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WHAT ONE NEEDS TO KNOW FOR THE ASSESSMENT OF TIMBER STRUCTURES 5. Biological damage and estimation of cross sections Cross section dimensions are in principle easy to assess, although in the case of irregular round elements this may require some simplification. If biological degradation of timber is identified its overall and local effects on strength and stiffness of members and joints must be judged. This is normally a great source of problems for building interventors and a frequent justification for integral replacement of structures that would otherwise be kept in service. In the case of fungal attack, the actual strength loss varies with wood species, specific fungus, time and environmental conditions. The affected volume may be assessed by non-destructive methods. Models to predict strength and stiffness reduction have been tried, but the allocation of equivalent values for the affected section in a practical situation involves a high degree of uncertainty. Consequently, it is common to assume in practice a negligible contribution of those cross sections seriously affected by decay and to completely or partially replace those members or strengthen them as if they were already lost. In those cases where decay is incipient and it seems to affect just the surface of a member, that member may be kept in place if it is clearly overdesigned by <strong>report</strong> to calculated stresses. Timber affected by insects is a totally different matter, as the decrease of timber members’ properties result from a reduction of material in the cross section (rather than from a chemical modification of the wood cells). When attacked by beetles, it is common that only a surface layer of sapwood is destroyed. The depth of the attacked layer may be assessed with the help of a simple knife or screw-driver or by other nondestructive techniques. A reduced cross section may then be estimated. Alike the procedure followed for timber quality, safety checking may be carried out by assuming the same cross section reduction for all members in the structure, followed by a more detailed assessment and refined verification for specific members of higher importance or higher calculated stress levels. This procedure may however be too conservative, since even the damaged layer is likely to contribute to the load carrying capacity of that cross section. Although less frequent, insect attack may instead take the form of a diffuse damage throughout most of the cross section. This has been observed in softwood members with a small percentage of heartwood. Considering a reduced cross section is meaningless in this case. Alternatively, a reduced quality due to a reduced apparent density may be assumed for such cases. However, both approaches regarding insect damage involve a high degree of uncertainty since the real influence of insect attack in the strength and stiffness of timber members has yet to be clarified. 6. Conclusions The lack of proper methods to evaluate the strength of timber members on-site is a strong drawback shown by timber structures, as compared to concrete or steel structures. The assessment of old timber structures requires specific expertise and knowledge, given the need to quantify individual timber members and joints mechanical properties and effective strength. Regarding the timber members assessment, possible approaches basically include load tests and grading approach Load tests rely on existing correlations between the timber strength and stiffness in structural dimensions. Such correlations only exist for a few species. Even then, one has to assume that the same correlation found for new timber also applies for timber that grew in different conditions and that was subjected to unknown loading and environment through its service life. In the above “direct evaluation” of individual timber elements strength and stiffness, one assumes that the correlation between species + origin + (quality) grade and strength properties established for new timber also applies to old aged timber. However, this is yet to be confirmed. Besides, grading operations may be difficult in practice when there is no visual access to the ends and some faces of the timber, or when it is dusty or stained. Grading randomly chosen members may be an alternative in these situations but it introduces even another source of uncertainty. 300
WOOD SCIENCE FOR CONSERVATION OF CULTURAL HERITAGE The estimation of the effective cross section dimensions represents a huge problem. Options like assuming a nil contribution of decayed cross sections, or a reduced cross section in the case of beetles surface layer attack have been widely used in practice but may be too conservative. Although a number of approaches may nowadays be used to assess old timber structures, they involve steps that still require scientific validation. Wood Science should help to reduce the present degree of uncertainty associated to some necessary assumptions, thus improving safety while avoiding too conservative evaluations that may obstacle the maintenance of still safe structures. 301
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Proceedings e report 67
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Wood science for conservation of cu
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SUMMARY Introduction ..............
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WOOD SCIENCE FOR CONSERVATION OF CU
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INTRODUCTION These proceedings of a
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(A) MATERIAL PROPERTIES
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SORPTION OF MOISTURE AND DIMENSIONA
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VIBRATIONAL PROPERTIES OF TROPICAL
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CREEP PROPERTIES OF HEAT TREATED WO
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MEASUREMENT OF THE ELASTIC PROPERTI
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ELASTIC PROPERTIES OF MINUTE SAMPLE
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4. Results Young's modulus (10^4 Mp
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PREDICTION OF LINEAR DIMENSIONAL CH
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DIMENSIONAL CHANGE OF UNRESTRICTED
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DIMENSIONAL CHANGE OF UNRESTRICTED
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AGEING OF WOOD - DESCRIBED BY THE A
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90 30 0 3,0E-03 - 2,0E 2,0E-03 - 1,
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4. Application HYGRO-LOCK INTEGRATI
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RESEARCH ON THE AGING OF WOOD IN RI
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RESEARCH ON THE AGING OF WOOD IN RI
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Xylarium Database RESEARCH ON THE A
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AGING WOOD FROM CULTURAL PROPERTIES
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AGING WOOD FROM CULTURAL PROPERTIES
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STRENGTH AND MOE OF POPLAR WOOD (PO
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STRENGTH AND MOE OF POPLAR WOOD ACR
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STRENGTH AND MOE OF POPLAR WOOD ACR
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PHOTODEGRADATION AND THERMAL DEGRAD
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ROMANIAN WOODEN CHURCHES WALL PAINT
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DISINFECTION AND CONSOLIDATION BY I
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3. Results and discussion WOOD SCIE
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FUNGAL DECONTAMINATION BY COLD PLAS
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STUDIES ON INSECT DAMAGES OF WOODEN
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TERMITE INFESTATION RISK IN PORTUGU
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BIODETERIORATION OF CULTURAL HERITA
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4. Causes of biodeterioration WOOD
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DEGRADATION OF MELANIN AND BIOCIDES
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MOULD ON ORGANS AND CULTURAL HERITA
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RESEARCH STUDY ON THE EFFECTS OF TH
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4. Methodology WOOD SCIENCE FOR CON
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NON DESTRUCTIVE IMAGING FOR WOOD ID
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METHODS OF NON-DESTRUCTIVE WOOD TES
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MEASUREMENT AND SIMULATION OF DIMEN
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IN THE HEART OF THE LIMBA TREE (TER
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3. Conclusions WOOD SCIENCE FOR CON
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2. Materials and methods WOOD SCIEN
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3. Chemometrics WOOD SCIENCE FOR CO
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3. Results and discussion WOOD SCIE
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NIR SPECTROSCOPIC MONITORING OF WAT
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NUMERICAL SIMULATION OF THE STRENGT
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3. Methods and models WOOD SCIENCE
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SIMPLE ELECTRONIC SPECKLE PATTERN I
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5. Conclusions WOOD SCIENCE FOR CON
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Average r adiated presure field (dB
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EXPERIMENTAL AND NUMERICAL MECHANIC
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EFFECT OF THERMAL TREATMENT ON STRU
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The aims of this work are to: WOOD
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ultramarine [bad] titanium white [m
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4. Conclusions chrome yellow WOOD S
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Fig. 7 - A soaked lamella clamped i
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Page 253: (D) CONSERVATION
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Page 264 and 265: THE WRECK OF VROUW MARIA - CURRENT
Page 270 and 271: ROMANIAN ARCHITECTURAL WOODEN CULTU
Page 276 and 277: RESSURREIÇÃO DE CRISTO - PANEL FR
Page 278 and 279: ON 18 TH- AND 19 TH- CENTURY SACRIS
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Page 286 and 287: DEFIBRING OF HISTORICAL ROOF BEAM C
Page 292 and 293: EXAMINATION AND CONSERVATION OF TWO
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Page 296 and 297: THE CONSEQUENCES OF WOODEN STRUCTUR
Page 298 and 299: CONSEQUENCES OF WOODEN STRUCTURES C
Page 301 and 302: WHAT ONE NEEDS TO KNOW FOR THE ASSE
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Page 311 and 312: load (KN) Samples 6 5 4 3 2 1 WOOD
Page 313 and 314: STRUCTURAL BEHAVIOUR OF TRADITIONAL
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Page 329 and 330: INVESTIGATION ON THE UTILIZATION OF
Page 337 and 338: 3. Results and discussions WOOD SCI
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Page 345 and 346: ANNEX 1: FACTS ABOUT COST ACTION IE
Page 351 and 352: Keynote presentations WOOD SCIENCE
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