tendons

To grout or not to grout ?

A question frequently asked by designers is whether bonded or unbonded post-tensioning should be specified. It is not possible to give a definitive answer to this question.

In some countries, floors are post-tensioned almost exclusively by unbonded monostrands (USA, Thailand, South Africa), while others only permit bonded post-tensioning (eg Australia and Hong Kong). This shows that opinions on this matter cover the entire spectrum from absolutely in favour to absolutely against one or the other.

The reasons for this are related to factors such as local supply, design codes, availability of skilled labour, the cost of grouting, the relative costs for manufacturing, handling and placing, together with the relative costs of prestressing strand and reinforcing steel.

The best approach in making the choice is to look at the specific advantages of bonded and unbonded post-tensioning, and then to judge which aspects are more important in each particular case. It is worth keeping in mind that the overall aim should be to achieve a short construction time without compromising the quality.

Apart from floor systems, many other possible applications of post-tensioning in building structures can result in significant savings. The list includes moment-resisting frames, shear walls, service cores, transfer beams and plates, foundations, masonry walls, hangers and ties, with the following applications:

  • Floor systems
  • Moment-resisting frames
  • Transfer plates and beams
  • Post-tensioned foundations and ground anchors
  • Other applications, such as mega trusses for high-rise building stabilisation.

Floor systems

Better constructability, thinner slabs or wide spans and more freedom for owners and designers

Designers can choose from a multitude of different flooring systems.

Floor systems can be classified in different ways, such as in-situ versus precast, single span versus multi-span, slab-on-beams versus flat slab and one-way versus two-way systems.

Each of these is further sub-divided into different groups, depending on whether or not beams are used and, if so, the type of beams. They may be of the wide, shallow type often referred to as ‘band beams’, or conventional narrow beams, which also include supporting walls.

Each of these groups can then be further subdivided by:

  • Slab type - flat solid slab or flat voided slab (both with or without drop panels), ribbed slab, waffle slab;
  • Beam type - solid or voided beams (both with or without drop panels);
  • Construction method, with different combinations of in-situ and partly precast construction, including the use of steel trough decking as composite slab formwork.

Moment-resisting frames

Rigidly connected columns and beams to resist moments and shears from lateral and gravity loads

Different arrangements are possible:

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Fig. 3.2: Frames in a High-Rise Building
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Fig. 3.1: Frames in a Low-Rise Building
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Fig. 3.4: Partly Precast Frame
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Fig. 3.5: Partly Precast Frame: Section through Beam

Transfer beams and transfer plates

Transferring the loads from one framing layout to a different one

In many high-rise hotel and office buildings, large column-free lobbies are required at ground level - often extending over several floors - while the hotel or office floors above have columns and walls at much closer spacing. The transition from the small support grid to the large column spacing in the lobby is either by means of transfer beams or a transfer plate. These beams and plates usually require considerable supports in order to transfer the highly concentrated forces from the columns and walls of the upper levels to the lower supports. Part of the loading, including the self-weight of the beam, is balanced by the upward-acting deviation forces from the parabolic tendons. Deflection is thus reduced considerably. The in-plane compressive stress provided by the post-tensioning tendons also improves the cracking behaviour of the beam.

Post-tensioning of transfer beams and plates offers considerable advantages:

  • Significant reduction in reinforcement, which simplifies steel fixing and reduces construction times
  • A reduction in many cases of the beam depth or plate thickness, in turn reducing the total building height and weight, saving materials
  • Greater stiffness and hence better behaviour in terms of cracking and deflection

Post-tensioned foundations and ground anchors

Rafts and strip foundations

Transferring the loads from the building to the ground

Fig. 3.17 Ground Anchors and Tension Piles

The principle of a raft foundation is very similar to that of a floor slab turned upside down. The distributed soil pressure acts at the bottom surface and is held in equilibrium by the downward-acting concentrated forces from columns and walls. Similarly, a strip foundation acts like a beam turned upside-down.

Ground anchors

An efficient way to mobilise underground loads

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Ground anchors and tension piles are special forms of post-tensioned foundations. They play an important role in resisting large overturning moments due to wind or earthquake loads, or in providing sufficient safety against buoyancy uplift.

Other applications

Underground tension members can also be used to provide the horizontal tie at the bases of arch or shell structures, or inclined columns.

Fig. 3.18: Under-Ground Tension Ties

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