Barrel vaults are perhaps the most useful of the shell structures because they can span upt o 150 feet with a minimum of material. They are very efficient structures because the use the arch form to reduce stresses and thicknesses in the transverse direction. Barrel vaults are essentially deep concrete beams with very thin web members and may be designed as such by the ordinary methods of reinforced concrete. The curve of the cross section of the barrel is usually a circle. However, any other form maybe used, such as the ellipse, a parabola, or a funicular curve which fits the thrust line of the applied load. Each curve has its particular structural and esthetic qualities.

A number of terms have been developed to describe cylindrical shells. If the span is large in comparison to the width, the form is called a long shell. If the length is short, it is called a short shell. An arbitrary ratio for long shells is a span/radius ratio of 5. A short shell has a span/radius ration less than 1 and shells between these limits are called intermediate shells. Short shells are a different structural type and are described in a later chapter.