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.