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truss configurations

Useful information about truss

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."

Explanation of truss terminologies

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The pitched or common truss is characterized by its triangular shape. It is mostly used for construction. Some common trusses are named according to their web configuration, such as the King Post, Fan, or Howe truss.

 

The chord size and web configuration are determined by span, load, and spacing. All truss designs are optimized to provide the most economical application.

 

The parallel chord or flat truss gets its name from having parallel top and bottom chords. This type is often used for floor construction.

Roof and parallel chord trusses

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 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 the outside edge of truss bottom chord made to ensure uniform nominal H3 (usually ¼ 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 H3

Horizontal distance between the 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.

 

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).

 

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.

 

Dual pitch truss

A truss that has two different pitches on its top chord.

 

Fascia

Trim board applied to ends of overhang.

 

Force diagram

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

 

Heel

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

 

Heel cut

See Butt Cut.

 

Interior bearing truss

Truss with structural support in the interior truss H3 as well as at end points.

 

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 H3

The horizontal projection of the bottom chord of the truss.

 

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.

 

Plumb cut

Top chord cut to provide for vertical (plumb) installation of fascia.

 

Purlin

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

 

Reaction

Total load transmitted to its support by a given truss.

 

Saddle

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

 

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.

 

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 approved by and having the seal of a registered professional architect or engineer.

 

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.).

 

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 at a panel point or between panel points.

 

Split truss

Trusses used where fireplace intersects the truss H3, 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 H3 or as a monopitch truss if less than one-half the H3.

 

Square cut

End of top chord cut perpendicular to the slope of member.

 

Tension

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

 

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).

 

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).

The terms below are typically used to describe the various parts of a metal-plate-connected wood truss. The truss profile, span, heel height, overall height, overhang, and web configuration depend on the design conditions and will differ by application.

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