Published: 07 December, 2007
Dr John Williams, senior technical consultant with TRADA Technology, looks at how timber heritage structures can be repaired and restored
There are many strong arguments for restoring old timbers in bridges and other structures. We are fortunate in this country to be surrounded by history. We must surely protect this heritage and, by reusing timber that has already been felled and processed, we are helping protect our forest resources. As the green revolution gathers momentum, what better act of recycling is there than to reuse our historic buildings and the materials within them? Furthermore, old timbers from slower grown trees will often be denser and stronger than anything we can buy today, while repair and refurbishment instead of rebuilding can save time and costs.
When we carry out an inspection and evaluation we ask the developer, contractor or engineer to set out clearly what is expected. They may say Give me a quote for treating any decay in the building, or How bad is the decay, where is it and what should I do next?. Often we hear the statement, You get what you pay for!. This is especially true when undertaking building investigations. We provide the client with information about the state of wood in the building, the risk of it decaying and how best to prevent degrade.
The condition survey is a vital first step and, for the client, a useful tool in determining contingencies and calculating how much money should be put aside for reinforcement, repair or refurbishment.
Decay detection has become more sophisticated since BT patented the TAMO method thump and move on for establishing the condition of thousands of telegraph poles. We carry out decay detection drilling with the help of a resistograph. Its very fine probe pushes through a small area of wood, so the damage left is minimal, making it near non-destructive. Knowing where wood is most at risk of decay is also important. We drill into the bearing ends and use long insulated probes to work out moisture content inside the timber, the part the client should be most interested in.
John Williams on Hammersmith Bridge - in situ consultancy carries its own riskFungal decay is one of the main risks in old timber structures. For decay to occur, there need to be persistent levels of wetting. You might also see mould. Although it doesnt cause decay, it is a good indicator that environmental conditions are likely to support decay.
Dry rot, most typically found in damp, poorly ventilated areas, affects hidden timbers, for example, wallplates, joists, beam ends or lintels. Historically, it has been wrongly considered as a building cancer and, certainly in the past, remedial measures may have been more destructive than the fungus itself. Why drill into brickwork and pump it full of water when all you need to do is dry the building out? True, dry rot can devastate buildings if you dont carry out essential maintenance. But for the dry rot fungus to travel, it needs specific conditions; not just damp, but high levels of relative humidity more than 90% for prolonged periods. Generally speaking, that can only occur in a building that has a major defect, not in one that is well constructed, ventilated and maintained.
Wet rots are caused by several species and, like dry rots, can cause severe damage. But unlike dry rot, wet rot cannot travel through a building, unless it is soaking wet. It is typically found around the points of moisture ingress such as defective guttering.
Insect attack is another risk. Our survey will identify the type of wood-boring insect in the building and whether there is an active infestation. Where insects have attacked wood, it is important to ascertain if a significant amount of section has been lost and, if so, what effect that will have on the bearing strength of the original timber.
Damage caused by timber being used as a dartboardThe next step is to assess the strength of the timbers in situ. The principles detailed in modern visual strength grading standards, BS 4978 and BS 5756, are used. It is also essential to identify the timber species so that the correct strength class can be assigned. And as timber dries it becomes stronger. Modern codes for permissible stresses are based upon 16-20% moisture content (mc). Timber in older buildings provided they are dry may be as low as 10-12%mc, so automatically there is an opportunity to increase the strength of timbers.
These grading rules are simple to apply, but they are, in my view, punitive when trying to assess the strength of historic timber. TRADA Technology has been evaluating the alternative of ultrasound testing of structural timbers in older buildings. It is a non-destructive method and works by measuring the speed of an ultrasonic stress wave along a timber member. This is then correlated to the actual Modulus of Elasticity and Modulus of Rupture values obtained by destructive testing, in the laboratory, of samples for which ultrasound data was also collected.
We have carried out extensive calibration testing on the equipment and good correlation has been established for a number of common species including oak, spruce, Scots pine, larch and Douglas fir. We are still evaluating the kit, with interesting results. We have obtained data to show that stresses calculated for old seasoned pine can exceed published permissible stresses for oak!
Misdiagnosis can lead to expensive remedial treatmentFor repair and restoration we prioritise the actions needed:
Low priority: where the timber structure is in good condition and at low risk of any decay.
Medium priority: where there is some decay, maybe some water ingress, but no need for immediate remedial action.
High priority: where there is obvious decay in critical locations and works are needed urgently.
It is rare for identical situations to exist in older timber structures. Repair and restoration should be chosen, designed and implemented on a case-by-case basis. The strategy can involve anything from repairs to individual members, to the connecting joints or mechanical fasteners, or to the whole structure. We can employ traditional carpentry methods, resin repairs or introduce alternative support, but often we recommend a combination of techniques.
The conservation and restoration of historic timbers, therefore, calls for a holistic and multi-disciplined approach but isnt our heritage worth it?Keywords: TRADA heritage repair restoration
Is timber well placed to benefit from the opportunities presented by the London 2012 Olympics?