Experimental analysis on small-scale finger-jointed specimens at elevated temperatures
Finger joints have to meet strength performance requirements for the use in structural engineered wood products such as glued-laminated timber beams (glulam). Among these, the thermal stability of the adhesive used in the finger joint is an important criterion to determine the suitability of the connection. The influence of adhesives on the load-carrying capacity of glulam beams and finger-jointed members can be assessed by large-scale fire tests; however, there are obvious benefits in using small-scale specimens tested at elevated temperatures as an alternative. In Europe, there is currently no small-scale test available to test the fire performance of adhesives in structural timber members. The work presented in this paper addresses this issue and was supported by members of a European standardisation committee to develop such a small-scale test. This paper presents, as a first step, the results of a series of tensile tests on small-scale finger-jointed specimens tested at elevated temperatures. The tests were performed with different types of adhesives which are currently also tested in large-scale fire tests. In the small-scale tests here, structural adhesives showed nearly no weaknesses with increasing temperature up to 140 °C. Further, MUF and PRF adhesives showed mostly wood failure even up to temperatures of 220 °C. In the next step of this investigation, the results obtained from the small-scale tests at elevated temperature will be compared to large-scale fire test results in order to find a possible link between both testing methods.
Components and Consequences of Cross-Laminated Timber Delamination
Mass timber products such as cross-laminated timber are increasingly being used in tall timber building designs. The high-density wood used in mass timber products has a natural self-extinguishment capacity which chars only under exposure from an external heat flux. This makes timber a suitable building material giving timber buildings the potential to withstand full fire burnout of the building furnishings. However, ply delamination poses a significant challenge as the loss of the char layer exposes the unburnt wood and prevents the insulating char from regulating the heat flux to the unburnt wood. Even further, ply delamination means the structurally designed composite action of the mass timber product has been lost, and the ply could fall off into the compartment fire increasing the room fuel load. Ply delamination is extremely complex involving the resolution of heat transfer and structural equations as well as variables which are functions of temperature, moisture content, and many other factors. To date, the structural and thermal effects of cross-laminated timber delamination have not been comprehensively studied. This paper seeks to review the current research into delamination and highlight the research gaps that still exist. The current market focuses primarily on charring and eliminates the thermal penetration depth at the expense of losing understanding on a major failure mode of engineered timber. More detailed research into delamination is necessary to confidently ensure tall timber buildings meet the stakeholder’s goals of providing a safe building which can withstand complete fire burnout.
Evaluating the strength of finger-jointed lumber of Gmelina arborea in Costa Rica
AbstractThe use of finger-jointed lumber of Gmelina arborea (gmelina) for the production of glue-laminated beams is becoming more important to the wood industry in Costa Rica. At the request of the local wood industry, two different kinds of adhesives, thermoplastic polyvinyl emulsion (catalyzed PVA) and 1-part polyurethane adhesive (PU) were tested on finger joints of gmelina wood and their strength properties compared under dry, boiled, and saturated conditions. The results showed that under the boiled and saturated conditions, the PVA adhesive exhibited low tensile strength and poor distribution of failures while PU showed much more promise. Based on our preliminary results, the PU adhesive can be successfully used for finger-joint boards of gmelina.
Palabras clave: Adhesivos, Melina, Modos de falla, Vigas laminadas
Resumen. El uso de la madera unida con juntas de dedo (finger-jointed) para la producción de vigas laminadas-encoladas está tomando mayor importancia en la industria maderera de Costa Rica. Atendiendo la solicitud de la industria, sobre la calidad de los finger joints de la madera de melina, se ensayaron dos tipos diferentes de adhesivos, una emulsión de polivinilo catalizado (thermoplastic polyvinyl emulsion, catalizado, PVA) y un adhesivo de poliuretano (polyurethane adhesive, PU) de un componente, para comparar sus características de resistencia bajo condiciones secas, de ebullición y de saturación. Los resultados muestran que bajo condiciones de ebullición y saturación, el adhesivo PVA tiene una baja resistencia a la tensión y una deficiente distribución de fallas, mientras que el PU mostró resultados mucho más prometedores. Basados en los resultados preliminares, el adhesivo PU puede ser utilizado exitosamente para unir las tablas de melina con juntas de dedo.