Pratt bridge and a Pratt roof truss: How the load is applied and why the diagonal members look different

Imagine the name "Pratt" means one thing: "Long members handle the pull (Tension), and short members handle the push (Compression)." This is the core efficiency goal, especially when building with steel.

---

The Pratt Truss Principle: The Load and The Flip

The fundamental structural difference between a bridge truss and a roof truss is where the weight is located.

1. The Pratt Bridge Truss

Think of a bridge that cars drive over.

• Location of the Load: The weight (cars, road deck, self-weight) is applied to the bottom of the truss, where the traffic is. This horizontal line of members is called the bottom chord.

• The Reaction: When you push down on the bottom chord, the entire structure sags slightly. This creates a pushing force (compression) in the top chord and a pulling force (tension) in the bottom chord.

• The Diagonals (The Web): The internal diagonal members are what transfer the shear force (the tendency to slide sideways) from the center out to the supports.

  • To efficiently handle the outward tension (the "pull") created by the load, the long diagonal members are oriented to slant inward toward the center of the bridge.

  • This geometry ensures that the long, slender diagonals are constantly stretched (in Tension), which steel handles very well. The shorter vertical members are then left to take the squashing (in Compression).

2. The Pratt Roof Truss

Now, think of a simple pitched roof over a building.

• Location of the Load: The weight (roof tiles, snow, etc.) is applied to the top, sloped chord.

• The Flip: If you took the efficient Pratt bridge design and just placed a roof on its top chord, the interior diagonals would actually flip their role and go into compression (squashing), making it inefficient, almost like a Howe truss.

• The Engineering Solution (The Pratt Inversion): To maintain the "Pratt efficiency" (long diagonals in tension, short verticals in compression) under a top load, the engineer must structurally invert the web pattern.

  • The truss itself looks like a standard pitched roof, but the pattern of diagonals is now slanting outward toward the ends (which is the pattern that defines the Howe in a bridge context).

  • However, because the load is on the top chord, this outward-sloping diagonal is now correctly oriented to take the Tension (the pull), while the vertical members take the Compression (the push).

Simple Analogy: The Suitcase Handle

Imagine you have a suitcase with a diagonal strap inside.

1. Bridge Pratt: You hold the suitcase by the top. You put the weight in the bottom (the load). The diagonal strap is positioned so that as the bottom sags, the strap is pulled tight (Tension).

2. Roof Pratt: You flip the suitcase over and carry it by the bottom (the horizontal chord). The weight is now on the top (the roof). To keep that same strap in Tension (the efficient Pratt way), you would have to re-position or flip the strap's anchor points. The final result is the appearance of a reversed diagonal, but the internal force (Tension) is maintained on the diagonal member.

In summary: The names "Pratt" and "Howe" refer to the internal force distribution (Tension on Diagonals for Pratt, Compression on Diagonals for Howe), not a fixed visual geometry. The geometry appears to "flip" when going from a bottom-loaded bridge to a top-loaded roof simply to keep the diagonal member in its preferred state of tension.

Continue Reading

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.

Continue Reading

Detailed Comparison: Pratt Truss vs. Howe Truss

Detailed Comparison: Pratt Truss vs. Howe Truss

The Pratt and Howe trusses are both characterized by a pattern of vertical posts and inclined diagonal members, often called an "N" or "K" pattern. The essential difference lies in the direction of the diagonals, which fundamentally changes the way internal forces are carried.

1. The Key Difference: Diagonal Direction

The easiest way to tell them apart is to look at the slope of the main diagonal members as you move from the supports toward the center of the span.

Truss Type	How the Diagonals Slope (The Visual Test)
Pratt Truss	The diagonals slant Down and In (towards the center, like a V or A pointing up).
Howe Truss	The diagonals slant Down and Out (away from the center, forming A shapes at the supports).

2. The Structural Mechanics (Internal Forces)

For a simply supported truss (like a standard bridge or roof) carrying a downward gravity load, the direction of the diagonals dictates which members are in Tension (pulled apart) and which are in Compression (pushed together).

Member Type	Pratt Truss	Howe Truss
Diagonal Members	Tension (Pulled apart)	Compression (Pushed together)
Vertical Members	Compression (Pushed together)	Tension (Pulled apart)
Top Chord (Upper Horizontal)	Compression (Same for both)	Compression (Same for both)
Bottom Chord (Lower Horizontal)	Tension (Same for both)	Tension (Same for both)

---

3. The Design and Efficiency Logic

The primary reason for preferring one over the other relates to the material used and the problem of buckling.

Aspect	Pratt Truss: The Steel-Optimized Design	Howe Truss: The Original Wood Design
Compression Member	The vertical members (posts) are in compression.	The diagonal members are in compression.
Member Length	Verticals are shorter than the diagonals (geometrically).	Diagonals are longer than the verticals.
Efficiency	Highly Efficient for Steel: Buckling is a major concern for long, slender members under compression. By placing the compression force on the shorter vertical members, the risk of buckling is minimized, and less material (thinner/lighter steel) can be used for the long tension diagonals.	Less Efficient for Steel: Placing the compression force on the longer diagonal members makes the truss susceptible to buckling, requiring the diagonals to be thicker and heavier than in a Pratt truss. Historically, the Howe truss was efficient for all-wood construction, as wood resists compression better than tension, and the metal verticals could be thin rods.

---

How to Identify on the Field or in Books

You can reliably identify the truss type by following a single diagonal member from the top chord down to the bottom chord (or vice versa).

A. Pratt Truss Identification Rule (The "Tension Diagonal" Rule)

• Rule: A Pratt truss diagonal always runs to the nearest support (or away from the center of the span) when starting from the bottom chord.

• Visual Check: The diagonals slant inward toward the center of the span.

  • Imagine a rain droplet falling down a diagonal: it travels down and in.

B. Howe Truss Identification Rule (The "Compression Diagonal" Rule)

• Rule: A Howe truss diagonal always runs away from the nearest support (or toward the center of the span) when starting from the bottom chord.

• Visual Check: The diagonals slant outward away from the center of the span.

  • Imagine a rain droplet falling down a diagonal: it travels down and out

Continue Reading