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Connections for CLT diaphragms in steel-framed buildings
... Works dealing with light timber floors were published in [54], [55] and [56]. Research on CLT floors is quite recent and was presented in [57] and [58]. The originality of our solution lies in the fact that modular prefabricated lightweight composite elements were developed and arranged in order to rapidly erect buildings. ...
Wood-frame is the most common construction type for residential buildings in North America. However, there is a limit to the height of the building using a traditional wood-frame structure. Cross-laminated timber (CLT) provides possible solutions to mid-rise and high-rise wood buildings. CLT offers many advantages such as improved dimensional stability, a quicker erection time and good performance in case of fire. In order to introduce the cross-laminated timber products to the North American market, it is important to gain a comprehensive understanding of its structural properties. This paper focuses on the seismic performance of CLT connections. Over the last few years FPInnovations of Canada has conducted a test program to determine the structural properties of CLT panels and its application in shear walls. The test program comprised of more than 100 connection tests which followed the loading procedures of CUREE and ISO test protocols as specified in ASTM Standards ASTM E 2126-09 (2009). These tests were performed parallel and perpendicular to the grain of the outer layer, respectively. The impact of different connections on the seismic performance of CLT walls was investigated in a second phase on full size shearwall. CLT panels are relatively stiff and thus energy dissipation must be accomplished through the ductile behaviour of connections between different shear wall elements and the connections to the story below. A literature review on previous research work related to damage prediction and assessment for wood frame structures was performed. Different approaches for damage indices were compared and discussed. This paper describes how the energy-based cumulative damage assessment model was calibrated to the CLT connection and shear wall test data in order to investigate the damage under monotonic and cyclic loading. Comparison of different wall setup provided a deeper insight into the damage estimation of CLT shear walls and determination of the key parameters in the damage formulation. This represents a first published attempt to apply the damage indices to estimate the seismic behaviour of CLT shear walls.
The paper focuses on the influence of modelling different types of connections in multi-storey cross-lam timber buildings when performing a linear modal response spectrum analysis and nonlinear static analysis assessed with a modified N2 method. The main parameter that defines the response of a building when performing a linear modal analysis is its stiffness. As shown in the paper, the stiffness of cross-lam buildings is predominantly governed by the stiffness of the connections between timber panels. Since the connections of walls (angular brackets or hold-downs) behave non-symmetrically in the vertical direction being characterized by a rather low stiffness in tension compared to compression due to the contact, it is necessary to transform this non-linear wall-rocking behaviour into an equivalent linear behaviour that is required in the linear modal analysis. The paper provides some information on how to model this behaviour. By using a nonlinear static analysis and the N2 method it is shown how different connection types, vertical load on walls and friction between walls and inter-story plates influence the building's global ductility and seismic resistance in terms, for example, of maximum peak ground acceleration. The results presented herein can be used as a basis for further nonlinear dynamic analyses and provide some information for future standard development regarding the influence of connection details and other boundary conditions on the seismic response of cross-lam buildings. Also basic values for overstrength factors of BMF 105 angular brackets and 8 mm self-tapping screws are presented and the importance of using the overstrength concept in design is demonstrated on a case study.
The seismic response of existing un-reinforced masonry (URM) buildings is strongly dependent on the characteristics of wooden floors and, in particular, on their in-plane stiffness and on the quality of connection between the floors and the URM elements. It is generally well-recognized that an adequate in-plane-stiffness and proper connections can significantly improve the three-dimensional response of these buildings, obtaining a better distribution and transfer of forces to the lateral load resisting walls. However, the extensive damage observed during past earthquakes on URM buildings of different types have highlighted serious shortcomings in typical retrofit interventions adopted in the past and based on stiffening the diaphragm. Recent numerical investigations have also confirmed that increasing the stiffness of the diaphragm is not necessarily going to lead to an improved response, but could actually result to detrimental effects. The evaluation of the in-plane stiffness of timber floors in their as-built and retrofitted configuration is still an open question and a delicate issue, with design guidelines and previous research results providing incomplete and sometimes controversial suggestions to practicing engineers involved in the assessment and/or retrofit of these type of structures. In this contribution, the role of the in-plane stiffness of timber floors in the seismic response of URM buildings is critically discussed, based on the relatively limited available experimental and numerical evidences. A framework for a performance-based assessment and retrofit strategy of URM buildings, capable of accounting for the effects of a flexible diaphragm on the response prior to and after the retrofit intervention, is then proposed. By controlling the in-plane stiffness of the diaphragm, adopting a specific strengthening (or weakening) intervention, the displacements, accelerations and internal force demands can be maintained within targeted levels. This will protect undesired local mechanisms and aim for a more appropriate hierarchy of strength within the whole system.
The present study tested the charring rates for cross-laminated timber panels (CLT) exposed to standard and parametric fires. A series of fire tests were performed on horizontal cross-laminated timber panels exposed to EN 1991-1-2 Standard and Parametric temperature-time curves, and a "Swedish" curve. A large gas-fired horizontal furnace was used in the experiments. The char depth was measured through all stages of the fires. The objectives were; to examine the charring rate of CLT panels and whether the charring changed through the fire stages, to investigate if the charring rates for the various temperature-time curves were different, and if the rates were affected by the panel thicknesses. Timber panels of three different thicknesses were tested to investigate a thickness effect. The results illustrate that the charring rate for CLT panels exposed to various fires can vary greatly. Fast temperature growth gave faster charring rates. The thickness of the panels did not have an unambiguous effect on the charring rate. The charring rate of wood exposed to fires of long durations became constant after a while.
In October 2007 a series of seismic tests were carried out on a 7-storey building made of cross laminated (XLam) wooden panels in natural scale on a shaking table E-Defence in Japan within the SOFIE project. The paper presents calculation procedure, prediction of dynamic behaviour of the tested structure excited by the earthquake record "Kobe JMA 1995" and comparison between predicted, that means calculated and measured response. Due to blind prediction approach some construction details were not known before dynamic time history response calculation. Therefore some assumptions, engineering judgment and rough static analyses were needed to define all construction parts which were in modelling approach assumed as important and could have had influence on dynamic response of the analyzed structure. The most important assumptions related to the definition of the stiffness and load bearing capacity of mechanical connections, types of anchors and their positions in each floor level, were determined on the basis of static analysis where the structure was loaded with equivalent horizontal seismic forces and then were used in dynamic analysis. A mathematical model was developed in program SAP2000 where modal and time history analyses were carried out. Comparison of calculated and measured results is described and evaluated on the basis of the model assumptions and its simplification.
p>Discussion is focused on connections between massive Cross-Laminated-Timber (XLam) floor slabs and steel skeletons of hybrid structures. Using the example of a 24-storey building, it is shown that such connections can be made using simple fasteners like large screws to enforce composite action in the critical case of XLam slabs acting as diaphragms during lateral seismic or wind events. Results of laboratory tests on connections between XLam slabs and structural steel are presented. It is concluded that hybrid steel-XLam structures can be superior to hybrid steel-RC structures.</p