Could you tell us about yourself and how you became a timber expert?
I came from university straight to Arup as a structural engineer. My first project was a timber structure working with Renzo Piano. After that project I did maybe 10 years of general engineering, but that first project was how I got interested in timber. Then in 2001 I was invited by Arup’s then timber expert to specialize in timber when he was about to retire. I suppose in all of these things, there’s a lot of chance and luck.
How do you promote a great co-operation between architect & engineer?
Often in a steel or concrete building, the structure will be hidden behind finishes. There’s almost a sense in which the engineer designs the structure and then the architect comes along and beautifies it. In a visible timber structure you can see all the connections, so architect and engineer are forced to work closelytogether which is more fun for both parties. For the relationship to work you need mutual respect. For example, working together with Alison Brooks on The Smile (see issue #3, 2016 of Trä!). Alison said: “Look, it’s really important that we make the foundation smaller so that it’s less visible.” From an engineering point of view that made it very, very difficult but I knew it was important to really help emphasize the shape of the Smile and so we worked nights and weekends to find a solution.
How does a project begin?
I have to say if the engineer isn't involved from the first day, it won't be a timber structure. To make an economical timber building it’s really important to get the right structural form from the start.
You argue the case that there shouldn’t be CLT walls everywhere, could you elaborate?
In an office building, you really want open plan spaces. You can't do it all in CLT. That's where a mixture of an open frame, which could be a glulam frame or it could be a steel frame, combined with CLT floor slabs becomes the logical option.
I also believe that we shouldn't be building residential buildings where every single wall is CLT even the walls within the apartments. For example for HAUT, an18 storey building in Amsterdam, we are planning to use CLT for the party walls between apartments and they also happen to be the structural walls and arealso used for the acoustic separation. Then what we're saying is the internal walls should be just lightweight studs, non-structural. That means that you've got a flexible building which can be adapted in the future.
We should be using the CLT for the walls that really matter, and where we need the strength. I've had this discussion with several architects and I believe I've won the discussion. I'm trying to change that in the industry really.
What about hybrid floor structures?
First of all they help you get longer spans. CLT is great but, economically you're really limited to about a 6 metre span. Otherwise it just gets too thick and heavy and expensive. If you want to go 8–9 meters, timber concrete composite slabs make a lot of sense. Also the mass of the concrete helps reduce the vibration and improve the acoustic separation. Plus it helps to create fire separation.
How do you view hardwood versus softwood CLT?
The mass market is always going to be the softwood CLT. That will always be the cheapest solution. But if you want a different appearance and improved strength to thickness ratio, then we should be considering hardwood CLT here as part of the mix.
Where do you get your passion from?
What I really like in a project is the team effort where we're all working together, pulling in the same direction, to come out with the best overall solution and have fun at the same time. I've said it quite publicly. The Smile is the most exciting project I've worked on. That is because of the people on the team.
Interview David Valldeby
