This state-of-the-art report has been prepared within COST Action FP1402 Basis of structural timber design from research to standards, Working Group 3 Connections. The Action was established to create an expert network that is able to develop and establish the specific information needed for standardization committee decisions. Its main objective is to overcome the gap between broadly available scientific results and the specific information needed by standardization committees. This necessitates an expert network that links practice with research, i.e. technological developments with scientific background. COST presents the ideal basis to foster this type of joint effort. Chapter 8 Connections presents an integral part of Eurocode 5 and is in need of revision. This state-of-the-art report shall provide code writers with background information necessary for the development of the so-called Second Generation of the Eurocodes, now aimed to be produced in 2022.
Project contact is Christian Dagenais at Université Laval
Summary
The use of materials in a building is traditionally determined from its combustibility (via ULC S114 or ULC S135) and by its flame propagation index (via ULC S102). The ULC S102 Flame Spread Test, developed in 1943, has historically reduced risk through its method of classifying materials. However, this test does not provide quantitative information on the combustion properties of materials, such as heat flow. The latter is one of the most important variables in the development of a fire. Thus, a new approach would be preferable in order to review the classification of materials according to ULC S102 and ULC S135 (cone calorimeter). The objective of this project is to develop a new approach to classifying materials based on cone calorimeter test results. These results can subsequently be used in numerical modeling as part of a fire safety engineering design. A significant amount of cone calorimeter (ULC S135) testing of materials currently evaluated according to ULC S102 will be required.
The use of cross-laminated timber (CLT) in multi-story buildings is increasing due to the potential of wood to reduce green house gas emissions and the high load-bearing capacity of CLT. Compression perpendicular to the grain (CPG) in CLT is an important design aspect, especially in multi-storied platform-type CLT buildings, where CPG stress develops in CLT floors due to loads from the roof or from upper floors. Here, CPG of CLT wall-to-floor connections are studied by means of finite element modeling with elasto-plastic material behavior based on a previously validated Quadratic multi-surface (QMS) failure criterion. Model predictions were first compared with experiments on CLT connections, before the model was used in a parameter study, to investigate the influence of wall and floor thicknesses, the annual ring pattern of the boards and the number of layers in the CLT elements. The finite element model agreed well with experimental findings. Connection stiffness was overestimated, while the strength was only slightly underestimated. The parameter study revealed that the wall thickness effect on the stiffness and strength of the connection was strongest for the practically most relevant wall thicknesses between 80 and about 160 mm. It also showed that an increasing floor thickness leads to higher stiffness and strength, due to the load dispersion effect. The increase was found to be stronger for smaller wall thicknesses. The influence of the annual ring orientation, or the pith location, was assessed as well and showed that boards cut closer to the pith yielded lower stiffness and strength. The findings of the parameter study were fitted with regression equations. Finally, a dimensionless ratio of the wall-to-floor thickness was used for deriving regression equations for stiffness and strength, as well as for load and stiffness increase factors, which could be used for the engineering design of CLT connections.
The present research report was written as a PhD thesis (ETH Dissertation Nr. 22815) by Flavio Wanninger and shows the results of a comprehensive experimental and numerical analysis on the structural behaviour of post-tensioned timber frame, in particular with focus on the momentrotation-behaviour of the developed post-tensioned beam-column timber joint using hardwood and the long-term behaviour of the system. The results of the experimental and numerical investigations provide reliable data for the development and validation of calculation models for the design of post-tensioned timber frames with hardwood for vertical and horizontal loads and taking into account the long-term behaviour. The objective of the research project is the development and implementation of post-tensioned timber frame structures into the practice and fits well into the overall research strategy of the institute on the development of innovative solutions for timber structures.
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.
This research investigates the in-plane seismic performance of glulam frames with buckling restrained braces (BRBs) through experimental testing, numerical modelling and design approach development.
With the advancement of engineered wood products (EWPs) and digital fabrication technology, there is an increasing interest and implementation of EWPs for mid-rise and high- rise buildings (also called mass timber buildings) around the world. However, the elastic modulus of timber is only around one-third of reinforced concrete and one-twentieth of structural steel. Additionally, limited ductility is assumed during mass timber building design due to the possibility of timber’s brittle failure in tension. Seismic considerations usually govern the design of lateral force resisting systems (LFRS) in earthquake-prone countries like New Zealand. The relatively lower elastic modulus and limited ductility of timber may cause uneconomical member sizes and increase the number of LFRS (e.g. shear walls and braces). These limitations motivated this research with the main objective to improve the seismic performance of mass timber building using a timber-steel hybrid system.
Experimental tests were conducted for BRB-braced glulam frames (BRBGFs). Following the capacity design approach, BRBs were designed as ductile elements while timber members and BRB-timber interface connections were designed as non-ductile elements. Two 8 m wide and 3.6 m high full-scale BRBGFs were built and tested under cyclic loading. Dowelled connections with inserted steel plates were used in one specimen to connect the glulam members and BRBs, while screwed connections with steel side plates were used in the other specimen. The test results showed that replacing the traditional timber braces with BRBs significantly increased the energy dissipation capacity and minimized the damage in the connections as well as glulam members. The BRBGF with the dowelled connections (S-D) had more initial slips than the BRBGF with the screwed connections (S-S), but both specimens had comparable performance after the serviceability limit state (SLS) load level.
Component-based numerical models were developed in OpenSees to investigate BRBGFs with general configurations. The test data of S-D and S-S were first used to calibrate the numerical models. Then, parametric studies were conducted to investigate the influence of connection stiffness and initial slips on the cyclic performance of BRBGFs. It was shown that the component-based numerical models represented the force-drift responses, accumulated energy dissipation and BRB deformations of S-D and S-S well. When the connection relative overstrength factor os,con was over the BRB overstrength factor os,BRB, the connections were sufficiently stiff to engage BRBs. The strength and stiffness of BRBs and initial slips caused by manufacturer tolerances had a negligible effect on the ultimate strength and energy dissipation under cyclic loading.
A direct displacement-based design (DDBD) approach was developed for the BRBGF system to avoid the complicated process of numerical modelling and facilitate the application of the hybrid system. The critical parameters for extending the DDBD approach to the BRBGF system were first discussed including the displacement profile, yield drift, connection stiffness, hysteresis damping ratio and displacement reduction factor in. Then, the component-based numerical modelling method was used to build one-bay one-storey BRBGFs and verify the critical parameters by pushover analyses and nonlinear time-history analyses (NLTHA). Moreover, the DDBD approach was used to design a set of BRBGF buildings with three, six, and nine storeys. The multi-storey BRBGF models were built in the OpenSees and analysed under a set of ground motions to verify the DDBD approach. The pushover analyses showed that the stiffness of BRB-timber connections needed to be considered when estimating the yield drift of BRBGFs. The NLTHA of one-bay one-storey BRBGFs showed that the relationship between in and ductility factor µ for the Takeda fat model was also suitable for BRBGFs on the conservative side. The NLTHA results of the multi-storey BRBGF models confirmed that the DDBD approach effectively controlled the inter-storey drift ratios of the BRBGF system under seismic loads.
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