It is all around us, but we mostly take it for granted. It is one of the first items exploited by man for building, weapons and so much else, yet we still find its behaviour unpredictable and mysterious.
Predictability is the essence of engineering but, where timber is concerned, we have long used a combination of special experience and general empiricism to make use of the only structural raw material available to us that regenerates itself ad infinitum. Sadly, though, that regeneration takes time — and time is what we do not seem to have in the modern world.
In Ireland, you can sometimes find magnificent examples of the massive oaks that once covered much of the island, but they are few and far between. Of those we still have, many are 200-plus years old. In the late 1500s and early 1600s, oak was felled and taken in vast amounts to Britain (especially Devon), mainly for ship-building. This was a major and exhaustive endeavour by such adventurers as Raleigh and Drake, but the wasteland left behind was not re-sown with broadleaf and oak, but was converted to grazing.
The result is that only relatively recently have we seen any replanting of forestry and, as intimated above, mainly only the faster-growing timber has been planted so that investors can see returns in their lifetime. Oak is seen as uncommercial, since its benefits and rewards are stretched to being enjoyed only after several generations. This is not the same as the attitude in, for instance, France where re-planting exceeds felling 2:1 for broadleaf species.
Oak is not the only native timber now rarely seen, but even some of the softwoods such as Pinus Radiatus do not now have the structural characteristics of those examples that were felled prior to 1900. Modern structural timber is governed by the CEN standard grading system (IS EN 338): C14, C16 and so on, which we are all familiar with, but even these grades are really only indicative and are dependent on ambient moisture levels.
All the more reason, therefore, to preserve what we have in situ where we find old timber in the inventory of our structures in Ireland.
Oak is uncommon to find except in the medieval buildings of Ireland (although it is found as lintels in 19th-century structures) and pine in its various forms is prevalent. On the continent and especially in Britain, it is a different story and in older buildings, oak is famously much used.
However, the survival of wood is precarious, since it is an organic material and prone to attack by fungi and wood-boring insects. The main clue to keeping timber effective is to keep it dry.
It is not that our ancestors were unaware of the need to protect timber in exposed applications: the Greeks used olive oil to soak vulnerable timber and the Romans used tar on their ships to preserve and seal the planking of the hulls, while the Chinese used tung oil.
Their problem was that if the protection was not maintained and rot set in, the timber would fail and there was nothing that they could do about it.
Modern approach to wood preservation
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Dry rot in the floor and the stud cladding in a newly refurbished house[/caption]
In the past century, a bewildering array of treatments for timber have appeared. These tend to be either anti-rot or anti-insect, but some are both. However, many of these treatments involve chemicals that are toxic to humans and the environment in general, as well as to fungus and insects.
Many are copper- and arsenic-based such as copper azole, copper napthenate and chromated copper arsenate, but there are also silicate-based treatments and complex propiconazole-tebuconazole-imidicloprid preservatives. Recent additions to the list include boron and acetylisation techniques.
Most of the treatments for timber were for pre-construction use and we can all recall the aroma of creosote used on timber in various applications, such as for fencing and railway sleepers. Only recently have systems appeared that are more benign and, interestingly, easier to apply
in situ.
Many of the treatments that were commonly in use only a decade ago are now labelled as hazardous. However, many of these older systems were popular because they were also cheap.
As engineers, we have to view structures in timber in terms of the contributions of the various members to the structure and we have to identify weaknesses. This may, once again, involve empiricism when it comes to assessing an older structure: for example, assessing how much of the original section is effective and whether that section can be taken as uniform throughout a member.
Assessment is particularly necessary in the case of timber attacked by wood-boring insects since, depending on the insect, the attack may be limited to the outer periphery of the section or it may be utterly destructive within the section. Of particular note are the problems faced with structures that have used wooden pegs as shear connectors. These can be totally eaten away and yet seem fine superficially. Their replacement with new is the only remedy.
The saving grace for timber, within structures, is its ability to adapt to load change and absorb extra stresses caused by failures elsewhere. The saddle-back roofs of older houses attest to the way in which the roof structure has relaxed but is still performing. The problem for the conservationist is whether to straighten the roof up or to preserve the sag as found while reinforcing damaged areas. There is a case to be made for both approaches.
What is paramount for the engineer is the need to ensure that the environment of any part of the structure is such that the propagation of rot is unlikely and the attack by insects is discouraged, if not entirely prevented. The chief provision here is simply the protection of the structure from damp (e.g. condensation) and from outright water impact.
Analysis and modern codes
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Massive hearth beam eaten away at both ends[/caption]
In-situ analysis of timber is both easy and hard. When a structure can be quickly demonstrated to be in good condition, analysis and re-calculation of the potential performance of the structure is relatively easy. Sampling of the wood can be done using small drills and these will give an approximation of the grade - which is frequently found to be superior to the grade which might have been used for the same structure today.
However, where a structure has been damaged and a degree of relaxation may have occurred, the analysis as found is hard enough in terms of estimating the re-distribution of stresses. But it gets harder when repairs are introduced, since all the repairs will carry factors that may reduce the overall strength of the original member and the structure.
What is important to remember is that on the older buildings, which have remained in place for perhaps well over 100 years, it is not necessary to go back to the drawing board and try to insert a new design to take the loads. It is only necessary to provide like-for-like element replacement, provided that the repaired structure can be shown to be capable of operating within its original parameters.
Some years ago, I dealt with a building where the back wall was collapsing and the roof had dropped by 600mm in one corner and one twisted main truss was only supported on one side by the purlin. After emergency propping and an examination of the timbers, I decided that it would be feasible to simply jack the roof back up.
This operation was successful and we then rebuilt the back wall underneath. The twisted roof returned exactly to its original shape. The previous surveyor of the building had condemned the entire structure and had recommended immediate demolition.
Given that the design of timber structures, under Eurocodes 1 and 5, starts from an assumption of particular gradings, a check calculation can be made for the same structure. If the grading used (with like-for-like sections) is less than that found or indicated in the existing structure, if the repairs made do not materially alter the behaviour of the structure and if defects such as shakes have been addressed, then a factor of safety can be assumed.
All the questions regarding the conservation of timber will be addressed during the upcoming seminar at Clyde Road on Wednesday 21 September entitled, ‘Essentials of Timber Conservation: Latest Developments, Techniques and Legislation’.
Tim Ryan, chartered engineer, is currently chair of the Conservation Group within Engineers Ireland and also Secretary of the Properties Committee of An Taisce. While he currently specialises in conservation engineering, Ryan also has considerable experience in civil engineering including piling, driven and bored; tunnelling in hard and soft rock, reinforced concrete structures in design and build but also in repair, sprayed concrete construction and marine structures.