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HO-scale Pratt truss bridge

Pratt diagonal rule + portal bracing

Through-truss railroad bridge with the Pratt diagonal rule and portal bracing.

HO-scale Pratt truss bridge

What it is

A through-truss railroad bridge in HO scale — train rides on a deck at the bottom chord, between two parallel truss walls. Six parts: trusses, transverse floor beams, plank deck, ties, rails, and portal X-bracing at the two entry portals. Default span is 200 mm (≈58 scale feet); panel count, span, height, and member thickness all parameterize.

Why visual feedback matters

The Pratt diagonal rule — top corner closer to bridge center, bottom corner closer to end — is an orientation invariant easy to get wrong by 90° or to mirror by accident. The iso view alone won’t catch it. Rendering the front view and counting \\\ on the left half and /// on the right half (meeting in a V at bottom-center) is the verification. Get the topX/botX swap wrong and you’ve built a Howe truss — visually similar from a distance, structurally a different load path.

Screenshots

Front view is the diagonal check: counting left-to-right the diagonals go `\\\` then `///`, meeting at the bottom-center post in the characteristic Pratt V.

Truss bridge iso
Iso
Truss bridge front
Front (Pratt diagonals)

Parameters

Parameters 7
NameDefaultDescription
span — mm End-to-end length.
panels — Panel count (intermediate posts = `panels - 1`).
trussHeight — mm Top chord to bottom chord.
deckWidth — mm Inside-truss gauge.
memberThickness — mm Frame member thickness.
chordThickness — mm Chord thickness.
tieCount — Number of ties on the deck.

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