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Ask an Expert

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'm hoping you can help me... what are the differences between a timber portal frame and a timber column and truss frame? I can't find any information for timber column and truss frames.

Portal frames are constructed as one structural unit with a rigid joint between columns and roof beams. Timber columns supporting roof trusses may not have a rigid conection between the two, in which case stresses will not be transferred from the roof trusses to the columns in the same way as in a portal frame. The resulting design may then be less economical than a portal frame and/or less able to achieve wide spans.

Q. I am trying to track down a copy of the National Association of Forestry Industries (NAFI) timber manuals. Specifically I am after the diaphragms and shear walls part in the timber data file series. There are links to some of the NAFI manuals on this site to do with cladding. Do you have links to all the documents?

The NAFI Timber Manual was published in 1989 as a 2-volume set. Some of the datafiles were revised in 2005, but the datafile on timber shearwalls and diaphragms was not updated and as far as we are aware is not available on the net, although it may be held in some libraries. Hard copies of the manual are still available for sale at $120 including postage within Australia, but some datafiles are now a little out of date.

Q. I’m a civil engineering student of Royal Melbourne Institute of Technology. I’m doing a research project which examines the design procedure for use of cross laminated timber in tall buildings, identify limitations and develop a design process. Contribution to knowledge would be a critical analysis of the design process and any possible improvements such as use of composite elements to address issues where a design criterion fails. Though I’ve gained some information, I still cannot establish the design procedures, as your know, there is not many CLT related articles, most of all, there is no standard for CLT in Australia. So I was wondering, if it is possible, as the leading company of the industry, could you please inspire me a little with how to establish the design procedures. Thank you very much.

Cross-laminated timber (CLT) is made by several different companies so its mechanical properties are provided by each manufacturer on a proprietary basis, depending on production techniques and the species of timber used. A CLT design guide is available from the Wood Solutions website at (refer Guide #16). The Guide deals mainly with the physical properties of the material rather than its structural properties, but might be helpful for your project. The Scandinavian company Stora Enso is a major producer of CLT and you might be able to obtain some more information from their Australian branch in Victoria.

Q. We want a bridge 700 wide with a 50 mm over hang on each side and 50 mm timber used the span in between the timbers would be under the 500. Looking at the 19mm spotted gum as decking. We were going to use the same timber as the bearers to make a H shape but with 2 extra cross beams. Then just drill and screw the decking to it.

Treated pine would be fine for the bearers spanning 4.0m, and we recommend a size of 190 x 45 in F7 grade material, dry after treatment, ie. seasoned timber.

Q. We are building a flat walk bridge over a pond in the yard. the span is 4meters. Should I be looking for hardwood or treated timber for the bearers ? We are using a spotted gum decking to run across the 2 bearers. Question is what size timber do I need for the bearers in hardwood or treated ? The old timbers were 70 x 100 but as it was so rotten I could not tell what type it was.

A small bridge in a private garden can be designed like a timber deck. You don't mention how wide the bridge is, but if the decking spans between the two bearers they could only be 500mm apart because that's the recommended span for 19mm hardwood decking. That's a pretty narrow bridge, so maybe you are using thicker decking that will span further. Otherwise it might be better to place joists between the bearers at 500mm centres and run the decking along the length of the bridge. If you would like to provide some more details, eg. how wide is the bridge, we can advise you further.

Q. We would like to use Glulam beams for an outside project where we are replacing stacked timber sleepers as the carriage for a 71cwt (3,607kg) cannon. Can you advise if this is suitable with the beams to be H3 hazard level and sizes 150 wide x 200 depth by approx. 2 metres length? To either side of the cannon.

Suitable beams include the cypress pine Durabeam which is rated Durability Class 1 outdoors above ground. Preservative treatment is not needed, but it will retain a better appearance if a suitable finish is applied. Laminated beams made from other naturally durable species are also available and you will find more information on the Glued Laminated Timber Association of Australia (GLTAA) website at We suggest you seek engineering advice regarding the required size, since this is beyond the scope of our service. Your engineer will need details showing how the beams are to be supported, eg. will they sit flat on a concrete slab, or be suspended above ground. The latter would be preferred to allow air circulation.

Q. Could you explain the difference between Stocklam and Glulam timber?

Glulam is a generic name, short for "glued laminated timber". However, manufacturers tend to use brand names to give them some product differentiation. We are not familar with the Stocklam product but suspect it is one company's version of glulam. In Australia there are various glulam products identified by brand names, eg. Tilling Timber's SmartLam, LTI's Durabeam, etc.

Q. When you are building a home and the timber required is F14 - does the timber have to be stamped or a certificate provided or what has to happen if you want to use timber off your property?

Australian Standard 1684, "Residential timber-framed construction", requires timber to have its stress grade "identified" but doesn't specify how this is to be done. It can be stamped, colour coded, or identified by a written certificate. If you wish to use timber from your own property it will be necessary to have it stress graded. This can be done by visual inspection in accordance with the relevant grading rules. There are various people in different parts of Australia who are experienced in grading timber.

Q. Repairing under decking. Have a span of 5.2mt to put new beam across. Need a beam of 300mm x50mm to match old layout. Can it be supported by 2 posts only & what timber would it require. Decking 5mt x 2mt.

Sounds as if the beam goes across the outer edge of the deck and the joists span 2 m and land on the beam. It would therefore be classed as a bearer. It's more usual to size bearers with a thickness of 70 mm or 90 mm, often made up of 2/35 or 2/45 sections. However, joists only need end bearing of 35 mm, so a 50 mm bearer is OK in that respect. In treated pine, 290 x 45 is nearest to your required size but would only span about 4 m as a single span, so you would need a post in the middle.

Q. I have a AS 1684 - 2006 with a span table CD which utilizes Adobe Reader 7 whereas the current AR is version X1. Can you please help as I am doing a design at home in which I have a design making use of a cathedral roof system and I need span table 30 & 31. You may have some idea of how to use the CD or simply allow me to download a copy of the span tables that use AR X1.

It's not possible to download the current (2010) AS 1684 span tables since they are copyright to Standards Australia. And it's not recommended to continue using the 2006 tables since the 2010 tables were adjusted in response to changes in the design values for the MGP grades. However, various timber organisations provide free software that allows you to calculate beam sizes. For more information go to the Wood Solutions website via this link


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.