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Dezi et. al (2010)
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Projectile motion
There are endless arrangements for trusses. Various truss types have been developed to meet specific engineering requirements and architectural preferences. So form meets function in a fascinating array of prescribed designs.
Trusses are grouped into two main categories:
- 2D trusses (Planar trusses) - located on the same plane
- 3D trusses (Space Frame truss)
Here, we'll just be focusing on Planar trusses.
The different types of Planar Trusses each have different load transfer mechanisms. Below are the most common that you may have seen around! We will look at:
- Pratt Truss
- Howe Truss
- Warren Truss
- Fink Truss
- King Post truss
- Sawtooth (or Butterfly) Truss
- K-truss
- Cantilever truss
Pratt Truss
What is a Pratt truss?
The orientation of the diagonals in the Pratt Truss slant downwards to the middle of the truss, distinguishing it from the Howe truss.
Pratt Braces:
- Vertical Braces are in Compression
- Diagonal Braces are in Tension
Pratt Chords (also same in Howe and Warren):
- Top Chords are in Compression
- Bottom Chords are in Tension
Timber is often used in the vertical compression members, while steel is often used in the diagonal tension members.
Applications of the Pratt Truss
Built in 1897, with a length of 77 m, it is one of the rare Pratt half-deck bridges left in the US. Hence, it has been placed on the National Register of Historic Places. (Source - Jet Lowe/Library of Congress)
Howe Truss
What is a Howe Truss?
The Howe truss is a reverse version of the Pratt truss. Even though the only change is to the orientation of the diagonal braces, it behaves structurally very different to the Pratt truss.
Howe Braces:
- Vertical Braces are in Tension
- Diagonal Braces are in Compression
Applications of the Howe Truss
This bridge is the longest continuous truss bridge in the world, using a mix of Howe truss and K-truss with crossing trusses connecting two side trusses of the bridge. It has a main span of 400 metres.
Built in 1892, With a length of 22 m, it is one of the rare purely Howe bridges left in the US, and it is a part of the National Register of Historic Places.
Warren Truss
What is a Warren Truss?
Warren truss is identified by arranging the diagonal members to form equilateral triangles. This even arrangement gives the truss the ability to effectively spread load across different members.
When increasing the vertical loading or span, vertical members can be added between each panel (to form a Pratt or Howe truss) to aid in withstanding compressive forces.
Applications of the Warren Truss
The bridge was built in 1913, and it is a Double-intersection Warren truss railway bridge spanning 169 metres. Here, you can see the X-shape diagonal braces of the bridge are two Warren trusses intersecting each other.
The stadium has the main trusses spanning 145 metres and a depth of 12.5 metres. The 650-tonnes roof trusses were installed using two 1,200 cranes and each member in the truss reach up to 6 metres in length. The truss is a space truss incorporating the Warren truss design.
So which should truss should I choose: Pratt, Howe or Warren?
There is not a simple answer! They each have their advantages and limitations:
- The Pratt and Howe Truss have similar advantages. They have a simple design, are cost-effectiveness and are commonly used in normal practice. They are both easy to build and transport.
- Geometrically, we can see that Warren Trusses use fewer materials than the Pratt and Howe Trusses due to the equilateral triangle pattern. This would lead to a reduced construction time.
- Howe Trusses are better at transferring the load; the compression chords see less force than a Pratt Truss for the same applied load and span, limiting the potential for buckling. See the contours below.
- (However) Pratt Trusses have the advantage of having a shorter compressive member; the verticals in compression are shorter than the diagonals in tension, limiting the potential of buckling.
👀 Let's compare the three trusses!
In the three contours below, the trusses have the same span, applied load and supports. Negative forces are in compression (cooler colours) and positive forces are in tension (warmer colours).
Pratt Truss
Howe Truss
Warren Truss
Other Truss Types
Trusses have many variations and shapes, let's explore some more below!
Fink truss
A Fink Truss is commonly used in roofs, especially in homes or pedestrian buildings. It could be seen as a variation of the Warren truss with a W shape in the braces.
King Post truss
The King Post Truss is normally made from timber, spanning up to 8m. It is commonly used to support the weight of expansive roofs or short span bridges.
Sawtooth (or Butterfly) Truss
This type of truss is typically used for short spans in industrial buildings. It comprises of alternating angled panes of either steep and shallow. The shallow panes are typically orientated to maximise natural light penetration.
K-truss
As the name suggests, the truss is formed by a vertical member and two oblique members in each panel, creating a 'K'. The additional members required in the design can incur additional costs, so K-trusses are rare.
Cantilever truss
Cantilever bridges are generally made with three spans, where the outer spans are cantilevering. They also typically form an arch shape due to the way their supports meet.