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Sizing Timber to AS1720.1

Sizing Timber to AS1720.1

Sizing Timber to AS1720.1

Hi guys

I have recently moved onto a timber design and want to understand what are the typical sizes that are called up in design.

If I where designing a flexural member with steel, I would size for deflections and strength, starting with the smallest size UB or PFC (depending on detailing and function) and would increase the size until I find a member that satisfies all criteria (i.e 360UB->410UB->460UB).

However with timber, there is so many different sizes and stress grades to select from. What approach should be taken when selecting a timber sizes, I am thinking of sticking to Seasoned Hardwood F17 and only under extreme circumstances will I venture to specialized products for spanning long-distances (such as hyne beams).

Do others have standard timber design procedures or rules of thumb that they like to follow? All comments and tech papers  will be appreciated.

RE: Sizing Timber to AS1720.1


Really depends on your market/project. We have builders and architects requesting different materials and products on a case by case basis. I have found the 'design in Hyne' software very useful as it provides a list of products that can be sorted by price - just make sure you check to make sure it is using the right design coefficients. At the preliminary stages we normally provide a few different options for framing (eg for joists MGP, F14, F17, Hyne beams etc..) this allows the client to make the call. I have found that F14 hardwood is more readily available/preferred to the F17 and as such is usually my starting point if the decision to go with hardwood has already been made.


RE: Sizing Timber to AS1720.1

The best solution will depend on the market/region. Where I am I would rarely specify hardwood.
MGP pine, LVL, hyne type I beams, truss floor joists are typically used.
It's best to speak to some suppliers and builders in your region to determine what is commonly available.

There is design software available from www.chhsoftware.com and www.tilling.com.au that I've found useful, especially for domestic type construction.

RE: Sizing Timber to AS1720.1

hardwood (F14 is my preference) external, everything that apsix suggested for internal. Hyne is just down the road so they are a very good reference like OzEng80 suggested, and a big supplier.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that them like it

RE: Sizing Timber to AS1720.1


This is definately some thing I can help you with, even after doing timber in 2 other countries since.

MGP10 is the cheapest, it is kiln dries and equivalent to about F8 (look up the appendix in the code)

Only if this isnt practical then look at a higher grade.

A few tips for the timber virgin:

Think of steel then divide it by ten to get an idea.
Moment connections are rarely practical.
Choose your species ewisely for durable timber
Stick to the standard sizes
Allow for dressing tolerances in your sizes (from memory 5mm on depth 2mm on width)
Be aware that cross grain compression often is the critical factor for compression members.
Allow for wood shrinkage in details.
Take note of potential timber splitting.
Not all timber treatments are water resistant

A few references:


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