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.
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).
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.
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