Taller Timber Buildings

I have had a long fascination with tall buildings. Whenever I hear about a new building project, the one statistic I really want to know is how many storeys high the building is. On some deep level I am awed by the extent to which the engineer has beaten gravity.

People were amazed and just a bit scared by the first concrete and steel skyscrapers that were built over 100 years ago.  I suppose there was a fear that they might collapse under their own weight.  It's a good thing that structural engineers know what they are doing, because almost nobody worries about skyscrapers collapsing any longer.

Reinforced concrete (and some steel) skyscrapers have become the norm in cities all around the world, but they are now being challenged by the oldest engineering material known to humankind, WOOD !

The tallest timber building in the world at the moment is known as Brock Commons.  It is 18 storeys and located in Vancouver (see above and below).  The lift cores of this building are made from reinforced concrete, but the columns and all the floors are made from solid glued-laminated pine timber.

Here in Brisbane, we have Australia's tallest timber building under construction right now (see below).  The 25 King St building is a Lend Lease project with ten storeys of timber sitting atop a reinforced concrete podium.  Even the cores on this building are made from timber!  Watching the construction is quite fascinating.  The wall panels, columns and beams are assembled on the ground and then lifted into position and bolted together.  The timber for this project has been prefabricated in Austria and shipped down under.

One of the really neat things about this method of construction is that you can immediately erect the facade and begin fitout works under the top level.  As you can see from the photo above, the glazing is fitted even before the next level is framed out.  You simply can't work that fast on a conventional reinforced concrete project because you have to wait 28 days for the concrete to cure before removing the temporary props!  Here's another image, this time from inside the building.

The architects have chosen to make wood the real star of this office building.  The columns and the soffit will remain fully exposed, but the floors will be covered over by a shallow false floor to hide the electrical and data services.

Fire is the question everybody has on the tip of their tongue, but you needn't worry.  It turns out that wood performs surprisingly well in a fire.  It takes a decent amount of heat to ignite timber in the first instance, particularly if we're talking about a large mass of wood.  The following figure shows that wood loses its strength more gradually compared to steel or aluminium.  I covered this issue in my last blog entry To the World Trade Centre Truthers.  If you live in a steel framed house, it will collapse more quickly in a fire compared to a timber framed house.

When wood burns, it develops a layer of char on its surface.  This char layer, which has very little oxygen present, acts as a physical barrier to the progress of the fire into the wood.  With all the scientific research done by fire engineers, we know the rate of charring is about 0.7 mm / min (see below).  This is why the engineers of 25 King St have designed the structure with extra thick columns and slabs.  They have allowed for the structure to burn for a period of time and remain structurally safe.  Having said that, the fire suppression systems should stop any fire from getting out of control.

The next problem to address in timber buildings is dimensional stability.  Anyone who has lived in an old timber building knows that the structure settles over time (known as creep deformation) and moves as the seasons change (known as shrink / swell characteristics).  You might have noticed that a door will close nicely during summer but sticks during winter, or you might have noticed that some walls aren't exactly vertical any more.  These changes occur for a couple of reasons.  Firstly, timber swells when the humidity is high and shrinks when the humidity is low.  Secondly, under stress, the fibres in timber slide past each other causing permanent changes in the deformed shape of the timber.  You can see this quite easily in old timber lintels which retain some sag even after the load has been removed.  This is why you should never re-use a recycled timber lintel upside down: the accumulated dislocations could rapidly come undone and your beam may deflect quite a bit more than you expect.

The problem of dimensional stability has been solved quite nicely with a timber product called cross laminated timber (CLT).  CLT is made by laying timber in multiple layers with each layer at right angles to the layer above and below it (see below).  The timber is glued and pressed together until the glue sets.  CLT doesn't swell or shrink much and is remarkably resistant to creep deformation.

The dimensional stability of engineered timber products like CLT and laminated veneer lumber (LVL) make it possible to build much taller buildings than the 18 storeys that have been built to date.  There are plans afoot to build an 80 storey timber skyscraper in London (see below).  Now that would be an awesome project to work on!

But what about the environment?  Aren't you opposed to chopping down all those trees?  Actually, no, I'm very much in favour of plantation timber.  Austria has had a sustainable forestry industry for 1000 years and, over that time, their spruce trees have sucked down CO2 continuously.  That's more than I can say for cement or steel production.  Every tonne of cement produces about 820 kg of CO2 equivalent (Flower & Sanjayan, 2007).  Cement is the most important ingredient in concrete.  A little math and you discover that there is about half a tonne of CO2 equivalent emissions for every tonne of concrete produced.  By comparison, every tonne of plantation timber (dry weight) sequesters about one tonne of CO2.  When you take into account the emissions from milling and transport of the timber, you still find that each tonne of kiln-dried plantation timber has more than 600 kg of CO2 equivalent sequestered (WoodSolutions, 2017).  This is why I want to see timber being used in buildings and, although this could be what is driving the recent interest in building of tall timber buildings, I think that developers are far more interested in timber construction because it is cheaper and faster than concrete construction.

Flower, D.J.M., & Sanjayan, J.G. (2007). Green house gas emissions due to concrete manufacture. International Journal of Life Cycle Analysis, 12(5), 282-288.
WoodSolutions. (2017). Environmental Product Declaration - Softwood Timber. Forest and Wood Products Association: Sydney, NSW.