Torsion

Turning Torso, Malmo, Sweden. This tower was designed to appear as if in a state of torsion. Actually it is not, but the appearance is a good example of what would happen theoretically if a turning force was applied to one end of the tower.

Torsion is the state of strain in a material that has been twisted by an applied torque. It will occur whenever a structural element is subject to a twisting force.

Torsion can be seen in a circular-section rubber bar inscribed with rectangles and which is held at each end, with one hand twisting in relation to the other: the rectangles become skewed – or distorted. The state of strain that has distorted the rectangles is called torsion and consists of pure shear. The torsion that exists in the rubber bar will try to make it revert to its original shape.

Torsion develops shear stresses and is equivalent to tension and compression at right angles. This can be seen when wringing a wet towel – the water is squeezed out by torsion-induced compression.

[edit] Torsion is expressed in:

• Pascal (Pa) – the SI Unit for newtons/m2, or
• Pounds per square inch (psi).

[edit] Related articles on Designing Buildings

• Bearing capacity.
• Bending moment.
• Biaxial bending.
• Lateral loads.
• Limit state design.
• Moment.
• Point of contraflexure.
• Racking.
• Settlement.
• Shear wall.
• Structural principles.
• Torsion springs.
• Types of structural load.