Structural timbers & engineering

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Question: Where do I get the requirements/formulas for calculating wind loads and subsequent timber/fittings sizes/specs to put on a plan for a 2 storey timber house with timber floor and cladding on stumps? How do I show them on the plan to be legal?

Answer: There are two steps required. First, someone needs to allocate a "wind classification" to your building site. This can be done in accordance with Australian Standard 4055-2006, Wind loads for housing. In some parts of Australia local Councils have mapped the wind classifications, while in other parts individual site assessment is needed. If your site does not fall within a mapped area the engineer who does your footing design might be able to work out the wind classification for you. Once that is done, you can refer to the relevant part of Australian Standard 1684-2010, Residential timber-framed construction to find out what tie-down, bracing and timber sizes are required. You will find that the Timber Solutions software helps in this process. It can be downloaded from www.timber.org.au.


Question: Hi, I'm an architect and I'm from Brazil. I'm really interested in how to work with timber. In Brazil it's not common to use timber in civil construction, but here you guys are really good at that. Where can I learn about timber construction?

Answer: We have a series of one day forums coming up that might interest you. They are aimed at building design professionals and we are running them under the name WoodSolutions. You can find out more at www.woodsolutions.com.au


Question: Hi, Currently designing an aquatic centre. A CSR representative has recommended a 140mm high treated timber packer over our Z-purlins. What type of pine (H6?) is suitable for this chemical laden environment? Should we go with a hardwood?

Answer: We suggest you avoid preservatives containing copper (CCA and ACQ) because of the possibility of enhanced corrosion of the Z purlins. This can occur if traces of copper leach out of the wood. We assume ventilation will prevent a build-up of condensation, in which case there should not be a high risk of decay. Nevertheless, it would be advisable to use preservative treated softwood or a naturally durable hardwood. If you choose treated softwood, we suggest LOSP as the preservative. The highest Hazard Class for LOSP is H3, suitable for outdoors above ground. If you consider condensation will maintain a higher moisture level than in a weather-exposed situation outdoors above ground, then a naturally durable hardwood would be a better choice. However, hardwoods in a moisture-laden environment are prone to leaching tannin. Your best option is therefore to control condensation and use LOSP-treated softwood. Chemical attack should not be a problem. Assuming the water is chlorinated, pH in a pool is generally between 7 and 8, i.e. only slightly alkaline. We have no special knowledge of pool chemicals, so if pH is likely to be outside this range, or other chemicals are involved, feel free to seek further advice.


Question: Could someone please advise me in regards to what timber we can specify over copper roof sheeting for an air conditioning platform?

Answer: Treated pine is your best choice. The corrosive effect of copper-based preservatives such as CCA and ACQ arises from contact between dissimilar metals, which occurs when traces of copper leach from the timber and come into contact with metal fasteners. Obviously there is no problem if copper leaches from the timber onto copper roofing. However, it will be necessary to make sure the air conditioning unit itself is not in direct contact with the treated pine platform, eg. by seating it on damp-course material or similar. Note that it is recommended to treat cut ends, holes and notches in treated pine with a sealer, in case the treatment zone is breached. A suitable product is Ecoseal Green, manufactured by Thomson White Australia in a handy aerosol, but there may be others.


Question: I am building a Granny flat for my mother out of Baltic Pine & European Spruce. My engineer needs to know what the stress grade of the timber is, is it Durability class 2 and treated to H3. It's all double Dutch to me. Something about BCA , help appreciated!

Answer: It can all be a bit confusing. Let's start with the stress grade. Your engineer needs to know this if he is going to advise you what sizes to use because the stress grade will tell him the strength properties of the timber. Grading timber is like grading any other natural product - it involves sorting out the good from the bad. If the timber has been graded its grade should be marked on each piece. If it hasn't been graded, someone will have to do it for you. Baltic pine and spruce fall into Durability Class 4, which means they are not suitable for permanent exposure to the weather unless preservative treated. The term "H3" indicates the level to which the timber has been treated - H3 is suitable for use outdoors, above-ground. If your timber has been treated to H3 it should be marked on the timber, like the stress grade, or certified by some documentation from the supplier. However, if the timber is only to be used inside the structure then its durability and/or preservative treatment would seem irrelevant.


Question: Is Glulam a suitable structural material to use in a climate where annual temperatures vary between 20-40 degrees C? If so, would this extremely hot, dry climate have adverse effects on the quality, structural properties or life-span of the timber?

Answer: We were not sure whether the glulam was going to be used indoors or outdoors. There are a number of issues that apply to using glulam outdoors, but we will assume that this is an indoor application. If so, it seems unlikely that the temperature will reach 40°C otherwise it would be uncomfortable for the occupants. In any case, it is the interaction between temperature and moisture content that is important rather than temperature alone. Relatively high temperature, combined with high moisture levels in the wood, is more significant, such as might be encountered in tropical climates or in some industrial buildings. Accordingly the Timber Structures Code (AS 1720.1) states that "for covered timber structures under ambient conditions, no modification for strength need be made for the effect of temperature....except that where seasoned timber is used in structures erected in coastal regions of Queensland north of latitude 25°S, and all other regions of Australia north of latitude 16°S, the strength shall be modified by a factor of 0.9".


Question: How long can house frames be exposed to the weather because my house structural pine timber frame has been exposed to the rain and sun for over 4 months?

Answer: It's hard to put a figure on it because any deterioration in the timber will depend on how severely exposed it is, how much rain has fallen and so on. However, the time would be measured in years rather than months before any significant problems would be expected. Mould growth may appear on the surface of the wood in the short term, but this has no effect on its strength. Wood rot takes much longer to become established. Nevertheless, it's a good idea to get the roof on as soon as possible (if it isn't already on). If you live in a wet part of Australia the timber must also be allowed to dry out before wall linings are installed.


Question: I am considering buying F27 keruing timber to construct a pergola, can you please tell me if this timber is suitable.

Answer: According to Australian Standard 5604, Timber - Natural durability ratings, keruing is rated Class 3 Durability outdoors, above ground. If the pergola is fully exposed to the weather (i.e. has no roof) we would prefer to see a timber of Class 1 or 2 Durability used, or a preservative treated timber. If the pergola has a roof, and water is not likely to enter joints, then any timber with the required strength will be suitable.


Question: In addition to my last Question the rafter span is 4-7m and I intend to use 170x35 F27 keruing members at 800 spacings. I realise that you can't provide designs etc., but for costing purposes does this seem about right?

F27 grade keruing of 170 x 35 is OK for a clear rafter span of up to 7 m at 800 mm spacing, assuming only a light roof load of 5 kg/m². This would allow for shade cloth, polycarbonate, or other lightweight claddings, but not metal deck roofing. We have assumed the keruing is kiln dried (seasoned). While 170 x 35 is structurally adequate, we would recommend 170 x 45 if available - it's easier to nail into if you are fixing roof covering from the top, and also likely to be more stable in service. We also assume the timber has been correctly graded, as this is a relatively high stress grade.


Question: Designer has specified 140x45 MGP 10 Treated Pine Joists for a pergola. All timber yards I have contacted do not stock Treated MGP, Theirs is rated as F7 is this the same or similar structural rating? Is Treated Pine available as MGP 10 ? Thanks

Answer: There is some treated MGP10 material in the marketplace, but F5 and F7 are far more commonly available grades. MGP10 and F7 are similar, but not exactly equivalent. Generally, MGP10 material will achieve slightly greater spans in a given size. For example, 140 x 45 MGP10 spaced at 1200 mm centres will span 4.4 m with a 10 kg/sqm roof load, whereas 140 x 45 F7 will span 4.1 m under the same conditions. It might be best to ask your designer to re-check the spans according to the F7 tables.


Question: Q 2. Same hall same timber poles as Q 1 - specified 'red ironbark narrow leaf. Certifier is concearned with durability of structure long term from moisture in concrete...is there any information on strength/suitability without H treatment? thanks.

Answer: I think we addressed most of your issues in the previous answer. However, just to add the point that the "H" classes of preservative treatment are achieved by pressure impregnation. Some softwoods, such as radiata pine, are relatively permeable and/or have a wide sapwood band which is easily treated. Pinus species have low natural durability in general, and therefore cannot be used in hazardous situations without treatment. The heartwood of ironbark is too dense to pressure treat effectively. In any case ironbark has a high level of natural durability and is therefore not reliant on treatment when used outdoors or in the ground.


Question: Hi, Good day. I am currently writing a structural report to strengthen the existing glu-lam rafters in a local church in Darwin. I have not much experience on how to strengthen the roof rafters with min disruption. Could you please advise me?

Answer: We cannot provide structural engineering advice on this website, but we can give you some general guidelines. If the rafters need strengthening because they were under-designed, perhaps another rafter could be installed alongside, or bolted to, the existing rafters. If the rafters need strengthening because they have been damaged in some way (eg. fungal decay or insect attack) it is essential to deal with the cause of the problem before attempting remedial work. For example, the source of the moisture that caused decay must be traced, or the insect infestation must be treated. Once that has been done, the remedial work required will depend on the extent of the damage. Typical repair methods include reinforcement with steel plates or brackets, epoxy injection, and so on. If the member is extensively damaged, complete replacement may be necessary. Further information about these repair techniques can be found on the net.


Question: I'm a student, and currently we are designing a surf life saving clubhouse at South Melbourne beach, using sustainable design principles. What would be the best structural timber to use, given the club is on the beach, touching ground?

Answer: Wood products perform well in a marine environment, which is why many seaside structures are traditionally built from timber. Think about jetties, retaining walls and steps down to the beach - not to mention boats! Although the marine environment doesn't present any additional hazard for timber, it still needs protection against moisture. For a structure right on the beach, we suggest either preservative treated pine or a durable hardwood. In this context "durable" means resistant to wood rot and insect attack. Commonly used timbers have been rated according to their durability and are listed in Australian Standard 5604, Timber - Natural durability ratings. You will need to specify fasteners with care, since salt is very aggressive to metals. Stainless steel is the only practical choice in this situation.


Question: Adelaide has been very wet for the last two months. My first floor timber frames were constructed in mid July. It will probably be some time before the roof goes on because of the nature of my build. Should I be concerned about the framing timbers?

Answer: Pine house framing is not usually preservative treated since it is not exposed to the risk of wood rot under normal circumstances, once the building is enclosed. While it is not advisable for untreated pine to be permanently damp, even without treatment it can stand up to weather exposure for a year or two with no risk of decay. However, it has probably absorbed a significant amount of water from recent heavy rain. This means the internal walls should not be lined until the timber has had time to dry - preferably for 2-3 weeks, depending on the weather.

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