# Technical Dictionary for structural mechanics

Draft 12.08.20

analysis model the mathematical representation of the behaviour
of a system.

**axial force** a force that is in the line of a longitudinal axis of
a member; internal force action at a section of a member the resultant
of which acts at the centroid of the section and at right angles to the
section.

**axis** a line, normally straight, used for reference

**bending action** load transfer via bending moments

**bending moment** an internal moment at a section of a member that
acts about an axis that is in the plane of the section.

**centroidal axis** an axis in the plane of an area that passes
through the centroid of the area.

**compression** the effect on a body of a force that tends to shorten
it.

**cut** an imaginary separation of parts of a structure used in
equilibrium calculations; to make such a separation

**deflection** translation of a body in relation to an external
reference system.

**deformation** the relative movement of parts of a structure that
result from a change in shape..

degree of statical indeterminacy the number of force actions in a
frame system that can be released in order to make it statically
determinate.

direct force a force in a direction that is perpendicular to the
surface on which it is considered to act: see axial force.

**direct strain** the ratio of the change, due to deformation,
in displacement between two points in a body and the displacement
between the points before deformation

**direct stress** direct force per unit area

**displacement** a vector that defines the movement of a point in
relation to a set of axes; a translation or a rotation

**equilibrant** the single force that balances a set of forces; the
equal and opposite of the resultant.

**equilibrium** in physics: the condition of static
equilibrium; in engineering mechanics: the condition of static
equilibrium or of dynamic equilibrium

** free body diagram** a diagram that shows the forces at the
cuts that define a part of a structure.

**flang**e the horizontal parts of an I section

**force** that which tends to cause or to resist
motion of a body. The SI unit for force is the Newton defined as the
force that will cause a mass of 1 kilogram to accelerate by 1 metre per
second per second. more

**force action** a direct force, a shear force, a moment or a
torque

**fully fixed support**
a support for a structure that restrains movement in the coordinate
directions and for rotation

**global axis** an axis that relates to the geometry of a system.

**gravity force** a force that causes the reciprocal attraction
of bodies under the effect of their masses: in general,
gravity force is defined by Newton’s universal law of gravitation but in
mechanics ‘gravity force’ is normally defined as the force exerted by
the mass of the Earth on a mass M at the surface of the earth. The
gravity force for a mass M at the surface of the Earth (based on
Newton’s universal law of gravity) is defined by: Fg =
Mg where g is the local gravity constant.

internal force action a force at a cut in a member.

**I section** a section commonly used for members of steel
structures

joint the position at which members are connected in a structure.

**lever arm** the perpendicular distance from the line of action of a
force to the line of an axis about which the value of a moment of the
force is to be calculated.

**local gravity constant** - *g* the constant, based on
Newton’s Universal Law of Gravitation, used to calculate the gravity
force between the mass of the Earth and a mass M at the surface of the
Earth: the gravity force is: Fg = Mg; g = 9.8
m/sec/sec; *g* is sometimes denoted as ‘little g’ or as the
‘acceleration due to gravity’

**longitudinal axis** an axis in the line of a member normally at the
centroid of the area of the member

**member** part of a structure normally defined between a pair of
joints

**moment** the tendency of a force to rotate an object; the
value of a moment is the product of the value of the force times the
lever arm about an axis; see bending moment, torque.

**moment connection** a
connection between members of a frame where there is no relative
rotation between the ends of the members and hence moments can be
transmitted at the member ends

**Newton’s universal law of gravitation** the
relationship that defines the attractive force - Fgravity between 2
masses m1 and m2 distance r apart: Fgravity = G x m1 x
m2/r2 G is the universal gravity constant (G
= 6.67384 × 10-11 m3 kg-1 s-2)

**pin connection** a joint in a
frame where the ends of the connected members are free to rotate
independently

**pin support ** a support for a
structure that restrains movement in the coordinate directions but
does not restrain rotations

**resistance** capacity
of a member or component, or a cross-section of a member or
component of a structure. to withstand actions without mechanical
failure e.g. bending resistance, buckling resistance, tension
resistance’ (definition from EN 1990)

**resultant** a single force that is equivalent to a
set of forces: equal and opposite to the equilibrant.

**resultant **force action 1. see
resultant 2. an internal force action that
is mainly due to applied forces that are not close to the position
at which it is defined; for example, a bending moment is a resultant
force action because it is not caused by local forces;

** roller support** a support
that restrians only horizontal movement

**rotation** a vector that represents the angular movement
about an axis; normally measured in degrees or radians.

**scalar** a variable defined only by its magnitude

**second moment of area** property of an area

**section** the area revealed at a cut

**shear force** a force in a direction that is parallel to
the surface on which it is considered to act; internal force action
at a section of a member that acts parallel to the section.

shear strain the change, due to deformation, in the angle
between two lines on a body

**shear stress** shear force per unit area

spring force the force exerted by a body due to strain
energy.

**statics** the mechanics of bodies under the influence of
forces that do not cause them to move.

**statical equivalence** a force action or set of force
actions that balances a set of forces, equal and opposite to the
equilibrant

**static equilibrium** the condition that the vector sum of
the static forces on a body is zero

static force a force in a context where there is
no motion

**statically determinate**
a frame structure for which the release of one restraining force
action would convert it to a mechanism. i.e. to being unable to
support load

**strain** direct strain or shear strain

strength of materials models that predict the
resistance properties of a material as part of a structure.

**stress** force per unit area

**structure** ‘organised combination of connected parts
designed to cary loads and provide adequate rigidity’ (EN
1900)

**structural analysis** the use of structural mechanics to
predict the behaviour of structures under load.

**structural mechanics** the mathematical logic and
procedures used both for structural analysis and for technical
assessment.

**technical assessment** the use of design rules, that are mainly
based on structural mechanics and set out in code of practice
provisions, to assess the adequacy of structural members and
components.

**tension** the effect on a body of a force that tends to shorten
it

**validation** consideration of the potential of a process to
satisfy the requirements; consideration of whether a process has
satisfied the requirements.

**vector** a variable that has properties of magnitude, position
and direction and conforms to the rules of vector algebra.

**vector algebra** a set of rules for manipulating vectors

vector component a vector that together with other components
can represent a single vector, based on vector algebra.

**vector sum** the sum of a set of vectors.

**web** the vertical part of an I section

**Young’s modulus** the the ratio of direct stress to
direct strain in a material, assumed to be linear.