Roof Trusses Loading and Engineering

 

AXIAL FORCE: A push (compression) or pull (tension) acting along the length of a member. Usually measured in Newtons (N) or kilonewtons (kN).

AXIAL STRESS: The axial force acting at a point along the length of a member, divided by the cross-sectional area of the member. Usually measured in megapascals (MPa) or Newtons per square millimeter (N/mm2).

BENDING MOMENT: A measure of the bending effect on a member due to forces acting
perpendicular to the length of the member. The bending moment at a given point along a
member equals the algebraic sum of all perpendicular forces, either to the left or right of thepoint, multiplied by their corresponding distances from the point. Usually measured in Newton millimeters (Nmm).

BENDING STRESS: The force per square millimeter of area acting axially at a point along the length of a member, resulting from a bending moment applied at that point. Usually measured in Newtons per square millimeter (N/mm2).

BRACING: Timber members placed perpendicular or diagonally between trusses to prevent movement due to lateral forces (such as wind) or buckling forces due to Compression. Suggestions for good bracing practice are given in SANS 10243: “The Design, Manufacture and Erection of Timber Roof Trusses” and in the ITC Roof Erectors Handbook, Volumes 1 and 2.

CAMBER: An upward vertical displacement built into a truss to compensate for deflection.

COMPRESSION MEMBER: A structural member that has purely axial compressive forces in the direction of the grain – forces that shorten or compress the member.

CONCENTRATED LOAD: Additional loading, e.g. roof-mounted air conditioners or geysers, applied at a given point.

DEAD LOAD: Any permanent load applied to a truss such as sheathing, roof covering, ceiling, purlins, and the self-weight of the truss.

DEFLECTION: Downward vertical movement of a truss due to dead or live loads applied to it.

ENGINEERING DRAWINGS: Engineered drawings that are checked, approved and signed by a Professional engineer (Pr. Eng).

LATERAL BRACE: A member placed and connected at right angles to top or bottom chords or web member of a truss.

LOAD BEARING WALLS: A wall constructed for the specific purpose of effectively supporting loads, such as the roof.

REACTION: The force at a support point of a truss that is equal but opposite to the sum of the dead and live loads applied at that point.

REGISTERED PERSON: A person who is registered with the Engineering Council of South Africa.

RESPONSIBLE PERSON: A competent person who takes responsibility for designing, erecting, manufacturing or inspecting a roof structure.

RUNNER or BINDER: Continuous bracing member that runs the length of the roof to provide lateral stability for web members or chords of trusses in conjunction with triangulated cross-bracing.

SELF-WEIGHT LOAD: The load that consists of the weight of all the members of the structure itself, plus the weight of all finishes, including permanent partitions that are to be supported by any member of the structure.

SHEAR FORCE: The resultant of all forces acting parallel to the cross-section through a structural member, acting on one side of the cross-section.

SHORT-TERM LOADING: Imposed loads of short duration, such as wind or hail loads.

TENSION MEMBER: A structural member that has purely axial tensile forces in the direction of the grain – forces that stretch or elongate the member.

TRIANGULATION: Webs and chords forming triangles to ensure the stability of a structural frame such as a truss.

TRUSS SPACING: Space between the centre lines of adjacent trusses. This distance x the total length of the truss, including overhangs, is the area of roof covering carried by each truss.

 

Roof Trusses Dimensioning

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