»Tall buildings can be built faster and more economically«

Trä meets William F Baker

William F. Baker, structural engineering partner at Skidmore, Owings & Merrill (SOM) in Chicago, USA, is probably best known for the design of the load-bearing structure for Burj Khalifa, the world’s tallest building. But he is also responsible for SOM’s Timber Tower Research Project, a study aimed at paving the way for high-rise wooden buildings in the USA.

What are your thoughts about tall wooden buildings?
Mass-timber can offer tall buildings real benefits beyond sustainability. Mass-timber is a lightweight material which can be prefabricated and constructed quickly. These benefits could allow tall buildings to be built faster and more economically when using mass-timber wisely.

Can you explain the problem of uplift?
Mass-timber has approximately 25% of the density of concrete. This creates a lightweight building which is more easily overcome by wind and seismic forces, causing parts of the building to lift off of the ground. In addition, resisting uplift with mass-timber is challenging because it places high stresses on the connections, which are the weak points of a timber structure.

Are there different solutions to counter this?
There are many solutions to solve this problem. Perhaps the most sensible solution is to provide alternate materials for the core such as concrete or steel. Those materials weigh more which decreases the uplift problem and can easily resist any remaining uplift.

Is a hybrid design always the best option?
No, the hybrid design only makes sense for a mid-rise building or taller. Small buildings like houses clearly don’t need a hybrid solution and other buildings where CLT can be »tilted up« might be better as »all timber« if schedule savings are significant.

What do you think the timeframe is for achieving the regulatory changes necessary to allow tall wooden buildings in the USA?
This is very tough to say. Americans are generally less receptive to performance based fire designs which are the most appropriate solution for timber structures. It may be some time before sweeping regulatory changes are made.

Any plans from SOM to realize timber towers?
We continue to work with our clients to find appropriate uses for timber structures, but cannot mention these projects on behalf of our clients at this time.

SOM’s study »Timber Tower Research Project« can be downloaded here.

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