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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. Is there a typical deflection head detail for timber stud framing? I have a non-loadbearing timber frame wall being constructed beneath a bondek slab supported on steel beams and columns. How can I efficiently transfer horizontal loads from wind and earthquake from the timber top plate to the steel frame while not imparting vertical load when the steel deflects with live load/creep over time. Thanks

In all-timber construction a clearance is required between the top of a window or door frame, and a loadbearing member such as a lintel, to make sure that when the lintel deflects under load it won't transfer that vertical load to the frame. Australian Standard 1684 calls for a minimum clearance of 15mm. The window or door frame is nailed into the studs either side, after being plumbed with packers or folding wedges, but is not nailed up into the lintel or framing member above. Presumably something similar will work with a steel frame.

Q. I'm in need of some specific information regarding the bracing of a cathedral roof frame. If you have any specifications available that I might forward to council or a company that may have advice on such matters it would really help. Thanks for your time.

Wind load on a cathedral roof is considered to be transferred into bracing walls via the ceiling diaphragm. AS 1684, Residential timber-framed construction, does not specify what the maximum roof slope is for raked ceiling diaphragms, but as the Code is limited to a roof slope of 35° it is recommended that this limit is also applied to raked ceiling diaphragms.

Q. Hi There, According to the standards we use in Australia like MGP10 or F7, Russian timbers have other standards like GOST 26002 etc. So I was wondering is there any table or guide to equalise Russian standards with ours? Please advise, Best Regards, Ryan

We are not familiar in detail with Russian grading rules, but our understanding is that the export grading rules in GOST 26002 define the quality of timber in terms of allowable knot sizes and other characteristics, but don't relate these factors to any specific stress grade or suite of structural properties. While GOST 26002 provides a useful basis for importers to understand what they are buying, it would seem somewhat difficult to directly relate Russian grades to Australian stress grades. The best approach would seem to be to stress grade the timber to Australian standards after it reaches Australia.

Q. Hi, I need to know what size timbers i need for a 7m (L) x 6m (w) lean to with polycarbonite sheeting on top. Just supported at each end of the length with 1m spacing for rafters. I would prefer HW if available.

Boral has F27 kiln-dried hardwood available in lengths up to 6m. According to Australian Standard 1684, a size of 190 x 45 will span 6.0m at 1.0m centres, supporting a lightweight roof. We assume the span is in the 6.0m direction, and sawn timber generally only comes in lengths up to 6.0m anyway.

Q. For the treated pine structural timbers --- what is the relationship between the mechanical grading and the old Stress gradings of F5, F7 F11 Is there a comparison table relating the stress gradings to the mechanical Gradings?

A data sheet published by Timber Queensland explains it this way:

"MGP10, MGP12 and MGP15 exceed the requirements of F5, F8 and F11 respectively, therefore these MGP grades may be substituted when projects are designed/specified using ‘F’ grade design tables. However, where MGP grades are specified ‘F’ graded timber must not be used without re-calculating sizes. ‘F’ grades can be allocated by visual grading, machine stress grading or proof grading. MGP grades can only be allocated by machine stress grading."
 
Separate span tables are provided in the Supplements to Australian Standard 1684 for F-grades and MGP grades in recognition of the fact that their properties are different.

Q. I have a 270 x 90 RS Oregon beam running through my room, this is currently supported by a 900 wide stud wall (along the beam) I wish to remove that stud wall for a kitchen renovation. The roof is metal deck with 200 x 50 at 600 cts. If the beam without the stud wall will not support the roof, would there be a hardwood column around 90 to fit under the existing beam and say 125mm wide to take the place of the stud wall. Thanks and regards-Mike

It's always hard to answer these questions without knowing all the details. For example, we weren't sure of the span of the oregon beam, or whether there was any ceiling load on it, or just the metal roof. And if the beam supports a ceiling, what is the span of the ceiling joists, and where is the stud wall located, eg. at one end, or in the middle? Maybe to be on the safe side, put a column in anyway. The size of 125 x 90 is not a standard hardwood size and is a big, heavy section of timber. A 90 x 90 will be much easier to obtain. Alternatively, if you prefer a proper structural assessment it might be wise to consult an engineer.

Q. We are building our own home our roofing is sandwich panelling so no roof trusses or rafters and we have 50x175 Red Iron bark beams our largest span is 4.5m. We want to know if these beams will be in agreement with Australian standards. We have already built our Earth Walls to height because we acted on advice that these beams would be right. But now we have to show that we have done the right thing we are having trouble finding the information to back it up. We know we can get an engineer to look into it but we are trying to save money. Any advice you can give us would be greatly appreciated.

We weren't sure what grade of timber you have or whether it is seasoned or unseasoned. However, if we assume it's unseasoned and a conservative grade of F11, we can say that your 175 x 50 red ironbark placed at 600mm centres will safely span 4.5m supporting a roof load of up to 20kg/m².

Q. I'm a structural engineer and I'm reviewing the capacity of various timber telecommunication poles around Australia and NZ. Is there a typical species of timber that is used for timber poles that I can reference in my calculations? As a reference, the sites range from 10 to 30 years old.

That's hard to answer because the species of timber would be different in different States, and at different times over the last 30 years, and their durability would vary depending on whether they were preservative treated or not. Incipient decay in older poles might also have reduced their strength. In Victoria messmate (Eucalyptus obliqua) was a common pole species, usually treated with creosote. In other States locally available species were used, eg. jarrah in Western Australia, ironbark in Queensland and New South Wales, and (to a limited extent) preservative treated pine in South Australia where there is no significant production of hardwood. Back in the days when CSIRO carried out major timber research, six very detailed reports were produced between 1961 and 1968 on the strength of Australian pole timbers. They were published as Division of Forest Products Technological Papers Nos. 15, 22, 23, 32, 50 and 53. Perhaps the relevant authorities in each State have more detailed records on exactly which species were used.

Q. I have purchased a pre fabricated timber framed pavilion to be built as a free standing structure Class 10 in our backyard in Adelaide SA. The council have indicated that the structure doesn't conform to Timber Development Association (TDA) engineered design guidelines. So my first question is what and where are these on the timber TDA website? My understanding is that the structure should be assessed against the timber framing code AS1684.2? Is the TDA a nationally recognised body or just in NSW?

If the local Council wants you to comply with a set of guidelines we think it would be reasonable to ask the Council to provide you with a copy of the guidelines they are referring to. Until 2009 there was a Timber Development Association in Adelaide and it's possible the Council has a publication that was circulated at that time by TDA of SA. On the other hand they might be referring to a publication of TDA of NSW. While AS 1684.2 covers most timber framed domestic structures, some that don't fall within its scope need special design. Perhaps there is a data sheet in the Council's possession that relates to your pavilion.

Q. Could you please tell me what the 4 species groups of timber referred to in the Forests and Wood products span tables for recycled wood are?

If you download the grading rules for recycled timber and refer to Table B1 you will find that most common hardwoods have been put into a species group (A to D). Softwoods are not covered by the recycled grading rules although guidance is included so structural members can be designed from first principles by engineers. You will find the grading rules on the net if you write Interim Industry Standard Recycled Timber in your browser.

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Did you know?

About 6% of Australia’s 147 million hectares of native forests are public forests potentially available for timber harvesting. Timber is harvested from about 1% of those public native forests each year.