Information on ductile and brittle failure modes is critical for proper design of timber connections in Crosslaminated Timber (CLT). While considerable research has been conducted in Europe and Canada on the ductile performance of connections in CLT, little is known about the brittle behaviour. This paper presents new information from testing programs and analysis performed in Canada and in New Zealand on the brittle performance of dowel-type fasteners in CLT. The testing programs have been designed to trigger brittle failure modes based on minimum end distances and fasteners spacings specified in the Canadian timber design standard. Timber rivets and bolts/dowels are covered under this study. At the time of writing of this abstract, the testing program is advancing and results will be available at the time of paper submission.
In the last 15 years timber-concrete composite (TCC) systems have gained market share around the world. To facilitate acceptance of this construction method and to set basis for building TCC bridges in the Province of Quebec, the authors conducted a test program on TCC beams with continuous shear connectors. It included push-out...
The introduction of Cross-laminated Timber (CLT) as an engineered timber product has played a significant role in considerable progress of timber construction in recent years. Extensive research has been conducted in Europe and more recently in Canada to evaluate the fastening capacity of different types of fasteners in CLT. While ductile capacities calculated using the yield limit equations are quite reliable for fastener resistance in connections, however, they do not take into account the possible brittle failure mode of the connection which could be the governing failure mode in multi-fastener joints. Therefore, a stiffness-based design approach which has already been developed by the authors and verified in LVL, glulam and lumber has been adapted to determine the block-tear out resistance of connections in CLT by considering the effect of perpendicular layers. The comparison between the test results on riveted connections conducted at the University of Auckland (UoA) and the Karlsruhe Institute of Technology (KIT) and the predictions using the new model and the one developed for uniformly layered timber products show that the proposed model provides higher predictive accuracy and can be used as a design provision to control the brittle failure of wood in CLT connections.
Highly loaded and large span timber beams are often used for halls, public buildings or bridges.
Reinforcement of beams may be required to extend the life of the structure, due to deterioration or damage to the material/product or change of use. The paper summarises methods to repair or enhance the structural performance of timber beams. The main materials/products cross sections and geometries used for timber beam are presented. Furthermore, their general failure modes are described and typical retrofitting and reinforcement techniques are given. The techniques include wood to wood replacements, use of mechanical fasteners and additional strengthening materials/products.
In existing wood strength prediction models for parallel to grain failure in timber connections using dowel-type fasteners, different methods consider the minimum, maximum, or summation of the tensile and shear capacities of the failed wood block planes. It is postulated that these methods are not appropriate since the stiffness of the adjacent wood loading the tensile and shear planes differs, and this leads to uneven load distribution among the resisting planes. A closed-form analytical method to determine the load-carrying capacity of wood under parallel-to-grain loading in small-dowel-type connections in timber products is thus proposed. For the wood strength, the stiffness of the adjacent loading volumes and strength of the failure planes subjected to nonuniform shear and tension stresses are considered. The effective wood thickness for the brittle failure mode is derived and related to the elastic deformation of the fastener. A mixed failure mode is also defined (a mixture of brittle and ductile) and depends on the governing ductile failure mode of the fastener. To help the designer, an algorithm is presented that allows the designer to calculate the resistances associated with predictions of the different possible brittle, ductile, and mixed failure modes. The proposed stiffness-based model has already been verified in brittle and mixed failure modes of timber rivet connections. In the research reported in this paper, an extended application is proposed for other small-dowel-type fasteners such as nails and screws. Results of nailed joint tests on laminated veneer lumber (LVL) and the test data available from the literature on glulam confirm the validity of this new method, and show that it can be used as a design provision for wood load-carrying capacity prediction of small-dowel-type timber connections.
In the present paper, the bending behavior of Cross Laminated Timber panels is investigated by means of the linear elastic exact solution from Pagano (1970; 1969). The resulting stresses are the input for a wood failure criterion, which can point out the first-crack load and the respective dominant failure mode. Heterogeneous layers are modeled as equivalent and homogeneous layers. This simplified and deterministic modeling gives results in good agreement with a reference experimental test. A comparison is made with respect to the panel’s global stiffness and failure stages within the apparent elastic stage. Finally, parameter studies are carried out, in order to quantify CLT limitations and advantages. The effect of varying properties like the panel’s slenderness, orientation of transverse layers and number of layers for a fixed total thickness are investigated.
The use of engineered timber products such as cross-laminated timber (CLT) is of increasing interest to architects and designers due to their desirable aesthetic, environmental, and structural properties. A key factor preventing widespread uptake of these materials is the uncertainty regarding their performance in fire. Currently, the predominant approach to quantifying the structural fire resistance of timber elements is the charring rate, which allows estimation of residual cross-section and hence strength. The charring rate is usually determined by testing timber specimens in a furnace by exposure to a ‘standard fire’. However, it is recognized that the resulting charring rates are not necessarily appropriate for non-standard fire exposures or for characterizing the structural response in a real timber building. The effect of heating rate on the charring rate of CLT samples is investigated. The charring rate resulting from three heating scenarios (constant, simulated ‘standard fire’ and quadratically increasing) was calculated using interpolation of in-depth temperature measurements during exposure to heating from a mobile array of radiant panels, or in a Fire Propagation Apparatus (FPA). Charring rate is shown to vary both spatially and temporally, and as a function of heating rate within the range 0.36–0.79 mm/min. The charring rate for tests carried out under simulated ‘standard fire’ exposures were shown to agree with the available literature, thus partially verifying the new testing approach; however under other heating scenarios the Eurocode charring rate guidance was found to be unconservative for some of the heat flux exposures in this study. A novel charring rate model is presented based on the experimental results. The potential implications of this study for structural fire resistance analysis and design of timber structures are discussed. The analysis demonstrates that heating rate, sample size and orientation, and test setup have significant effects on charring rate and the overall pyrolysis, and thus need to be further evaluated to further facilitate the use of structural timber in design.
Wood as building material is gaining more and more attention in the 21st century due to its positive attributes such as light weight, renewability, low carbon footprint and fast construction period. Cross-laminated timber (CLT), as one of the new engineered wood products, requires more research emphasis since its mechanical performance can allow CLT to be utilized in massive timber structures. This thesis focuses on revealing one of the key failure mechanisms of CLT, which is usually referred to as the rolling shear failure. The scientific research conducted in this thesis combined both analytical modelling and experimental material testing. The stresses in CLT cross-layers obtained from a finite-element model were analyzed to differentiate various failure modes possible. Tension perpendicular to grain stress was found to cause cross-layer failure in combined with the rolling shear stress. Experimentally, specimens prepared from 5-layer CLT panels were tested under center-point bending condition. Detailed failure mechanism of CLT cross-layers were recorded with high speed camera to capture the instant when initial failure happened. It is evident that some of the specimens failed in tension perpendicular to grain which verified the modelling results. Variables such as the rate of loading and the manufacturing clamping pressure were designed in experiments to compare their influence to the failure of CLT specimens. In this research, the failure of CLT cross-layer was updated to a combined consequence of both rolling shear stress and tension perpendicular to grain stress. Future research topics and product improvement potentials were given by the end of this thesis.
This paper outlines a series of experimental tests of LVL box beams designed to fail in shear. Some beams utilised post-tensioning systems to increase the flexural strength and decrease deflection. Fire conditions were simulated using either an ISO 834 furnace test or by mechanically reducing the section dimensions on three-sides of the beam to replicate charring. Comparisons with a simplified calculation method for the fire performance of post-tensioned timber box beams are made and discussed. This paper gives special focus to the shear performance of LVL box beams because previous research had identified that the inclusion of post-tensioning may increase the likelihood of shear failure occurring in LVL box beams, especially in fire conditions.