Frequently Used Truss Terms
AXIAL FORCE - A push (compression) or pull (tension) acting along the length of a member. Usually measured in pounds.
AXIAL STRESS - The axial force acting at a point along the length of a member, divided by the crosssectional area of the member. Usually measured in pounds per square inch.
BEARING - Structural support of a truss, usually walls, hangers, or posts.
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 the given point along a member equals the sum of all perpendicular forces, either to the left or right of the point, times their corresponding distances from the point. Usually measured in inchpounds.
BENDING STRESS - The force per square inch of area acting at a point along the length of a member, resulting from the bending moment applied at that point. Usually measured in pounds per square inch (psi).
BOTTOM CHORD - A horizontal or inclined (scissors truss) member that establishes the lower edge of a truss. Usually carrying combined tension and bending stresses.
BUILTUP BEAM - A single unit composed of multiple wood members having the same thickness but not necessarily the same depth, which provides greater loadcarrying capability as well as greater resistance to deflection.
BUTT CUT - Slight vertical cut at outside end of truss bottom chord made to insure uniform nominal span and tight joints.
CAMBER - An upward vertical displacement built into a truss, usually to offset deflection.
CANTILEVER - The part of a structural member that extends beyond a support with no support at the other end.
CLEAR SPAN - Horizontal distance between interior edges of supports.
COMBINED STRESS - The combination of axial and bending stresses acting on a member simultaneously, such as occurs in the top chord (compression + bending) or bottom chord (tension + bending) of a truss.
CONCENTRATED LOAD - An additional load centered at a given point. An example is a crane or hoist hanging from the bottom chord at a panel or mechanical equipment supported by the top chord.
DEAD LOAD - Permanent loads that are constant on the truss, e.g.; the weight of the truss itself, purlins, sheathing, roofing, ceiling, etc.
DEFLECTION - Downward vertical displacement of a truss due to loads.
LEVEL RETURN - Lumber filler placed horizontally from the end of an overhang to the outside wall to form a soffit framing.
LIVE LOAD - Any load which is not of permanent nature, such as snow, wind, movable concentrated loads, furniture, etc. Live loads are generally of short duration.
NOMINAL SPAN - Horizontal distance between outside edges of the outermost supports.
OVERHANG - The extension of the top chord of a truss beyond the bearing support.
PANEL - The chord segment defined by two successive joints.
PANEL LENGTH - The centerline distance between joints measured along the chords.
PANEL POINT - The point of intersection where a web (or webs) meets a chord.
PEAK - Point on a truss where the sloped top chords meet. PLUMB CUT Top chord end cut perpendicular to the building floor line provided for vertical installation of facia.
PURLIN - A horizontal member in a roof perpendicular to the truss top chord used to support the decking.
QUARTER POINT - Point on a Fink (T43) or Howe (T44) truss where the webs connect to the top chord. Also one fourth the distance between two joints from either joint.
REACTION - Forces acting on a truss through its supports that are equal but opposite to the sum of all dead and live loads.
SLOPE (Pitch) - The inches of vertical rise in 12 inches of horizontal run for inclined members, generally expressed as 3/12, 4/12, 6/12, etc.
SPLICE POINT - (top and bottom chord splice) The point at which two chord members are joined together to form a single member. It may occur between panel points or at a panel point.
SPLIT TRUSS - Trusses used where a fireplace, skylight, etc. intersects the truss span, parallel or perpendicular to the truss.
SQUARE CUT - A cut perpendicular to the slope of the member at its end.
STRESS DIAGRAM - Graphical depiction of axial forces and moments as they interact within the members of a truss.
THIRD POINT - Point on a Fink (T43), truss where the webs connect to the bottom chord.
TOP CHORD - An inclined or horizontal member that establishes the upper edge of a truss, usually carrying combined compression and bending stresses.
DURATION OF LOAD FACTOR - An adjustment in the allowable stress in a wood member, based on the duration of the load causing the stress. The shorter the duration of the load, the higher the percent increase in allowable stress.
HEEL - Point on a truss at which the top and bottom chords intersect.
JOIST - A parallel chord truss with the least chord dimension in the vertical plane.
LATERAL BRACE - A member installed and connect at right angles to a chord or web member of a truss to resist lateral movement.
TRUSS - A prebuilt component that functions as a structural support member. A truss employs one or more triangles in its construction.
WEBS - Members that join the top and bottom chords to form the triangular patterns that give truss action, usually carrying tension or compression stresses (no bending).