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Structural timbers & engineering

Like to know what wood to use building a pergola or framing a barn? How about the difference between tung oil and polyurethane as a floor finish? How to revive decking or deal with merbau stains? Or how to meet the building code or bushfire standards? Or ask about the environmental advantages of wood or forest certification?

Q. I recall seeing in AS1684 many years ago that you cannot restrain the wall and ceiling junction. I also recall the use of triple grips and L brackets which allowed the trusses to move up and down when attached to the top plate. I guess what I am looking for is further clarification on clause 8.22 (below in bold) in that the ceiling linings cannot be fixed if they do not permit the vertical movement of the bottom chord of the trusses For example, I have attached a plasterboard manufacturer’s specification where they do not rigidly fix their cornice to the wall which allows ceiling movement/flexing. AS1684 Clause 8.22 For trussed roofs, where nominal fixings are permitted as above, the nominal fixings should permit vertical movement of the trusses. See Table 8.23, Items (a) and (i). Support of trusses. Trusses shall not be supported off internal walls unless the wall and the truss are specifically designed for the purpose. Whilst I understand clause ( as above) I am suggesting that restrained wall and ceiling junctions do transfer loads ( albeit small) as the trusses go through the creep process With this in mind my question is Can I fasten a 42 x 42 DAR pine moulding to the plasterboard at the internal wall and ceiling junctions, effectively not permitting the trusses to move vertically?

The special details shown in AS 1684 and AS 4440 for attaching trusses to internal walls are designed to restrain the bottom chords of trusses from lateral (horizontal) movement, while allowing deflection to take place. Clearance of 10mm is required for this purpose. Note that these connections are only required where a bracing wall is required to be restrained. Where a non-loadbearing wall is stable in its own right no stabilising fixing is required. In answer to your question, a moulding that prevented the trusses from deflecting would not satisfy the requirements of the two relevant Australian Standards.

Q. I am after a simple collar tie design for an open carport roof. The rafters are at 1200 centres, every rafter is tied, spanning 3.5m, carrying a steel clad roof with no ceiling. Is there a simple sizing for the rafters, for the collar ties, & for the bolts between the collar ties & the rafters. I intend to use MGP10 or something similar.

For a roof mass of 10kg/m² (steel roof 0.5mm thick + purlins) a rafter size of 120 x 45 can span up to 3.6m at 1200mm centres in MGP10 grade. The length of the collar ties will depend on the roof pitch and their precise height in the roof space. They will be required on every pair of rafters at a height above the top plate not greater than two-thirds the rise of the roof. Collar ties are to be fixed to the rafters with an M10 bolt, or if they are less than 4.2m long, 3/75 x 3.05mm gun nails can be used. Collar ties are to be 90 x 45 or 120 x 35, but since you are using 120 x 45 rafters you may wish to use 120 x 45 collar ties.

Q. I'm after a timber framing manual that covers three (3) story construction, if you can point me in the right direction that would be great.

A manual dealing with construction details for 3-storey timber-framed multi-residential buildings is available from the Wood Solutions website via this link: The manual is number 2 in the Technical Design Guide series and shows BCA-compliant fire and sound control details. However, it doesn't include framing sizes. Timber sizes, bracing and tie-down details need to be designed by a consulting engineer.

Q. I need to know what size of beam I would need for an opening of 6 metre and 900mm deep in a sandstone wall.

We sometimes give sizes for a simple member such as a floor joist, but when it needs more complex input, as in this case where an assessment of the load on the beam is needed, it goes beyond our service. It’s also necessary to know the type and stress grade of timber you intend to use for the beam. There are some websites that allow people to calculate timber sizes taking all these factors into account, but unless you are familiar with the process it might be best to consult a professional. If you feel confident about tackling it yourself, go to the Wood Solutions website via this link where you will find free software for calculating timber sizes.

Q. I was hoping you could suggest a good timber design textbook I could buy. I'd like it to cover the fundamentals, but preferably also cover new developments (e.g. massive timber).

We assume you are looking for a book on structural design rather than architectural design. Perhaps Standards Australia's "Timber Design Handbook" HB 108-2013 would be useful. The book interprets AS 1720.1-2010, "Timber Structures, Part 1: Design methods" and provides comprehensive guidance to "develop the understanding and confidence to efficiently and effectively design in timber". The 2013 edition reflects the changes in the Australian Building Industry since the release of the last 1988 edition, in particular the focus on timber and wood products as key materials in sustainable construction. If you are looking for a more general reference on the properties of timber, "Wood in Australia" by Keith Bootle is very informative.

Q. For connecting a wooden rafter 140x35 MPG10 to a steel cleat plate with say 2x16mm bolts, what are the hole positions and spacing requirements through the wood?

The Timber Structures Code specifies that end distances for bolts shall be not less than 5D (bolt diameter) in compression joints and joints subject to bending. Edge distance should not be less than 4D.

Q. Can you please tell me the span for 90mm X 45mm Blue Pine and 90mm X 35mm Blue Pine?

We would need more information to answer your question. The allowable span depends on (a) the stress grade of the timber, (b) how much load it is supporting, and (c) the spacing of the timbers, ie. how far apart they are. A common stress grade is MGP10, and the load depends on where the timber is situated in the building (roof rafter, floor joist, wall stud, etc.) If you have all these details you can find the span from one of the software programs that are available on the Wood Solutions website via this link

Q. I want to construct a pergola, open with no roof, between my house and garage. Distance is 5.35m (wall to wall). I was considering using 190 x 45mm F7 MGP10 joists which would sit on a ledger at each end, probably in joist hangers. Separating the joists I would use the same material to prevent the joists bowing, probably notched and toe-nailed to the joists. My question is will the joists sag over such a long length under their own weight. I've looked at structural tables but they don't really apply since my loads are minimal.

Your size of 190 x 45 F7 treated pine will be OK for a 5.35m span at approx 1.2m centres. In assessing the size we have assumed a nominal load of 10kg/m² which allows for the self-weight of the timber, and the timber that the main beams are supporting.

Q. Maximum spacing of bearers if using 90x 45 treated pine joists?

Treated pine 90 x 45 joists of F5 grade, spaced at 450mm centres, can span 1.0m as a single span, or 1.6 m if they are continuous over two spans, ie. 3.2m long and supported each end and in the middle. So the bearer spacing would be 1.0m or 1.6m as applicable.

Q. I have a hardwood timber deck that is free standing around a pool. I have had an engineer look at the structure as I seem to have excessive lateral movement. He has suggested that I use 12mm steel rod and cross brace between the the outer posts and then every second set of posts in between. I am having trouble finding connections to timber for using a 12mm reidbar tension system. Is it possible to use 10mm 6x36 wire rope with an eye nut connected by threaded rod through the top and bottom of each post (100mx100) with a clevis/clevis turnbuckle and d-shackle. I know this will provide tension but will it be suitable for compression as well if cross braced? It is also on sloping land so the connections are between posts at different levels. Or is it instead better to use a reid brace system, but I am having trouble finding a suitable cleat connection I can attach to the posts for the reid brace to attach to.

A steel cable won't provide any compression bracing but the principle of cross-bracing is that whichever way the load is applied, one of the braces goes into tension. So a single cable wouldn't do the job, but if they are crossed they should be OK. However, being unfamiliar with the details of your job we strongly recommend that you run this past your engineer. Since you are proposing something different from the engineer's recommendation it would be a good idea to get his approval.


Did you know?

Australia’s native forests, timber plantations and wood products are net absorbers of greenhouse gases, sequestering 56.5 million tonnes of carbon dioxide in 2005, reducing Australia’s total greenhouse gas emissions by nearly 10%.