Stone and brick have been used for the construction of barrel vaults since ancient times in temples, audience halls, and other monumental buildings in Mesopotamia, Persia, and in particularly the Roman Empire.
Braced barrel vaults consist of developable surfaces generated by the repetitive use of a curve known as ‘directrix’ over a generator straight line. The directrix may be a circular arc, an ellipse, a catenary, a parabola or a cycloid. Most of the braced barrel vaults built in practice are part of a right circular cylinder, which may be either supported by columns or simply springing from the ground surface.
Generation of Braced Barrel Vaults
The semi-circular barrel vaults have the clear advantage of facilitating water drainage and providing strong architectural form recognition.
Under loads, braced barrel vaults may behave in two different modes: Arch, and beam; depending mainly on the location of supports.
In the arch mode, the braced barrel vault behaves as an arch when supported along the sides.
Braced Barrel Vault Arch Mode Behavior
The braced barrel vault behaves in the beam mode when it is supported at its ends. In this case the longitudinal compression forces occur near the crown and longitudinal tensile forces towards the free edge.
Braced Barrel Vault Beam Mode Behavior
If the braced barrel vault is supported at the four corners, it behaves as a combined beam and arch under the loads. In this case, it acts as a series of arches in the cross-section direction and as a beam longitudinally.
Braced Barrel Vault Arch/Beam Mode Behavior
Similar to the flat double-layer grids (FDLG), double-layer braced barrel vaults (DLBBV) consist of a top and bottom layer connected to each other by bracing members. The top/bottom layers are also called the ‘chord members’. All the flat double-layer configurations can also be used for the DLBBV. The following shows a few such patterns:
Square-on-Square Offset DLBBV
Diagonal-on-Diagonal Offset DLBBV
Square-on-Diagonal Offset DLBBV
When span is large, braced barrel vaults can be a much more economical choice than flat spatial structures. For example, a double-layer braced barrel vault with a span of 270 ft is almost equal in cost to a flat double-layer grid with only a 150 ft span.
The height/span ratio of a barrel vault determines the magnitude of the thrust (horizontal reaction force at the base). The shallower the arch, the greater is the horizontal thrust. The thrust is usually resisted by internal ties or external braces.
Lateral Thrust
As in the case of flat double-layer grids(FDLGs), the joint rigidities (moment connections versus pin-connections) have minor effects on the structural behavior of double-layer braced barrel vaults(DLBBVs). The effects of wind loads on DLBBVs are more significant than on the FDLGs, and they are the controlling factor in their design. For more information, click here.
