Struts and ties

Little Belt: a truss bridge in Denmark

The McDonnell Planetarium by Gyo Obata in St Louis, Missouri, USA, a concrete shell structure

A masonry arch
1. Keystone 2. Voussoir 3. Extrados 4. Impost 5. Intrados 6. Rise 7. Clear span 8. Abutment

A truss is a structure comprising two types of structural element, ie struts and ties. A strut is a relatively lightweight column and a tie is a slender element designed to withstand tension forces. In a pin-jointed truss (where all joints are essentially hinges), the individual elements of a truss theoretically carry only axial load. From experiments it can be shown that even trusses with rigid joints will behave as though the joints are pinned. Trusses are usually utilised to span large distances, where it would be uneconomical and unattractive to use solid beams.

Plates

Plates carry bending in two directions. A concrete flat slab is an example of a plate. Plates are understood by using continuum mechanics, but due to the complexity involved they are most often designed using a codified empirical approach, or computer analysis. They can also be designed with yield line theory, where an assumed collapse mechanism is analysed to give an upper bound on the collapse load (see Plasticity). This is rarely used in practice.

Shells

Shells derive their strength from their form, and carry forces in compression in two directions. A dome is an example of a shell. They can be designed by making a hanging-chain model, which will act as a catenary in pure tension, and inverting the form to achieve pure compression.

Arches

Arches carry forces in compression in one direction only, which is why it is appropriate to build arches out of masonry. They are designed by ensuring that the line of thrust of the force remains within the depth of the arch.