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 analyzed to give an upper bound on the collapse load. This technique is used in practice but because the method provides an upper-bound (i.e. an unsafe prediction of the collapse load) for poorly conceived collapse mechanisms, great care is needed to ensure that the assumed collapse mechanism is realistic. 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. It is mainly used to increase the bountifulness of any structure. Catenaries Catenaries derive their strength from their form and carry transverse forces in pure tension by deflecting (just as a tightrope will sag when someone walks on it). They are almost always cable or fabric structures. A fabric structure acts as a catenary in two directions. Materials Structural engineering depends on the knowledge of materials and their properties, in order to understand how different materials support and resist loads. It also involves a knowledge of Corrosion engineering to avoid for example galvanic coupling of dissimilar materials. Common structural materials are: Iron: wrought iron, cast iron Concrete: reinforced concrete, prestressed concrete Alloy: steel, stainless steel Masonry Timber: hardwood, softwood Aluminium Composite materials: plywood Other structural materials: adobe, bamboo, carbon fibre, fiber reinforced plastic, mudbrick, roofing materials See also Glossary of structural engineering Aircraft structures Architects Architectural engineering Building officials Building services engineering Civil engineering Construction engineering Corrosion engineering Earthquake engineering Forensic engineering Index of structural engineering articles List of bridge disasters List of structural engineers Mechanical engineering Nanostructure Prestressed structure Structurae Structural engineer Structural engineering software Structural fracture mechanics Structural failure Structural robustness Structural steel Structural testing Notes References Hibbeler, R. C. (2010). Structural Analysis. Prentice-Hall. Blank, Alan; McEvoy, Michael; Plank, Roger (1993). Architecture and Construction in Steel. Taylor & Francis. . Hewson,