2016, Articolo in rivista, ENG
Polastri A., Pozza L.
A crucial issue in the design of a mid-rise Cross Laminated Timber (CLT) building under horizontal seismic action, is the definition of the principal elastic vibration period of an entire superstructure. Such vibration period depends on the mass distribution and on the global stiffness of the buildings. In a CLT structure the global stiffness of the buildings is highly sensitive to deformability of the connection elements. Consequently for a precise control of the vibration period of the building it is crucial to define the stiffness of each connections used to assemble a superstructure. A design procedure suitable for a reliable definition of the connection stiffness is proposed referring to code provisions and experimental tests. Discussion addresses primary issues associated with the usage of proposed procedure for numerical modeling of case study tall CLT buildings is reported.
2015, Contributo in atti di convegno, ENG
Polastri A., Pozza L., Loss C., Smith I.
In last twenty years the CLT panels have become quite widely employed to build multi-storey residential and mercantile buildings. These buildings are often characterised by the presence of many internal and perimeter shear walls. Such structures have been widely studied through experimental tests and numerical simulations. One of the most comprehensive experimental research on seismic behaviour of CLT buildings was carried out by CNR-IVALSA, Italy, under the SOFIE Project [1-2]. Other important activities have been conducted at University of Trento, Italy [3]. European seismic performance related tests have been conducted at the University of Ljubljana, Slovenia where the behaviour of 2-D CLT shear walls having various load and boundary conditions was assessed [4]. FPInnovations in Canada has undertaken tests to determine the structural properties and seismic resistance of CLT shear walls and small-scale 3-D structures [5-6]. Recently innovative connection solutions have been studied in order to create panel to panel, or panel to other material joints [7]. The innovative jointing method results in point-to-point mechanical connections designed to only connect corners of individual CLT panels in ways that fulfil both hold-down and the lateral shear function; and is an alternative to traditional connections made using shear and hold-down anchors [8]. Different connectors have been tested in order to define the best way to joint CLT panels to steel structures [9]. Failure mechanism in large shear wall systems have been characterised in multiple studies [10]. For examples, multi-storey buildings having beam-and-column frameworks resisting effects of gravity loads, and cross-bracing or core substructures and exterior CLT shear walls to resist effects of lateral earthquake or wind loads have been studied [11]. Advantages of such systems can include creation of large open interior spaces, high structural efficiency, and material economies. From a structural design and analysis perspective point-to-point interconnection between CLT panels or point-to point connections at their boundaries leads to less ambiguity in load paths than exists for other approaches. It also lessens the chances that structural systems will not fail in unintended ways if overloaded by force flows thought superstructure elements of from superstructures to foundations. Although buildings of the new typology have already been built there has not been full study of the structural behaviour. The most crucial structural performance aspects that have not been fully studied are those related to construction of CLT building cores as replacements for one constructed from materials like reinforced concrete or masonry. Pertinent issues relate to vertical continuity between storeys, connections between building core elements and elevated floors, and core to foundation connections. The in-plane deformability of floor diaphragms is also a factor known to greatly influence global responses of completed building superstructures subjected to lateral loads and therefore requires close attention [12]. This paper related to elimination of the deficiencies. The behaviour of multi-storey buildings braced with cores and CLT shear walls is examined based on numerical analyses. Two procedure for calibrating numerical analysis models are proposed using information from Eurocode 5 [13] and specific experimental test data. This includes calibration of parameters that characterise connections between CLT panels and other CLT panels, building cores and shear walls. The aim is to make the characterizations of behaviours of connections that reflect how those connections perform within complete multi-storey superstructures, rather than in isolation or as parts of substructures. The earthquake action for cases studied was according to Eurocode 8 [14] and using the appropriate behaviour factor (q factor). Results of analyses of entire buildings are presented in terms of principal elastic periods, base shear and up-lift forces. Discussion addresses key issues associated with behaviour of such systems and modelling them. In particular, it is to be noted that available seismic codes do not provide guidance on most crucial aspects of how to design such structural systems. The analyses mid-rise CLT buildings demonstrates importance of representing their seismic force resisting systems in proper ways, including paying close attention to the schematization of base and inter-storey connections. Obtained results permit creation of appropriate guidelines and rules for design of the aforementioned types of hybrid buildings incorporating CLT wall panels. This takes into considering the large variation that can exist in the elastic lateral vibration periods due to differences between code and experimentally calibrated estimates of connection stiffness.
2013, Articolo in rivista, ENG
Follesa M., Christovasilis I.P., Vassallo D., Fragiacomo M., Ceccotti A.
Cross Laminated Timber (CLT) structures are nowadays increasingly used worldwide and mostly in Europe where the system originated. However, in spite of this diffusion which led to the construction of a great number of multi-storey buildings all over Europe, still Eurocodes are almost completely missing provisions for CLT designers, especially regarding the seismic design. Nevertheless, Eurocode 8 requires in most cases, due to the regularity criteria being not fulfilled for most of the buildings, the use of the modal response spectrum analysis method, i.e. the linear dynamic analysis. This method requires the correct estimation of the lateral stiffness of the building in order to accurately calculate the design seismic forces in the building, which may be significantly underestimated or overestimated depending on the size of the building and the shape of the design spectrum. This can be done by modelling each connection with different methods that are often based on available test results, which are not easily accessible by a practicing engineer. This paper provides a design approach for dynamic linear modelling of CLT structures using SAP 2000. Equations are proposed based on available design codes and literature references, and used to design a 3-storey case study building. Further provisions for the seismic design of CLT buildings which are not included in Eurocode 8 are also given. Finally, the proposed design model is also compared with the results of the shaking table tests conducted in 2006 in Japan by CNR-IVALSA on a three-storey CLT building.