9 Key Advantages and Disadvantages of K-Truss Bridge | K Truss Bridge Compression & Tension Forces

What is a K-Truss Bridge? | Advantages and Disadvantages of K-Truss Bridge | K Truss Bridge Vs. Parker Truss | Examples of K-Truss Bridge

What is a K-Truss Bridge?

The K-truss is named after the K shape by the vertical member and two oblique members in each panel.

The primary idea behind creating a bridge in the K-truss shape is that it is made up of shorter elements, and because of this, it can withstand buckling from compression to a large degree.

The K truss is designed to divide vertical elements into smaller parts. This is due to the compression of the vertical members.

The shorter a member is, the better it can withstand compression buckling. The K truss was not widely used in the United States, most likely due to its complexity.

A K-Truss is usually used for reinforcing members with high axial compression, not so much for bending; this is because the diagonals take almost nothing of the axial force.

Since the diagonals usually have a lower bending stiffness than the chords, they give in more easily. That results in fewer secondary tensions in the truss.

Examples of K-Truss Bridge

. Examples of K-Truss Bridge include: The Südbrücke rail bridge over the Rhine in Mainz, Germany, the I-895 (Baltimore Harbor Tunnel Thruway) bridge in Baltimore, Maryland, the Long–Allen Bridge in Morgan City, Louisiana (Morgan City Bridge) with three 600-foot-long spans, and the Wax Lake Outlet bridge in Calumet, Louisiana

K Truss Bridge Vs. Parker Truss

The K Truss design was a variant from the Parker truss design. The Parker was derived from the Pratt truss. The K truss is designed to divide vertical elements into smaller parts.

This is due to the compression of the vertical members. The shorter a member is, the better it can withstand compression buckling.

Advantages and Disadvantages of K-Truss Bridge

Advantages of K-Truss Bridge

• Reduced compression in vertical members: K-truss layout enables the vertical members to be self-supporting. The compression of the deck members is not required because they are stiff at their base.
• Possible reduction in steel and cost if designed efficiently: The deflection and the moment in the vertical members are parallel to each other. This avoids the requirements of excessive steel at joints, which may otherwise be necessary to prevent buckling.
• Simplicity of erection: Because of the short diagonal members and their tapered or curved layout, it is possible to erect a K-truss bridge with comparatively less labour and material than would be required for a Warren Truss.
• Reduced bending in horizontal members: Horizontal members that extend over more than two spans will bend less and require smaller braces than those using beam and pier supports exclusively because less bending occurs in areas where trusses join together, all resulting in a reduced required member size (i.e., smaller girders).
• Increased Efficiency; High efficiency because of the longer diagonal members is in tension and the shorter vertical members are in compression

Disadvantages of K-Truss Bridge

• Slightly more complex: The simple Warren Truss design is easy to analyse, design, and build. This is also true of the Pratt Truss design. The K-Truss has some additional complexity because of its two diagonals per panel.
• Increased deflection: Deflection increases with the cube of the span length for a beam bridge. That means that a long span K-Truss will sag more than would be the case with a Warren or Pratt Truss having the same span and total load.
• Increased constructability due to additional members: The construction of a K-Truss is more complex because there are more members. This increases the labor requirement and the cost of materials.
• Another significant disadvantage of K-truss is that a member can be in compression under one load scenario and in tension under another. This may imply that the structure cannot be designed optimally.
• Not Popular: Because of its complexity, it is disliked among designers.

K-Truss Bridge FAQs

Who designed the K truss bridge?

The K-Truss truss configuration was invented by Phelps Johnson, president of the Dominion Bridge Company of Montreal, Quebec, Canada, as part of the design effort for the second (existing) Quebec Bridge. Phelps was born in 1849 in the United States and worked for the Wrought Iron Bridge Company for a time.

Are K-truss bridges strong?

The main concept behind building a bridge in the K-truss form is that it is made up of shorter elements, and because of this, it can withstand buckling from compression to a significant degree.

What is the compression on a K-Truss?

The compression on K-truss is determined by the ratio of length of the short diagonal member to the long diagonal member.

Is it possible to get a longer section for vertical members?

It is possible to increase the length of trusses by adding one or two diagonals to spans. This would reduce deflection, but adversely affect horizontal stability as well as height.

What is the best way to reinforce a bridge with K-Truss?

K-trusses are inherently stronger than Warren or Pratt trusses, but they are also heavier and more complex to build. The structural behaviour of the different types of trusses is similar.

Therefore, any type of truss will perform well when properly supported. The principle method used for reinforcing a bridge with K-trusses is by the addition of piers (vertical columns is usually built-in groups called ‘pier packs’.

As the pier pack can be used to carry vertical loads, both compression and tension may be transferred directly into the ground

This may prevent the need for as much vertical steel to be provided to resist tension and compression forces.

What caused the K truss to be unpopular?

This is due to the compression of the vertical members, and the shorter a member is, the better it can resist buckling from compression.

The K truss was not widely used in the United States, most likely due to its complexity.

What are k truss bridge pros and cons?

•  Pro: The cost of materials and labor is lower than a Pratt truss bridge.
•   Con: The K truss requires more steel to be provided to support the vertical members. This increases the total steel required for both sides of the bridge and therefore, it may increase the costs of building a bridge.

What is the load distribution in K-Truss Bridge?

The steel in the joints is not required to support the members when they are in tension. So, this results in low weight on the long members and high weight on the short members.

What happens to forces of compression and tension when trusses join K truss?

When a K-truss supports a horizontal load, forces of compression and tension are transferred through the bridge’s diagonals from member to member whilst also transferring across bridge spans.