Added on 29/12/2011Article

Fire Damaged Oak


By Chris Skellern

What would you do with it?


This Oak stands in a strip of public parkland which is surrounded by council housing estates on all sides (many of the houses are now privately owned). The park is an important recreation point for this area.

The Oak tree stands at about 11-13m tall with a stem diameter of approximately 1080mm at a height of 1m. The tree has a reasonably good shape and is growing in a sheltered area (this is because most of the park lies below the surrounding levels). It has a large cavity in it stem, which appears to have been regularly packed with rubbish and set alight, probably by bored youths. The extent of the damage can be seen in the following photographs.



This photograph also shows, at the bottom right corner of the stem, a burnt area where the burned cavity has erupted through.



Here's the view of its eastern side.



And the western side.



I measured 900mm across the widest part of the cavity and 1100mm along its vertical. The depth of the cavity is variable but approximately 500-700mm. This gives a wall thickness of approximately 90mm on each side and 100-200mm on the rear.

The tree appears to have recently received crown works as many fresh pruning wounds are visible within its crown. This may have been a deadwood removal process, or a thin or possibly a reduction. This suggests the council responsible for it, is aware of its state and has prescribed some manner of remedial works. This also means they are happy to retain it with its current structural condition which of course is a judgment they must justify in the event of failure & injury to person.

I can apply the works of Claus Mattheck to this situation and calculate the t/R ratio by dividing the wall thickness by its radius (where t=wall thickness of the cavity and R=the external radius of the stem).

His research has found that hollow trees with a t/R ratio of more than 0.3 don't normally fail as a result of a hollow stem buckling ('hosepipe kinking') but that failures occur very frequently on ratios below 0.3 (but there are some trees which still survived with such a low value). Also by reducing the sail area of a trees crown a lower t/R value could be tolerated. All calculations assume the wall thickness is sound.

If I use a 't' value of 90mm and a radius value of 540 (which are present for this tree) then the t/R ratio calculates to 0.166. Matthecks research suggests this would be a potential failure.

Out of interest, if I use a 't' value of 150mm and a radius of 500mm I get a t/R ratio of 0.3.

Additionally, there is the issue of the open cavity. Mattheck states that where there is an opening occupying 120° or more of the stems circumference, failure from bending fracture or cross sectional flattening is probable.

My assessment of the cavity opening is that it does indeed occupy a space equal to, or larger than 120° (120° is one third of the circumference).

In my own mind, if the tree was to be retained I would prescribe some system to close the cavity such as a steel corset or wooden plug. This would prevent further damage because as it stands further fire damage is very likely. Imagine the late night scene where a gang of bored kids with nothing much to do grasp the idea of packing the cavity with litter, twigs etc and having a bonfire. Its going to happen. The tree with its blackened cavity is a magnet to such ideas.

So, what would you do with it? Keep it as it is, or fell it? Bearing in mind the cost of constant monitoring (with its history of fire damage I think it needs at least 3+ visits a year). Is it worth retaining for the longer term? It will always be a focal point of attention for further damage. Is it actually safe now? Consider the legal implications of it failing and causing injury, could it be seen as negligence. Remember it stands in a heavily used recreational park.

My own view, is to remove it immediately and replant with 1-2 new trees.