What is a crescent truss?
A crescent truss is an arched lattice truss in which both the top chord and the bottom chord are curved. The two chords converge towards the supports, so the truss takes the shape of a sickle or crescent moon. In Dutch it is called a sikkelspant; in 19th-century English sources it is also described as a sickle girder.
Origins and use
Crescent trusses were used mainly in the second half of the 19th century as wrought or cast iron roof trusses for large column-free spans: railway train sheds, market halls, factories and exhibition buildings. A well-known example is the 19th-century train shed at Liverpool Lime Street, whose roof used trusses in the form of sickle girders. The web members between the chords make the truss very stiff for the amount of material used.
Difference from an arched truss and a Polonceau truss
- Arched truss: the general term for curved trusses. Often only the top chord is curved, with a straight bottom chord or a tie rod. The crescent truss is a specific form with two curved chords.
- Polonceau truss: an iron triangular truss with straight rafters, each propped by a strut and tie rods. No arch, but a pitched-roof shape.
- Parallel-chord truss: straight top and bottom chords; simpler to make, less elegant.
Application: crescent trusses in conversions
Historic iron sheds are often reused as market halls, event venues or offices. Points to watch:
- Corrosion at the joints. Moisture and dirt collect where members meet and at the bearings. Check these areas specifically for rust and loss of section.
- Rivets. Historic trusses are riveted, not welded or bolted. Replace loose or corroded rivets only in consultation with a structural engineer experienced in historic ironwork.
- Identify the material. Cast iron, wrought iron and early steel behave differently, and welding old iron is not straightforward.
- Fire resistance. Unprotected iron loses strength quickly in a fire. A change of use often brings new requirements; an intumescent coating or a fire strategy can keep the trusses exposed.
- Listed status. Many of these structures are protected. Check this before making plans, as alterations will then need consent.
Always have the capacity of a historic truss reassessed before adding loads such as services, insulation or solar panels.
Related terms
- Arched truss
- Polonceau truss
- Roof truss
- Truss
- Rivet
- Fire resistance
- Listed building
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