# Dictionary Structural engineering

**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

N**ewton’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 can satisfy 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.