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
Many of these elements can be classified according to form (straight, plane / curve) and dimensionality (one-dimensional / two-dimensional):
|(predominantly) bending||beam||continuous arch||plate, concrete slab||lamina, dome|
|(predominant) tensile stress||rope||Catenary||shell|
|(predominant) compression||pier, column||Load-bearing wall|
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.
- 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.