Bridge Truss Calculator

Pratt, Howe, and Warren — the three bridge layouts every truss design starts from. Pick a profile, set span and deck load, and read the full member-force table before building a single popsicle stick.

Bridge Build Dashboard — Plan Your Truss Bridge in 5 Steps

Choose a profile, set the numbers, and the dashboard writes your build plan — with a one-click preloaded model that matches it.

Span (ft)Truss height (ft)Bays per sideDeck load per joint (kip)

Setup progress20%

1
Open the Pratt bridge preset
The calculator starts the Pratt profile at your span of 48 ft and a height of 12 ft — that is a 4.0-to-1 span-to-depth ratio, the band most through-truss footbridges sit in (8:1 to 12:1).
2
Check the panel layout
Your setup draws 8 panels at 6.0 ft each. Panels are the repeating trapezoids between joints — every panel you add trades member count for shorter unsupported lengths in the chords.
3
Load the deck joints
Deck load enters at the bottom chord. Set 5 kip at each of the 7 interior bottom-chord joints for 35 kip total — the calculator applies this automatically in the preloaded model.
4
Read the three tables
Reactions first (each support should carry about 18 kip vertical on this symmetric layout), then member forces: chords heaviest near midspan, diagonals near the supports doing the shear work.
5
Compare with the other profiles
Re-run the identical numbers with the other two bridge types using the comparison links below — the force tables tell you exactly which members flip between tension and compression in each layout.

▶ Open the preloaded Pratt model

Pratt vs Howe vs Warren: The Bridge Layout Matrix

Three layouts, one idea — turn deck bending into member tension and compression. The matrix below compares them dimension by dimension as this calculator draws them; the tension/compression row is the one that decides which material each layout historically loved.

Dimension	Pratt	Howe	Warren
Diagonal direction (this tool's layout)	Slope downward toward midspan	Slope upward toward midspan	Alternating zig-zag between chords
Classic force pattern under deck load	Diagonals in tension, verticals in compression	Diagonals in compression, verticals in tension	Diagonals alternate tension / compression panel by panel
Best structural job	Steel bridges — tension diagonals are slim and cheap	Timber bridges — wood hates tension across grain; short compression diagonals suit it	Long spans with the fewest members — lightest frame per foot
Member count at 48 ft	31 members (8 panels)	31 members (8 panels)	23 members (10 zig-zag bays)
Where students meet it	The default 'truss bridge' in textbooks — joint method examples	Covered bridges and heritage timber spans	Railway spans and model-bridge competitions
Watch-out	Long compression verticals — check buckling, not just force	Compression diagonals near midspan are the longest — buckling governs	Zero-force members appear under some load spots — a feature for learning, a puzzle otherwise

Same Bridge, Same Load, Three Force Tables

The fairest comparison is a controlled one: all three models below span 48 ft, stand 12 ft tall, and carry 35 kip of deck load — 5 kip at each of seven bottom-chord joints on the Pratt and Howe, 7 kip at each of five joints on the Warren. Open any one, read its force table, then open the next; the chord forces will land close together, and the webs will tell three different stories.

Model 1

Pratt Bridge — 48 ft

8 panels, diagonals diving toward midspan. Watch the web diagonals sit in tension — the Pratt’s signature.

▶ Load model Model 2

Howe Bridge — 48 ft

Same panels, diagonals climbing. Every diagonal the Pratt holds in tension, the Howe squeezes in compression.

▶ Load model Model 3

Warren Bridge — 48 ft

No verticals, 8 fewer members, zig-zag doing all the shearing — the lightest frame of the three.

▶ Load model

💡 What “same load” proves: member forces follow geometry, not vibes. When all three bridges show near-identical chord forces, students see that the top chord must squeeze and the bottom chord must stretch no matter how the middle is webbed — the profile choice is really about which members do the shearing, and what material each member type prefers.

Truss Bridges in the Classroom: A Ready-Made Structural Lesson

Historic through-truss bridge crossing a river with its steel framework visible

Every real through-truss is this calculator's force table made physicalPhoto: Unsplash

The compare-predict-verify loop

Bridge trusses are the classic teaching subject because their stories are legible: chords visibly thicken toward midspan on real bridges, diagonals lean in the direction that suits the material, and failure history — the Silver Bridge, the Quebec Bridge — hangs on single-member forces. The calculator turns each of those stories into an experiment students run in under a minute.

The share-link button on every model hands out identical starting points: paste the same preloaded Warren into thirty browsers and every student predicts, verifies, and modifies from the same baseline. No installs, no licenses, no 2D drafting prerequisite — which is precisely why it works on the first day of a structures unit instead of the fourth week.

Lesson 1 — Read the pattern

Load the Pratt model, sort the force table by magnitude, and ask: why do the four biggest numbers all live near midspan? The answer — chord bending, shear in the webs — is the entire beam analogy in one screenshot.

Lesson 2 — Flip the profile

Swap Pratt for Howe at identical loads and mark every member whose sign changed. Students discover the tension/compression duality themselves, and the buckling conversation starts with their own data.

Lesson 3 — Break it safely

Double the load on one off-center deck joint and find which member spikes. It is the nearest thing to a collapse investigation a classroom can run without a collapse — and it teaches load path like nothing else.

Bridge Truss Calculator FAQ

Which bridge truss is strongest — Pratt, Howe, or Warren?▼

For the same span, height, and load, all three carry the deck — the difference is which members work in tension versus compression. The classic answer: a Pratt bridge is the strongest per pound in steel because its diagonals take tension (slim members do that job cheaply) while only its short verticals take compression; a Howe flips that pattern, which suited timber bridges whose joints were strongest in compression; a Warren removes verticals entirely and spreads force along alternating diagonals, giving the most material-efficient frame at long spans. Run the three comparison models on this page and you will see it directly: the chord forces are nearly identical across all three — the web members are where the philosophies differ.

How do I calculate the forces in a truss bridge?▼

The calculator does the full joint-by-joint solve for you — but here is what it is doing, in plain terms: first it finds the two support reactions from the deck load (about half the total at each end for a symmetric layout); then, at each joint, it enforces one balance rule — every force up must equal every force down, every force left must equal every force right. With two balance rules per joint and enough members to satisfy them, each member's force falls out one joint at a time. The 'with steps' version of that process is exactly what the solver's report shows: reactions, then displacements, then a force line for every member. To check any line by hand, use the method of joints at the end posts, where the numbers are simplest.

Why do the bottom chords of a deck-loaded bridge always show tension?▼

Because bending the whole bridge tries to pull its belly apart. A deck-loaded truss bends like a beam over the two supports: the top edge squeezes together, the bottom edge stretches apart. The top chord collects the squeeze (compression — highest near midspan), the bottom chord collects the stretch (tension — also highest near midspan), and the web members shuttle the forces between the two chords at every panel. This is why the 48 ft models on this page show bottom-chord tension peaking near the middle and why a bottom chord failure is the classic catastrophic mode in truss bridge history lessons.

How do I model a truss bridge for a school project?▼

Three moves, all doable in one class period. First, load a preset from this page — Pratt, Howe, or Warren at 48 ft with a 35 kip deck — and have students predict which member carries the largest force before anyone clicks Calculate. Second, test the prediction with the force table and sort members by force: the table's sort plus the diagram's red/blue coloring makes tension and compression visible without any math. Third, change one thing — delete a web member, double the deck load on one joint, steepen the height — and compare tables. That compare-predict-verify loop is the entire structural course in miniature, and the URL share button hands each student the exact same starting model.

What span-to-depth ratio should a truss bridge use?▼

The working band for through-truss footbridges and light rail spans is 8:1 to 12:1 — a 48 ft bridge stands 4 to 6 ft tall at the supports. Shallower than 10:1, chord forces grow fast because the chords' lever arm shrinks: halve the height and you roughly double the chord force for the same load. Deeper than about 8:1 you are buying frame height you no longer need, plus wind load and bracing to match. The dashboard above computes the ratio as you type; aim the slider at the band and watch the chord forces flatten out — the flattening is the geometry doing the work.