This paper presents a direct displacement-based design (DDBD) approach for the buckling restrained braces (BRBs) braced glue-laminated timber (glulam) frame (BRBGF) structures. First, the critical design parameters of the DDBD approach were derived for BRBGFs. Then, using experimentally verified numerical models, pushover analyses and nonlinear time-history analyses (NLTHAs) were conducted on a series of one-storey BRBGFs to calibrate the stiffness adjustment factor for BRB-timber connections and the spectral displacement reduction factor . Finally, the DDBD approach was verified as a prospective approach for the seismic design of multi-storey BRBGF buildings by NLTHAs of the case study buildings.
Various design approaches for establishing the resistance of connections in cross-laminated timber (CLT) structures have been developed and adopted in timber design standards worldwide. Although the fundamental principles are similar, the new design provisions for CLT connections have been aligned in some standards with the existing design philosophy and format adopted for sawn timber and glulam using traditional fasteners such as dowels, nails, and wood screws for consistency and simplicity, in the other standards, alternate approaches have been developed. This article presents a snap shot of the various design approaches for connections in CLT adopted in Europe, Canada, the United States, and New Zealand. The intent is for the reader to have a better knowledge of the underpinning assumptions, principles, and the adopted design rules in each of these standards.
An experimental and analytical study on rotational behaviour of glulam beam-column moment connections with self-drilling dowels (SDD) was conducted. Connection properties including strength, stiffness, ductility and energy dissipation were experimentally evaluated by testing seven full-scale connection specimens with and without self-tapping screw (STS) reinforcement along timber perpendicular to grain. All the connections showed high initial stiffness and high moment capacity when compared with the test results of bolted connections reported in literature. The unreinforced connections had relatively low ductility due to timber splitting despite the increased fastener edge distance. The STS reinforcement effectively reduced timber splitting tendency and encouraged the yielding of more SDD, leading to slightly increased moment capacity, but significantly improved ductility. A modified analytical model (MAM) was then proposed to predict strength and rotation of the SDD moment connections based on force and moment equilibrium of the glulam members. Improved prediction accuracy was achieved for the SDD moment connections when compared with the past analytical methods.
Cross-laminated Timber (CLT) is gaining popularity in Australasia as a building material for multi-storey structures. For multi-storey timber buildings located in seismic areas, designing strong but ductile hold-downs for CLT shear walls can be challenging and requires careful structural connection design. In this study, dowelled connections in New Zealand Douglas-Fir (D.Fir) CLT with inserted steel plates were experimentally investigated as a solution for hold-downs in multi-storey timber buildings. The dowel group spacing was varied for CLT3 (3-ply, 135 mm thick), CLT5 (5-ply, 175 mm thick) and CLT7 (7-ply, 275 mm thick) D.Fir CLT to investigate the spacing impact on ductility of the hold-down connections under both monotonic and quasi-static cyclic loading. These results were also compared with past similar testing of dowelled connections in 5-ply (150 mm) Radiata Pine CLT. A total of 12 monotonic and 36 quasi-static cyclic tests were carried out and it was observed that increased dowel spacing increases ductility with similar strength when compared to past more dense dowel spacing tests. Furthermore, to deter the onset of tension perpendicular to grain brittle failure, fully threaded screws and nuts were added to the dowelled connection and the impact of this is discussed.
This paper presents the modeling of coupling effect of tension and shear loading on Cross Laminated Timber (CLT) connections using a finite element based algorithm called HYST. The model idealizes the connections as a “Pseudo Nail” - elastoplastic beam elements (the nail) surrounded by compression-only spring elements (steel sheath and wood embedment). A gap size factor and an unloading stiffness degradation index of the spring elements under cyclic loading were integrated into the optimized HYST algorithm to consider the coupling effect. The model was calibrated to compare with 32 configurations of CLT angle bracket and hold-down connections tests: in tension with co-existent constant shear force, and in shear with co-existent tension force. The results showed that the proposed model can fully capture the coupling effect of typical CLT connections, considering strength degradation, unloading and reloading stiffness degradation, and pinching effect. The model provided a useful tool for nailbased timber connections and a mechanism-based explanation to understand the hysteretic behaviour of CLT connections under bi-axial loading.
Rolling shear (RS) strength may govern load carrying capacity of cross laminated timber (CLT) subjected to high out-of-plane loading because high RS stresses may be induced in cross layers and wood typically has low RS strength. This study investigates RS strength properties of none-edge-glued CLT via experimental testing (short-span bending tests and modified planar shear tests) and numerical modelling. CLT specimens with different manufacturing parameters including two timber species (New Zealand grown Douglas-fir and Radiata pine), three lamination thickness (20 mm, 35 mm, and 45 mm) and various lamination aspect ratios (4.1~9.8) were studied. The lamination aspect ratio was found to have a substantial impact on RS strength of CLT. Higher aspect ratios led to a significant increase of RS strength and an approximately linear relationship could be established. With similar lamination aspect ratios, the Radiata pine CLT had higher RS strength than the Douglas-fir CLT. The two different test methods, however, yielded comparable RS strength assessments. Numerical models were further developed to study the influence of the test configurations and gaps in the cross layers on stress distributions in the cross layers. It was also found the compressive stresses perpendicular to grain in cross layers had negligible influence on the RS strength evaluations.
Particle Tracking Velocimetry (PTV) is a quantitative field measuring technique originally designed to track individual particles in fluid flows. In this study, PTV was applied for the first time in the context of large scale timber connection testing. The suitability of PTV in structural applications was assessed by tracking the attachment points of string potentiometers and comparing the PTV displacements to those obtained by the potentiometers. Furthermore, it was found that PTV was able to capture crack growth and compute the resulting displacement field in the connection area.
This paper presents an experimental study on dowelled connections in Cross Laminated Timber (CLT) and Laminated Veneer Lumber (LVL) using 20 mm mild steel dowels and internal steel plates. Connections designed to fail in brittle row shear and group tear-out were tested under monotonic loading to assess the validity of analytical models from literature and code provisions. Connections designed to provide non-linearity before failure and thus produce ductility were tested under both monotonic and cyclic loading to study the influence of cyclic loading on ductility and the possibility of mode cross-over. It was found that cross layers in CLT improve ductility. Furthermore, mode cross-over from ductile response to brittle failure was observed in both CLT and LVL connections. Nevertheless, a good amount of ductility was achieved in all layouts (except the LVL connections designed for group tear-out failure) before cross-over to brittle failure occurred.
Steel-timber hybrid structural systems offer a modern solution for building multi-story structures with more environmentally-friendly features. This paper presents a comprehensive seismic performance assessment for a kind of multi-story steel-timber hybrid structure. In such a hybrid structure, steel moment resisting frames are infilled with prefabricated light wood frame shear walls to serve as the lateral load resisting system (LLRS). In this paper, drift-based performance objectives under various seismic hazard levels were proposed based on experimental observations. Then, a numerical model of the hybrid structure considering damage accumulation and stiffness degradation was developed and verified by experimental results, and nonlinear time-history analyses were conducted to establish a database of seismic responses. The numerical results further serve as a technical basis for estimating the structure's fundamental period and evaluating post-yielding behavior and failure probabilities of the hybrid structure under various seismic hazard levels. A load sharing parameter was defined to describe the wall-frame lateral force distribution, and a formula was proposed and calibrated by the time-history analytical results to estimate the load sharing parameter. Moreover, earthquake-induced non-structural damage and residual deformation were also evaluated, showing that if designed properly, desirable seismic performance with acceptable repair effort can be obtained for the proposed steel-timber hybrid structural system.
This paper presents an evaluation of overstrength based on an experimental study on dowelled connections in cross-laminated timber (CLT). Connection overstrength needs to be well understood in order to ensure that ductile system behaviour and energy dissipation can be achieved under seismic loading. Overstrength is defined as the difference between the code-based strength, using characteristic material strengths, and the 95th 4 percentile of the true strength distribution. Many aspects contribute to total connection overstrength, which makes its definition challenging. In this study, half-hole embedment tests were performed on CLT to establish embedment strength properties and three point bending tests were performed to determine the fastener yield moment. Different connection layouts, making use of mild steel dowels and an internal steel plate, were tested under monotonic and cyclic loading to evaluate theoretically determined overstrength values and study the influence of cyclic loading on overstrength. Experimental results were compared with strength predictions from code provisions and analytical models for ductile response under monotonic loading. It was found that cyclic loading does not significantly influence overstrength for connections that respond in a mixed-mode ductile way indicating that in future more expedient monotonic test campaigns could be used. This work also provides further experimental data and theoretical considerations necessary for the estimation of a generally applicable overstrength factor for dowelled CLT connections.