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Structural systems Timber I Beams and Parallel Chord Trusses

Timber I beams and parallel chord trusses are Engineered Timber Products that are gaining in popularity. They are typically used as purlins, rafters or floor joists in the place of solid timber.

Timber I beams consist of timber chords, which is typically solid timber or Laminated Veneer Lumber (LVL), connected with a timber web. The timber web is a sheet material, usually plywood although other materials such as Oriented Strand Board (OSB) are used.

Hyjoist beams in a midfloor

Hyjoist beams

Photo: Courtesy Carter Holt Harvey.

Parallel chord trusses consist of timber chords fixed together with either timber webs or a proprietary light gauge steel web.

The advantages of these systems are:

  • longer spans and higher loads are achievable
  • more efficient use of the material
  • quick installation and light to handle
  • fast installation of services
  • available in long lengths
  • cost advantages

These products are not used as heavy duty beams and lintels.

Section sizes – I beams

Hyjoist
Hyne I Beam
Lumberworx I Beam
SectionSizeSectionSizeSectionSize
HJ200 45200 x 45HI20070200 x 70LIB200.68200 x 68
HJ240 63240 x 63HI24570245 x 70LIB240.68240 x 68
HJ240 90240 x 90HI30070300 x 70LIB300.68300 x 68
HJ300 63300 x 63LIB360.68360 x 68
HJ300 90300 x 90
HJ360 63360 x 63
HJ400 9-400 x 90

Section sizes - Parallel Chord Trusses

Posi-Strut
Pryda Span
Longreach
SectionSizeSectionSizeSectionSize
PS20-21x07217 x 70PS 25260 x 90FT200200 x 90
PS20-21x09217 x 90PS 30310 x 90FT250250 x 90
PS20-21x14217 x 140PS 40422 x 90FT300300 x 90
PS25-25x07249 x 70FT350350 x 90
PS25-25x09249 x 90FT400400 x 90
PS25-25x14249 x 140
PS30-30x07303 x 70
Proprietary I beams
The proprietary I beams available in New Zealand are Carter Holt Harvey’s Hyjoist, Engineered Timber Solution’s Hyne I-Beam and the Lumberworx I-Beam. The Hyjoist and Lumberworxs products both utilise Laminated Veneer Lumber (LVL) chords with plywood webs; the Hyne I beam uses solid timber chords with Oriented Strand Board (OSB) webs. Discrete depths are available and vary between 200mm and 400mm. Lengths are available up to 12m and the product can be cut to length on site with carpenter’s tools. More information including span tables is available on the manufacturers’ websites. Because of the unique nature of these products it is important that the manufacturers’ recommendations for their use are followed. For example the fixing recommendations published in the manufacturers’ documentation must be followed.

Ibeams during installation. Note the use of temporary top flange bracing

ParallelChordTrusses

Photo courtesy Timber Design Guide, 2007

Parallel Chord Trusses
There are two types of timber parallel chord trusses – metal webbed trusses and timber webbed trusses. Metal Webbed Trusses Metal webbed trusses utilize timber chords, held in place with light gauge metal V shaped webs. The webs are manufactured with teeth and the teeth are pressed onto the chords by timber truss fabricators to form the parallel chord truss. Vertical timber webs are used at each end and at fixing points through the truss. Discrete depths between 200 and 400mm are available.

Posi-STRUT parallel chord truss

Posi-strutMitek

Photo courtesy MiTek

A feature of metal web trusses is the open web; this enables services to be run through without drilling. Another feature is that the products are made to measure based on the actual site dimensions. This speeds up installation, avoiding the need for cutting on side, but means that the length can’t be easily adjusted if the site measurements are incorrect. The two proprietary systems available are MiTek Posi-STRUT and Pryda Span.

Timber Webbed Trusses

Timber webbed trusses use timber chords and webs. The webs are cut to accurately fit and fixed into the truss with toothed metal plate connectors. The benefit of the timber webbed truss is that any depth of truss can be manufactured. The main disadvantage is that the extra material and material handling can result in a more expensive truss. Pryda have marketed their Pryda Longreach system for some years now, however parallel chord timber webbed trusses are available from any Timber Truss Fabricator.

Longreach floor trusses with services running through

Longreachfloortrusses

Photo Courtesy Pryda

Availability

Timber I beams are available through builder’s merchants such as PlaceMakers, Carters, Bunnings, Mitre 10 and the ITM group. Parallel chord trusses are also available through the same merchants, however the product is manufactured by an associated timber truss manufacturer, using the steel components supplied by MiTek or Pryda. Visit the Suppliers database

Design

Selection charts for typical floor joist, rafter and purlin situations is available from the manufacturers’ websites. Carter Holt Harvey and Engineered Timber Solutions have software available for designing I beams and Pryda has a floor truss design program available. Material strength properties and guidelines on specific design are also typically available. When designing I beams the shear deflection is a significant component of the deflection. The manufacturers guidelines show how to incorporate this into the serviceability check. Parallel chord truss members perform as a truss and can be modeled as such, however the suppliers of these systems consider them to be proprietary products and have not released design strengths for their webs.