Repair Details

When we turn to the tactics of repair, timber has always seemed to be a fairly contentious material the methods of repair have attracted rather strongly held positions. There has also been a developing of skills and the two together seem to have resulted in some swings in approach. (This might be true of conservation in general, it just being my own perspective that makes me think that it is more problematic for timber.) Repair methods in timber can be loosely divided into three kinds:

- Those using steel

- Those using modified traditional methods

- Those using epoxy resins with or without some reinforcement.

The kind of repairs once commonly carried out by engineers, cutting back the decayed timber and putting in supplementary steelwork (Fig. 1), was later deprecated by those who favoured a more 'traditional' approach, although that usually meant carpentry methods supplemented by modern steel fasteners. It has more recently been recognized that while such an approach is more visually attractive, and so appropriate for medieval timber frames, where the results are both visible, and have to form a junction with secondary materials, it does involve a greater loss of historic fabric. In that respect the use of supplementary steelwork is perhaps to be preferred where the repair is not seen. Repairs using epoxy resins had a 'bad press' for a while and suffer from a regrettable lack of research into their long-term performance, but lack of research also affects other areas of repair work.

One of the major repair projects carried out early in the 20th century was to the roof of Westminster Hall (Baines, 1914) where substantial decay had been found. That used steel connectors, but in locations

Figure 1. Repairs using steel carried out to the heel joint of a Wren roof truss at Hampton Court.
Figure 2. A truss in the Banqueting House, Whitehall with unnecessary reinforcing.

where it would not be visible. One wonders whether there was general concern at the time for the adequacy of timber structures simply because they were timber. The roof of the Banqueting House, Whitehall, built by Soane in 1824 to replace Inigo Jones's original, was a fine combination of timber and iron. Nevertheless, sometime in the inter-war period it was felt necessary to add steel to all the joints (Fig. 2) and to insert steel ties to take over the job that the tie beams seem to have been doing adequately till then. Clearly the reinforcing shown in Figure 2 is having no useful effect. The steel structure added to the roof over the hall at Greenwich Hospital also seems to be an unnecessary supplement to the original structure because in neither case is there obvious structural distress. But excessive use of steel can still be seen in use today because it is the material that engineers are familiar with and so may instinctively turn to even though timber repairs would be perfectly satisfactory.

In the 1970s the use of epoxy-resins as either a consolidant, or to replace lost timber appeared to offer a solution for those who wanted the minimum loss of historic fabric, but this ran into technical difficulties. Early research in the US on this method, where they were interested in it for repairing softwood trusses in the roofs of aircraft hangars, showed the difficulty of controlling the run of what was a liquid and the obvious change in the structural characteristics of joints. However, it is questionable whether or not this work was noticed by those in Britain who were enthusiastic for the use of the material and there are some unfortunate results of its early and inexpert use. Baguely Hall, on the outskirts of Manchester, is an example of the kind of thing that can go wrong. Epoxy-resin was used to repair the main fames of this fine medieval hall but was allowed to run over the surface of timbers and is now a permanent disfigurement. Many early repairs using this material were carried out by a firm who appeared to have little technical knowledge and even less interest in the proper training of their operatives. The result was a number of failures, sometimes because of the use of the technique in unsuitable locations where exposure to the weather seems to have resulted in accelerated decay of timber adjacent to the epoxy-resin repair.

The difficulty then was that with considerable anecdotal evidence about failures following the use of epoxy-resin, and with little scientific evidence, there was deep concern among some over the long-term performance of such repairs. This was especially so where they might be used in timbers with high or fluctuating moisture content. If there were to be deterioration of the adjacent timber it would have an effect on the strength of the joint. One might also be suspicious of its behaviour when used as an adhesive to join steel reinforcing plates to timber and the effects of temperature changes. As the steel must move more than the timber what is happening at the interface between the materials? Here we have no information and can only wait and see.

Opinions on the use of the material seem to still be divided between those with long memories and an instinctive distrust of these techniques and those for whom it is a valuable tool. TRADA (2001) have published a book on its use and at the Whitbread Brewery, London, Hockley and Dawson used it to effect in the repair what may well be the largest surviving eighteenth century trusses. More recently the Weald and Downland Open Air Museum has been developing skills in the use of epoxy resins for repairing structures that have been dismantled. (Their techniques are not suitable for in-situ work.) Richard Harris points out that this material allows one to make repairs that retain a lot more of the historic fabric. He has also argued that if something is going to be lost through decay and epoxy-resin will prolong its life, then it is sensible to use it - but this is an argument normally applied to non-structural elements.

5 FASTENERS AND DESIGN CODES

Once the forces in a structure have been determined, and unless there is particular concern over deflections, our engineering problems become a series of connections with members in between; member stresses are generally low. Unfortunately the jointing methods used in new structures are not always appropriate for historic structures. Moreover, historic structures use details that are not used in modern carpentry and therefore for which there is no guidance within design codes framed for new buildings, using modern materials and modern methods. This was never so clear as during the lacuna when the newly introduced British Standard (BS5268) in 1984 failed to include oak as a structural material and conservation engineers had to continue to use the obsolete code (CP112). While that issue has been resolved, there are still questions over the performance of traditional joints and the values that one might use for metal fasteners.

The British code of practice is even inadequate in the information given for steel fasteners. One example is that safe loads for screws in shear are only given up to 10 mm diameter when carpenters may well wish to use 12 mm diameter screws. (Even that is an improvement on the earlier editions of the code where loads were only given up to 8 mm diameter.) An engineer who is unfamiliar with timber may be unwilling to go beyond the limits of the table, particularly as the formulae for determining the loads from first principles are rather daunting. Bolts can be seen used in circumstances where screws would have been better but where M10s were presumably inadequate. Moreover it might not be clear to those unfamiliar with the code that the spacings and edge and end distances for bolts rather than screws apply to these larger sized screws.

There is the serious question about how we should view the allowable loads on bolts and screws used in green timbers that will dry in service. The problem for conservators is complicated by the fact that one side of the joint will be dry timber and the other side green. I have no idea what reasoning or experimental results lie behind rather draconian reduction factor required by the code. If it is to allow for the possibility of splitting as the timber dries one would have thought that such an event might well reduce the capacity of the joint to zero.

6 TRADITIONAL JOINTS

The terminology is a little loose here because there are traditional joints, either surviving in existing structures or used in repair work that are no different from

Figure 3. In loading to determine shear capacity of timber the forces are applied at each end.

Figure 3. In loading to determine shear capacity of timber the forces are applied at each end.

Figure 4. In atypical heel joint restrain is provided by the tie beam behind the notch.

joints contemporary with the original construction. At the same time there are those joints that have a superficial resemblance to traditional joints but which rely on modern fasteners, the most common being the scarf joint. The difficulty is that any tests to determine working loads for such joints must be affected by the workmanship in the carpentry. One can envisage the problem of relating test results to the performance of historic joints, both with very uncertain standards of workmanship and the effects of time and chance -or perhaps that should be wind and weather.

One wonders whether the standard test for shear resistance is a suitable basis for the design of traditional carpentry joints relying upon shear along the grain? The test loads the timber at either end so that one might expect a sensibly uniform shear stress along the plane of failure - the dotted line in Figure 3.

The joint where timber is loaded in shear parallel to the grain is a typical heel joint where the normal condition in practice is for restraint to be provided by the tie beam behind a notch (Fig. 4) or a mortice and tenon joint. In such configurations one would expect the shear stress along the plane of failure to fall off towards the unloaded end; but in what way? A complicating factor in some buildings is that this timber may be rather exposed and have a higher than normal moisture content.

Pegged joints is one of those areas where there is an overlap between new-framed construction and conservation, the question being whether such joints can be relied upon to transmit tensile forces. The difficulty is that pegs do not behave in the same way as steel bolts or dowels so that Johansen's equations cannot be applied to find the allowable loads. Experimental work shows that the pegs fail in shear close to the tenon/mortice

Figure 5. A pegged mortice and tenon joint. The peg will fail at the interface between the two timbers.

Figure 5. A pegged mortice and tenon joint. The peg will fail at the interface between the two timbers.

Figure 6. Forces on a lap-dovetail joint used between tie beams and wall plates.

boundary without the kind of rotation that occurs in bolts.

Unfortunately what work has been done so far has produced conflicting results. Drawing on the work of Jonathan Shanks carried out at Bath (Shanks & Walker, 2005), Ross et al (2007) have given possible working loads for pegs in oak, whereas a formula derived from quite different tests by Schmidt (2004) at the University of Wyoming suggests a working loads of less than half their figures. Given such a wide discrepancy, prudence suggests one should use the lower figure for design, but science surely requires some explanation for this large difference. The Bath tests were carried out on complete joints while those in Wyoming in more 'traditional' laboratory experiment that allowed the effects of varying density in both pegs and member timbers to be explored. What surprises me is that the Bath tests that used carpenter-made joints, and presumably subject to variations of workmanship, nevertheless resulted in the higher figure.

The behaviour of dovetailjoints is another problematic area as these rely on timber loaded across the grain (Fig. 6). When there is drying shrinkage in the timbers there is a change in the angles of the two parts of the joint resulting in a reduced contact area and a correspondingly higher compressive stress. The stress might be well above the elastic limit with collapse of some of the cell walls. However there is no readily available data from which one can estimate the movement that might result.

A serious problem occurs when existing structures requiring repair incorporate details that are 'prohibited' by the present code. An example occurred where rafters had birdsmouth cuts over the supporting plate with the depth of the cuts exceeding half the depth of

Figure 7. Birdsmouthed rafters.
Figure 8. A simple half-lap scarf with bolts in shear.

the rafters (Fig. 7). This is larger than the depth of a notch allowed by the code but any change would have affected the geometry of the roof. Admittedly some had split, but these were in the minority. In the event a case had to be carefully made to allow their retention of the sound rafters and the repair of those that had split.

Scarf joints are commonly used in repairs where bending moments have to be carried. A simple arrangement is a half lap with bolts or other fasteners acting in shear (Fig. 7) but a more traditional joint is often preferred for visual reasons (Fig. 8). If a moment is applied to this, one end will go into tension and the preferred fixing is usually one or two coach screws acting in withdrawal. (Screws are preferred to bolts so that they are visible from one side only.) However, there is currently no guidance on the detailing for such a screw fixing and as carpenters will usually want to sink the heads below the surface, the small amount of room available is evident from the drawing.

Carpenters adopt a simple rule of making the length of the joint about three times the depth of the section. A few years ago TRADA looked at a number of different methods of making such joints deriving measures of 'efficiency' for each, i.e. comparing their resistance moment with that of solid timber of the same section. This simple, some might say simplistic, approach to the assessment of joint behaviour does not tell one how the

Figure 9. A more traditional scarf joint with screws in withdrawal at the tension end of the joint.

joint is behaving in practice, and the fastening methods were not those that would recommend themselves to carpenters.

I have left the most difficult issue till last, which is resistance to wind loads. One might assume that building that has stood for many years, possibly centuries, has stood the test of time. While that is not unreasonable for masonry structures with large masses it is more problematic with lighter timber structures that might have been subject to some deterioration, especially if there have been periods of neglected maintenance. Moreover we are aware that climate change will result in higher wind speeds in future, again more of a problem for timber structures rather than masonry. The result is that it might be difficult to demonstrate that an existing structure can cope with the required design loads. Of course, in attempting to do this one will often consider the structure acting alone whereas the frames may have always relied to some extent upon the infill material. It is, of course, possible to use the resistance of infill panels where these are of a construction recognised by present design codes, but we know little or nothing about the resistance provided by historic lath and plaster or wattle and daub infills. Here we might consider Japanese work that has explored the resistance of traditional infilling material in the context of earthquake loading. Whether such an approach can be extended from large and infrequent loading to more modest but very frequent loading is a moot point and one that would need to be considered before embarking on research in this direction.

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