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11 Dec 2018

I was surprised to discover that the world record for the tallest natural Christmas tree has been intact since 1950. At 212 feet (67.36 metres) tall, said tree was erected at the newly built Northgate Shopping Centre in Seattle. Northgate was one of the first designated shopping ‘malls’ and management wanted to create a stir for its first Christmas. It certainly did this when it brought in (with much disruption to the neighbourhood traffic) the enormous Douglas Fir that had been cut just 20 miles away. On I learnt from the reminiscences of Jim Douglas (the President of Northgate Centres Inc at that time) that the tree could be seen from 0.6 miles away, carried 3,600 lights and required its own generator to prevent the mall from being plunged into darkness. The budget for installing the tree was blown, but with a feature in LIFE magazine, it proved to be a very lucrative marketing success!

But I digress…this is a blog about structures and engineering…just how is a tree of this stature kept upright, stable and safe?

The Newgate tree was secured by heavy guidewires installed in 4 directions to hold the tree upright. To learn more about this I read an article on describing how the Christmas tree at the Rockefeller Centre in New York is secured each year with this same method. Two cables are attached to the tree, the main load line on the top and a lesser cable on the bottom. A huge crane then uses these cables to lift the tree into a horizontal position…still following…the main load line is then used to lift the top of the tree until it stands upright.

Once the trunk is secured in a metal stand, the bottom cable is disconnected, the top one being left attached to the crane. Workers then nimbly climb to the top of the tree and secure 4 anchoring cables which are attached to 4 anchoring points on the ground. The tension of these cables is adjusted until the tree is straight, and then the main crane cable is disconnected. Hey Presto! The tree stands vertical and strong, all ready for it’s lights and decorations. But this isn’t magic, it’s physics - the very foundation of structural engineering – no complicated technology needed for this job that has been done the same way over many years.