What is a Polonceau truss?
A Polonceau truss, also called a French truss, is a roof truss formed by two trussed rafters whose lower points are connected by a horizontal tie rod. It is named after the French engineer Camille Polonceau, who first applied the type in 1839.
In essence it is a doubly trussed beam: each rafter is propped down at mid-length by a short strut, and those struts are linked to each other and to the feet of the truss by tie rods.
How the forces run
What makes it distinctive is the division of roles between members:
- The rafters carry compression and bending.
- The short struts at right angles to the rafter take compression and prop the rafter at mid-length, halving its buckling and bending length.
- The tie rods, including the main horizontal tie, carry tension only.
That allowed the 19th-century builder to put each material exactly where it performed best: cast iron (strong in compression, brittle in tension) for the struts, and wrought or rolled iron for the ties. The result is a structure that achieves a large span with remarkably little material — and that made the type economically attractive.
Where you find them
The Polonceau truss was used wherever large column-free spaces were required: station train sheds, factory buildings, market halls, gymnasia, auction halls and engine sheds. In north-western Europe you meet the type mainly in buildings from roughly 1850 to 1940. Variants with more than one level of trussing (the “compound” or double Polonceau truss) exist for even greater spans.
Related types found in the same buildings are the Belgian truss and the English truss; those are true trusses with a continuous bottom chord, whereas the Polonceau truss deliberately has no continuous bottom chord beneath the rafters.
Points for repair
- The tie rod is the heart of it. Cutting, drilling or weakening the horizontal tie lets the truss spread. Even hanging a ceiling from the tie changes the force pattern.
- Check the connections. The joints are usually riveted or adjusted with cotters and nuts. Slack in a joint means additional deflection and distortion.
- Corrosion at the feet. The bearing point on the wall or column is the most vulnerable spot, especially in damp masonry.
- Fire resistance. Unprotected iron and steel lose strength rapidly in a fire. In the reuse of industrial heritage this is often the hardest question to resolve.
Related terms
- Roof truss
- Belgian truss
- Roof structure
- Fink truss
- Tie rod
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