Compression

[edit] Simple compression

Simple compression occurs when the particles of a material are pushed against each other. As a state of stress, it is the opposite of tension, where particles are pulled apart.

When a column supports a load (or weight) from above, it is said to be under compressive stress; it also shortens – a typical consequence of compression. The same applies to a strut in a truss or bridge which is designed to work solely in compression.

Compressive shortening is proportional to the load per unit area (of a column) and is typical of compression, just as lengthening is typical of tension. In addition to the compressive shortening which takes place along the longitudinal axis (usually the centre line), there is also lengthening that occurs at right angles to the longitudinal axis. In other words, the column gets shorter and fatter.

Strictly speaking, a compression member such as a column or strut is subject only to axial compressive forces ie the load is applied through the member’s centre and along the longitudinal axis. The stress in the compressed member is given by the load over the cross-sectional area.

Simple compression is a common phenomenon in building structures as all loads and forces have eventually to be directed into the ground. Thus, they occur in Greek temples as much as in Manhattan skyscrapers.

[edit] Strength of materials

Concrete, masonry are materials with high compressive strengths but they are also weak in resisting tension. Steel has a high compressive strength and also a high tensile strength and can resist the same compressive forces as concrete or masonry but with a slimmer profile.

[edit] Buckling

When a material has sufficient compressive strength to allow the use of smaller cross-sections, the result may be lower costs but this could have side effects. Increasing the slenderness ratio (the column length is many times greater compared to the cross-sectional area), may lead to buckling. If the load is great combined with a relatively small cross-sectional area, the column may buckle as it is easier for it to bend outwards (or buckle), rather than shorten. Buckling may also be exacerbated by eccentric loads.

Whether a column reaches its buckling load limit will depend on numerous factors, including its length, the type of material, the cross-sectional shape and how it is restrained at its ends.

[edit] Compressive structures

Being built mostly from masonry, Greek and Roman temples, and Romanesque and Gothic cathedrals, are structures almost entirely under compression.

An arch in brickwork or stonework has simple, uniform compression and no bending (and therefore little or no tension). The thrust of the arch – compressive forces diverging down and either side of the keystone – is absorbed by the abutments on either side. For more information see: Arch

The flying buttress in a Gothic cathedral channels forces of compression from the roof and walls down into the foundations. For more information see: Flying buttress

When under load, a dome develops compressive stresses along its meridians; these can be thought of as an infinite number of arches connecting opposite points on the ground circumference. The dome will also develop compressive or tensile stresses around its hoops (lines of ‘latitude’). For more information see: Dome

[edit] Related articles on Designing Buildings

• Arch.
• Barrel vault.
• Concrete.
• Dome.
• Elements of structure in buildings
• Engineer.
• Flying buttress.
• Shear strength.
• Steel.
• Stress.
• Structural engineer.
• Structural principles.
• Strut.
• Substructure.
• Superstructure.
• Tensile strength.
• Tension.
• Ties.
• Tower.
• Types of structure.
• Vibration Compaction Technology.
• Voussoir.