Why Post-tensioning and Pre-tensioning?

Why Post-Tensioning?

The use of post-tensioning allows engineers to use thinner concrete sections, longer spans, stiffer to resist lateral loads and resist the effects of shrinkage and swelling.
Concrete has ‘compressive’ strength i.e it can carry its own weight within a structure. And when ‘live’ loads such as vehicles on a bridge are applied the concrete tends to deflect or sag which leads to cracking, thus weakening the structure.

Why Post-Tensioning is done?

We know Concrete lacks ‘tensile’ strength. That’s why steel reinforcing bars – ‘rebar’ – are often embedded in the concrete to limit the width of cracks. However, rebar provides only passive reinforcement – that is, it does not bear any load or force until the concrete has already cracked.
Now, this is the time when Post Tensioning comes into act. Post-Tensioning systems provide active reinforcement. The function of post-tensioning is to place the concrete structure under compression in those regions where load causes tensile stress. Post-tensioning applies a compressive stress to the material, which offsets the tensile stress the concrete might face under loading.

Post-Tensioning

How it's actually done?

Post-tensioned bridge decks are generally composed of in situ concrete in which ducts have been cast in the required positions. When the concrete has acquired sufficient strength, the tendons are threaded through the ducts and tensioned by hydraulic jacks acting against the ends of the member. The ends of the tendons are then anchored.
Tendons are then bonded to the concrete by injecting grout into the ducts after the stressing has been completed.
It is possible to use pre-cast concrete units which are post-tensioned together on site to form the bridge deck.
Generally, it is more economical to use post-tensioned construction for continuous structures rather than in-situ reinforced concrete at spans greater than 20 meters. For simply supported spans it may be economic to use a post-tensioned deck at spans greater than 20 meters.

Advantages.

It reduces or eliminates shrinkage cracking-therefore no joints, or fewer joints, are needed
Cracks that do form are held tightly together
It allows slabs and other structural members to be thinner
It allows us to build slabs on expansive or soft soils
It lets us design longer spans in elevated members, like floors or beams

Common Applications.

Post-tensioning, or PT, has become increasingly popular over the past 30 years or so as the technology has been perfected. At one time there were problems with corrosion of the cables, especially in deicing-salt-laden parking structures, but better materials and construction methods (plus good training and certification programs) have eliminated most problems.

What is Pre-tensioning?

Pre-tensioning The tension is applied to the tendons before casting of the concrete. The precompression is transmitted from steel to concrete through bond over the transmission length near the ends.

Why Pre-tensioning?

We know that concrete is good for compression and develops cracks when tensile strength is applied. 

The basic concept behind it is rebars are excellent in tension and when we apply tension to rebars and release, it applies compression to the concrete. So we will definitely look forward to compression as much as possible but sometimes concrete is subjected to tension.

For example, any cantilever section hanging out on one end and other end attached to the column will get subjected to tension at the hanging out end as it will be pushed down. Now imagine if we Pre-tension it, the tensile strength of the rebars will apply the compression force to the concrete.

Pre-Tensioning.

How Pre-tensioning is done?

Pre-tensioning is accomplished by stressing wires or strands, called tendons, to predetermined amount by stretching them between two anchorages prior to placing concrete. The concrete is then placed and tendons become bonded to concrete throughout their length. After the concrete has hardened, the tendons are released by cutting them at the anchorages. The tendons tend to regain their original length by shortening and in this process transfer through bond a compressive stress to the concrete. The tendons are usually stressed by the use of hydraulic jacks. The stress in tendons is maintained during the placing and curing of concrete by anchoring the ends of the tendons to abutments that may be as much as 200m apart. The abutments and other formwork used in this procedure are called prestressing bench or bed.

K-Truss Designs (Bridge structures)

Fig: -Balsa wood bridge, used for 'K-Truss' design with seven vertical members and 6 'K-Trusses' per side.

The basic concept of designing a bridge in the K-truss structure is that it is comprised of shorter members and due to the presence of shorter members it can resist buckling from compression to a great extent. But due to its complexity, it is quite unpopular among designers. Another very important cons of K-Truss is a member may be in compression under one load scenario and in tension under another. This can mean the structure may not be able to be optimally designer.

An example of a K-Truss setup and its reaction under an applied load is shown below. Compressive members are shown as green and tension as red.

But what actually makes K-Truss unique?

 It is clear that the truss type is a cross between a Parker and a Pennsylvania petit but feature two subdivided diagonal beams per panel that meet at the centre of the vertical beam, featuring the letter “K” in the alphabet. 

There are two types of K-trusses that exist: 

• one that features the subdivided beams going outwards away from the centre of the span, creating a rhombus shape at the centre of the span. 
•  other type features subdivided beams going inwards, towards the centre of the span, creating the letter “X”. 

This truss design is one of three that feature diagonal beams resembling a letter in an alphabet. The other two are the Warren (with the W-shape) and the Howe lattice or double-intersecting Warren, which feature the letter X. Technically, a two-panel Warren truss design, resembling the letter V also counts in the mix.

History:-

According to information collected to date, the K-truss came into existence in the United States during the age of Standardisation in the 1920s. During that time, fancier but structurally deficient truss designs, such as the Thacher, Kellogg and Whipple trusses, were either phased out or modified with heavier truss beams and riveted connections with the goal of handling heavier volumes of traffic.

In India Digha–Sonpur bridge is a K-truss Bridge across river Ganges, connecting Digha Ghat in Patna and Pahleja Ghat in Sonpur. The bridge was completed in August 2015. The bridge provides easy Roadway and Railway link between Northern and Southern parts of Bihar.

Earlier, Rajendra Setu was the only bridge that carries railway tracks across the Ganges in the state of Bihar. It was opened in 1959. Initially sanctioned as a rail bridge, the Ganga Rail Bridge project was converted to a rail-cum-road bridge in 2006. The total cost of the project was put at 13,890 million, out of which 8,350 million was for the rail part, and 5,540 million was for the road part. It was expected to be completed in five years. When completed, the 4,556 metres (14,948 ft) bridge is amongst the longer bridges in India. The total length of construction, including approaches, would be 20 km. Rail part of the bridge was inaugurated in 12th March 2016.

Advantages:-

• Reduced compression in vertical members.
• Possible reduction in steel and cost if designed efficiently.

Disadvantages:-

• Slightly more complex.
• Increased constructibility due to additional members.