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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 designing a Heavy Duty Platform, and require some technical information on say, 1 square metre laminated panels approx. 50mm thick. Weight? Cost? Strength? Durability? Number required = 2-3,000 Regards Mike McDonnell

The properties of the laminated panels will depend on the type of timber they are made from. You can select from a wide range of timber species including radiata pine, cypress pine, brush box and jarrah to name a few. Your best way forward would be to contact a laminated timber producer and explain your requirements, eg. will the panels be weather-exposed, what loads are they to support, will they be subject to wheeled traffic, etc. A good starting point would be the website of the Glued Laminated Timber Association of Australia (GLTAA) at this address: www.gltaa.com.

Q. I am a structural engineer and have been requested to review the capacity of an existing and quite old timber framed floor for a change in use from an office space to a function room / dance floor. I am looking for an expert timber grader who can visually inspect the timber and advise on either a suitable F Grade, taking into account the floor's age, or provide a visual grading to the interim industry standard for grading recycled timber. Do you know of any suitable professionals in the Brisbane area who provide such services? Thanks for your assistance Rob Knight

We don't know of any graders in Queensland although it's likely that there are some qualified people around. Perhaps Timber Queensland can put you on the right track. Their phone number is (07) 3254 1989. Failing that NSW State Forests no longer provide 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/.

Q. Situation:- Previous owner widened part of the existing deck and roof area (deck No problems) Roofing support beam is sagging too small timber for new span (used 125x38 cca pine to span 4.8 mtrs) How big should the beam be to support steel sheet roof over this distance and is there any way to remedy this situation without replacing existing beam or adding a support post Deck area is now approx 4.8 x 4.8 mtrs roof over hangs approx 600 on sides

We assume the beam in question is what we would call a verandah beam or lintel beam, spanning 4.8m and supporting rafters which also span 4.8m. In this situation a 125 x 38 pine beam is way undersize and actually dangerous if someone climbed on the roof, since it might not support the weight of a person. No wonder it's sagging! The size required in F5 grade CCA-treated pine is 240 x 45. Putting another 125 x 35 alongside won't be enough. If you don't want to replace the beam maybe you could install a steel beam behind the timber to maintain the timber look - otherwise it's really a replacement job.

Q. I have a 4810 opening to a garage can you please advise as to the legal size beam required originally I was using a 240x45 F17 but I have been told this is not suitable what should I be using (not load bearing).

If the beam is not supporting any load at all, then technically there is no "legal size", since it doesn't come within the scope of the Building Code of Australia (BCA). You can use any size that's capable of supporting its own weight without sagging. For a span of 4.8m we consider that an F17 240x45 would be quite suitable, assuming it's KD hardwood.

Q. I was wondering if you have any guides on the use of timber in enclosed public swimming pools? We've got a project we'd like to use timber on but aren't able to find much specific guidance on durability requirements with respect to both the timber and the fixings.

An up-to-date guide on the use of timber in swimming pool enclosures has been published by TRADA in the UK and is available free to TRADA members, or for £12 to non-members. The guide can be downloaded via this link: http://www.trada.co.uk/news/view/DAA88198-DFF3-49E5-98F4-CC8A40D7F6C4/. The TRADA Guide makes the point that "any potential risk to timber does not come from the pool environment but from condensation and/or gross wetting." It is therefore critical to avoid condensation forming on (for example) the underside of the roof and dripping back onto the timber. Similarly, it is critical to avoid column bases coming into contact with splasing from the pool, by setting them up from the floor. With regard to fixings, stainless steel is the usual choice. However, there have been some failures due to corrosion and this topic is explored in more detail in another UK guide titled "Stainless Steel in Swimming Pool Buildings". It can be downloaded free of charge via this link: http://www.nickelinstitute.org/~/Media/Files/TechnicalLiterature/StainlessSteelinSwimmingPoolBuidlings_12010_.pdf.

Q. I am a product developer designing a new kid's toy. One of the components used I would like to make from MDF. My current prototypes work well with MDF but I am concerned about the long-term creep properties exhibited by MDF.

There is a useful discussion of creep in particleboard and MDF in a manual titled "Particleboard & MDF Structural Shelving Design Manual" published by the Engineered Wood Products Association of Australasia (EWPAA). Refer to the section headed "Long Term Effects".

Q. Will 25mm grey ironbark laid onto 400 mm joists meet the load requirements 5KPa and 13 Kn? This allows for light vehicular access.

We take it the joists are 400mm apart. The ironbark decking will probably be unseasoned, in which case it would need to be a bit more substantial. According to our information, unseasoned hardwood of F17 grade or better can span up to 481mm in a size of 120 x 45 under a uniformly distributed load of 5kPa and a point load of 13kN as appropriate for light vehicular traffic.

Q. What size timber and spacing is required for a 5m span supporting a 5 degree sloped iron sheet roof, ceiling, and allowance for a skylight which is basically a 1200mm window. Timber preferably oregon. NSW Non-cyclonic area. Alternative: What difference would it make to add a crossing beam 2.4m from the exterior wall which would span 4.3m perpendicular to the ceiling beams, if required to add the crossing beam can you please provide a size for that as well.

Assuming the oregon is F7 grade (your supplier can advise) you would need a size of 275 x 50 to span 5.0m at 1200mm centres. This allows for a load of 30kg/m² which includes the weight of sheet roofing 0.75mm thick, 13mm plasterboard ceiling, battens and insulation. If you had an intermediate beam, also in F7 oregon, it would need to be 275 x 75, spanning 4.3m, but your rafter size would reduce to 100 x 50.

Q. I would like to know the size of timber beam to span 5 meters between posts of my pergola I would like to build. It would have 3 rafters, length would be 1.8m attached to the beam with some shade battens also.

Assuming you are going to use treated pine, the usual stress grade is F5 and a suitable beam size would be 240 x 45, or two 190 x 35 nailed or bolted together to form a single beam.

Q. We are renovating a kitchen that was the verandah on a 1924 weatherboard house. The kitchen was very poorly constructed and most of the walls will have to be reinforced. In the middle of the 5m square room is an original verandah post, which we want to remove by spanning a beam right across the 5m. We could put in two beams, but one will have to go where the verandah post is, as the roof has two different pitches which meet there. Advice so far has been confusing and conflicting! Can you advise us on what type and size of timber would be suitable? Or - could we use C-section steel?

We don't usually provide structural sizes as it's hard to be sure we have all the relevant details. For example to determine the size of the new beam one needs to know how much load is contributed by the two roof pitches. This information is required for structural design purposes whether you use timber or steel. If you decide on timber, laminated veneer lumber (LVL) would be an appropriate material, or if the beam will be exposed to view, a glue laminated beam ("glulam") will have a more decorative appearance. Various timber organisations provide software that allows you to calculate beam sizes - for more information go to the Wood Solutions website via this link: http://www.woodsolutions.com.au/Resources/span-tables-software-resources.  However, in your case it sounds a bit complicated and it might be a good idea to talk to a consulting engineer.

WoodSolutions

Did you know?

About 16% of Australia’s 147 million hectares of native forests are in nature conservation reserves.