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)

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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.

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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

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