Structural Elements 101 –  San Antonio, TX

Structural engineering is a field of engineering dealing with the analysis and design of structures that support or resist loads economically. Structural engineering is usually considered a specialty within civil engineering, but it can also be studied in its own right.
Structural engineers are most commonly involved in the design of buildings and large nonbuilding structures but they can also be involved in the design of machinery, medical equipment, vehicles or any item where structural integrity affects the item’s function or safety. Structural engineers must ensure their designs satisfy given design criteria, predicated on safety (e.g. structures must not collapse without due warning) or serviceability and performance (e.g. building sway must not cause discomfort to the occupants).
Structural engineering theory is based upon physical laws and empirical knowledge of the structural performance of different geometries and materials. Structural engineering design utilises a relatively small number of basic structural elements to build up structural systems that can be very complex. Structural engineers are responsible for making creative and efficient use of funds, structural elements and materials to meet these goals.
Examples of Structural Elements
  • Columns
  • Beams
  • Plates
  • Arches
  • Shells
  • Catenaries

Many of these elements can be classified according to form (straight, plane / curve) and dimensionality (one-dimensional / two-dimensional):

One-dimensional Two-dimensional
straight curve plane curve
(predominantly) bending beam continuous arch plate, concrete slab lamina, dome
(predominant) tensile stress rope Catenary shell
(predominant) compression pier, column Load-bearing wall


Columns are elements that carry only axial force – either tension or compression – or both axial force and bending (which is technically called a beam-column but practically, just a column). The design of a column must check the axial capacity of the element, and the buckling capacity.

The buckling capacity is the capacity of the element to withstand the propensity to buckle. Its capacity depends upon its geometry, material, and the effective length of the column, which depends upon the restraint conditions at the top and bottom of the column. The effective length is K * l where l is the real length of the column.

The capacity of a column to carry axial load depends on the degree of bending it is subjected to, and vice versa. This is represented on an interaction chart and is a complex non-linear relationship.


A beam may be defined as an element in which one dimemsion is much greater than the other two and the applied loads are usually normal to the main axis of the element. Beams and columns are called line elements and are often represented by simple lines in structural modeling.
  • cantilevered (supported at one end only with a fixed connection)
  • simply supported (supported vertically at each end; horizontally on only one to withstand friction, and able to rotate at the supports)
  • continuous (supported by three or more supports)
  • a combination of the above (ex. supported at one end and in the middle)

Beams are elements which carry pure bending only. Bending causes one section of a beam (divided along its length) to go into compression and the other section into tension. The compression section must be designed to resist buckling and crushing, while the tension section must be able to adequately resist the tension.

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 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 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 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.


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.