Why are Laminated beam sizes changing ?
It’s all to do with recovery.
The prime component for graded laminated beams has always been 38mm thick rough pine timber.
In the old days timber was cheap. Mills supplied for example a rough 38x152mm and it often measured 40mm + in thickness.
This allowed the laminator (us included) to get a good clean planed finish at 33mm (important for glue adhesion) to make up a lamination of 7 to get 231mm, 9 to get 297mm and 11 to get 363mm in thickness. This was how the SABS made the specifications and standardised sizes.
Standing log pricing (the price of a tree in a timber plantation) has skyrocketed over the last 10 years. Now in 2021 timber and transport is expensive. (and everything else…)
Mills are optimizing recovery and sizing all timber to get maximum payloads on trucks – it is cheaper to transport sized timber. And sized timber for normal structural use makes up probably 80% of the market. It is now actually more expensive to get rough timber than sized !
The sizing issue:
The SABS thickness specification for sized 38mm timber is 36mm. The problem is this varies due to bulk manufacturing and is close to 34mm sometimes – the mills are squeezing every bit out.
The structural market doesn’t really notice this.
Why does this affect laminating ? 34mm is bigger than 33mm ?
To laminate timber a clean FRESH surface is required. It is important for glue to have clean dust free open cells on the glued face for good adhesion.
We and other laminators will replane sized timber just before laminating.
To replane 34mm sized timber is not possible – there is not enough “meat” to plane off and have a good surface.
To accommodate this the laminae (individual strips of a laminated beam) are now being planed slightly thinner. This results in a narrower finished beam. The alternative is increasing the beams by 1 laminae but this makes the beam wider. This is not ideal for space constraints and is a more expensive option.
This table shows old sizes (up to July 2021) and the new sizes moving forward as new stock is made.
|Old size||New size|
If you have any queries on this please let us know. This change does affect design and span tables slightly and will require recalculation by the responsible engineer.