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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. The plan specifies 200 x 65 Hyne LGL loaded joist but I want to use hardwood. What would the equivalent be?

Hyne's LGL is produced in a single structural grade with the notation STR, according to their website. Australian/New Zealand Standard 1328.2 "Glue laminated structural timber" provides for a range of GL grades, which allows glue laminated products from different manufacturers to be interchangeable. Apparently Hyne have chosen not to use the GL grading system, but to use the provision in the Standard which allows manufacturers to declare their own grade. Since we don't know the properties of Hyne's STR grade it's not possible to nominate an equivalent product. However, various software programs are available on the net to calculate beam sizes and you can find the major ones on the Wood Solutions website if you paste this link into your browser -

Q. If timber wall framing is within a kilometre of coastal area, do the nails used on joining framing have to be galvanised, as the framing is going to be enclosed but until roof is on it has some exposure to the salt & rain.

The "coastal zone" is generally defined as an area within 1 km of the coast line, while structures within 10 km of breaking surf may require special corrosion protection. However, these requirements relate primarily to exposed steel such as roofing, or structural members exposed to the elements as might occur under an elevated house. AS 1684.1 "Residential timber-framed construction" simply states that "the level of corrosion protection provided shall take into consideration weather exposure, timber treatment, moisture and presence of salt". In our view nails in internal framing well back from the sea front would not need to be galvanised if a sheet roof, or sarked tiled roof is applied. Closer to the coast, and/or in buildings that are not well sealed, galvanised nails may be warranted.

Q. I am a structural engineer providing advice to a timber landscape feature for the town of Cardwell, QLD. The feature involves mounting 6 x timber piles (1.6 to 3m long) that were originally part of a jetty constructed in 1872. The buried piles were 'found' during recent construction works in Cardwell and are intended to be mounted as a landscape feature. We understand that they are either Turpentine or Yellow Stringy-bark (or a mixture of both). The mounting concept seeks to show a maximum possible surface area of timber hence a desire to minimise the structural fixings (to steel or stainless steel posts). We are looking at an option of adhesively bonding posts to holes drilled into the timber. I am looking to explore the feasibility of this option. Is there any data available to assist the selection of a suitable structural adhesive for bonding steel to timber (eg epoxy?)? Are the timber species in question suitable for such a connection? I have found the technical timber data on the website for these two timber species. It notes that Turpentine is 'difficult to glue' whereas polyurethane glues are suitable for bonding yellow stringy-bark. I presume these comments relate to timber-timber gluing and not to steel-timber bonds.

The "difficult to glue" comment about turpentine wood relates to timber-to-timber gluing. We have no experience in gluing timber to steel, but it seems 2-part epoxies are generally used. However, some caution would be needed regarding the moisture resistance of the glue bond if the structure is exposed to the weather. The difficulty in gluing turpentine arises from its high density and possibly extractives in the wood. These issues are usually dealt with by (a) machining the surface to be glued with a sharp blade to avoid local crushing of wood cells, immediately before gluing if possible, and (b) wiping the surface with a solvent to remove any oils. However, we emphasise that we have no expertise on the many types of adhesive available, and suggest you seek further advice from an adhesive manufacturer.

Q. The ridge beam is only supported at each end. I believe it is safe to assume that the ridge beam was unseasoned when the house was built. Given the house was built about 40 years ago, would it be reasonable to assume that the timber would have dried to 15% or less or are there too many unknowns to reach this conclusion? Finally, would a Colorbond roof with pine lining and insulation have a roof load of approximately 20 kg/m2?

In a normal indoor environment the moisture content of timber is in the range 9-12% so we can consider it to be "seasoned". AS 1170.1 (Structural Design Actions) gives some generic weights of materials and corrugated sheet roofing of 0.8 mm thickness is rated at 10 kg/m². It's hard to know exactly what the weight of corrugated roofing was 40 years ago, but if you are considering today's Colorbond it's lighter and not more than 6.3 kg/m². Pine ceiling lining 12 mm thick would weigh 6.6 kg/m², based on a design density of 550 kg/m³. So by the time you add in insulation, purlins and rafters, 20 kg/m² is around the mark.

Q. Could you please advise the maximum span of an Oregon F7 ridge beam having the following dimensions: 250 mm x 75 mm? We have a cathedral roof, sheet roofing and a roof load width of approximately 4,000 mm. The span tables I have seen for Oregon ridge beams are only available up a beam width of 42 mm.

Assuming the oregon is unseasoned at the time the roof load is applied, and supported only at each end, an F7 grade 250 x 75 ridge beam could span 3.7 m, supporting a roof load of 20 kg/m². There's an improvement if the timber is dried to a moisture content of 15% or less - the span is then 4.2 m.

Q. I have cracking in old hardwood recycled timber perimeter floor beams. The beams ring the entry verandah. Cracking is full length of the beams on the top face approximately 30 to 40mm inside the external edge. Size of cracks is 3mm to 18mm. Filling is not effective. It is very unsightly. Is this defective? Where is the standard reference for me to look at what is acceptable size and shape of cracking in timber?

We weren't sure if the beams in question developed cracks under load, or whether they had cracks when they were installed, ie. whether it was a structural issue or an appearance issue. There is an industry standard available that provides guidance on grading recycled timber, but it deals with grading for strength ("stress grading") rather than grading for appearance. A copy can be downloaded free of charge - to find it, write FWPA Project Number PN06.1039 in your browser.

Q. Good morning. I am in the process of buying Douglas Fir timber from the US. I am having trouble trying to establish the equivalent US grade for our Australian standard F7 grade. Would you be able to assist me???

Sounds like a simple question, but the answer is a bit complicated. If you are buying timber according to the Export R List, a widely used reference, the grades listed - eg. No. 4 Clear (D Clears) - are based on appearance, not stress grade. The sizes are intended for resawing into smaller sizes, in which case it's not possible to apply a stress grade. While timber can be visually stress graded, the process must be carried out on the finished size. It's not only the size of knots that affects the stress grade, it's also their position, so a knot that might have been in the centre of the piece could finish up on the edge after resawing, where it has a greater effect on strength. If you can let us know which grading rules your US supplier is using, whether you are importing large sections for resawing or finished sizes ("dimension lumber"), whether the timber will be green or kiln-dried, and whether the timber will be stress-graded to US standards (or stress-rated as it's termed in the US), we may be able to advise you further.

Q. Could you please let me know if an F7 Oregon beam measuring 240mm x 70mm can span 4500m when used as a ridge beam supporting a cathedral ceiling/roof made of colorbond and pine lining?

The size required for a ridge beam depends on the "roof load width" (ie. the load that the ridge supports) as well as the distance between points of support. A ridge beam of 240 x 70 spanning 4500mm could only support a roof load width of 1500mm. This is measured as the length of the rafters either side of the ridge, divided by two. Each rafter could therefore only have a span of 1500mm which is fairly short. Even a ridge beam of 300 x 75 would only allow a roof load width (or rafter span) of 3000mm. So we feel you might be better off using a glue laminated beam, which has greater spanning capacity size for size than unseasoned oregon.

Q. I am planning to install a narrow mezzanine which will provide walkway access to some existing unused roof space. Other than being used as a walkway, there will be no weight added to the mezzanine. The supporting beam of the mezzanine will span 2.8 metres and will be fixed to walls at each end. The mezzanine will be just 880 mm wide (ie, from front to back) and its flooring joists will be supported at the back by a single timber cleat that will be attached to an existing wall. I am keen to use either a single hardwood (KD) beam or an LVL beam, but cannot find a suitable span table to establish the ideal dimensions - could you suggest suitable dimensions for the beam?

We don't usually provide structural sizes as it's hard to be sure we have all the relevant details. For example to establish the joist sizes we would need to know their centre-to-centre spacing. However, various timber organisations provide software that allows you to calculate beam sizes. It sounds as if the member you describe as the "supporting beam" would be classed as a floor bearer, while the joists will be short lengths spanning 880 mm. This would give a floor load width of 440 mm. For more information go to the Wood Solutions website via this link

Q. I need to calculate the capacity of a bolted timber member to steel cleat connection. The load is acting perpendicular to the grain and bolt is in single shear. 1720.1 does not specifically address this type of timber to steel connection, but Table 4.10A would suggest that b effective equals twice the timber thickness and Qskp = Qkp. Would it be correct to adopt these values in this case?

The answer is “yes”, since the timber-to-steel connection is still a two member joint, even though one member is steel. The effect on the timber member is essentially the same. However, it would be helpful if AS 1720.1 specifically addressed this situation as it's not uncommon.


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%.