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Middle East Engineering - Structural Glossaries -
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**Acceleration**: A
vector quantity equal to the rate that velocity changes
with time.

**Applied force**: see external force.

**Axial force**: A system of internal forces whose
resultant is a force acting along the longitudinal axis
of a structural member or assembly.

**Bending moment**: A system of internal forces whose
resultant is a moment. This term is most commonly used
to refer to internal forces in beams.

**Body force**: An external force acting throughout
the mass of a body. Gravity is a body force. An inertial
force is a body force.

**Brittle**: A brittle structure or material exhibits
low ductility, meaning that it exhibits very little
inelastic deformation before complete failure.

**Center of Gravity**: The location of the resultant
of gravity forces on an object or objects: sometimes
called center of mass.

**Centroid**: Similar to the concept of center of
gravity, except that it applies to a two dimensional
shape rather than an object. For a given shape, the
centroid location corresponds to the center of gravity
for a thin flat plate of that shape, made from a
homogeneous material.

**Component **(of a vector): Any vector can be
expressed as a collection of vectors whose sum is equal
to the original vector. Each vector in this collection
is a component of the original vector. It is common to
express a vector in terms of components which are
parallel to the x and y axes.

**Concentrated force**: A force considered to act
along a single line in space. Concentrated forces are
useful mathematical idealizations, but cannot be found
in the real world, where all forces are either body
forces acting over a volume or surface forces acting
over an area.

**Concentrated load**: An external force which a
concentrated force.

**Connection**: Connection is similar to the concept
of support, except that connection refers to a
relationship between members in a structural model. A
connection restrains degrees of freedom of one member
with respect to another. For each restrained degree of
freedom, there is a corresponding force transferred from
one member to the other; forces associated with
unrestrained degrees of freedom are zero. See fixed
connection and pin connection.

**Couple**: A system of forces composed of two equal
forces of opposite direction, offset by a distance. A
couple is statically equivalent to a moment whose
magnitude equals the magnitude of the force times the
offset distance.

**Deflection**: This word usually carries the same
meaning as displacement, although it is sometimes used
in place of deformation.

**Deformation**: A change in the shape of an object
or material.

**Degree of Freedom**: A displacement quantity which
defines the shape and location of an object. In the two
dimensional plane, a rigid object has three degrees of
freedom: two translations and one rotation. In three
dimensional space, a rigid object has six degrees of
freedom (three translations and three rotations).

**Displacement**: A change in position. A
displacement may be a translation a rotation or a
combination of those.

**Distributed load**: An external force which acts
over a region of length, surface, or area: essentially
any external force which is not a concentrated force.

**Ductility**: Ductility generally refers to the
amount of inelastic deformation which a material or
structure experiences before complete failure.
Quantitatively, ductility can be defined as the ratio of
the total displacement or strain at failure, divided by
the displacement or strain at the elastic limit.

**Dynamic equilibrium**: Equilibrium which includes
inertial forces.

**Elastic**: A material or structure is said to
behave elastically if it returns to its original
geometry upon unloading.

**Elastic energy**: The energy stored in deformed
elastic material (e.g., a watch spring). Elastic energy
equals where k is the stiffness, and is the associated
deflection. Elastic energy is sometimes called elastic
potential energy because it can be recovered when the
object returns to its original shape; see potential
energy.

**Elastic limit**: The point beyond which the
deformations of a structure or material are no longer
purely elastic.

**E-Modulus**: see modulus of elasticity.

**Energy**: A property of a body related to its
ability to move a force through a distance opposite the
force's direction; energy is the product of the
magnitude of the force times the distance. Energy may
take several forms: see kinetic energy, potential
energy, and elastic energy.

**Equilibrium**: An object is in equilibrium if the
resultant of the system of forces acting on it has zero
magnitude. See static equilibrium and dynamic
equilibrium.

**External force**: A surface force or body force
acting on an object. External forces are sometimes
called applied forces.

**Fixed connection**: In two dimensions, a fixed
connection between two members restrains all three
degrees of freedom of the connected member with respect
to one another. A fixed connection is sometimes called a
rigid connection or moment-resisting connection.

**Fixed support**: In two dimensions, a fixed support
restrains three degrees of freedom: two translations and
one rotation.

**Flexibility**: Flexibility is the inverse of
stiffness. When a force is applied to a structure, there
is a displacement in the direction of the force;
flexibility is the ratio of the displacement divided by
the force. High flexibility means that a small load
produces a large displacement.

**Flexure**: Bending deformation, i.e., deformation
by increasing curvature.

**Force**: A directed interaction between two objects
that tends to change the momentum of both.Since a force
has both direction and magnitude, it can be expressed as
a vector

**Force System**: see system of forces.

**Funicular**: A funicular shape is one similar to
that taken by a suspended chain or string subjected to a
particular loading.

**Gravity**: An attractive force between two objects;
each object accelerates at a rate equal to the
attractive force divided by the object's mass. Objects
near the surface of the earth tend to accelerate toward
the earth's center at a rate of ; this value is often
called the gravitational constant and denoted as

**Inelastic**: Not surprisingly, the opposite of
elastic. A deformation of a structure or material under
load is described as inelastic when the deformation
remains after the load is removed. The term plastic is
often used with the same meaning.

**Inertia**: The tendency of an object at rest to
remain at rest, and of an object in motion to remain in
motion.

**Inertial Force**: A fictitious force used for
convenience in visualizing the effects of forces on
bodies in motion. For an accelerating body, the inertial
force is considered as a body force whose resultant acts
at the object's center of gravity in a direction
opposite the acceleration. The magnitude of the force is
the mass of the object times the magnitude of the
acceleration.

**Internal force**: Forces which hold an object
together when external forces or other loads are
applied. Internal forces are sometimes called resisting
forces since they resist the effects of external forces.

**Internal hinge**: see pin connection.

**Kinetic Energy**: The energy of a moving mass;
equal to . Where m is mass and v is the magnitude of the
velocity.

**Linear**: A structure is said to behave linearly
when its the deformation response is directly
proportional to the loading (i.e. doubling the load
doubles the displacement response). For a material,
linear means that the stress is directly proportional to
the strain.

**Line of Action**: The line of action of a force is
the infinite line defined by extending along the
direction of the force from the point where the force
acts.

**Linear Elastic**: A force-displacement relationship
which is both linear and elastic. For a structure, this
means the deformation is proportional to the loading,
and deformations disappear on unloading. For a material,
the concept is the same except strain substitutes for
deformation, and stress substitutes for load.

**Load**: An external force. The term load is
sometimes used to describe more general actions such as
temperature differentials or movements such as
foundation settlements.

**Mass**: A property of an object measured by the
degree that it resists acceleration.

Magnitude: A scalar value having physical units.

**Modulus of elasticity**: The proportional constant
between stress and strain for material with linear
elastic behavior: calculated as stress divided by
strain. Modulus of elasticity can be interpreted as the
slope of the stress-strain graph. It is usually denoted
as E, sometimes known as Young's Modulus Y, or
E-Modulus.

**Moment**: The resultant of a system of forces
causing rotation without translation. A moment can be
expressed as a couple

**Moment of inertia**: Moment of inertia has two
distinct but related meanings: 1) it is a property of a
an object relating to the magnitude of the moment
required to rotate the object and overcome its inertia.
2) A property of a two dimensional cross section shape
with respect to an axis, usually an axis through the
centroid of the shape.

**Moment Release**: see pin connection.

**Moment resisting-connection**: see fixed
connection.

**Normal strain**: Strain measuring the intensity of
deformation along an axis. Normal strain is usually
denoted by . Average normal strain between two points is
calculated as , where L is the original distance between
the points, and L is the change in that distance. Normal
strain is often simply called strain.

**Normal stress**: Stress acting perpendicular to an
imaginary plane cutting through an object. Normal stress
has two senses: compression and tension. Normal stress
is often simply called stress.

**Pin connection**: In two dimensions, a pin
connection restrains two translation degrees of freedom
but does not restrain rotation. Since the rotation
degree of freedom is unrestrained at a pin connection,
it transfers no moment.

**Pin support**: In two dimensions, a pin support
restrains two translation degrees of freedom but does
not restrain rotation. When considering reaction forces,
a pin support is usually considered to have two force
components: one each about the x and y axes
respectively.

**Plastic**: see inelastic.

**Potential Energy**: The energy stored in a raised
object (e.g. the weights in a grandfather clock).
Potential energy equals mgh, where m is mass, g is the
acceleration of gravity, and h is the vertical distance
from a reference location. It is called potential energy
because the energy can be regained when the object is
lowered. This type of potential energy is sometimes
called gravitational potential energy in order to
distinguish it from elastic potential energy: see
elastic energy.

**Pressure**: Pressure is a similar idea to stress,
the force intensity at a point, except that pressure
means something acting on the surface of an object
rather than within the material of the object. When
discussing the pressure within a fluid, the meaning is
equivalent to stress.

**Racking**: The distortion of a rectangular shape to
a skewed parallelogram.

Reaction: A reaction is a force exerted by a support on
an object: sometimes called support reaction. Using this
definition, a reaction is an external force.

**Resisting force**: see internal force.

**Resultant**: The resultant of a system of forces is
a single force or moment whose magnitude, direction, and
location make it statically equivalent to the system of
forces.

**Rigid**: An idealized concept meaning something
which does not deform under loading. In fact, all
objects deform under loading, but in modelling it can be
useful to idealize very stiff objects as rigid.

**Rigid connection**: see fixed connection.

**Roller support**: In two dimensions, a roller
support restrains one translation degree of freedom.

**Rotation**: Motion of an object where the path of
every point is a circle or circular arc. A rotation is
defined by a point and vector which determine the axis
of rotation. The direction of the vector is the
direction of the axis and the magnitude of the vector is
the angle of rotation.

**Scalar**: A mathematical entity which has a numeric
value but no direction (in contrast to a vector).

**Section Modulus**: A property of a cross sectional
shape, which depends on shape, and orientation. Section
modulus is usually denoted S, and S = I/c, where I =
moment of inertia about an axis through the centroid,
and c is the distance from the centroid to the extreme
edge of the section.

**Shear**: An system of internal forces whose
resultant is a force acting perpendicular to the
longitudinal axis of a structural member or assembly:
sometimes called shear force.

**Shear stress**: Stress acting parallel to an
imaginary plane cut through an object.

**Shear strain**: Strain measuring the intensity of
racking in the material. Shear strain is measured as the
change in angle of the corners of a small square of
material.

**Shear modulus**: The ratio of shear stress divided
by the corresponding shear strain in a linear elastic
material.

**Stability**: Stability is best defined as the
opposite of instability, which is the occurrence of
large structural deformations which are not the result
of material failure.

**Static equilibrium**: Equilibrium which does not
include inertial forces.

**Statically determinate**: A statically determinate
structure is one where there is only one distribution of
internal forces and reactions which satisfies
equilibrium. In a statically determinate structure,
internal forces and reactions can be determined by
considering nothing more than equations of equilibrium.

**Statically equivalent**: Two force systems are
statically equivalent when their resultants are equal.
Physically, this means that the force systems tend to
impart the same motion when applied to an object; note
that the distribution of resulting internal forces in
the object may be different.

**Statically indeterminate**: A statically
indeterminate structure is one where there is more than
one distribution of internal forces and/or reactions
which satisfies equilibrium.

**Stiffness**: This is a general term which may be
applied to materials or structures. When a force is
applied to a structure, there is a displacement in the
direction of the force; stiffness is the ratio of the
force divided by the displacement. High stiffness means
that a large force produces a small displacement. When
discussing the stiffness of a material, the concept is
the same, except that stress substitutes for force, and
strain substitutes for displacement; see modulus of
elasticity.

**Strain**: The intensity of deformation at a point
in an object. See normal strain and shear strain.

**Strength**: A very general term that may be applied
to a material or a structure. In a material, strength
refers to a level of stress at which there is a
significant change in the state of the material, e.g.,
yielding or rupture. In a structure, strength refers to
a level of level of loading which produces a significant
change in the state of the structure, e.g., inelastic
deformations, buckling, or collapse.

**Stress resultant**: A system of forces which is
statically equivalent to a stress distribution over an
area.

**Stress**: The intensity of internal force acting at
a point in an object. Stress is measured in units of
force per area. See shear stress and normal stress.

**Structural model**: An idealization for analysis
purposes of a real or conceived structure. A structural
model includes boundaries limiting the scope of the
analysis. Supports occur at these boundaries,
representing things which hold the structure in place.

**Support**: A support contributes to keeping a
structure in place by restraining one or more degrees of
freedom. In a structural model, supports represent
boundary entities which are not included in the model
itself, e.g., foundations, abutments, or the earth
itself. For each restrained translation degree of
freedom at a support, there is a corresponding reaction
force; for each restrained rotation degree of freedom,
there is a reaction moment.

**Surface force**: A force applied to the surface of
an object.

**System of Forces**: One or more forces and/or
moments acting simultaneously.

**Translation**: Motion of an object where the path
of every point is a straight line.

**Transmissibility**: The principle stating that a
force has the same external effect on an object
regardless of where it acts along its line of action.

**Vector**: A mathematical entity having a magnitude
and a direction in space.

**Velocity**: A vector quantity equal to the rate
that position changes with time.

**Weight**: The force on an object resulting from
gravity.

**Yield stress**: A material loaded beyond its yield
stress, no longer exhibits linear elastic behavior.
Metals, particularly mild steel, generally have a very
well defined yield stress compared to other materials.
Yield stress is sometimes called yield strength.

**Yield strain**: A material deformed beyond its
yield strain, no longer exhibits linear elastic
behavior. See yield stress.

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