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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 am looking for a timber supplier that supplies sawn seasoned structural hardwood with a joint group of JD3 or better. Could you please advise me of any suppliers (near NSW) that may supply sections around 330x85?

That's a difficult size to supply because kiln-drying is generally limited to a maximum thickness of 45mm. It's not impossible to dry heavier sections, but it takes longer and they are more prone to splitting. However, a size close to the one you want could be made up by nail-laminating 2/290 x 45 to make a 290 x 90. Boral produces blackbutt in kiln-dried sizes up to 290 x 45 and the Joint Group is JD2. Otherwise, the most likely source of a 330 x 85 section is from recycled timber.

Q. Could you tell me the timber dimensions required for trusses to clear span 12m with 5m bays mounted on 4.5m hardwood posts.

We don't usually get involved in structural design. It requires detailed knowledge of the proposed structure, including type of timber, loads supported by the trusses, and whether they are nailplated trusses or bolted trusses. Site factors also have to be taken into account, such as the wind load on the building. If they are to be nailplated trusses a truss company should be able to help with the design as long as you can provide the above details. If you are looking for bolted trusses, you will need to engage a consulting engineer.

Q. I have a number of floor joists from a demolished house which was 30 years old that I wish to use in a new dwelling. The sizes are 165x50 and 145x50, very heavy hardwood. The engineer on the project has classed them as F14 and has severely over engineered the flooring system without looking at the timber. How can I get the timber graded?

If you suspect the floor system in your new house has been over-engineered you may wish to check by referring to the span tables for recycled timber, available via this link: http://www.woodsolutions.com.au/Articles/Resources/Recycled-Hardwood-span-tables. Note that you will need to know the species of timber to select the correct tables, but if the engineer has allocated a stress grade presumably the species of timber is known. If you feel the engineer has under-estimated the strength of the timber, and perhaps taken a conservative approach without seeing it, you might want to engage someone who is experienced in grading timber. Various people provide this service depending which State you are in. In Victoria you could contact John Hay on Mob. 0407 043 185. In New South Wales State Forests no longer provides an inspection service, but their former inspectors have formed a private company and you can access their details via this link: http://www.timberinspection.com.au/. In South Australia you can contact a company called Timber Consulting Pty. Ltd. on (08) 8373 5285. To find out the cost you will need to contact the relevant grading service.

Q. I am inspecting an old warehouse that has some timber columns and trusses. We have found that the timber columns have rotted and deteriorated at the base, and I was wondering if there were any products available to strengthen them. I am looking for something that could be injected into the timber to make the columns stronger. If there are no products like this, are there any collars that can be wrapped around the timber columns? Also would you be able to identify the type of timber used in this shed or the species? I have attached a few photos of the site. More photos can be provided if required. I hope you can help me with this information! Thank you!

It's sometimes possible to repair timber structures using steel or fibreglass rods bound into the remaining wood with epoxy resin. This is generally a procedure for heritage buildings where the original building fabric needs to be preserved, since the cost and complexity of the technique can exceed the cost of installing a new timber member. It's also best carried out by a contractor with some experience. You will find more information on the net about this technique if you write "epoxy repair of structural timber" in your browser. Regarding identification of the timber, it's very difficult from a photograph, but a positive identification can be made by examining a sample microscopically. Each species has a distinctive cell structure and specialists in this field use such characteristics to identify different species. A noted expert is Dr. Jugo Ilic whose consultancy is called Know Your Wood. For details of fees and required sample size send an email to knowyourwood1@gmail.com.

Q. We are considering the use of a glulam structural frame in an industrial building storing salt at high temperatures. We reckon the temperature in the roof space will be 60 deg C. As it is salt, it is corrosive so wood offers an advantage over steel. However, suppliers have advised at these temperatures the glue in the glulam is a problem.

We were surprised that concerns had been raised about the ability of glulam to withstand heat. It is not uncommon for large-section glulam to be sandblasted and re-used after a building fire, where temperatures would be very much higher than 60°C. Examples of this can be found on the net. Block shear tests on gluelines after fires in the US have not shown any significant loss in glueline strength. However, you should really put the question to the glulam manufacturers you are dealing with, and ask why they are concerned, ie. what evidence do they have of a problem when glulam is exposed to heat. Perhaps there are concerns relating specifically to the adhesive in question. Casein-glued joints are likely to delaminate when exposed to fire, but we would assume that structural glulam from an Australian producer would be fabricated with a resorcinol-type adhesive, which is heat-resistant.

Q. What size hardwood beam would take a 6m span as a roof purlin?

The term "roof purlin" normally means the member that is supported by the rafters, and in turn supports the roof cladding. It would be unusual for purlins to span 6m because that would mean the rafters were 6m apart. We would also need details such as the type of roof cladding (metal or tiles), and the type and grade of hardwood, before we could answer your question. When you have these details you are welcome to visit our site again, or you may be able to obtain the answer from the span tables and software programs available at this location: http://www.woodsolutions.com.au/Articles/Resources/span-tables-software-resources.

Q. I would like to use LVL for house frame work instead of natural beams. Would you be able to provide some info for interchanging between them?

To design a house frame in LVL you will need to refer to one of the free software programs available on the net at this address: http://www.woodsolutions.com.au/Articles/Resources/span-tables-software-resources. For example, Carter Holt Harvey's DesignIT program allows the user to select wall frame and roof frame member sizes in a range of timber products, including LVL.

Q. What is the definition of structural timber? I believe products like timber cladding, decking, linings, flooring are classed as 'non-structural timber' unless they are a primary building element used as a brace - and these are calculated into the design. Is this correct, and is there a definition in the building code of 'structural' and 'non-structural' timber?

The Building Code of Australia (BCA) defines a structural member as follows: "Structural member means a component or part of an assembly which provides vertical or lateral support to a building or structure". There is no definition of structural timber in the Residential Timber-Framed Construction Code, but there is a requirement that "All structural timber used in conjunction with this Standard [AS 1684] shall be stress-graded in accordance with the relevant Australian Standard". It could be inferred that products that are not required to be stress-graded, such as the ones you mention, are therefore non-structural.

Q. Looking for advice whether 19mm thick decking, cladding, flooring for residential use is classed as 'structural' or 'non-structural' timber.

We consider that decking and flooring are structural products since they span between supports and carry loads. They don't normally go through a stress grading process, but recommendations on spanning capacity are based on the strength and stiffness associated with their visual grade. Generally we would not consider timber cladding to have any significant structural role, unless it forms part of the external wall bracing system.

Q. Please HELP – where do I find Span tables for roof battens for Metal AND TILED roofs – larger than std 900 crs?

Table 32 in the supplements to Australian Standard 1684, "Residential timber-framed construction", gives allowable spans for roof battens at spacings up to 1200mm. The specific supplement you need depends on the stress grade of the timber. For example if the roof in question is on a site with an N1/N2 wind classification, and the battens are MGP10 pine, 45 x 90mm battens spaced at 1200mm are suitable for a sheet roof on rafters at the usual 1200mm spacing. Note that the battens are assumed to be installed "on flat". For a tiled roof, hardwood battens are more usual, and again sizes and spans can be sourced from Table 32 of the relevant supplement.

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Did you know?

A government report showed there is no evidence proving that harvesting timber from native forests has reduced overall forest biodiversity or led to the extinction of any species of plant or animal.