Last updated Friday, January 9, 1998, at 9:33 AM Copyright © 1996, 1997 Kirk Martini

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**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.

**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.

**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.

**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 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 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.

**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.

**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.

**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 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 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.

**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.

**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.

**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.

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

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

Last updated Friday, January 9, 1998, at 9:33 AM

Copyright © 1996, 1997 Kirk Martini

Please send comments or questions to Martini@virginia.edu