This is the first in a series of articles reviewing specific structural engineering basics and design processes. In this introductory article, simple structural engineering elements are classified to form the basis of more complex structures.
Never Push On a Rope
Creating a safe, durable, cost effective structural engineering design is usually not a trivial process. A thorough understanding of stresses, strains, material properties, environmental effects, computer modeling, physics, mathematics, construction, manufacturing, architecture, and related engineering concepts are typically required to successfully execute a design. And while many structures appear incredibly complex, often they are constructed using multiple quantities of only a few basic elements.
A useful collection of these structural engineering basics includes beams, columns, cables, and plates. Beams, columns, and cables are considered to be elements much longer in a single dimension, i.e., length vs. width and depth. Plates, including shells, have two dimensions greater than the third. The type of load each element carries helps to define them as well. Beams are typically compressively loaded to bend normal to the main axis, while columns carry axial tensile or compressive loads. Cables (including wires, ropes, and chains) carry only axial tensile loads and are considered flexible elements. Plates carry bending and shells carry compressive loads in two dimensions.
Catenaries Don't Have Feathers
Using these simple elements in complex geometries creates many useful structural forms. For example, a chain hanging freely but with ends anchored across two columns creates a catenary curve. Catenary suspension bridges, such as a simple foot bridge, place the load directly on this catenary element. Suspension bridges which support the deck with vertical chains or cables attached between the deck and the flexible suspension cable force the catenary shape into a parabolic form. In both cases the suspension cable experiences tensile stresses. However, inverting the catenary shape creates an ideal arch, a spanning structural form which can withstand purely compressive loads. Building on this form with rigid elements, such as stone blocks, has created some of the most graceful and enduring structures throughout history. Over 75 years ago Catalan architect Antoni Gaudí is said to have used this technique to design some of the arches and vaults in the Church of Colònia Güell. By creating catenaries using small suspended chains and viewing them in a mirror, accurate models could be viewed of the stonework required for constructing the building. Not a bad technique for the period, but modern design and analysis uses computational techniques is a more accurate and efficient method.
And Never Pull On Dirt
Consequently before a single cable or block is put in place, a fair amount of structural analysis takes place to ensure these basic elements are assembled correctly using properly selected and specified materials. Using concrete as an anchoring medium for the end of a suspension cable, for example, would be highly inappropriate as concrete has very poor tensile strength. Attaching the cable to a steel eyebolt embedded in concrete tangentially opposite of the tensile stress running in the cable would be more appropriate, as this would load the concrete in compression. And so forth. Future articles in this series will further examine the construction of complex elements using these basic structural elements.