/ˈbɛndɪŋ/ created by applying a force at right angles onto a beam or other structural element. Causes sagging of the structure. If the structure fails in bending, or sags too much, the structure will become a catenary structure, if possible. See tension below. If the structure cannot convert to a catenary structure it will collapse.

Beam /biːm/ a structural element that spans from support to support and transfers load to those supports by bending.


/ ˈtɛnʃən/ a stretching force, tends to lengthen the structural member. Failure is sudden if the material is brittle, but can be proceeded by excessive elongation if the material is ductile. Because of this tension structures made from brittle materials are generally avoided.

Tensile Structure /ˈtɛn.saɪ(ə)l ˈstɹʌktʃə(ɹ)/ A structure that supports load purely by tension forces. They include catenaries, membranes and wheels.

Catenary /kəˈtiːnəɹi/ a structure that supports load purely by tension, like a washing line. The structure resists load by deflecting and tensioning up. The line will naturally form a parabolic curve, like a ball thrown will follow a parabolic curve.

The tension in the line pulls the load up. The more load applied to the catenary the more tension will develop and the more it will deflect. If you pull the catenary, that is tension the catenary, the line will straighten. To reduce the deflection of the catenary more tension needs to be applied in the line.

The structure (the washing line in this case) is normally incapable of resisting any bending. For the structure to support the load the ends need to be anchored into a structure. Any beam that is incapable of resisting load through bending will form a catenary (if the ends of the beam are secured in place). This will be accompanied by extreme deflection, but not structural failure. Beams in buildings are normally tied into their support structure, so a catenary can form as a fail safe system.

A slack line is achieved with low levels of tension in the slack line. Hence the large deflection of the line, but it can be secured easy as there are only minor forces at the anchor points.

A tightrope is achieved with high levels of tension which is why it is flatter, and requires more substantial support structures. It is impossible to have a flat tightrope, because this will require an infinite amount of tension to be applied to the tightrope.

Suspension Bridge /səˈspɛnʃən bɹɪd͡ʒ/ Suspension bridges are supported by the main support cable. This is a catenary structure as well. The Millenium bridge is a suspension bridge, but due to sight lines requirements to St. Paul’s, and the surrounding buildings, the support towers have to be low and the cables had to be relatively flat. This led to large forces in the suspension cables, which is why there are four cables and there needs to be substantial anchorage into the ground at either end of the bridge.

Membrane /ˈmembɹeɪn/

A membrane is formed by weaving multiple cantenaries in multiple directions to form a structural fabric. The Dome in Greenwich is a membrane structure although it doesn’t hang, but is pulled up and effectively hangs up instead of down.


/kɒm.pɹɛʃ.ən/ A force that squashes or compresses an element. Can cause buckling, or if the element is stable, failure will be explosive and sudden. Compression structures include columns, arches and domes.

Buckling /ˈbʌk.lɪŋ/ occurs when a member is in compression and moves laterally to the force applied: for instance when you compress a ruler. The lateral movement, or buckling, will always happen in the less stiff direction: the direction that it is easiest to bend. A member that is symmetrical and equally stiff in both directions cannot buckle. The buckling of a column shares behaviour and mathematics with the vibration of a string.


/ɑ˞tʃ/ compression structure. In the opposite of a catenary an arch is in pure compression, but only if is formed as a parabolic curve. In essence it is a catenary upside down. If the arch is formed in any other shape it will induce bending.

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