 # 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.

flange 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.