Structure
Contenido
Las primeras columnas se construyeron con piedra, algunas de una sola pieza de piedra. Las columnas monolíticas se encuentran entre las piedras más pesadas utilizadas en la arquitectura.
Otras columnas de piedra se crean a partir de múltiples secciones de piedra, con mortero o con juntas en seco (grapas). En muchos sitios clásicos, las secciones (tambores) de columnas se tallaron con un orificio central o una depresión para poder unirlas con pasadores (clavijas) "Clavija (carpintería)") de madera, piedra o metal.
El diseño de la mayoría de las columnas clásicas incorpora entasis (la inclusión de una ligera curva hacia afuera en los lados) más una reducción en el diámetro a lo largo de la altura de la columna, de modo que la parte superior sea tan solo el 83 % del diámetro inferior. Esta reducción imita los efectos de paralaje que el ojo espera ver, y tiende a hacer que las columnas parezcan más altas y rectas de lo que son, mientras que el éntasis se suma a ese efecto.
Hay flautas y filetes que suben por el fuste de las columnas. La flauta es la parte de la columna que está indentada con forma semicircular. El filete de la columna es la parte entre cada una de las flautas de las columnas de orden jónico. El ancho de la ranura cambia en todas las columnas cónicas a medida que sube por el fuste y permanece igual en todas las columnas no cónicas. Esto se hizo con las columnas para agregarles interés visual. El jónico y el corintio son los únicos órdenes que tienen filetes y flautas. El estilo dórico tiene flautas pero no filetes. Las flautas dóricas están conectadas en un punto afilado donde se ubican los filetes en las columnas de orden jónico y corintio.
Nomenclature
Most classical columns arise from a base, which rests on the stylobate, or foundation&action=edit&redlink=1 "Foundation (engineering) (not yet written)"), except those of the Doric order, which usually rest directly on the stylobate. The base may consist of several elements, starting with a wide, square slab known as a plinth "Plinth (architecture)"). The simplest bases consist of the plinth alone, sometimes separated from the column by a convex circular cushion known as a torus. The most elaborate bases include two tori, separated by a concave section or channel known as a scotia (Scotiae) or trochilus. Scotiae could also occur in pairs, separated by a convex section called an astragalus, or bead, narrower than a torus. Sometimes these sections were accompanied by even narrower convex sections, known as annulets&action=edit&redlink=1 "Annulet (architecture) (not yet drafted)") or fillets.[3][4].
At the top of the axis there is a capital, on which the roof or other architectural elements rest. In the case of Doric columns, the capital usually consists of a round, tapering cushion, or echinus, supporting a square slab, known as an abax or abacus. The Ionic capitals feature a pair of volutes "Volute (architecture)"), while the Corinthian capitals are decorated with reliefs in the form of acanthus leaves. Any type of capital could be accompanied by the same moldings as the base.[3][4] In the case of free-standing columns, the decorative elements on the shaft are known as finials.
Modern columns can be constructed of steel, poured or precast concrete, or brick, left bare or covered with an architectural covering or sheet metal. Used to support an arch, an impost&action=edit&redlink=1 "Impost (architecture) (not yet drafted)"), or pillar, is the upper member of a column. The lower part of the arch, called the jump, rests on the fascia.
Balance, instability and loads
As the magnitude of the axial load on a perfectly straight slender column with elastic material properties increases, this ideal column passes through three states: stable equilibrium, neutral equilibrium, and instability. The straight column under load is in stable equilibrium if a lateral force, applied between the two ends of the column, produces a small lateral deflection which disappears and the column returns to its straight shape when the lateral force is removed. If the load on the column is gradually increased, a condition is reached in which the straight equilibrium shape becomes the so-called neutral equilibrium, and a small lateral force will produce a deflection that does not disappear and the column remains in this slightly bent shape when the lateral force is removed. The load at which neutral equilibrium of a column is reached is called the critical or buckling load. The state of instability is reached when a slight increase in spinal loading causes increasing and uncontrollable lateral deflections leading to total collapse.
For an axially loaded straight column with any end support condition, the static equilibrium equation, in differential equation form, can be solved for the deflected shape and critical load of the column. Under hinged, fixed, or free-end support conditions, the flexed shape in neutral equilibrium of an initially straight column with uniform cross section throughout its length always follows a partial or compound sinusoidal curve shape, and the critical load is given by.
where E = elastic modulus of the material, I = the minimum moment of inertia of the cross section, and L = actual length of the column between its two extreme supports. A variant of (1) is given by.
where r = radius "Radius (geometry)") of gyration of the cross section of the column that is equal to the square root of (I/A), K = ratio of the longest half of the sine wave "Sine (trigonometry)") to the actual length of the column, E = tangent modulus in stress F, and KL = effective length (length of an equivalent hinged-hinged column). From Equation (2) it can be seen that the buckling resistance of a column is inversely proportional to the square of its length.
When the critical stress, F (F =P/A, where A = cross-sectional area of the column), is greater than the proportional limit of the material, the column is experiencing inelastic buckling. Since at this stress the slope of the material's stress-strain curve, E (called the tangent modulus"), is less than that below the proportional limit, the critical load in inelastic buckling is reduced. More complex formulas and procedures apply for such cases, but in its simplest form, the formula for the critical buckling load is given as Equation (3),.
Types of column failures
There are different types of structural failures in columns, these can be caused by different criteria, for example: corrosion, poor quality of construction materials, poor casting processes (in the case of concrete)/installation (in the case of steel), among others. The main types of failure that can be observed in columns are compression failure, failure due to adhesion of reinforcing rods, failure due to flexure-compression, failure due to torsion, failure due to the short column effect, shear stresses, among others. The level of danger that each poses within a structural system varies, from structural malfunction to collapse. When a structural member suffers damage, it must be monitored to avoid collapse of the member itself.
Extensions
When a column is too long to be built or transported in one piece, it must be extended or spliced at the construction site. A "reinforced concrete column" is extended by having steel rebar extend a few inches or feet above the top of the concrete, then the next level of rebar is placed to overlap and the next level's concrete is poured. A steel column is extended by welding or bolting butt plates to the flanges and webs or walls of the columns to provide a few inches of load transfer from the top to the bottom section of the column. A wood column is usually It is extended by using a steel tube or wrapped sheet metal plate bolted to the two connecting wood sections.
Bases
A column carrying the load to a foundation must have a means to transfer the load without overloading the foundation material. Reinforced concrete and masonry columns are usually constructed directly on concrete foundations. When sitting on a concrete foundation, a steel column must have a base plate to distribute the load over a larger area and therefore reduce the bearing pressure. The base plate is a thick rectangular steel plate, usually welded to the lower end of the column.