Prefabricated trusses have changed roof framing by providing stronger, carefully engineered rafters and joists formed into a single unit for installation. Truss design is calculated according to the width, pitch and loads on the roof. There are at least two dozen styles, ranging from a basic king post with a single internal brace that spans up to 16 feet, to a three-piece center bar with multiple braces that can cover up to 100 feet.
No Single Strongest Truss
No single style of truss is intrinsically stronger than another. The most common styles, Fink or W and Howe or K, along with a related type called fan, are the basis for most truss styles. All have angled top rafter chords and horizontal bottom beams or chords. Fink uses four diagonal braces in a W pattern, while Howe uses a center post, two other vertical braces and two angled supports. Fan is essentially a Fink with vertical braces added inside each W side.
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The simplest truss is a king post, with two top chords, a bottom chord and a single upright brace in the center. A queen post adds a diagonal brace on either side of the center post. A queen spanning the same distance as a king would be stronger, because of the additional braces, but a king post is structurally adequate for spans up to 16 feet wide. A queen post would be used for very short spans in circumstances of very heavy roof material or snow accumulations.
Trusses are engineered based on the load they must support. There are three types. Dead load is the weight of the roof itself and all related components, incluidng the truss members, decking, shingles or other covering. Live load is the addition from weather, rain and mainly snow and ice. The pitch of the roof and type of covering affect this. A steep roof sheds snow and ice better than a low slope. A slick metal roof sheds snow better than rough shingles. Wind load is the force of wind on a vertical structure, a factor in areas subject to high winds.
There is no "strongest" truss, but rather, one that is most appropriate for a specific application. There are four basic types of truss design: dropped chord, raised chord, parallel chord and scissors. Dropped chord uses a beam on two load-bearing walls and can restrict interior space. Raised chord is the most common type, providing excellent structural support and greater energy efficiency. Parallel chords are used on flat or low-slope roofs. Scissors create vaulted interior ceilings and are designed to bear additional weight of heavy snows. They are frequently used where beams are exposed inside.
All Are Triangles
All trusses are based on triangles, the shape that provides the most structural stability under compression, downward pressure, or tension, lateral or bending pressure. No other shape can equal that strength. Companies that build trusses factor in the size, pitch and loads for each building and locale, then design a truss to meet those structural requirements and fulfill the design needs for interior space and appearance.
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