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The substance of being hollow

Thanks to uniform stress, a hollow tree doesn’t always mean a weak tree. This is worth considering if a client has asked you whether a tree should be removed because it is hollow or has a hole in it

As you read this, you’re probably sitting in a building with walls and hollow internal spaces called rooms. You could be in your work vehicle; a hollow structure with outside walls and an inside space called the cab. You may well be drinking from a cup, can or bottle; a hollow vessel. You push a wheelbarrow, drive a car, ride a bicycle - all connected to the ground with tyres; hollow rubber things filled with air.

Every day you are surrounded by, use and/or depend on hollow things. These hollow things are not just hollow, as in mostly empty on the inside, but they are also structural. Hollow load-bearing things that support hollow structures that hold hollow things – if you think about it, many of the things that we consider to be solid are not entirely solid. A wall of concrete is just an outer shell used to create a space. We literally live in hollow things. So why am I pointing out the obvious, and why should you care?

Apply your knowledge to the natural world

Being surrounded by hollow engineered structures is one of those instances where we know something at one level, but don’t use that knowledge on the other. We don’t apply what we know.

Consider an old tree growing on some property or in a park somewhere. Someone may drive up to that tree in a mostly hollow car, look at the tree, find a cavity and decide that the tree is rotten because it’s hollow. They will

then drive back to their hollow office in their hollow car, sit at their mostly hollow desk and write an email. They may send that email to someone who is sitting inside a hollow room and the email might say, ‘that the tree is hollow, and because it’s hollow it will fall down’.

While we are imagining – let’s say that the person that wrote the hollow email just focused on the cavity and thought ‘there is a big hole in the side of the tree and the tree will fall down because there is a hole in it.’ When they wrote this insightful email they entered their office through a door – a large hole in a wall. They sent their letter of concern to a person, who upon reading it, gazed out through a hole in the side of their building (a window) in despair and thought ‘heck there is a big hole in the side of a tree and the tree will fall down because there is a hole in it.’

So not only are we surrounded by hollow things that don’t fall down, but many of the hollow things that don’t fall down have holes in them too. Who would have thought?

So, how are you going to use this insightful bit of information that you already knew? How are you going to apply your knowledge in a way that might be useful?

The next time you find a tree with a big hole in it or you tap a tree and it sounds hollow, don’t instantly assume that it will fall down. It might, but it might not.

Uniform stress matters

In nature, strength is created through uniform stress (the axiom of uniform stress). Engineers have been learning from nature and they too design structures that are incredibly strong yet hollow by applying the concept of uniform stress. It’s all about the shape and thickness of the shell wall – the outside layer that supports everything.

If you are so inclined and still retain a sense of balance (I don’t necessarily recommend you do this), place an empty aluminium can upright on flat ground (the can will have to be empty, but otherwise undented or misshapen in any way), then with grace and elegance place one foot on top of the can, then slowly and smoothly lift your other foot off the ground so all of your weight is transferred onto the foot that is on top of the can. It’s possible that an empty aluminium drink can support the weight of your entire body – if the weight is applied evenly and if the stress is uniform.

From there you should hop off, but if you were to tempt fate (and again I don’t recommend you actually try this), while balancing on top of the can, bend down and tap one side of it (do this with a pen or short stick, so your fingers are well clear and not at risk of being severed!). The can will crumple, it will completely implode, collapsing in a millisecond under the weight of your body.

By tapping the can, you have created a localised point of stress and the can will fail under the change of load. There are many reasons why you shouldn’t do this experiment – least of all losing a finger and falling on your face – so again, I don’t recommend that you do it. But trust me it can be done.

Hollow doesn’t always mean weak

There are many engineered structures and natural examples (like the shaft of a feather) that create and retain strength through maintaining uniform stress. Having nothing on the inside, (being hollow) is unimportant as long as the outside can handle the load.

When it comes to trees, many of them are hollow. When it comes to trees with holes in them (trees with cavities), if the tree can grow sufficient timber around the hole, then the outside can handle the load.

So, in conclusion, don’t think that all hollow trees, or trees with holes in them will fall down - they might, but they might not. If you’re unsure, make sure to get advice from an expert. More information on what type of experts might be useful will be in the next issue.

Mark Roberts is a qualified arborist and tree risk assessor with 25 years’ experience. Mark is also former President of the International Society of Arboriculture, and a past President of the NZ Arboricultural Association. Mark now heads an arboricultural collective based in Dunedin:


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