A
column in
structural engineering is a vertical
structural element that transmits, through
compression, the weight of the
structure above to other structural elements below. For the purpose
of
wind or
earthquake engineering,
columns may be designed to resist lateral forces.
Other
compression members are
often termed "columns" because of the similar stress conditions.
Columns are frequently used to support
beam or
arches on which
the upper parts of walls or ceilings rest. In architecture "column"
refers to such a structural element that also has certain
proportional and decorative features. A column might also be a
decorative or triumphant feature but need not be supporting any
structure e.g. a statue on top.
History
In the architecture of
ancient Egypt
as early as
2600 BC the architect
Imhotep made use of stone columns whose surface was
carved to reflect the organic form of bundled reeds; in later
Egyptian architecture faceted cylinders were also common.
Some of
the most elaborate columns in the ancient world were those of
Persia especially the massive stone
columns erected in Persepolis. They included double-bull structures in
their capitals.
The Hall of
Hundred Columns at Persepolis, measuring 70 × 70 meters was
built by the Achaemenid king Darius I (524–486 BC).
Many of the ancient Persian columns are standing, some being more
than 30 meters tall.
The impost (or pier) is the topmost member of a column. The
bottom-most part of the arch, called the springing, rests on the
impost.
Structure
Modern column grid in a car
park.
Early columns were constructed of stone, some out of a single piece
of stone, usually by turning on a lathe-like apparatus.
Single-piece columns are among the heaviest stones used in
architecture. Other stone columns are created out of multiple
sections of stone, mortared or dry-fit together. In many classical
sites, sectioned columns were carved with a center hole or
depression so that they could be pegged together, using stone or
metal pins. The design of most classical columns incorporates
entasis (the inclusion of a slight outward
curve in the sides) plus a reduction in diameter along the height
of the column, so that the top is as little as 83% of the bottom
diameter. This reduction mimics the parallax effects which the eye
expects to see, and tends to make columns look taller and
straighter than they are while entasis adds to that effect.
Modern columns are constructed out of steel, poured or precast
concrete, or brick. They may then be clad in an architectural
covering (or veneer), or left bare.
Equilibrium, instability, and loads
As the axial load on a perfectly straight slender column with
elastic material properties is increased in magnitude, 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 form when the lateral force
is removed. If the column load is gradually increased, a condition
is reached in which the straight form of equilibrium becomes
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 form 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 of the column load
causes uncontrollably growing lateral deflections leading to
complete collapse.
For an axially loaded straight column with any end support
conditions, the equation of static equilibrium, in the form of a
differential equation, can be solved for the deflected shape and
critical load of the column. With hinged, fixed or free end support
conditions the deflected shape in neutral equilibrium of an
initially straight column with uniform cross section throughout its
length always follows a partial or composite sinusoidal curve
shape, and the critical load is given by
f_{cr}\equiv\frac{\pi^2\textit{E}I_{min}}{{L}^2}\qquad (1)
where
E =
modulus of elasticity of
the material,
I_{min} = the minimal moment of
inertia of the cross section, and
L = actual length of the
column between its two end supports. A variant of (1) is given
by
f_{cr}\equiv\frac{\pi^{2}E_T}{(\frac{KL}{r})^{2}}\qquad (2)
Table showing values of K for
structural columns of various end conditions (adapted from Manual
of Steel Construction, 8th edition, American Institute of
Steel Construction, Table C1.8.1)
where
r =
radius of gyration of
[column]cross-section which is equal to the square root of (I/A),
K = ratio of the longest half
sine
wave to the actual column length, and
KL = effective
length (length of an equivalent hinged-hinged column). From
Equation (2) it can be noted that the buckling strength of a
column is inversely proportional to the square of its length.
When the critical stress,
F_{cr}
(
F_{cr} =
P_{cr}/
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_{t}
(called the
tangent modulus), is smaller
than that below the proportional limit, the critical load at
inelastic buckling is reduced. More complex formulas and procedures
apply for such cases, but in its simplest form the critical
buckling load formula is given as Equation (3),
f_{cr}\equiv{F_y}-\frac{F^{2}_{y}}{4\pi^{2}E}\left(\frac{KL}{r^2}\right)\qquad
(3)
where
E_{t} = tangent modulus at the
stress
F_{cr}
A column with a cross section that lacks symmetry may suffer
torsional buckling (sudden twisting) before, or in combination
with, lateral buckling. The presence of the twisting deformations
renders both theoretical analyses and practical designs rather
complex.
Eccentricity of the load, or imperfections such as initial
crookedness, decreases column strength. If the axial load on the
column is not concentric, that is, its line of action is not
precisely coincident with the centroidal axis of the column, the
column is characterized as eccentrically loaded. The eccentricity
of the load, or an initial curvature, subjects the column to
immediate bending. The increased stresses due to the combined
axial-plus-flexural stresses result in a reduced load-carrying
ability.
Extensions
When a column is too long to be built or transported in one piece,
it has to be extended or spliced at the construction site. A
reinforced concrete column is extended by having the steel
reinforcing bars protrude a few inches or feet above the top of the
concrete, then placing the next level of reinforcing bars to
overlap, and pouring the concrete of the next level. A steel column
is extended by welding or bolting splice plates on the flanges and
webs or walls of the columns to provide a few inches or feet of
load transfer from the upper to the lower column section. A timber
column is usually extended by the use of a steel tube or
wrapped-around sheet-metal plate bolted onto the two connecting
timber sections
Foundations
A column that carries the load down to a foundation must have means
to transfer the load without overstressing the foundation material.
Reinforced concrete and masonry columns are generally built
directly on top of concrete foundations. A steel column, when
seated on a concrete foundation, must have a base plate to spread
the load over a larger area and thereby reduce the bearing
pressure. The base plate is a thick rectangular steel plate usually
welded to the bottom end of the column.
Classical orders
The
Roman author
Vitruvius, relying on the writings (now lost) of
Greek authors, tells us that the
ancient
Greeks believed that
their Doric order developed from techniques for building in wood in
which the earlier smoothed tree trunk was replaced by a stone
cylinder.
Doric order
The
Doric order is the oldest and
simplest of the classical orders. It is composed of a vertical
cylinder that is wider at the
bottom. It generally has neither a base nor a detailed
capital. It is instead often topped
with an inverted
frustum of a shallow cone
or a cylindrical band of carvings.
It is often referred to as the masculine
order because it is represented in the bottom level of the Colosseum and the Parthenon, and was therefore considered to be able to hold
more weight. The height-to-thickness ratio is about 8:1. The
shaft of a Doric Column is always fluted.
The Greek Doric, developed in the western Dorian region of Greece,
is the heaviest and most massive of the orders. It rises from the
stylobate without any base; it is from four to six times as tall as
its diameter; it has twenty broad flutes; the capital consists
simply of a banded necking swelling out into a smooth echinus ,
which carries a flat square abacus; the Doric entablature is also
the heaviest, being about one-fourth the height column. The Greek
Doric order was not used after c. 100 B.C. until its “rediscovery”
in the mid-eighteenth century.
Tuscan order
The
Tuscan order, also known as Roman
Doric, is also a simple design, the base and capital both being
series of cylindrical disks of alternating diameter. The shaft is
almost never fluted. The proportions vary, but are generally
similar to Doric columns. Height to width ratio is about 7:1.
Ionic order
The
Ionic column is considerably more
complex than the Doric or Tuscan. It usually has a base and the
shaft is often fluted (it has grooves carved up its length). On
ALOHA the top is a capital in the characteristic shape of a
scroll, called a
volute, or scroll, at the four corners. The
height-to-thickness ratio is around 9:1. Due to the more refined
proportions and scroll capitals, the Ionic column is sometimes
associated with academic buildings.
Ionic capital
Corinthian order
The
Corinthian order is named for the
Greek city-state of Corinth, to which it was connected in the period.
However,
according to the architectural historian Vitruvius, the column was
created by the sculptor Callimachus, probably an Athenian, who drew
acanthus leaves growing around a
votive basket. In fact, the oldest known Corinthian capital
was found in Bassae, dated at
427 BC. It is
sometimes called the feminine order because it is on the top level
of the Colosseum and holding up the least weight, and also has the
slenderest ratio of thickness to height. Height to width ratio is
about 10:1.
Composite order
The
Composite order draws its name
from the capital being a composite of the Ionic and Corinthian
capitals. The acanthus of the Corinthian column already has a
scroll-like element, so the distinction is sometimes subtle.
Generally the Composite is similar to the Corinthian in proportion
and employment, often in the upper tiers of colonnades. Height to
width ratio is about 11:1 or 12:1.
Solomonic
Capital of Solomonic Column
Solomonic columns were inventions
of
Baroque architects in Europe. They were
not used in antiquity, but were called “Solomonic” by baroque
architects because they were based on a description of columns in
the great
temple of King
Solomon in the
Old
Testament. A Solomonic column begins on a base and ends in a
capital, just like a classical column, but the shaft twists around
the usual parameters of a column, producing a dramatic, serpentine
effect of movement.
The most famous use of Solomonic columns is
in the baldacchino designed by Bernini for Saint Peter’s Basilica in the Vatican City.
Notable columns
See also