Barrel Shell
A curved, cylindrical thin shell that spans like an arch.
Quick Answer
A barrel shell is a thin, cylindrically curved roof or floor structure, typically reinforced concrete, that spans between end supports like a curved beam. Its curved surface carries loads mainly by in-plane forces, so it can be very thin for its span. Edge beams and end supports hold the shape in place.
The Full Picture
A barrel shell is a section of a cylinder used as a roof. Think of a half-pipe laid on its side: the curve lets the thin surface carry load mainly through forces within its own plane, a behavior called membrane action, instead of through bending as a flat slab does. That is why a shell can be remarkably thin compared with the area it covers.
Barrel shells are most often cast-in-place reinforced concrete, though the same geometry can be built in other materials. A shell can span its length between end diaphragms or frames, with edge beams along the long sides to collect the forces and keep the curved surface from spreading. Several barrels placed side by side form a series of long vaults across a large floor area. Shell roofs were popular for hangars, markets and industrial buildings in the 20th century, and are less common now because of the cost of curved formwork.
Building one is mainly a formwork exercise. The contractor needs curved forms, often reusable if the same barrel profile repeats, along with shoring, reinforcing in two directions, and careful placement so the thin concrete reaches uniform thickness. Edge beams and end frames are heavier concrete elements with their own reinforcement. Waterproofing, insulation and drainage also have to be fitted to a curved surface, which affects roofing choices.
In estimating terms, a barrel shell is priced differently from a flat roof. Formwork contact area is often the largest cost driver, ahead of concrete volume, and the quantity of reinforcing depends on the shell and edge beam design. Because the details are specialized, a general contractor usually relies on the structural drawings and a specialty subcontractor or experienced formwork supplier to confirm methods and reuse.
Real Examples
Common Misconceptions
People assume: A barrel shell is the same as a barrel vault.
Actually: They are closely related shapes, but a barrel vault is traditionally a masonry or thick arch form, while a barrel shell is a thin shell that depends on its reinforcement and edge members. The terms overlap in casual use.
People assume: Because the shell is thin, it is cheap.
Actually: Concrete volume can be low, but curved formwork, shoring, two-way reinforcing and waterproofing on a curved surface often make the roof costly per square foot.
People assume: Openings can be cut anywhere in a shell.
Actually: The shell carries load through its surface, so large openings disturb the load path and need the engineer's design and reinforcing around the edges.
Frequently Asked Questions
How does a barrel shell carry load?
Its curved surface carries loads mainly through in-plane forces rather than bending, which lets the shell be thin. End supports and edge beams resist the thrust and keep the shape from spreading.
What is a barrel shell made of?
Usually reinforced concrete, cast in place over curved forms. Other materials and construction methods exist, but concrete is the typical choice for the type.
Why are barrel shells uncommon today?
Curved formwork is labor-intensive, and flatter framing systems are faster and easier to detail and insulate. Shells still appear where the architecture or the long clear span justifies the cost.
What is the main cost driver for a shell roof?
Formwork contact area and shoring are usually the largest items, followed by reinforcing and waterproofing. Form reuse across repeated barrels can reduce the cost per bay.