tecnologia de la madeera mecanica .pdf

8/15/2019 Tecnologia de la madeera mecanica .pdf

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La tabla 1, adaptada de Sotomayor (1987), presenta un esquema general para identificaruna característica mecánica y su significado físico.

Tabla 1. Esquema para identificar una característica mecánica y su significado físico.

EspecieCaracterísticas

físicas dereferencia

Característicamecánicaanalizada

Solicitación aplicada ObservacionesTipo Orientación Velocidad

Nombre botánico

Nombrecomún

DensidadModulo deElasticidad

Modulo deRigidez

Modulo deRuptura

Resistencia allimite elástico

Flexión

Tensión

Compresión

Cortante

Relajamiento

Flujo

RadialEstática

Dinámica

De caráctertecnológico

Variabilidadnatural

Deterioro

Contenido dehumedad Tangencial

Temperatura Longitudinal

Del examen de los textos sobre construcciones con madera de Robles y Echenique-Manrique (1983) y de Sotomayor (1987), amén de la recopilación de informacióntecnológica presentada por Zizumbo (1998), notamos que las características mecánicas propuestas para la industria de la construcción no prevén los casos de diseño para regionesde riesgo a sismos. Igualmente, la inalterabilidad de estas características a través del tiempono está muy estudiada aun en publicaciones especializadas. Una propuesta interesante deinvestigación básica en la filial experimental sobre estructuras de madera, es el estudio delcomportamiento dinámico y diferido de materiales compuestos de madera, relacionado concondiciones de servicio excepcionales tales como vibraciones y temperaturas elevadas.

¿CÓMO SE DETERMINAN LAS CARACTERÍSTICAS MECÁNICASDE LA MADERA?

La madera proviene de árboles que han crecido en ecosistemas forestales y en climas quevan desde la tundra, en latitudes extremas, hasta regiones de selva cálida – húmeda cerca delas costas y trópicos. Como consecuencia, existe una gran diversidad cualitativa de madera, producto de esta pluralidad de ecosistemas y particularidades climáticas.

El ingeniero, el arquitecto y el usuario de la madera como elemento estructural, requierende información confiable y estandarizada de la resistencia mecánica del material,(American Society of Civil Engineers, 1975; Sotomayor, 1987). Esto con la finalidad deoptimizar los procesos constructivos y de diseño, así como de mejorar la calidad y lafabricación de productos donde la madera juega un papel substancial.

Para resolver esta paradoja, las características tecnológicas de la madera son obtenidasaplicando ensayos normalizados y en otras ocasiones los investigadores utilizan procedimientos o ensayos exploratorios destinados a dar respuesta a dificultades particulares de diseño o análisis.


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Los ensayos normalizados son los procedimientos aceptados por la comunidad científica eindustrial que garantizan la calidad de las conclusiones al utilizar rigurosos muestreosestadísticos y controlando los factores o variables que pudieran distorsionar los resultados.

Estas prácticas establecen las reglas acerca de cómo la madera debe ser preparada y

sometida a experimentos para medir su comportamiento en condiciones de laboratorio ocampo. Las normas son prescritas por instituciones académicas, asociaciones acreditadas yotras veces por las mismas industrias. Las normas utilizadas frecuentemente son las normasASTM (American Society for Testing and Materials, 2000), las normas DIN (DeutchesInstitut Für Normung, 1988) y las normas ISO (International Organization forStandardization, 1975).

Los informes relativos a las características de resistencia mecánica de diferentes especies demadera provienen de varios autores que han publicado una enorme cantidad de informaciónreferente a la caracterización tecnológica de la madera. Todos ellos han utilizado distintasnormas de ensayo y diferentes metodologías de evaluación, dificultando con ello suutilización.

Un esfuerzo en la normalización de criterios de clasificación y en la estandarización para lacaracterización mecánica de la madera, promete tener un buen porvenir en la investigacióntecnológica de especies con posibilidades de industrialización.

La tabla 2, presenta valores de características mecánicas de la madera de especiesmexicanas (Sotomayor 1987). Para cada especie se anota la densidad de la madera comocaracterística física de referencia. Los datos fueron determinados condiciones ideales delaboratorio y utilizando ensayos mecánicos normalizados. Las probetas de madera tenían pequeñas dimensiones, no presentaban anomalías de crecimiento, estaban libres de defectosy su contenido de humedad fue del 12 %.

Tabla 2. Características mecánicas de la madera de especies mexicanas.Especie

Nombre botánico Nombre común

Densidad

kg / cm3

GRT

kg / cm2

Flexión Estática

MOEkg / cm2

MORkg / cm2

RLEkg / cm2

Swietenia macrophyllaCaoba 0.42 1 800 95 850 535 220

Calophyllum brasilienseBarí 0.52 1 860 110 130 800 520

Pinus leiophyllaPino chino 0.46 905 107 000 800 480

Pinus ponderosaPino ponderosa 0.41 760 94 900 820 290Quercus coccolobifolia

Encino rojo 0.60 2 340 118 120 955 624

Quercus martineziiEncino blanco 0.62 2 470 122 050 945 646

R: dirección radial; T: dirección tangencial


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¿DÓNDE Y CÓMO SE EMPLEAN LAS CARACTERÍSTICAS MECÁNICASDE LA MADERA?

La construcción con madera se respalda en las Ciencias de la Ingeniería. Las característicasmecánicas de la madera se utilizan durante el diseño y calculo de estructuras tales como

puentes, armazones para techos, pisos y muros, o particularmente en el estudio deelementos específicos de estructuras, como por ejemplo, pisos, puertas, escaleras yventanas.

De acuerdo con las proposiciones de Breyer (1980) y del American Institute of TimberConstruction (1974), la aplicación práctica se desarrolla de la manera siguiente:

A partir de la determinación de una cota mecánica resultado de un ensayo normalizado, porejemplo: el Modulo de Elasticidad en flexión estática, se le asocia una serie de ajustes paracorregir la influencia de las condiciones de servicio tales como la humedad, la temperaturay la duración de la carga.

Estas correcciones se hacen aritméticamente utilizando coeficientes que aumentan odisminuyen el valor numérico de la característica en cuestión. Una vez obtenido un valorajustado, se procede a la rectificación tomando en cuenta las dimensiones del elemento adiseñar. Finalmente esta operación sufre una última transformación para considerar todoslos factores que pueden influir debido a la calidad tecnológica de la madera.

Este procedimiento facilita proyectar las dimensiones correctas de la estructura, así como laselección conveniente de la especie y la calidad de la madera a utilizar, optimizando así eltiempo y costo del proceso constructivo.

La ecuación general que contiene los coeficientes usados para ajustar los datos provenientesde pruebas mecánicas, y que sirve también para calcular un esfuerzo permisible, esconforme a la relación sugerida por la American Society of Civil Engineering (1975),Bodig y Jayne (1982) y la American Society for Testing and Materials (2000), y tiene laforma:

F* = Fx k T x k s x k p x k d x k g x k c x k f (1)

Donde:

F* = Esfuerzo permisible ajustado a un diseño en condiciones estructurales de servicio

particulares. Este valor es requisito en los reglamentos y las normas oficiales relacionadascon los aspectos de diseño y uso de la madera en la industria de la construcción.

F = Valor del esfuerzo permisible listado en la bibliografía, establecido bajo un criterio probabilístico de ocurrencia y una norma estándar utilizada para su determinaciónexperimental. Este valor es el concepto empleado a lo largo del manuscrito, llamadocaracterística mecánica para una especie en particular.


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k T = Factor de corrección para la duración de la carga, debido al comportamiento mecánicodiferido de la madera. Generalizando se puede afirmar que la rigidez de la madera es mayorcuando la velocidad de solicitación es grande y su resistencia puede verse disminuida con eltranscurso del tiempo. Este coeficiente prevé el diseño a través del tiempo y advierte posibles alteraciones futuras en la estabilidad estructural del proyecto.

k s = Factor de corrección para seguridad, relacionado con la variabilidad natural de lamadera, pues la información tabulada proviene de pruebas con probetas de madera de pequeñas dimensiones, libres de defectos y en condiciones ideales de ensayo. Por elcontrario, la madera comercial no esta exenta de peculiaridades tecnológicas. Además, unavez sometida a condiciones reales en una construcción, la madera es susceptible dealteraciones importantes.

k p = Factor de corrección para condiciones especiales de servicio, relacionado con lageometría de la pieza y el tipo de ensayo del cual proviene el esfuerzo permisible calculado.

k d = Factor de corrección para defectos, función del tipo, magnitud, frecuencia ycombinación de irregularidades tales como nudos, desviación del grano, rajaduras,ondulación y degradación biológica.

k g = Factor de corrección para la calificación estructural, relacionado con una apreciaciónde la densidad de la madera, de su velocidad de crecimiento y de la presencia de tejido demadera tardía.

k c = Factor de corrección para el contenido de humedad del material de ensayo,normalmente 12 % en el contenido de humedad.

k f = Factor de corrección para contenido de humedad en servicio, dependiendo de laexposición a la intemperie o a fuentes cercanas de humedad.

Los valores de los coeficientes no son fijos. Para condiciones de trabajo específicas, paradiferentes especies y para diseños con productos compuestos, la relación (1) debeadecuarse. Cada factor de esta ecuación, se transforma para cada caso en particular. Esrecomendable la consulta de la bibliografía al respecto: Breyer (1989) y Faherty yWilliamson (1989), donde se proponen tablas con valores con arreglo a la calidad de lamadera y particularidades del proyecto de diseño.

Otras aplicaciones importantes del calculo propuesto por la ecuación (1), es el diseño devigas laminadas y placas estructurales y en general, en soluciones arquitectónicas para proyectos complejos.

Existen además otros métodos o formas de cálculo para madera y sus derivados, como se puede consultar en el manual editado por el American Institute of Timber Construction(1974). Sin embargo, los datos utilizados provienen mayoritariamente de madera ideal,


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cuando el componente utilizado realmente por el constructor es la madera comercial, producida industrialmente y a veces, conteniendo anomalías de crecimiento.

Una propuesta interesante de investigación tecnológica en Ciencias de la Madera es elestudio de madera de calidad, presentación y dimensiones comerciales comúnmente

encontrados en el mercado de la madera. El usuario de la madera podrá así beneficiarse conlos resultados de investigación más adaptados a sus necesidades y por consiguiente,estimular el uso extensivo del material en la industria.

La investigación sobre mecánica de la madera es una veta inagotable de sorpresas y nuevosretos. La Biomecánica, es decir, el estudio del comportamiento mecánico de la madera vivacuando esta forma todavía parte del árbol, es una interesante línea de investigación básica para jóvenes investigadores en la especialidad. Los resultados biomecánicos de la madera podrían explicar muchas de las interrogantes actuales sobre la materia.

CONCLUSIONES

Una primera conclusión de este esbozo acerca de las características mecánicas de la maderay su aplicación en la industria de la construcción, es que la madera tiene una vocaciónnatural para su incorporación en el proceso constructivo. Sus características tecnológicas le permiten funcionar mecánicamente como un material de uso estructural. Estos datos provienen de ensayos normalizados y cuando son correctamente ajustados, son confiables para el cálculo estructural.

Es importante señalar que, si bien los criterios de diseño y calculo tienen por vocación prevenir las posibles alteraciones en la resistencia de la madera una vez instalada enservicio, la madera requiere necesariamente de mantenimiento continuo para preservardentro de limites razonables, su integridad estructural

La industria de la construcción requiere de información confiable y estandarizada sobre lacalidad tecnológica de la madera, para así poder incorporar este material al proceso deedificación. Esta conclusión implica a su vez proponer que la investigación delcomportamiento mecánico de la madera debe orientarse hacia las necesidades propias de laIndustria.

BIBLIOGRAFÍA

American Institute of Timber Construction. 1974. Timber Construction Manual. 2nd Edition. Wiley, New York, USA.

American Society for Testing and Materials. 2000. Annual Book of ASTM Standards.Section Four; Construction, Volume 04.10, Wood. USA. 707 p.

American Society of Civil Engineers. 1975. Wood Structures: A Design Guide andCommentary. New York, USA.


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Bodig, J.; Jayne, B.A. 1982. Mechanics of Wood Composites. Van Nostrand ReinholdCompany. USA. 711 p.

Breyer, D.E. 1980. Design of Wood Structures. McGraw-Hill, Inc. USA.

Brown, H.P.; Panshin A.J.; Forsaith C.C. 1952. Textbook of Wood technology. Volume II.McGraw-Hill, Inc. USA. 783 p.

Deutches Institut Für Normung (DIN). 1988. Normen über Holz. Beuth Verlag. Berlin-Köln. B.R.D. 240 Seiten.

Faherty, K F.; Williamson, T.G. 1989. Wood Engineering and Construction Handbook.Second Edition. McGraw-Hill, Inc. USA.

Guitard, D. 1987. Mécanique du Materiau Bois et Composites. Cepadues-Editions, France.238 p.

International Organization for Standardization (ISO). 1975. ISO 3129-1975 (E). Wood – Sampling methods and general requirements for physical and mechanical tests. Publicadoen Internet, disponible en www.iso.ch/iso/en/ISOOnline.openerpage.

Kollmann, F.F.P.; Côté, W.A. 1968. Principles of Wood Science and Technology. VolumeI: Solid Wood. Springer-Verlag, Berlin, Germany. 592 p.

Kollmann, F.F.P.; Kuenzi, E.W.; Stamm, A.J. 1975. Principles of Wood Science andTechnology. Volume II: Wood Based Materials. Springer-Verlag, Berlin, Germany. 703 p.

Mattheck, C.; Kubler, H. 1997. Wood – The internal Optimization of Trees. Springer, NewYork, USA. 129 p.

Panshin, A.J.; De Zeeuw, C. 1964. Textbook of Wood Technology. Volume I. McGraw-Hill, Inc. USA. 705 pp.

Robles Fernández-Villegas, F.; Echenique-Manrique, R. 1983. Estructuras de Madera.Editorial LIMUSA. México. 367 p.

Sotomayor Castellanos, J.R. 1987. Calidad de la Madera para la Industria de laConstrucción. Consideraciones Tecnológicas, Industriales y Comerciales. Cámara Nacionalde la Industria de la Construcción. Morelia, Michoacán, México. 141 páginas y anexos.

Zizumbo Cortés, F. 1998. Estudio Tecnológico de Pithecellobium ebano (Benth.) Berlan.,como fundamento para su aprovechamiento racional. Tesis de Maestría. Facultad deIngeniería en Tecnología de la Madera. Universidad Michoacana de San Nicolás deHidalgo. Morelia, Michoacán. 159 páginas y anexos.


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ANCIENT WOOD STRUCTURES EVALUATION METHODOLOGY

Javier Ramón Sotomayor CastellanosFaculty of Engineering in Wood Technology.

Universidad Michoacana de San Nicolás de Hidalgo, MÉXICO.

José Cruz de LeónFaculty of Engineering in Wood Technology.

Universidad Michoacana de San Nicolás de Hidalgo, MÉXICO.

SUMMARY

During the evaluation of ancient wood structures already erected, the restorer takes theresponsibility of working with a framework built using old technology. This paper reviewsliterature and criteria pertaining to wood structures, with an emphasis on diagnosis of thewood technological condition, the mechanical state of the structure and its preservation,consolidation and maintenance. The aim of this essay is to contribute to the establishmentof methodological criteria for the evaluation of wood functioning as a structural element inhistorical and cultural patrimony edifices. The objective of the methodology is to help torestorer with the background necessary to realize the restoration tasks and maintain thehistorical and cultural value of the structure. The methodology has three components: thediagnosis of the state of the structure; the evaluation of the structural system and therestoration treatments required to preserve the system’s vitality. This synthesis includes thekey elements necessary to carry on anatomic, physical and mechanical examinations ofwood. It includes as well, the therapeutic criteria for restoration and conservation. Someconcepts and technical propositions could be applied – within certain limits- to the study ofancient wood furniture, window and door frames, wooden art objects, archeologicalobjects, paintings on wood boards and wooden altars. Laboratory and experimental tests onthe validity of the fundamental methodology’s hypothesis should be adapted to everyspecific case in study.

Keywords: Wood Structures, Wood Decay, Wood Restoration, Wood Preservation, WoodStructures Evaluation, Technical Diagnosis, Wood Structures Maintenance.

INTRODUCTION

During the evaluation of structures already erected, the restorer takes the responsibility ofworking with a framework built using old technology, and many times the projectemployed empiric design criteria. To achieve the renovation, it is often necessary to applymodern technology, but in some cases it is better to conform to the native treatmentsapplied when the original edifice was created. The employment of many of thesetechniques is difficult nowadays because many have been forgotten. The aim of this essayis to contribute to the establishment of methodological criteria for the evaluation of woodfunctioning as a structural element in historical and cultural patrimony buildings.

The objective of the methodology is to help the restaurateur with the background necessaryto realize the restoration tasks and maintain the historical and cultural value of the structure.


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In addition, the methodological aim is to secure the structural stability of the edifice and toguarantee the users’ safety. Also, the method proposes – for every case of study- amaintenance program to avoid any future deterioration of the wood. The outcome of thetechnique employed should provide the stabilization structure for its contemporary use. Atthe same time, the restorative methods should corroborate the structure’s original and

technological condition.The methodology has three components: the diagnosis of the state of the structure; theevaluation of the structural system and the restoration treatments required to preserve thesystem’s vitality. (Figure 1). This paper is presented using tables and co nceptual maps tosynthesize the criteria of evaluation and the information for restoration strategies. Thedescription of techniques and methods are fully explained in the references cited.

Figure 1. Ancient wood structures evaluation methodology.

DIAGNOSIS

The objective of the diagnosis is to identify the technical condition of the structure inrelation to the past and present service conditions. The diagnosis is divided into threeapproaches: constructive antecedents, system characteristics, and execution needs (Figure1). Wood structures evaluation and restoration techniques references can be found in[4][13][16][25][26][27][28][35][36][38][42][46][49][55].

The destructive agencies that attack wood can be classified as follows [22]: mechanicalwear, decomposition by fire or prolonged heating, chemical attack, fungal decay, insectattack and attack by marine borers, birds, etc. Figure 2, adapted from [46], shows stressing

EVALUATION

DIAGNOSIS

Constructiveantecedents

Executionneeds

Systemcharacteristics

RESTORATION

Technologicalwood

condition

Contiguoussystems

condition

Mechanicalstate of thestructure

Preservationtreatments

Maintenancetasks

Consolidationworks


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elements, influence factors and weathering effects that contribute to wood-finishing performance, and should be taken into account for diagnostic purposes.

Figure 2. Stressing elements, influence factors, and weathering effects in wood-finishing performance.

Constructive antecedents

Wood is a biological material used in construction. As a consequence, wood exposed toadverse conditions for a period of time can deteriorate both aesthetically and functionally[37]. This corollary has unfavorable consequences for user safety, structural stability andfor the historic and cultural value of the building.Constructive antecedents’ analysis references can be found in [1][4][36].

Time factors, biodeterioration and changes in the use of the building modify the initialdesign condition [19]. In regard to the constructive antecedents, it is recommendable tostudy the original structural and architectural layouts. It will help to compare the ancientand the present characteristics of the structure with the objective of not destroying theintegrity of the structure, its aesthetics, and its research potential [20][21]. Further, it willhelp to identify problems or modifications done to the building that have affected the wood.

Regional weather information, seismic and flood histories help to understand the periodicor sporadic physical factors which have acted on the structure and the consequentappearance of biological agents stressing the wood. Insect attack often follows fungal decayand both types of damage may be found in the same piece of wood. Conditions leading tofungal decay often encourage attack by insects, and partially decayed wood appears to be particularly attractive to many kind of beetles. Since the treatment required to eradicateinsects is different from that needed to arrest fungal decay, it is important to be able to

Weathering effects

Stressing elements

Photo-irradiation

Thermal radiation

Moisture changes Micro-organisms

Wind, Rain Fall, Dust, Pollution

Wood performancefinished or unfinished

Influence factors- Water exposure- Time- Wood properties- Quality of design- Properties of finish- Maintenance

Photochemicaldegradation Dimensional changes:deformations, cracks,leaching, hydrolysis

Heating Discoloration Erosion


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distinguish between the two types of attack, and to determine which is still active when thecurative measures will be applied [22]. There are occasions when the damage caused bychemical attack may simulate fungal decay. An emphasis should therefore be put on theneed for accurate diagnosis of the cause of deterioration before any remedial measures aretaken.

System characteristics

From a technical point of view, a wood structure is a mechanical system formed byelements (beams and columns) and connectors (metallic or other), forming a constructiveframework with the capacity to support deformations and structural loads, independently ofthe design criteria [1][7][8][31][48]. An ancient wood structure is defined in this paper as aconstitutive part in a building with considerable antiquity that could be classified as a partof the historic and cultural heritage. Characterization references about structural woodsystems can be found in [3][8][19]. This mechanism interacts with other componentsadjacent to the structure and these constituting the entire edification. The description refersto roofs, plafonds, walls, floors and escalators, as well as to isolated elements acting as beams and columns. Other items that could be considered to be wood structures are bridges, platforms, mezzanines and decks.

A detailed inspection of the wood members and the structural connections withedification’s walls and columns is recommended. A preliminary identification of weakenedmembers or fragile connections and the extent of general system damage should beforecasted. A specialist should do this task.

The principal wood structure components to check are [39]: embed elements, unions andconnectors, ground contact parts, humidity and liquids conducts, roof covers and exteriorwindows and doors. The system characteristics to be outlined in the diagnostic report are:structure geometry, member dimensions and spans, apparent load capacity, lateralconnections with walls and basements and the association of wood with other materials.

Execution needs

Execution needs relate to financial requirements for material and wood to restore thesystem. A cost analysis will lead to different scenarios: if it is a live attack (presence ofdeterioration agents) or a dead attack, or if it is necessary to renovate or to substitute certainaccessories from the structure. A budget elaboration with the laboratory, specialist andviaticum fees should be carefully considered.

Also, at this time a first evaluation of techniques and the equipment required to carry outthe restoration in the evaluation phase is suggested. In addition, a plan concerning woodspecies and qualities necessary to repair or to substitute damaged members should beestablished.

The result of the diagnosis is a report containing the historical uses, reparations andconstructive modifications done to the building. The report should outline the systemcharacteristics and to propose the operation’s program containing the budget, techniques


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and the timing necessary to evaluate the full process. A photographic record file should gowith the dossier. Legal issues are also important to assure good restoration projectdevelopment.

EVALUATION

To carry out a technical evaluation of ancient structures built with wood, an abstraction isnecessary of the aesthetic, architectural, ambience and functional meanings of the edifice. Nevertheless, a wood structure not can be evaluated as an independent practice withoutconsidering other structural parts of the facility. The scope of this proposal is limited to thetechnological condition of wood, concentrating on the mechanical state of the structure.The objectives of the evaluation phase are to render precise the technological condition ofthe wood members, to calculate the mechanic state of the structure and to evaluate thestability of the contiguous system that could influence the framework in study. Theevaluation phase consists of three approaches: technological wood condition evaluation,structure mechanical state study and contiguous systems condition inspection. Evaluation practices information can be found in [24][43][44][47].

Table 1. Objectives, criteria and techniques used in wood structures evaluationObjectives Criteria Techniques Place

Technological wood condition

Specieidentification

Softwoods Hardwoods Macro-anatomic observation In situ / lab.

Genus Specie Variety Micro-anatomic observation LaboratoryUltra-cellular observation Laboratory

Particularitiesexamination

Growth speed Technical classification In situ / lab. Notches and holes presence Classification In situ / lab.Cracks and fissures presence Technical classification In situ / lab.Load history Technical investigation In situ / lab.Superficies state Laboratory analysis Laboratory

Decay inspection

Weathering – discoloration Technical test LaboratoryChemical attack – carbonization Chemical analysis LaboratoryBacteria Microbiologic analysis LaboratoryFungus Microbiologic analysis LaboratoryInsects Microbiologic analysis In situ / Lab.Chemical structure alteration Chemical analysis Laboratory

Mechanical state of the structure

Physical andmechanical state

verification

Moisture content M. C. determination In situ / Lab.

Stability analysis Structure Free body diagram LaboratoryEdification Members and junctions LaboratoryStrains and deformations Standards & code application In situ / Lab.Stresses Structural calculus In situ / Lab.

Loads Static loads Analysis In situ / Lab.Time effect Dynamic loads Analysis In situ / Lab.Contiguous systems condition

System revision Integral performance of theedification.Visual inspection andInstrumental inspection

In situ andLaboratory

Natural wood durability and variability properties are two factors to consider in ancientmaterial evaluation, along with tree growth conditions and natural anomalies in wooden


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elements which require technological considerations to assess wood quality [9][34].Additionally, ancient wood buildings present a wide variety of constructive genera,architectural styles and constructive system categories. In effect, it is difficult todeterminate the deterioration state of a structure. Every case of study should be evaluated inaccordance to specific criteria for each case. The evaluation phase output should to confirm

the structure degradation condition (type of attack identification and the spread magnitude),to determinate the appropriate restoration treatment. Table 1 summarizes objectives, criteriaand techniques useful in wood structures examination. This table also shows where testdevelopment is recommended: in situ using the equipment adequate, and if necessary, in alaboratory which specializes in wood analysis.

Technological wood condition

Concerning the wood’s technological condition, the first procedure is to identify the woodspecies constituting the structure using identification keys and archive material of woodsamples and complementing this information with wood micro-anatomy and imagetreatment techniques. Wood botanic specie identification is useful for determiningtechnological properties and strength characteristics with regard to the wood condition andmechanical vitality of the structure [12]. Wood does not deteriorate as a result of agingalone, but if timber is exposed to consistently damp conditions it will sooner or later,assuredly become decayed. Moisture content in critical frame positions is a parameter toconsider during evaluation. Hygrometers and acoustics techniques are useful [10]. Thetechnological condition of every wooden member should be evaluated. Also, weatheringeffects, decay presence, type of attack and its extent, physical properties (density, moisturecontent); dry defects (checks, twist, and fissures) should be checked.

The wood decay process is a consequence of physical factors: weathering (photo-degradation, leaching, hydrolysis, shrinkage and swelling), and wood moisture contentvariation [20][21]. Construction factors such as inappropriate wood construction design promotes wood deterioration [39], and pertinent wood specie and quality choice in the building project are also influencing deterioration factors in wood structures.

Wood bio-deterioration and colorlessness due to the action of microorganisms are biological factors [17][45][55]. Other factors that affect wood decay are the absence of orinadequate preservation treatment and deficient surface-finishing techniques.

The combined effect of all these factors on wood condition promotes poor vitality of thematerial forming the structure’s resistant elements, with its consequential loss of structuralstability and mechanical wood strength, and the loss of aesthetics [54]. Figure 3 shows asimplified diagram of the decay process of wood. Table 2 is adapted from [56], and showsthe mayor types of wood damage, their descriptions and the criteria of prevention orcontrol.

Wood decay detection techniques, wood evaluation, standard destructive tests and non-destructive tests are documented in the literature [2][18][32][41]. Acoustic and stress wavetechniques [24][43][44], computer assisted and traditional anatomy analyses [12][15], andthe presence of microorganisms in wood using chemical indicators techniques [45][52].


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Table 3, adapted from [56], presents the applicability of various wood-inspectiontechniques for detection of incipient, early, intermediate or advanced stages of internaldecay. When decayed timber is discovered in a structure, the first step is to find out whichfactors or agents have caused the problem. A through and careful survey must be made tofind out which fungus has been responsible and how far the fungus has spread.

Figure 3. Wood decay process.

Mechanic state of the structure

Regarding the mechanical state of the structure, modern reliability-based design criteria ofwood structures should be utilized [7][37][48]. Likewise, ancient wood quality must beevaluated the same as a new, sound and performing wood [54]. Wood mechanics andstructures behavior information can be found in [3][6][23][51].

Concerning the structure’s efficiency, the parameters to estimate are: load charges anddeflection requirements in building codes; span between frames forming the system;fasteners; interior and lateral supports; basement state; and framework embedded [53].Concerning wood performance, the principal modifications to evaluate are weight loss,dimension variation and rigidity decrease, changes in hygroscopicity and permeabilityattributes and in electrical and acoustic properties [33]. In fact, a comparative study of

original or optimal mechanical stability of the structure and the actual conditions will leadto an evaluation of the performance of the structure.

Contiguous systems condition

The inspection of the contiguous systems condition of the structure, such as walls, platforms, plafonds, doors, windows and stairs is recommended. Also, the verification ofelectrical, heating and air-conditioned systems could play an important roll in the

Constructive Factors

Wood surfaceerosion

Wood tissuedamage

Cracks, Holesand Galleries

Resistant sectionreduction

Aesthetic value loss

Mechanical overloads

Physical Factors

Structural instability Stress and Strain

Wood decay

Chemicalstructure decay

Cell walldeterioration

Biological Agentsarrival


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structure’s behavior. Additionally, noise control and fire security installations and thermalisolation and roof covers should be checked [8].

Evaluation methodology components are systematized in table 4. Evaluation criteria toqualify the technologicalwood condition, the structure’s efficiency and the contiguous

system’s vitality should be contrasted with the performance expected, and in that way, astrategy for restoration can be proposed [4][40].

Table 2 Major type of wood damage and their descriptionsType of damage Causal agent(s) General description Prevention or control

WeatheringUltraviolet light, oxidation,

swelling and shrinkage,leaching, and fungi

Unprotected surfaces develop a gray colorand roughened texture

Ultraviolet light-resistantcoatings

Thermaldecomposition High temperature

< 200˚C, uniform surface brittleness >200˚C, charcoal in absence of oxygen,

combustion around 275˚C Fire-retardant chemicals

Chemicaldecomposition Caustic chemicals

With acids wood turns brown, chars, and becomes brittle; with bases wood bleaches

and defibrillatesChemically resistant woods

Mechanicaldamage

Mechanical forcesrupturing wood tissues

Selective surface erosion in heavy frictionzones; non-allowable stress and strain

High-specific-gravity woods,edge grain, right design or

chemically hardened woods

Insect damage

Termites Localized honeycomb cavities, woodsoiled and filled with frass Insecticides orkeep wood dryBorers Tunnels, cavities and pinholes

Ants Localized honeycomb cavities, woodchannels clean

Marine borerdamage

Shipworms Interior tunnels with lime-coated wallsProtective surface barriers oruse wood preservativesPholads Large interior tunnels near surfaceGribbles Surface tunneling in tidal zone

Decay FungiWhite fibrous pockets or punky texture.

Brown fibrous pockets or cubical checking pattern.Soft surface embrittlement and

exfoliation in small fragments

Keep wood dry oruse wood preservatives

Molds Fungi Colored spores or mycelium on the woodsurfaceDry wood or

use protective chemicalsStains FungiSapwood discolored gray, black, brown, blue and intensified in ray parenchyma

Ray cell and cell-wall damage Bacteria

Soft surface, ray cells destroyed,microscopic tunnels in cell walls

Table 3 Applicability of inspection techniques to detect internal decay in woodStages of Internal Decay in Wood

Incipient Early Intermediate AdvancedSmall Scale Mechanical Tests

Acoustics TechniquesFull Scale Mechanical TestsComputer-aided Axial Tomography ScanningX-ray

ShigometerChemical Indicators

CulturingMicroscopic Examination

Drill BitIncrement Borer

Hammer


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RESTORATION

The objective of the restoration methodology component is to propose the proceduressuggested by the findings of the diagnosis and evaluation, in the attempt to recover or tomaintain the vitality of the structure. As a result, it should deliver a work program

suggesting the material and the equipment necessary and a budget, as well as the qualifiedtechnicians needed to carry out the tasks [26]. To apply restoration therapy it is necessary toconsider first of all the possible modification to the aesthetic, historic and cultural worth ofthe structure. If the treatment proposed could modify the cultural essence of the structureunder study, finding the right procedure to implement is recommended. This restorationmodule has three approaches: preservation, consolidation and maintenance, and they can beseen as different methods, signifying that a restoration strategy should follow only preservation or a maintenance procedure. On the other hand, restoration should be seen asan investment and an insurance policy. Wood restoration techniques are well reported in theliterature [30][50]. If decay is detected in a structural member before significant strengthreduction occurs, then remedial treatments can be used to arrest the decay [28].

Concerning the structure’s restoration proposal, the following criteria should be con sidered[46]: respect for the material and historic authenticity of the framework; the application ofreversible restoration techniques; maintaining the essential nature of the cultural good as ahistorical testimony and the scientific rationalization of the cultural interpretation for theobject.

Table 4. Ancient wood structures evaluation criteria.Feature or propriety Performance& criteria

Strategy or action proposedConsolidation Substitution Preservation Maintenance

T e c

h n o

l o g

i c a l w o o

d c o n

d i t i o n Weathering & Decay State

Incipient - - - NecessaryIntermediate Suitable - Advised -Advanced - Necessary - -

Wood Weight Lost(%)

Less that 5 Suitable - - -5 - 10 - - - NecessaryMore that 10 - Advised Advised -

Wood Moisture Content(%)

Less that 8 - - - -8 – 18 - - Suitable Necessary

More that 18 - - Necessary Necessary

Shrinkage and Swelling Acceptable - - Suitable AdvisedOut standards Necessary Advised - Necessary

Surface state & Appearance Colorless - - - AdvisedRoughened - Suitable Advised Advised

S t r u c t u r e

e f f i c i e n c y

General Stability Steady Suitable - - AdvisedUnsteady - Suitable - -

Stress & Deformations Allowable Advised - - -

Unallowable - - - NecessaryMaterial Discontinuity Fissures Advised - Advised NecessaryKnots-Holes Advised - - Necessary

C o n

t i g u o u s

s y s t e m s

Walls, Floors, Windows,Stairs & Basements

Deteriorated Advised Suitable Advised NecessaryReliable - - Suitable Suitable

Electrical and Heatingsystems

Damaged - Advised - NecessaryReliable - - - Advised

Aeration and SewerSystems

Damaged - Advised - NecessaryReliable - - - Advised


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Preservation treatments

Surface finishing or penetration preservation treatments are alternatives to approach the preservation strategy. The purpose of in-place or supplemental treatments is to prevent orarrest decay in existing structures [28]. Further, wood permeability properties and toxicity

factors should be taken into account [5]. Preservation technology information can be foundin [38][42][50].

Table 5, adapted from [39], presents several examples of specific cases of timber usage in building, the risk categories and methods of preservation. These risk categories are thePreservation Essential (PE), which applies when timber is exposed to a continuallyhazardous environment and can not be protected by design, when building regulations prescribe treatment, and when there is a high risk of decay or insect attack in structures, thecollapse of which would constitute a serious danger to persons or property. The preservation desirable (PD) applies where experience has shown that there is a high risk ofdecay, whether due to the nature of the design or the standard of workmanship andmaintenance, and when there is a substantial risk of decay or insect attack, which, if itoccurs, would be difficult and expensive to remedy. The preservation optional (PO) applieswhen there is a low risk of decay or insect attack, or where remedial action or replacementis simple. The criteria proposed in the table should be treated as guidance notes rather thaninviolate rules. The desirable degree of penetration for a particular risk category andtreatment is also proposed in table 5: Penetration Superficial (PS): 1-2 mm. from surface;Penetration Medium (PM): 50% into the transversal section; Penetration Deepest (PDS):90-100% into the transversal section.

The most used types of preservatives for wood structures can be listed as [30][50]: Oil- borne (Creosote, Pentachlorophenol, Tributylin oxide, among others), waterborne(Chromated cooper arsenate and ammoniacal copper zinc arsenate) and new biocides.Another approach to wood preservation is biocontrol using microorganisms rather thattoxic chemicals. Currently, all commercial wood preservative formulaes contain chemicalsthat are toxic to microorganisms and insects [50]. In order to be commercially viable wood preservatives, biocide formulations must have the following characteristics: costeffectiveness, good permanence in the wood under all conditions, no significant effect onthe strength properties of wood, low corrosivity to metal fasteners, good penetration properties, safe to handle and use, low mammalian toxicity and non detrimental effects onthe environment.

Preservation application techniques could be classified as follows [42]: fluid penetration,where the principal variables to control are treatment time, temperature and pressure;superficial treatments and immersion treatment (including diffusion and osmose processes); pressure and vacuum treatments; and double vacuum process, among others. Wood preservation technology and standards are documented in [2][5][27][28][29].


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Table 5. Examples of timber usage in buildings, risk categories, treatments of preservationand desirable degree of penetration.

Examples Risk category Acceptable treatment Penetration

Roof timbers in house longhorn beetle areas PE

CCA (5-3 kg/m ). MediumBoron diffusion. Medium

Double vacuum OS. Deepest

10 min OS immersion SuperficialTimber embedded in ground concrete PE CCA (5-3 kg/m ). MediumTimber in ground cills in contact with brickwork orconcrete below damp-proof course. PE CCA (5-3 kg/m

3). Medium

Timber ground plates directly on top of damp proof course PECCA (5-3 kg/m3). MediumBoron diffusion. Deepest

Double vacuum OS. DeepestPitched roof timbers in normal and domestic cases PO - SuperficialWall plates in normal and domestic cases PO - SuperficialTiling battens in normal and domestic cases PO - Superficial

Pitched roof timbers with high condensation risk PDCCA (5-3 kg/m). MediumBoron diffusion. Deepest

Double vacuum OS. Deepest

Wall plates in case with high condensation risk PDCCA (5-3 kg/m ). MediumBoron diffusion. Deepest

Double vacuum OS. DeepestTiling battens in roofs with high condensation risk PO - Medium

Exposed timbers in chemical or corrosive atmospheres PE CCA MediumDouble vacuum OS Deepest

Flat roof joist, shallow depth, ordinary ceiling, little or noventilation PD

CCA (5-3 kg/m3). MediumBoron diffusion. Deepest

Double vacuum OS. DeepestDitto but with ample through-ventilation of roof space PO - SuperficialFlat roof joist, deep beams, with ventilation of roof PO - SuperficialGround floor joist PO Double vacuum OS. MediumTimber in external and internal stud walls PO Double vacuum OS. MediumPE: Preservation Essential; PO: Optional; PD: Desirable. CCA: Cooper-Chrome-Arsenic; OS: Organic Sales.PS: Penetration Superficial: 1-2 mm; Medium: 30-50% transversal section; Deepest: 90-100% transversal section.

Consolidation works

The ideal purpose of the consolidation works is to bring to the optimal design strength performance the real structure’s stabi lity, deterioration conditions and the state of theallowable stress and strain [23][31]. However, the building’s contemporary function or itscultural value could reduce the reconstruction strategy’s scope. The shape rectificationshould try to reallocate the charges to correct deformations and in that way, avoid fragilestructure stability. Also, calibrating fasteners and fixing supports are recommendableassignments. Partial or total truss replacement is another strategy: their substitution could be with consolidated wood elements, plastic, metal or composite materials. If reparationrequires substituting an ancient wooden member, it should be done using wood fromsimilar specie with an equivalent performance and technological quality. Also, the legal building construction codes must be observed, and if the case requires, a specialengineering study should be proposed to the authorities.

Conservation chemical treatments for wooden members are classified as [36][46][47][50]:coatings, bulking treatments, lumen-filling treatments, drying methods; controlled, freeze-drying and solvent drying. Furthermore, internal consolidants such as organic, soluble and


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exothermal resins; water-soluble adhesives and mineral waxes, as well as seed oils, canhelp consolidate some dysfunction properties of deteriorated wood. Concerningwaterlogged wood, there are some treatments that can be adapted to recover ancient woodstructures, and they are [50]: on-site preservation (sodium borate, boric acid); classicimpregnation treatments (glycerin, aluminum potassium sulfate); consolidation using

water-borne chemical (sugar, salts, polyethylene glycol, tetraethyl ortho silicate, phenol);sucrose treatment; acetone-rosin treatment; alcohol-ether treatment; and in-situ polymerization treatment, among others. Remedial measures are [26] replacement ofmembers, reinforcement and fumigant treatments. Concerning dimensional treatments,there are those that reduce the extent of swelling and may or may not alter the rate of waterabsorption. Furthermore, techniques for dimensional stabilization of wood in use arereported in the literature [47], such as cross-lamination, water-resistant coatings,hygroscopicity reduction, cross-linking and bulking treatments.

Maintenance tasks

Maintenance tasks in a historical structural system avoid further wood deterioration andstructural deficiency, and at the same time preserve historical value. This can beaccomplished using a preventive maintenance program for decay agents and deteriorationfactors control. Building external enveloping protection to control environment variables(relative humidity, light, temperature and biological agents’ presence), and pollution agents(dust and air), will maintain the structure clean. Also, building internal modifications (forexample, appropriate draw and air circulation facilities) will maintain the structure’s health[36].

Much of the deterioration that occurs in a wood structure is due to failure to give wood the prior treatment and upkeep necessary for the particular purpose at hand. Decayed timbershould never be accepted as inevitable. If the wooden parts of any structure decay while therest of it is still serviceable, one must conclude that the wood was not adequately protectedor preserved. Some precautions should be included in the maintenance program [22], andthe condition of all guttering and down pipes should be regularly examined. It should beseen to that overflow pipes do not drip onto walls, that the soil level is not allowed to riseabove the damp-proof course in any part of the building, and that the opening in air bricksventilating the sub-floor spaces are kept free from obstruction.

The recommendations for preventing decay in buildings are [56]: use bright, kiln-driedlumber; protect lumber from wetting during construction; provide adequate roof overhang(> 0.60 m); provide gutters; use well-drained building sites; install preservative-treated still plates (> 0.20 m) above grade; install ground covers on soil in craw spaces; ventilate crawlspaces (opening 1/160 of surface area); flash wood where it is exposed in a horizontal position; use pressure-treated wood for exposed design features, such as posts or rails;maintain coatings, such as paints or stains, and recaulk joints regularly; periodically inspect building for signs of moisture; and use exterior finishes that shed water.


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CONCLUSION

This paper reviewed literature and criteria pertaining to ancient wood structures evaluation,with an emphasis on diagnosis, technological wood condition, mechanical state of thestructure and preservation, consolidation and maintenance. Based on our review and

experience, we conclude that this synthesis has the key elements necessary to carry onanatomic, physical and mechanical examinations of wood. It also includes the therapeuticcriteria for restoration and conservation. Some concepts and technical propositions could beapplied – within certain limits- to the study of ancient wood furniture, window and doorframes, wooden art objects, archeological objects, paintings in wood boards and woodenaltars. Laboratory and experimental tests of the validity of the fundamental methodology’shypothesis should be adapted to every specific case. Research efforts and their applicationin practical cases would advance state-of-the-art evaluation techniques considerably. Thismethodology involves three components that are made up of three factors. Each factor hastwo aims: one is synthesis, and should be considered independent. The other is analysis,which should be interrelated within the whole of the process.

Acknowledgments: The authors gratefully acknowledge Professor Kimberly A. Brooks-Lewis (Foreign Language Department, Universidad Michoacana de San Nicolás deHidalgo, México), for proofreading the English text.

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LABORATORIO DE MECÁNICA DE LA MADERADE LA DIVISIÓN DE ESTUDIOS DE POSGRADO

DE LA FACULTAD DE INGENIERÍA EN TECNOLOGÍA DE LA MADERA

El laboratorio de Mecánica de la madera tiene por misión realizar investigaciones sobre elcomportamiento mecánico de árboles, estructuras de madera, madera aserrada y de productos compuestos de madera.

En el laboratorio se realizan además las prácticas de la materia Física de la madera de laMaestría en Ciencias y Tecnología de la Madera y sirve también de laboratorio en la preparación de tesis de Licenciatura y de Maestría de la Facultad de Ingeniería enTecnología de la Madera.

Entre otros servicios, el laboratorio realiza estudios de caracterización mecánica pormétodos no destructivos de materiales de ingeniería y de productos forestales. Además secuenta con la experiencia para practicar trabajos In-Situ de inspección y de evaluación deestructuras de madera.

Por otra parte, el laboratorio tiene el equipo y el personal especializado para efectuarestudios de análisis de calidad de la madera en medio ambiente industrial. El laboratorioorganiza también seminarios y cursos de capacitación para profesionales en Ingeniería yArquitectura.

El equipo principal de investigación con que cuenta el laboratorio es:

- Maquina Universal de pruebas mecánicas Tinius Olsen®.- Equipo de ondas de esfuerzo Metriguard®.- Equipo de ondas de esfuerzo Fakopp®.- Equipo de ultrasonido Sylvatest®.- Sistemas y programas de computación para adquisición y tratamiento de datos.

El laboratorio cuenta igualmente con una biblioteca especializada en Mecánica deMateriales.

Los proyectos de investigación recientes en los cuales el laboratorio ha participado son:

-Evaluación mecánica de materiales compuestos de madera. 2002 – 2004.- Evaluación con métodos no destructivos de estructuras de madera en edificios antiguos devalor histórico y cultural. 2003 – 2007.

El laboratorio de Mecánica de la madera ha colaborado recientemente con La Universidaddel Estado de Oregon, USA., El Instituto de Investigaciones Forestales de Tsukuba, Japón ycon la Universidad Técnica de Munich, Alemania.

El responsable del laboratorio es el Doctor Javier Ramón Sotomayor Castellanos, ProfesorInvestigador de la Facultad de Ingeniería en Tecnología de la Madera.

tecnologia de la madeera mecanica .pdf

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