Pratt bridge and roof truss appear to have different diagonal directions Why ?

The reason a Pratt bridge truss and a Pratt roof truss appear to have different diagonal directions, even though they share the same name, is due to the principle of Inversion and the assumption of where the load is primarily applied.

In reality, the internal forces on the core members of a Pratt truss design remain the same, but the truss is simply flipped upside down to suit the application.

1. The Core Principle: The Load Path

The definition of a Pratt truss is not just its diagonal direction, but which members are in tension and compression under gravity loads:

$$\text{Pratt Truss Definition}⟹$$

$$\begin{array}{ll}\text{Diagonals }& \to \text{Tension (T) }\end{array}$$

                                   Verticals ​→Compression (C)​

This relationship is maintained whether the truss is used for a roof or a bridge, as long as the compression chord is on the top and the tension chord is on the bottom.

2. The Bridge Truss (Deck Below)

• Geometry: The load (traffic, road deck) is primarily applied to the bottom chord (or the bridge deck itself, which transfers to the bottom chord joints).

• Visual: The diagonals typically slant down and inward towards the center of the span.

• Force Path: The load pushes the bottom chord down, creating an outward shear force. The diagonals are aligned to resist this outward pull in Tension. The short verticals push back up in Compression.

3. The Roof Truss (Load Above)

• Geometry: The load (roofing material, snow, wind) is applied to the top chord. The entire truss is often inverted (flipped) from the bridge profile, but still retains the "Pratt" name because of the forces.

• Visual: The diagonals now appear to slant down and outward (like the Howe bridge truss).

• Force Path: The load pushes the top chord down. The diagonals are aligned to resist the internal outward pull toward the supports in Tension. The short verticals push up (or down, depending on the panel) in Compression.

Summary of the Difference (The Inversion)

The key to the confusion is the orientation of the structure in space.

Feature	Pratt Bridge Truss (Bottom Deck Load)	Pratt Roof Truss (Top Load)
Top Chord	Horizontal (Compression)	Sloped (Compression)
Bottom Chord	Horizontal (Tension)	Horizontal (Tension)
Web Diagonals	Down-and-In (Tension)	Down-and-Out (Tension)
Web Verticals	Upright (Compression)	Upright (Compression)
Reason for Appearance	The design is optimized to keep the long diagonals in tension, hence their direction relative to the support forces.	The basic bridge geometry is inverted and a pitch is added, but the members that are long (diagonals) are still in Tension, fulfilling the Pratt efficiency objective.

Conclusion on Naming

In structural engineering, the name (Pratt or Howe) is derived from the Force Pattern under primary gravity loading, specifically which web members are in tension and which are in compression:

• Pratt = Diagonals in Tension (Most efficient for slender steel)

• Howe = Diagonals in Compression (Efficient for wood, less for steel)

The visual appearance of a Pratt roof truss (with diagonals slanting outward) may look identical to a Howe bridge truss, but because the roof load is applied on the top chord, the internal forces align to keep the diagonals in tension, making it a structurally equivalent Pratt configuration.

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