Cross Laminated Timber (CLT) is a relatively new timber product used in construction that has gained popularity over the last decade. The product itself is constituted by multiple glued layers of juxtaposed boards, usually arranged in an orthogonal direction between one layer and the adjacent ones. This particular structure brings several benefits, such as the possibility to use the same product both for walls and slabs, since it can bear in-plane and out-of-plane loads. However, the mechanical behavior differs from usual timber products, and research is still ongoing to achieve common agreement on standard procedures for testing products and theories for evaluating stresses for safety verifications. This paper focuses on the in-plane shear behavior of CLT and analyzes the existing methods to evaluate shear stresses. An experimental part then presents a four-point bending test of CLT beams with a specific geometry to induce shear failure. Results are reported both for the elastic range test, measuring the Modulus of Elasticity, and for the failure test to investigate shear behavior with regard to different mechanisms. Previously exposed methods are used for the calculation of shear stresses and to analyze the correspondence between them, and the results are then compared with other existing tests and values in literature. A new test setup for future research is eventually proposed.
Friction-based dampers are a valid solution for non-invasive seismic retrofitting interventions of existing structures, particularly reinforced-concrete (RC) structures. The design of friction-based dampers is challenging: underestimating the slip force prevents the full use of the potential of the device, which attains the maximum admissible displacement earlier than expected. By contrast, overestimating the slip force may cause delayed triggering of the device when the structure has suffered extensive damage. Therefore, designing the appropriate slip force is an optimization problem. The optimal slip force guarantees the highest inter-story drift reduction. The authors formulated the optimization problem for designing a specific class of friction-based dampers, the asymmetric friction connection (AFC), devised as part of the ongoing multidisciplinary Horizon 2020 research project e-SAFE (Energy and Seismic AFfordable rEnovation solutions). The seismic retrofitting technology involves the external application of modular prefabricated cross-laminated timber (CLT) panels on existing external walls. Friction dampers connect the CLT panels to the beams of two consecutive floors. The friction depends on the mutual sliding of two metal plates, pressed against each other by preloaded bolts. This study determines the optimal slip force, which guarantees the best seismic performance of an RC structural archetype. The authors investigate the nonlinear dynamic response of a coupled mechanical system (RC frame-friction damper) under a set of strong-motion earthquakes, using non-differential hysteresis models calibrated on the experimental cyclic responses. The solution of the optimization leads to the proposal of a preliminary simplified design procedure, useful for practitioners.
The use of Cross Laminated Timber (CLT) as structural element for shear walls and floors in multi-storey buildings has become very popular in the recent years in Europe. In many situations it is necessary to characterize the mechanical properties for CLT-elements subjected to in-plane loads, such as in the case of walls elements under lateral loads, deep beams or lintel beams realized in CLT. Many recent research works have been devoted to the definition of a proper test arrangement in order to determine specific mechanical properties. At the present a harmonized European Standard is Under Approval (prEN 16351 Timber structures - Cross laminated timber - Requirements), reporting two different test methods for determining the in-plane characteristic of CLT panels: i) edgewise in four-point bending test, with a gaps of non-edge bonded layers or butt joints positioned on the neutral axis of the specimen; (ii) shearcompression test on the net cross section with an ad hoc set-up.
Engineered Wood Products like Cross-Laminated-Timber (CLT) are transforming capabilities of wood as a construction material, enabling architects and engineers to create innovative buildings. Using CLT can have many advantages compared with using traditional materials, not least of which is reducing total superstructure gravitational weights. Reducing gravitational weight can simplify and speed up construction processes and reduce foundation costs. Plus, being made from wood, CLT has desirable ‘green’ credentials like renewability of forest resources and carbon sequestration for the lifespans of buildings. However, like other lightweight structural systems, CLT buildings can be susceptible to high-amplitude motions during ambient or other dynamic force and displacement disturbances. Studies reported here address the dynamic behaviour of mid-rise multi-storey buildings constructed from massive CLT elements, with a focus on predicting lateral modal response characteristics of such buildings. The vehicle for this is detailed Finite Element (FE) models verified as accurate replicators of ambient dynamic motions of completed CLT buildings. Here applications of FE models relate to performances of buildings during seismic events. However, the intent is to also use them to predict motions of buildings during windstorms.
The outcome of an experimental campaign on the long-term behaviour of timber floors retrofitted with timber-to-timber composite methods is presented. Four diaphragm specimens, 5.2 m long (5 m span) were tested out-ofplane. Each specimen consisted of a solid wood-spruce joist strengthened with a crosslam panel. A layer of timber boards was placed in between the joist and the panel to simulate the existing flooring. The specimens, were subjected to uniformly distributed loading in a climatic controlled chamber. A patented procedure that enables to apply a pre-stressed state and a pre-camber to the composite floor joists by just using screw fasteners, was adopted. Different typologies and arrangements of screws were tested in order to maximize the performance (cost/effectiveness) that can be achieved by employing the above mentioned procedure. Uplift values of approximately 1/300th of the diaphragm span were registered at the end of the cambering procedure. After an initial testing phase (duration approximately equal to 3h) where the loading was consistent with the characteristic combination, the specimens were set for long-term testing under an imposed load equal to that specified by the quasi-permanent combination.
The intention of this STAR is a more detailed summary of the relevant work, done during the last 20 years on the topic of CLT. Additional references not included in the documents of SC5.T1 were added highlighting also CLT relevant publications not directly addressing content of the CLT draft version for EC 5. Apart from providing additional references for all topics and chapters of SC5.T1 draft documents edited by WG 2 / TG 1 & TG 3 within this STAR, further necessary scoentific work was identified and listed.
Cross Laminated Timber (CLT) and Light Timber Frame (LTF) shear walls are widespread constructive technologies in timber engineering. Despite the intrinsic differences, the lateral response of the two structural systems may be quite similar under specific connection layouts, boundary constraints, and size of the shear walls. This paper compares the experimental cyclic responses of CLT and LTF shear walls characterized by the same size 250×250cm, and loaded according to the EN 12512 protocol. The rigid-body rotation of the shear walls prevails over the deformation and rigid-body translation in the post-elastic displacement range. As a consequence, a capacity model of the two systems based on the sole hold-down response accurately seizes the observed cyclic response, despite ignoring the other resisting contributions. The authors examine the differences exhibited by the CLT and LTF shear walls and the related error corresponding to a capacity model based on the sole hold down restraints. Additionally, it is assessed the overstrength of the CLT panel and LTF sheathing to the shear walls collapse due to the hold-down failure. The estimated overstrength factor is the most meaningful difference between the two structural systems in the considered experimental layouts.
Cross-Laminated Timber (CLT) structures exhibit satisfactory performance under seismic conditions. This ispossible because of the high strength-to-weight ratio and in-plane stiffness of the CLT panels, and the capacity ofconnections to resist the loads with ductile deformations and limited impairment of strength. This study sum-marises a part of the activities conducted by the Working Group 2 of COST Action FP1402, by presenting an in-depth review of the research works that have analysed the seismic behaviour of CLT structural systems. Thefirstpart of the paper discusses the outcomes of the testing programmes carried out in the lastfifteen years anddescribes the modelling strategies recommended in the literature. The second part of the paper introduces theq-behaviour factor of CLT structures and provides capacity-based principles for their seismic design.
In high timber structures, cross-laminated timber panels are common structural elements. The wall and floor panels are typically connected with steel plates, angle brackets, hold-downs, and screws. Based on analytical research, it seems that panel-to-panel connections give additional stiffness due to structural redundancies resulting from...