Heritage Wood Assessment
Briefly, the steps required for the assessment of an historic timber structure and the planning and execution of any intervention are as follows:
1. A desk survey, which will deal with the history of the structure making clear its heritage value. This should also take into account the intentions of the building owner regarding its use and accepted alterations, so that the intended ultimate load and environmental conditions are clearly stated.
2. A preliminary visual survey. This is simply to obtain an overview of the structure that is sufficient to plan the next stage identifying what provisions need to be made to gain safe access to the timber structure.
3. A measured survey to determine the overall disposition of the structural members and locate the main problems. This survey should include principal dimensions and the nominal sizes of all members. It should also note any obvious signs of damage, decay or structural distress, which will need to be investigated in more detail at a subsequent stage.
4. A structural analysis to determine the overall forces and general levels of stress within the structure.
5. The preparation of a preliminary report which will specify any additional survey work that may be necessary. (One might need to do this simply to provide the client with a more accurate estimate of costs.) This will indicate what aspects of the structure require further investigation and what methods are recommended. It will draw upon the four tasks already carried out, identify the aspects of the structure that need to be preserved for their heritage value, identify areas of high stress and/or significant biological attack that need further measurement, note any ‘defects’ within the structure and identify the vulnerable areas.
Need for assessment
One would normally assume that a structure that has proved to be adequate in the past will continue to be structurally adequate requiring no detailed assessment of timber strengths, although due consideration might still be needed regarding extreme events such as hurricanes, snow storms, earthquakes, fires or other. However a structural assessment is certainly needed when:
i) There is to be a change in use of the structure and hence a possible change in loads.
ii) There has been significant decay or insect damage to the timbers, or the structure has suffered damage, e.g. due to fire.
iii) There has been mechanical damage or excessive deflection indicating overloading of the timbers in the past, inferior initial design or poor quality of the used materials. iv) There have been alterations/interventions to the structure during its lifetime that have resulted in a reduction of its strength or changes to the original structural system. It should be mentioned that there are occasional examples of poor initial design or workmanship. In such situations, or whenever past structural alterations or damage of timber and joints imply insufficient strength, measures must be taken to guarantee an adequate safety level and/or to limit public access.
Within the scope of these guidelines, the assessment of any existing timber structure has to be performed by desk work (historic/architectural survey, structural analysis, etc), on site inspections and other complementary on site measurements and laboratory tests. The general principle is that all inspections and surveys are non-destructive so that no part of the structure has to be removed or sampled except for small samples used for identification of timber species and biological decay agents. In any case the sampling should not modify the mechanical properties of the elements or have an impact on the other properties (e.g. aesthetical or historical) of the timber structure.
Necessary conditions for the inspection
The following conditions must be met:
a. Safety: the timber structure must provide reasonable safety level to walk on or walk under, otherwise propping or shoring is necessary;
b. Accessibility: the timber members must be made sufficiently accessible to allow for the assessment procedures to be carried out; Access will depend upon the nature of the structure itself and may be as basic as simple ladder or may require full scaffolding. Accessibility has a great influence on the choice of the inspection technique.
c. Lighting: proper light (quality and intensity) must be used to permit a correct visual examination of the joints, the timber members as a whole and the wood surface details. 6/26 Note that some control over the lighting may be necessary since surface features are sometimes easier to see with appropriate directional lighting rather than bright general light.
d. Cleaning: the surface of the wood must not be covered or concealed in any way by debris, dirt and dust. The surveyor must be adequately equipped to clean areas of the timbers as required by using dry processes (brushing, vacuum cleaning, air pressure, etc.). Note 1: In the case of decorated (painted) or covered timber members, wood surface accessibility and visibility may not be fully possible. In such cases, the inspection report must detail what information was not obtained and explain why. Note 2: Inspection is particularly important in highly stressed points of the structure, particularly areas subjected to high bending moments, near joints and at the supports. In all parts of the structure where a regular visual inspection is not possible, such as the ends of beams inserted into supporting walls, alternative inspection methods, e.g. by resistance drilling methods, or indirect assessment techniques should be carefully planned.