Sistemas de refuerzo y consolidación en estructuras de madera

La intervención en edificios con estructura de madera implica en muchos casos la necesidad de consolidar la estructura para alcanzar un nivel de seguridad aceptable por la normativa o cuando se produce un cambio de uso, se requiere el refuerzo de la estructura para asumir las nuevas cargas. Actualmente se emplean diversas técnicas con esta finalidad, por ejemplo: Sistemas de consolidación mediante refuerzos y formulaciones epoxi, Sistemas de reconstrucción de piezas mediante el encolado de láminas de madera, Sistemas mixtos de madera y hormigón, etcétera

Consolidación de estructuras de madera mediante refuerzos embebidos en formulaciones epoxi

Este trabajo consiste en el estudio del comportamiento mecánico de los métodos de consolidación de estructuras de madera basados en la aplicación de la tecnología de las resinas epoxi. Estos procedimientos permiten recuperar la capacidad mecánica de piezas deterioradas por ataques xilófagos u otras causas, mediante perfiles de refuerzo embebidos en la madera y conectados a ella a través de una formulación epoxi. El estudio se desarrolla sobre una base experimental y se puede dividir en los siguientes apartados: En la primera parte se trata de manera general de los materiales que intervienen en el sistema (madera, formulación y refuerzo), justificando la elección de los utilizados en los ensayos. Así, para la madera se emplearon tres especies distintas: Pino insigne, Pino laricio y Quercus robur; tres formulaciones epoxi de diferentes características; y el refuerzo utilizado está constituido por materiales compuestos (resina de poliéster y fibra de vidrio). A continuación se estudian las propiedades mecánicas de los materiales centrándose en aquellos que pueden tener más influencia en la forma de trabajo de estos sistemas, como puede ser el esfuerzo cortante. El tema siguiente consiste en el estudio de la adherencia tangencial entre la formulación epoxi y los dos materiales: madera y refuerzo. Analizando la influencia en la calidad de la unión de diversos factores como son el tratamiento y la rugosidad de la superficie, el tipo de formulación y su edad, la especie de madera y el tipo de producto del material compuesto. A continuación se estudia el comportamiento de los tres materiales actuando solidariamente en simulaciones de posibles refuerzos de vigas. Esta parte del trabajo se centra principalmente en la consolidación con refuerzos en forma de placas. Se ensayan soluciones de recuperación de apoyos y de tramos intermedios de vigas en pequeña escala. Así como la consolidación de vigas previamente rotas. En todos los casos se compara el comportamiento en resistencia y deformación de piezas antes y después de la consolidación. Y se establecen unos modelos teóricos de cálculo que se contrastan con los resultados experimentales. Se describe además, un procedimiento simplificado que permite dimensionar la longitud de anclaje de la placa en función de la geometría de la viga. Finalmente se trata de conocer el efecto que puede provocar en estos sistemas la variación de las condiciones de humedad y temperatura. Para ello se someten series de probetas o unos ciclos de condiciones extremas de temperatura y otros de humedad. La importancia de este fenómeno radica en la contracción volumétrica de la madera con la variación del contenido de humedad. Los movimientos diferenciales de origen higrotérmico entre los tres materiales provocan tensiones normales y tangenciales al encontrarse unidos entre sí. Los resultados experimentales se comparan con las estimaciones teóricas del fenómeno. Podemos concluir que estos métodos de consolidación basados en la tecnología de las resinas epoxi y los refuerzos con materiales compuestos, aquí tratados, permiten la recuperación de la capacidad portante de estructuras de madera deteriorados con seguridad suficiente

Bonding shear strength in timber and GFRP glued with epoxy adhesives

The bonding quality of epoxy glued timber and glass fibre reinforced polymers (GFRP) was evaluated by means of compression loading shear test. Three timber species (Radiata pine, Laricio pine and Oak) and two kinds of GFRP (plates and rods made with polyester resin reinforced with mat and roving glass fibre) were glued and tested using three epoxy formulations. The increase in shear strength with age after the setting of epoxy formulations and the effect of surface roughness on timber and GRP gluing (the planing of the surface of timber and the previous sanding of GRP) were studied. It can be concluded that the mechanical properties of these products make them suitable for use in the reinforcement of deteriorated timber structures, and that a rough timber surface is preferable to a planed one, while the previous sanding of GRP surfaces is not advantageous

In-situ density estimation by four nondestructive techniques on Norway spruce from built-in wood structures

Needle penetration resistance (NPR), screw withdrawal resistance (SWR), core drilling (CD) and drilling chips extraction (DCE) are nondestructive and semi-destructive techniques used to estimate density in timber structures. In most of the previous studies, these techniques were tested in clear sawn timber and clear specimens. The goal of the present paper is to study the relationship between density and these techniques by means of five different devices in whole pieces of timber from built-in engineering structures, which are from 12 4.5-m long structural timber joists of Norway spruce from a 19th century building in Barcelona (Catalonia, Spain). Although determination coefficients (R2) for density estimation models were lower than those from clear timber, the results obtained confirmed that these four techniques are suitable for in-situ density estimation of woods in buildings. The best results were obtained by CD (the bigger the bit, the higher the correlation), followed by DCE, and SWR. The worst correlation was found for NPR devices, but the results could be probably improved with more measurements.Peer ReviewedPostprint (published version

Prediction of bendig load capacity of timber beams by finite element method simulation of knots and grain deviation

A finite element model was used to simulate timberbeams with defects and predict their maximum load in bending. Taking into account the elastoplastic constitutive law of timber, the prediction of fracture load gives information about the mechanisms of timber failure, particularly with regard to the influence of knots, and their local graindeviation, on the fracture. A finite element model was constructed using the ANSYS element Plane42 in a plane stress 2D-analysis, which equates thickness to the width of the section to create a mesh which is as uniform as possible. Three sub-models reproduced the bending test according to UNE EN 408: i) timber with holes caused by knots; ii) timber with adherent knots which have structural continuity with the rest of the beam material; iii) timber with knots but with only partial contact between knot and beam which was artificially simulated by means of contact springs between the two materials. The model was validated using ten 45 × 145 × 3000 mm beams of Pinus sylvestris L. which presented knots and graindeviation. The fracture stress data obtained was compared with the results of numerical simulations, resulting in an adjustment error less of than 9.7

In situ acoustic methods to estimate the physical and mechanical aging of oriented strand board

Following the success achieved in previous research projects usin non-destructive methods to estimate the physical and mechanical aging of particle and fibre boards, this paper studies the relationships between aging, physical and mechanical changes, using non-destructive measurements of oriented strand board (OSB). 184 pieces of OSB board from a French source were tested to analyze its actual physical and mechanical properties. The same properties were estimated using acoustic non-destructive methods (ultrasound and stress wave velocity) during a physical laboratory aging test. Measurements were recorded of propagation wave velocity with the sensors aligned, edge to edge, and forming an angle of 45 degrees, with both sensors on the same face of the board. This is because aligned measures are not possible on site. The velocity results are always higher in 45 degree measurements. Given the results of statistical analysis, it can be concluded that there is a strong relationship between acoustic measurements and the decline in physical and mechanical properties of the panels due to aging. The authors propose several models to estimate the physical and mechanical properties of board, as well as their degree of aging. The best results are obtained using ultrasound, although the difference in comparison with the stress wave method is not very significant. A reliable prediction of the degree of deterioration (aging) of board is presented

NDT to identify biological damage in wood

Nondestructive techniques are widely used to assess existing timber structures. The models proposed for these methods are usually performed in the laboratory using small clear wood specimens. But in real situations many anomalies, defects and biological damage are found in wood. In these cases the existing models only indicate that the values are outside normality without providing any other information. To solve this problem, a study of non-destructive probing methods for wood was performed, testing the behaviour of four different techniques (penetration resistance, pullout resistance, drill resistance and chip drill extraction) on wood samples with different biological damage, simulating an in-situ test. The wood samples were obtained from existing Spanish timber structures with biotic damage caused by borer insects, termites, brown rot and white rot. The study concludes that all of the methods offer more or less detailed information about the degree of deterioration of wood, but that the first two methods (penetration and pullout resistance) cannot distinguish between pathologies. On the other hand, drill resistance and chip drill extraction make it possible to differentiate pathologies and even to identify species or damage location. Finally, the techniques used were compared to characterize their advantages and disadvantages

The influence of sensor placement on in-situ ultrasound wave velocity measurement.

Ultrasound wave velocity was measured in 30 pieces of Spanish Scots pine (Pinus sylvestris L.), 90 x 140 mm in cross-section and 4 m long. Five different sensor placement arrangements were used: end to end (V0), face to opposite face, edge to opposite edge, face to same face and edge to same edge. The pieces were successively shortened to 3, 2 and 1 m, in order to obtain these velocities and their ratios to reference value V0 for different lengths and angles with respect to the piece axis for the crossed measurements. The velocity obtained in crossed measurements is lower than V0. A correction coefficient for crossed velocities is proposed, depending on the angle, to adjust them to the V0 benchmark. The velocities measured on a surface, are also lower than V0, and their ratio with respect to V0 is close to 0.97 for distances equal to or greater than 18 times the depth of the beam