Howe Truss

Howe claims a truss with timber diagonals (in compression) and iron verticals (in tension) that can be tightened by nuts. Note the two materials and the adjustability.

Lay a single balsa stick across a gap and hang weight at the middle. Note how much it takes to bend or break. This is the beam you are going to beat with a frame.

Push the ends of a long thin stick toward each other. It bows sideways and buckles. Compression members must be stout and short; tension members can be thin.

Pull a string tight — it holds a big load. Push its ends together — it just goes slack. A string is a perfect tension member and useless in compression. Wood and iron each have a job.

Make two parallel horizontal members — the top chord and bottom chord — the length of your span, from balsa or basswood.

Fit stout wooden diagonals between the chords, angled so a downward load squeezes them. These are Howe's timber compression members.

Run wire (standing in for iron rods) as the vertical members, tying the chords together. Under load these are stretched — the tension members.

Twist or tension the wire verticals so the whole frame pulls tight. Howe's nuts do exactly this — you can pre-load and adjust the truss, taking up any slack in the timber.

Hang weight at the centre and read the force to fail it. The truss carries far more than the plain stick from step 2, using little more material. Record the ratio.

Load slowly and feel each member: diagonals are being pushed (they resist buckling), verticals are being pulled (they twang taut). Mark C or T on every member. That map is the whole design.

Replace a compression diagonal with wire. Load it — the wire goes slack and the truss collapses. The material must match the job: wood pushes, iron pulls.

Slacken one iron vertical. The truss droops at that panel. Tension in the verticals is holding the shape — which is why being able to re-tighten them mattered so much in service.

The patent. US 1,711, "Manner of constructing the truss-frames of bridges and other structures", granted 3 August 1840 to William Howe of Warren, Massachusetts, and reissued in 1850 as No. 175. The reissue matters — a granted patent could be re-examined and re-granted with amended claims, and Howe's was. He filed an earlier version (US 1,685) a month before.

The truss itself is ancient; Howe's contribution is the material pairing and the adjustability. Triangulated timber trusses go back to Greek and Roman roofs, and to Palladio's Renaissance bridges. What Howe did was assign the two forces in a truss to two different materials chosen for each: the diagonals are timber, which is cheap, plentiful and strong when squeezed; the verticals are iron rods, which are strong when stretched and — crucially — fitted with nuts so they can be tightened. Steps 3 and 4 are why this matters: a slender member buckles helplessly in compression but a thin rod or string is a superb tension member, so matching material to force lets each be as light as possible. And the adjustable rods let a builder pre-load the truss and re-tension it as green timber shrank and joints settled — a truss you could tune.

Why it won the railroad age. The 1840s–1870s were a frenzy of railroad building across America, and railroads needed bridges by the thousand — fast, cheap, from materials available anywhere, buildable and repairable by ordinary crews. The Howe truss fit perfectly: standard sawn timber and iron rods, simple connections, and that on-site adjustability. It was quicker to erect and easier to maintain than the all-timber trusses before it, and it carried the enormous, pounding loads of locomotives. Thousands were built, many as covered bridges (the roof was to protect the timber, not the travellers).

Where it sits in the family. The Howe truss has a famous mirror image — the Pratt truss (1844), which swaps the roles so the diagonals are in tension (iron) and the verticals in compression (later, steel). Which is better depends on the material economics of the day, and as iron and then steel got cheap, the Pratt and Warren trusses took over. But the core idea you built here — resolve a bending load into a frame of pure-tension and pure-compression members, and give each the right material — is the foundation of all structural framing, from steel roof trusses to the booms of cranes to the lattice of an electricity pylon.