The term spandrel refers to various architectural elements, including the triangular spaces above arches and the space between a window and the floor above. Spandrels can be load-bearing and support walls and roofs. Material selection is important for both aesthetic and functional purposes, such as insulation in case of fire. In bridge design, open span bridges have structural members supporting the roadway, while closed span bridges use earth fill. Moisture can cause problems in closed span bridges. Torsion is a concern in spandrel beam design.
The term spandrel has been adapted from its original use in medieval architecture to encompass a number of meanings, all related to architecture or construction. In medieval architecture, it denotes the roughly triangular spaces that would appear to the top right and left of an arch. With regards to modern arches, this term means the area between the arch and the boundary around it, such as a roadway above. In architecture, it can refer to the space between the top of a window on one floor and the bottom of the window directly above it on the next floor. Spandrels are load-bearing beams around the perimeter of a building that extend from column to column.
Arch bridges can be classified as open or closed depending on the configuration of the area between the arch and the road above it. The roadway on an open span bridge is supported by a system of structural members resting on the top of the arch. In closed span bridges, the roadway is supported by earth fill below and above the arch, leaving no open spaces in the area between the arch and the roadway. One potential failure mode of such bridges is excessive moisture in the filler material. This can be caused by leaking water lines, poorly conveyed storm runoff, or moisture ingress through a worn or cracked road surface.
In architectural design, the space between the top of a window on a story and the window sill directly above it may also be called the spandrel. It not only has an aesthetic function, but also provides protection by insulation in the event of a fire. The material selection can take into account both of these functions.
A stone or metal panel can be used to achieve a smooth, finished exterior appearance while hiding an unattractive building from view. Glass can be used for decorative effects with particular colors and shapes or to create the illusion that there is nothing in the space. Such spandrels are generally not load-bearing structural elements.
Spandrels are load-bearing structural members around the perimeter of a building floor. Not only can they support loads from the roof and other floors, they can also help support the walls of a building. For example, walls that are expected to experience significant forces from the wind can be anchored to these beams to better distribute the load.
While an interior beam can be loaded on both sides, a spandrel beam is only loaded on one side. This raises the possibility of the beam twisting in torsion, which is a concern that must be considered in the design of the structure. Strategic material selection can help mitigate this design problem by leveraging materials with compatible ductilities, compressive strengths, and other related material properties.
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