Terms

Allowable Stress Increase: A percentage increase in the stress permitted in a member, based on the length of time that the load causing the stress acts on the member. The shorter the duration of the load, the higher the percent increase in allowable stress.

Axial Force: A push (compression) or pull (tension) acting along the length of a member. Usually measured in pounds, kips (1,000 lbs.) or tons (2,000 lbs.), or metric equivalent.

Axial Stress: The axial force acting at a joint along the length of a member divided by the cross-sectional area of the member. Usually measured in pounds per square inch.

Bearing: A structural support, usually a wall that occurs at the top or bottom chord or between the end points of a roof or floor truss.

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 sum of all perpendicular forces, either to the left or right of the point, times their corresponding distances from the point.

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 or metric equivalent.

Bottom Chord: A horizontal or inclined (e.g. Scissors Truss) member that establishes the lower edge of a truss, In a conventional system, this is the ceiling joist.

Butt Cut: Slight vertical cut at the outside edge of truss bottom chord made to ensure uniform span and tight joints - usually 1/4 inch.

Camber: An upward vertical displacement built into a truss bottom chord to compensate for deflection due to dead load.

Cantilever: Extension of the bottom chord beyond its support, exclusive of overhang.

Check: A lengthwise separation of wood fibers, usually extending across the rings of annual growth, caused chiefly by strains produced in seasoning.

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: Superimposed load centered at a given point (e.g., roof-mounted air conditioner, furnace).

Connector Plate: Pre-punched metal toothed connectors located at the joints and splices of a truss and designed to hold the forces which occur at those locations.

Cripple Rafter: Infill rafter installed to continue the roof line - fixed to valley board in valley construction.

Dead Load: Any permanent load such as the weight of roofing, flooring, sheathing, insulation or ceiling material, as well as the weight of the truss itself.

Deflection: Downward vertical movement of a truss (when in place) due to dead and live loads.

Design Loads: The dead and live loads which a truss is engineered to support.

Engineer Certified Drawing: A truss design where loading requirements, lumber species, sizes, grades and connector plate requirements are detailed and a certified engineer's seal is affixed.

Girder Truss: Usually a multiple-ply truss designed to carry other trusses over an opening.

Heel: Point on a truss at which the top and bottom chords intersect.

Heel Cut: See Butt Cut.

Jack Rafter: Infill rafter installed to continue the roof-line - fixed from wall plate to hip board in hip end construction.

Joint: See Panel Point.

Lateral Brace: A member placed and connected at right angles to a chord or web member of a truss.

Level Return: A lumber filler placed horizontally from the end of an overhang to the outside wall to form a soffit.

Live Load: Any loading which is not of a permanent nature (e.g., snow, wind).

Moisture Content of Wood: The weight of the moisture in wood expressed as a percentage of its ovendry weight.

Overall Rise: Vertical distance from bottommost part of the bottom chord to uppermost point on peak.

Overhang: The extension of the top chord of a truss beyond the heel, measured horizontally.

Panel: The chord segment defined by two adjacent joints.

Panel Length: The centerline distance between joints measured horizontally.

Panel Point: The point where a web or webs intersect a chord.

Peak: Point on truss where the sloped top chords meet.

Pitch: Inches of vertical rise for each 12 inches of horizontal run.

Plumb Cut: Top chord end cut to provide for vertical (plumb) installation to fascia (face trim board).

Plumb Rise: Vertical overall measurements at the end of a truss where the top and bottom chords meet.

Purlin: A horizontal member attached to and placed perpendicular to the truss top chord to support the roofing.

Reaction: Forces acting on a truss, through its support, that are equal but opposite to the sum of the dead and live loads.

Ridge: Line formed by truss apexes.

Rise: Vertical distance from bottom most part of the bottom chord to inside of the peak.

Scab: Additional timber connected to a truss to effect a splice, extension or general reinforcement.

Shop Drawing: Detailed drawings of a roof truss or roof framing showing critical dimensions such as span,overhang, cantilever, slope, etc.

Slope: See Pitch.

Spacing: Centerline distance between trusses - usually 24" O.C. (on center).

Span: Horizontal distance between outside edges of the supports.