International Conference on Performance-based and Life-cycle Structural Engineering
December 9-11, 2015, Brisbane, Australia
Tall timber building designs have utilized cross-laminated timber (CLT) significantly over the past decade due the sustainable nature of timber and the many advantages of using an engineered mass timber product. Several design methods have been established to account for the composite action between the orthogonally adhered timber plies. These methods assume perfect bonding of the adjacent plies by the adhesive. CLT designs methods for timber in fire have also been formulated. These methods rely on the relatively constant charring rate of timber to calculate a sacrificial layer to be added onto the cross-sectional area. While these methods focus on the timber failure mode of reduced cross section by charring, the failure mode of ply delamination is often overlooked and understudied. Due to the reduction of shear and normal strength in the adhesive, the perfect bond assumption can be questioned and a deeper look into the mechanics of CLT composite action and interfacial stress needs be conducted. This paper seeks to highlight the various design methods for CLT design and identify the failure mode of delamination not present in the current design codes.
This paper addresses the quality of the interface- and edge-bonded joints in layers of cross-laminated timber (CLT) panels. The shear performance was studied to assess the suitability of two different adhesives, polyurethane (PUR) and phenol–resorcinol–formaldehyde (PRF), and to determine the optimum clamping pressure. Since there is no established testing procedure to determine the shear strength of the surface bonds between layers in a CLT panel, block shear tests of specimens in two different configurations were carried out, and further shear tests of edge-bonded specimen in two configurations were performed. Delamination tests were performed on samples which were subjected to accelerated aging to assess the durability of bonds in severe environmental conditions. Both tested adhesives produced boards with shear strength values within the edge-bonding requirements of prEN 16351 for all manufacturing pressures. While the PUR specimens had higher shear strength values, the PRF specimens demonstrated superior durability characteristics in the delamination tests. It seems that the test protocol introduced in this study for crosslam-bonded specimens, cut from a CLT panel, and placed in the shearing tool horizontally, accurately reflects the shearing strength of glue lines in CLT.
The paper presents results from the experimental testing of load-bearing timber–glass composite shear walls and beams. Shear wall specimens measuring 1200 × 2400 mm2 manufactured with three adhesives of varying stiffness were tested. Twelve specimens with a single 10 mm thick glass pane and one specimen with an additional insulating glass unit were produced. The testing procedures involved various loading conditions: pure vertical load and different combinations of shear and vertical loading. The test results showed that the adhesive had only a minor influence on the buckling load which was the main failure mechanism. 240 mm high and 4800 mm long timber–glass beams manufactured with adhesives of different stiffness were tested. For the webs, two types of glass were used: annealed float and heat-strengthened glass, in both cases 8 mm thick panes were used. In total, 12 beams were tested in four-point bending until failure. Despite the considerable difference in adhesive stiffness, beam bending stiffness was similar. Concerning load-bearing capacity, the beams with heat-strengthened glass were approximately 50% stronger than the beams made using annealed float glass.
Ongoing development of timber and timber products made from European hardwoods like ash and
beech influences the selection of acceptable methods for connecting these elements and thus demands validation and application of current design methods for softwood and glulam. For the last 20 years, despite many national and international research projects and practical applications of glued-in rods in timber structures, there is still no universal standard with respect to their design. The use of adhesives available for bonding rods and timber is limited to softwood. This work shows the performance of different timber species Norway spruce (Picea abies Karst.), European ash (Fraxinus excelsior L.) and European beech (Fagus silvatica L.) and engineered timber products (laminated veneer lumber made of Norway spruce and European beech) based on comprehensive pull-compression tests of glued-in rods. For characterizing the elastic and elasticplastic behavior, failure loads as well as stiffness and ductility were considered whereby the rod diameter and anchorage length were maintained constant. The aim of the research was to show that glued-in rods cannot only be used in softwoods and glulam members but also in hardwoods and in wood-based products such as LVL.
Cross laminated timber (CLT) has been developed to a worldwide well-known and versatile useable building material. Currently increasing rates in production volume and distribution can be observed. In fact CLT, thanks to its laminar structure making it well suited for use in construction, provides new horizons in timber engineering, in areas which had until now been the realm of mineral building materials like concrete and masonry.
After a short introduction, this paper aims to demonstrate current production processes used for rigid CLT. In section 2 the process steps are described and essential requirements, as well as pros and cons of various production techniques, are discussed. Latest results of R & D and of development and innovation in production technology are presented. In section 3 test and monitoring procedures in the area of the internal quality assurance, known as factory production control (FPC), are presented. Diverse regulations, in the form of technical approvals for CLT as well as in the CLT product standard prEN 16351 , are discussed. Additionally, some technological aspects of the product, CLT, together with a comparison of geometrical and production relevant parameters of current technical approvals in Europe are provided in section 4. In the final and main part of the paper, production and technology is presented in a condensed way. The outlook for current and future developments, as well as the ongoing establishment of the solid construction technique with CLT, is given. The product, CLT, comprises an enormous potential for timber engineering as well as for society as a whole. Standardisation and further innovation in production, prefabrication, joining technique, building physics and building construction make it possible for timber engineering to achieve worldwide success.
In order to explore bamboo glulam utilization in structure construction, the adhesive bonded steel connection of bamboo glulam was investigated in this study. By carrying out both-end pullout tests on glued-in threaded rods in bamboo glulam, the effects of depth and diameter of embedded rods in bamboo glulam on the pullout strength and the failure modes were discussed. Results showed that threaded rods fracture and adhesive interface failure were the two main different failure modes in the tests. The pullout peak load of both-end glued-in rods in bamboo glulam increased with the diameter and the embedded length of the threaded rods. To satisfy tensile load of the glued threaded rods (quality 4.8) used in the connections between engineering structural materials, the slenderness ratio (the ratio of depth and diameter of glued-in threaded rods) equal to 10 or over was necessary.
The Cradle-to-Cradle Certification at Platinum level, awarded to products which perfectly embody the principles of Cradle-to-Cradle design, is perhaps one of the most esteemed standards of excellence in sustainability circles. Currently, there is no Platinum-level product which can deliver the classic postand-beam structural system. This literature review investigates the possibility of a timber beam product filling in that gap, and the potential design specifications necessary to do it. Findings suggest that the resin component of current glulam beams harm the Cradle-to-Cradle assessment rating, therefore posing a challenge to find eco-friendly alternative. Potential candidates such as lignin and casein resin are studied, along with the novel technology of welded dowel-laminated timber.
Structural changes like deformations and crack growth in polymers, filled with electrically conductive particles can be measured by resistography. Accordingly, the polymeric adhesive layers in glued-laminated timber should be usable for characterization of the wooden structures and the integrity of the bondline. The described research of the last years – partly described on different conferences in 2012 to 2016, refined and extended - addresses the question, if electrically conductive adhesives can be used to characterize structural changes of wooden structures. Electrical conductive adhesives have been modified with carbon based fillers to use the bondline as a sensor in layered wood structures. Laboratory scaled samples were prepared and tested in different load and climate conditions to proof the usability of the conductive adhesive for measurement purposes. The results are showing a correlation between displacement and DC resistivity. Further, the signals also allow a separation among the different kinds of stress states. By varying the contact points of the resistivity measurement it was also possible to monitor the wood moisture.
Delamination resistance and tensile shear strength (TSS) are essential for structural adhesives used in timber industry. Thus these two factors were investigated on bonded ash (Fraxinus excelsior L.) to check the suitability of adhesively bonded ash as building material. For determination of the delamination resistance industrially bonded ash glulam was used. The specimens for the tensile shear tests where produced in the laboratory. Four different adhesives types and different pre-treatment were investigated. The samples for TSS were tested in dry and wet condition. 80% of the tested series met the requirements of the standards at dry, and only 30% passed at wet condition. None of the adhesives tested was able to pass the delamination test. No distinct influence of the different parameters studied is notable for most of the adhesive systems, only extended closed assembly time and lower mixing ratios seem to improve the bond quality of MUF. Additional chemical analyses, conducted to find evidence for the poor bonding performance, showed that fatty acid content, pH and acidic extractives are in between the range of beech (Fagus sylvatica L.) and Spruce (Picea abies Karst.). However the formic acid is an exception with a four times higher amount as the other two species investigated.
Massive timber panels (MTPs) has shown a great potential in construction of tall buildings. Evaluation of the face-bond strength of MTPs is of an interest to use of this kind of products. This study was aimed at developing an appropriate test procedure for evaluating the adhesive bond strength of cross-laminated laminated strand lumber (LSL). Short span bending tests were conducted on two-layer asymmetric cross-laminated LSL specimens, which were adhesively bonded using two-component polyurethane (PUR) and polyvinyl acetate (PVAc). For comparison, block shear specimens were tested as well. It was found that the 2-layer asymmetric cross-laminated specimen assembly under the short span bending could be used to differentiate between good and poor bond quality.