Truss Terminology

Allowable Stress: The amount of force per unit of area permitted in structural member. Values for allowable stresses of wood can be found in "National Design Specification Supplement Design Values for Wood Construction."

Allowable Stress Increase or Duration of Load Factor: 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 higher the percentage increases in the allowable stress.

Axial Force: A push (compression) or pull (tension) acting along the length of a member. Usually measured in pounds, kips (1000 lb.), tons (2000 lb.) or the metric equivalents.

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 pounds per square inch).

Beam Pocket: A void or cutout built into truss to allow beam support.

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

Bending Moment: A measure of the bending effect due to the live load and dead load on a given truss chord member.

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, usually carrying combined tension and bending stresses.

Built-up Beam: A single member composed of two wood members stacked on top of each other and fastened together with connector plates, for the purpose of crating additional strength and stiffness.

Butt Cut or Nub Cut: Slight vertical cut at outside edge of truss bottom chord made to ensure uniform nominal span (usually 1/4 inch).

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

Cantilever: The condition where both top and bottom chords extend beyond a support with no bearing at the extended end.

Chase Opening: An open panel in a floor truss for the purpose of running utilities through it, such as heating and air conditioning ducts.

Clear Span: Horizontal distance between interior edges or 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.

Common Truss: An engineered component shaped so as to have a near equal pitch on both sides of a center peak.

Compression: Force exerted on truss member that has a compressive or pushing effect on the member and its respective end joint.

Concentrated Load: Superimposed load centered at a given point (e.g., roof-mounted air conditioners).

Connector: A mechanical device for securing two or more Trusses, components, pieces, parts, or members together, including anchors, buckets, straps, wall ties, and fasteners.

Cricket: A ridge or drainage flume structure designed to divert roof framing. Generally found on the high sloped end of a chimney or the transition from one roof area to another.

Dead Load: Any permanent load such as the weight of the truss itself, purlins, sheathing, roofing, ceiling, etc.

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

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

Drag Strut: Typically a horizontal member, such as a truss or beam, which transfers shear from a diaphragm to a shear wall.

Dropped Gable: A gable truss that has its top chord lowered vertically to allow outlookers or a gable ladder to, for an overhang.

Dual Pitch Truss: A truss that has two different pitches on its top chord.

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.

Engineer Certified Drawings: A truss where loading, size and grades of material are called out and detailed and a certified engineer’s seal is affixed to that drawing.

Facia: Trim board applied to ends of overhang.

Force Diagram: Graphical solution of axial forces as they interact within the members of a truss.

Gable End: A truss with vertical studs, usually spaced at 16” o.c., The gable usually sits on an end wall and the studs help to provide support for the sheathing and resistance to wind.

Gambrel: A roof profile having two slopes on each side. The lower slope is usually steeper than the upper one.

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

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

Heel Cut: See Butt Cut.

Hip Set: A trussed system where three planes come in on a slope.

Hip Truss: A component of a hip roof system of roof Trusses affording symmetry of architectural appearance. The eave line extends to the same level around all sides of the building eliminating the use of gable ends. Normally the off site manufacture of hip Truss parts aids in speed and quality of field construction.

Interior Bearing Truss: Truss with structural support in the interior truss span as well as at end points.

Joist: A parallel chord truss with the least chord dimension in the vertical plane.

Lateral Brace: A member placed and connected at right angles to a chord or web of a truss for the purpose of providing lateral support.

Level Return: Lumber filler placed horizontally from the end of an overhang to the outside wall to for a soffit.

Live Load: Any loading which is not of a permanent nature, such as snow, wind, temporary construction loads, etc.

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 Length: The center line distance between joints measured along the chords.

Panel: The chord segment defined by two succeeding joints.

Panel Point: The point of intersection where a web (or webs) meets a chord.

Peak: Point on truss where the sloped top chords meet. The highest point of the truss.

Permanent Bracing: Bracing put on a roof system that is intended to remain permanently on the roof to reinforce the structure.

Piggyback: A cap truss provided which will sit on top of the trusses below (with purlins and blocking) usually when trusses are too tall to build, or too tall to deliver.

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

Plumb Cut: Top chord cut to provide for vertical (plumb) installation of facia.

Ply: One member of a multi-member truss or beam.

Purlin: A horizontal framing member used to support sheathing or decking between two main load carrying structural members.

Quarter Point: Point on a Fink or Howe 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.

Ridge: A ridge is the line formed when two planes meet, usually at truss peaks.

Rise: The vertical distance from the bottom of the bottom chord to bottom side of the top chord.

Saddle: An area where an additional roof slope and a ridge are created to facilitate drainage. Usually found behind vertical obstructions in the roof.

Scupper: An opening in a roof or parapet usually faced with metal flashing to drain water from the roof at a given point.

Sealed Drawings: Drawings prepared, checked, and/or approved by and having the seal of a registered professional architect or engineer.

Setback: The distance from the outside edge of the wall to the face of a girder.

Slope: (Pitch). The inches of vertical rise in 12 inches of horizontal run for inclined members (generally expressed as 3/12, 4/12, 5/12, etc).

Span: The length of the truss, measured from outside bearing to outside bearing, except in the case of cantilever conditions. If the truss is cantilevered beyond an outside bearing, the span length would include the length of the bottom chord beyond the outside wall.

Splice Point: (Top & Bottom chord splice). The point at which two chord members are joined together to form a single member. It may occur at a panel point or between panel points.

Split Truss: Trusses used where fireplace intersects the truss span, parallel or perpendicular to the truss in the middle or inside of the house. A split truss can be defined also as a stub truss if it is longer than one-half the span or as a monopitch truss if less than one-half the span.

Square Cut: End of top chord cut perpendicular to the slope of member.

Stress Diagram: Graphical depiction of axial forces and moments as they interact within the members of a truss.

Stress Rated Lumber: Lumber that has been graded either visually or by machine by an approved grading agency and assigned allowable working stress values. All lumber used in engineered wood products such as trusses must be stress rated.

Stub: When the truss is held back from its original span.

Temporary Bracing: Bracing added to the roof or floor system to brace it during erection and installation.

Tension: Forces being exerted on a truss member that creates a pulling apart of elongating effect.

Third Point: Point on a Fink, 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 compression and bending stresses.

Truss: An engineered pre-built structural component designed to carry superimposed dead and live loads. The truss members are coplanar and are usually assembled such that the members form triangles.

Uniform Load: A total load that is equally distributed over a given length, Usually expressed in pounds per lineal foot (plf).

Uplift: Wind, increased in speed, moving over a structure causing negative wind pressure (suction) to be placed inside an enclosed structure, creating uplift forces (upward pull) capable of blowing off the roof. Roofs are designed to resist only certain uplift caused when high winds travel over and across the roof.

Valley: A depression in a roof where two roof slopes meet.

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).