13 Key Pratt Truss Bridge Advantages and Disadvantages |Application of Pratt Truss Bridge| Pratt vs Howe Truss Bridge

What is Pratt Truss Bridge? |  Pratt Truss Bridge Advantages and Disadvantages |Application of Pratt Truss Bridge| Pratt vs Howe Truss Bridge

What is Pratt Truss Bridge?

Pratt truss bridge is one of the most frequent types of trusses and has vertical members as well as diagonals that slope down towards the center (opposite of the Howe truss).

A Pratt truss has vertical members and diagonals that slope down towards the center. Under balanced loading, the inner diagonals are tensed and the vertical elements are compressed.

If only tension components are utilized in the diagonals, crossing elements may be required towards the center to absorb concentrated live loads as they traverse the span.

It can be separated to form Y- and K-shaped patterns. Thomas and Caleb Pratt designed the Pratt truss in 1844.

This truss is suitable for spans of up to 250 feet (76 m) and was a popular configuration for railroad bridges as truss bridges transitioned from wood to metal.

They are statically determined bridges that are well suited to long spans. Between 1844 through the early twentieth century, they were popular in the United States.

Pratt truss bridges include the Maryland Governor’s Bridge, the Dearborn River High Bridge near Augusta, Montana, erected in 1897, and the Fair Oaks Bridge in Fair Oaks, California, built in 1907–09.

The Scenic Bridge near Tarkio, Montana, is a Pratt deck truss bridge with the highway on top of the truss.

History of Pratt Truss Bridge

The Pratt truss was invented in 1844 by Thomas and Caleb Pratt, who held a patent on it.

The Pratt, which was popular from the 1840s to the early twentieth century, has diagonals in tension and verticals in compression, with the exception of the hip verticals immediately opposite to the bridge’s slanted end pillars.

Pratt trusses were initially created as a combination of wood and iron trusses, but were quickly replaced with iron-only trusses.

The Pratt type survived both the move to iron construction and the second transition to steel usage.

During the nineteenth and early twentieth centuries, the Pratt truss spawned a significant variety of modifications and modified subtypes.

The majority of Maryland’s remaining metal truss bridges are Pratt through and pony trusses, including pin-connected and riveted versions.

Pratt Truss Bridge Advantages and Disadvantages

Pratt Truss Bridge Advantages

1. The design addresses all member’s behavior – diagonal members are in tension, vertical members in compression
2. Pratt Truss Bridge design is a cost-effective structure that is easier to build by unskilled labor, it has a low initial cost and is easy to transport.
3. The design of Pratt Truss Bridge can be applied in different design situations – the design can be modified to reduce the span-to-depth ratio.
4. The design of Pratt Truss Bridge has several built examples, and efficient manufacturing methods for its construction have been developed through various iterations.
5. Simple design – fewer members.
6. Structural behavior is well understood – members are simply in tension and compression; therefore, the behavior can be analyzed using simple analysis tools.
7. The bridge is straight and smooth – ideal for heavy traffic such as railroads, trucking and modern cars.
8. Utilizes standard parts, which means the design can be quickly assembled in a span-assembly shop to reduce field costs.
9. Well accepted and used design and is widely used in bridge building world.

Pratt Truss Bridge Disadvantages

1. Pratt Truss Bridge is not as advantageous if the load is not vertical (but diagonal); vertical and horizontal loads require different design considerations.
2. If not well designed, Pratt Truss Bridge can be susceptible to failure– which can result in high maintenance costs.
3. Not efficient in long span and shallow depth situations – in these situations other truss designs are preferred.
4. Aesthetically Pratt Truss Bridge is not as pleasing as other truss designs – although this can depend on personal taste.

Pratt Truss Bridge FAQs

1. What are the pros and cons of Pratt truss?

Pros of Pratt truss bridge

• It is a simple and efficient design which is easy to construct, and the cost of its construction is low.
• The bridge can be divided into smaller sections and assembled in a span-production shop to reduce field costs.
• The bridge has additional design parameters that make it appropriate for various applications, such as reducing span-to-depth ratio – this makes the bridge applicable in different situations which reduces the risk of failure.
• It is a well-recognized and used design which has been in existence for over a century.
• It has a good structural behavior which is well understood.

Cons of Pratt truss bridge

• It is not ideal for all situations, as it does not address vertical loads or shear loads – this limits its application in structures of these types.
• The design is susceptible to failure if not properly designed.
• It is not an aesthetic bridge – but this can be a subjective statement.

2. Why the bridge is called Pratt truss?

The bridge type was named after Thomas Willis Pratt and  his father Caleb (were well known engineer/architects in New England), who perfected it, patented it in 1844.

3. What are the types of a Pratt truss?

There are two types of this truss – the Pratt Through Truss – which has vertical members and through members, and the Pratt Lenticular Truss – which has diagonal members only. Both designs have been refined to meet application specific design requirements

4. How is a Pratt truss bridge built?

The process of constructing the bridge depends on the size of the structure.

Vertical members of the Pratt Truss are compressed, whereas diagonal members are tensioned.

This has a few effects: it reduces the cost of the structure owing to more efficient members, it reduces the self-weight, and it makes the structure easier to build.

The Pratt Truss kind of truss is best suited for horizontal spans when the load is mostly vertical.

5. What elements make a Pratt truss bridge strong?

There are three important elements in a Pratt truss that contribute to its strength: (1) the truss is reinforced by diagonal members, which are in tension; (2) the deck is under compression; (3) the bridge has a straight, smooth design.

6. What are the disadvantages of a Pratt truss?

The main disadvantage of Pratt Truss Bridge is that it does not address vertical or horizontal loading, therefore it cannot be used if these types of loads are present.

7. Is the Pratt Truss stronger with equilateral triangles or 45-45-90 triangles?

A Pratt Truss is referred to as “vertical members and diagonals sloping down towards the center.” This definition necessitates triangles with 90-degree angles, and it appears that a 45–45–90 arrangement would be ideal in this circumstance.

A Warren truss is formed by removing the vertical component from a Pratt truss and replacing it with equilateral triangles.

A Warren truss is more efficient to build since it has fewer members but is longer. Again, the majority of the strength comes from the construction material and spans.

Which truss is better Pratt vs Howe?

Although both truss bridges dissipated force far more effectively than the beam bridge, the Pratt truss diffused the load more efficiently than the Howe truss. Furthermore, on average, the Pratt truss deflected the least and retained the most, whereas the beam bridge deflected the most and held the least.