Initially, timber was considered only as an easily accessible and processable material in nature; however, its excellent properties have since become better understood. During the discovery of new building materials and thanks to new technological development processes, industrial processing technologies and gradually drastically decreasing forest areas, wood has become an increasingly neglected material. Load-bearing structures are made mostly of reinforced concrete or steel elements. However, ecological changes, the obvious problems associated with environmental pollution and climate change, are drawing increasing attention to the importance of environmental awareness. These factors are attracting increased attention to wood as a building material. The increased demand for timber as a building material offers the possibility of improving its mechanical and physical properties, and so new wood-based composite materials or new joints of timber structures are being developed to ensure a better load capacity and stiffness of the structure. Therefore, this article deals with the improvement of the frame connection of the timber frame column and a diaphragm beam using mechanical fasteners. In common practice, bolts or a combination of bolts and pins are used for this type of connection. The subject of the research and its motivation was to replace these commonly used fasteners with more modern ones to shorten and simplify the assembly time and to improve the load capacity and rigidity of this type of frame connection.
Timber structures are strongly depending on the design of connections, which are mostly constructed from steel components. However, these joints have a number of limitations such as the tendency to be heavy, proneness to corrosion and often poor aesthetic appearances. Therefore, this study aims to replace metallic joints by non-metallic materials. An experimental testing program was performed to investigate the use of glass fiber reinforced plastics (GFRP), densified veneer wood (DVW) and laminated veneer lumber (LVL) in the form of plates and dowels in different test configurations. Analytical and numerical models were developed to better understand the load-bearing behaviour and to perform static verifications. The models were validated based on the experimental results. The results demonstrate that the use of GFRP dowels in combination with GFRP plates can provide a robust connection system for contemporary applications.
The fiber-reinforced polymer is one kind of composite material made of synthetic fiber and resin, which has attracted research interests for the reinforcement of timber elements. In this study, 18 glued-laminated (glulam) beams, unreinforced or reinforced with internally embedded carbon fiber–reinforced polymer (CFRP) sheets, were tested under four-point bending loads. For the reinforced glulam beams, the influences of the strengthening ratio, the modulus of elasticity of the CFRP, and the CFRP arrangement on their bending performance were experimentally investigated. Subsequently, a finite element model developed was verified with the experimental results; furthermore, a general theoretical model considering the typical tensile failure mode was employed to predict the bending–resisting capacities of the reinforced glulam beams. It is found that the reinforced glulam beams are featured with relatively ductile bending failure, compared to the brittle tensile failure of the unreinforced ones. Besides, the compressive properties of the uppermost grain of the glulam can be fully utilized in the CFRP-reinforced beams. For the beams with a 0.040% strengthening ratio, the bending–resisting capacity and the maximum deflection can be enhanced approximately by 6.51 and 12.02%, respectively. The difference between the experimental results and the numerical results and that between the experimental results and analytical results are within 20 and 10%, respectively.
Project contact is Daniel Dowden at Michigan Technological University
Summary
This award will investigate a low-damage solution for cross-laminated timber (CLT) seismic force-resisting systems (SFRSs) using a novel uplift friction damper (UFD) device for seismically resilient mass-timber buildings. The UFD device will embrace the natural rocking wall behavior that is expected in tall CLT buildings, provide stable energy dissipation, and exhibit self-centering characteristics. Structural repair of buildings with these devices is expected to be minimal after a design level earthquake. Although CLT has emerged as a construction material that has revitalized the timber industry, there exists a lack of CLT-specific seismic energy dissipation devices that can integrate holistically with the natural kinematics of CLT-based SFRSs. CLT wall panels themselves do not provide any measurable seismic energy dissipation. As a payload to the large-scale, ten-story CLT building specimen to be tested on the Natural Hazards Engineering Research Infrastructure (NHERI) shake table at the University of California, San Diego, as part of NSF award 1636164, “Collaborative Research: A Resilience-based Seismic Design Methodology for Tall Wood Buildings,” this project will conduct a series of tests with the UFD devices installed on the CLT building specimen. These tests will bridge analytical and numerical models with the high fidelity test data collected with realistic boundary and earthquake loading conditions. The calibrated models will be incorporated in a probabilistic numerical framework to establish a design methodology for seismically resilient tall wood buildings, leading to a more diverse and eco-sustainable urban landscape. This project will provide local elementary school outreach activities, integrate participation of undergraduate minorities and underrepresented groups into the research activities, and foster graduate level curriculum innovations. Project data will be archived and made available publicly in the NSF-supported NHERI Data Depot (https://www.DesignSafe-CI.org). This award contributes to NSF's role in the National Earthquake Hazards Reduction Program (NEHRP).
The research objectives of this payload project are to: 1) bridge the fundamental mechanistic UFD models linking analytical and numerical models necessary for seismic response prediction of seismically resilient CLT-based SFRSs, 2) characterize the fundamental dynamic UFD behavior with validation and calibration through large-scale tests with realistic boundary conditions and earthquake loadings, and 3) integrate low-damage, friction-based damping system alternatives within a resilience-based seismic design methodology for tall wood buildings. To achieve these objectives, the test data collected will provide a critical pathway to reliably establish numerical and analytical models that extend the shake table test results to a broad range of archetype buildings. The seismic performance of mass-timber archetype building systems will be established through collapse risk assessment using incremental dynamic analyses. This will provide a first step in the longer term goal of establishing code-based seismic performance factors for CLT-based SFRSs.
International Journal of Advanced Structural Engineering
Summary
This paper investigates the mechanical performance of longitudinally cracked glulam columns under eccentric compression loads. Experimental investigation was conducted to explore the influence of initial cracks on the failure modes and load bearing capacity of glulam columns. Two different crack patterns named DC and IC, and two column lengths (i.e. 600 and 1100 mm) were considered in the experiments. It was indicated that these two crack patterns reduced the capacity of slender glulam columns and the difference of failure modes was observed between glulam columns with and without initial cracks. Further, a numerical model was developed and validated by the test results. With the application of cohesive zone material model, the propagation of initial cracks could be considered in the numerical modeling. A parametric study was carried out by the verified model and the influence of crack lengths and crack locations was further investigated. From the numerical analysis, it was found that through cracks reduced the capacity of glulam columns significantly. Also, crack location impacts the capacity of glulam columns and the extent of impact relates to the slenderness ratio of the columns, while cracks with different lengths have similar influence on the capacity of columns.
In this contribution bending and shear tests of cross laminated timber (CLT) plates under concentrated loads are presented. The so loaded structural members can fail either due to punching along a critical perimeter line in the vicinity of the concentrated load or in bending. Two test configurations were developed and investigated by linear elastic models. The obtained test results and observed failures as well as their correlation with the mechanical modeling are shown in this paper. The established numerical model was a 3D solid model with different material behavior for all acting stresses. The material behavior was implemented in a user subroutine for the FE program ABAQUS. By comparison of measured and computed load displacement curves numerical models could be discussed regarding their reliability and conclusions about missing input for an increasing accuracy of the model could be drawn.
A new numerical model able to account for the interaction between tension and shear forces on typical hold-down connections used in CLT structures is proposed and discussed, starting from results of an experimental campaign conducted at University of Bologna. A specifically developed method appropriate to evaluate the main strength and stiffness parameters from the experimental cyclic force-displacement curves is presented, and the corresponding trilinear backbone approximation is defined. Parameters obtained from tri-linear backbone curves were used to define the effect of the tension-shear interaction on the behaviour of hold-down connections, particularly as far as yielding and peak strength and stiffness parameters are concerned. In order to numerically reproduce the behaviour of connections, a coupled zero-length element is developed and presented. The model is implemented in OpenSees and adopted to model single connection element. The model is calibrated referring to experimental results of specimens loaded only in tension. Then the model is validated referring to tests with increasing level of tension-shear interaction. The proposed model is able to reproduce the actual behaviour of hold-down connection with coupled tension-shear forces under monotonic load conditions. Finally, a first proposal for accounting the hysteretic behaviour is presented, and some preliminary results are shown.
Traditional wood-wood connections, widely used in the past, have been progressively replaced by steel fasteners and bonding processes in modern timber constructions. However, the emergence of digital fabrication and innovative engineered timber products have offered new design possibilities for wood-wood connections. The design-to-production workflow has evolved considerably over the last few decades, such that a large number of connections with various geometries can now be easily produced. These connections have become a cost-competitive alternative for the edgewise connection of thin timber panels. Several challenges remain in order to broaden the use of this specific joining technique into common timber construction practice: (1) prove the applicability at the building scale, (2) propose a standardized construction system, (3) develop a convenient calculation model for practice, and (4) investigate the mechanical behavior of wood-wood connections. The first building implementation of digitally produced through-tenon connections for a folded-plate structure is presented in this work. Specific computational tools for the design and manufacture of more than 300 different plates were efficiently applied in a multi-stakeholder project environment. Cross-laminated timber panels were investigated for the first time, and the potential of such connections was demonstrated for different engineered timber products. Moreover, this work demonstrated the feasibility of this construction system at the building scale. For a more resilient and locally distributed construction process, a standardized system using through-tenon connections and commonly available small panels was developed to reconstitute basic housing components. Based on a case-study with industry partners, the fabrication and assembly processes were validated with prototypes made of oriented strand board. Their structural performance was investigated by means of a numerical model and a comparison with glued and nailed assemblies. The results showed that through-tenon connections are a viable alternative to commonly used mechanical fasteners. So far, the structural analysis of such construction systems has been mainly achieved with complex finite element models, not in line with the simplicity of basic housing elements. A convenient calculation model for practice, which can capture the semi-rigid behavior of the connections and predict the effective bending stiffness, was thus introduced and subjected to large-scale bending tests. The proposed model was in good agreement with the experimental results, highlighting the importance of the connection behavior. The in-plane behavior of through-tenon connections for several timber panel materials was characterized through an experimental campaign to determine the load-carrying capacity and slip modulus required for calculation models. Based on the test results, existing guidelines were evaluated to safely apply these connections in structural elements while a finite element model was developed to approximate their performance. This work constitutes a firm basis for the optimization of design guidelines and the creation of an extensive database on digitally produced wood-wood connections. Finally, this thesis provides a convenient design framework for the newly developed standardized timber construction system and a solid foundation for research into digitally produced wood-wood connections.
The diaphragmatic behaviour of floors represents one important requirement for earthquake resistant buildings since diaphragms connect the lateral load resisting systems at each floor level and transfer the seismic forces to them as a function of their in-plane stiffness. This paper presents an innovative hybrid timber-steel solution for floor diaphragms developed by coupling cross-laminated timber panels with cold-formed custom-shaped steel beams. The floor consists of prefabricated repeatable units which are fastened on-site using pre-loaded bolts and self-tapping screws, thus ensuring a fast and efficient installation. An experimentally validated numerical model is used to evaluate the influence of the; i) in-plane floor stiffness; ii) aspect ratio and shape of the building plan; and iii) relative stiffness and disposition of the shear walls, on the load distribution to the shear walls. The load transfer into walls and lateral deformation of the construction system primarily depend on the adopted layouts of shear walls, and for most cases an in-plane stiffness of floors two times larger than that of walls is recommended.
Multi-storey platform cross laminated timber (CLT) structures are becoming progressively desirable for engineers and owners. This is because they offer many significant advantages such as speed of fabrication, ease of construction, and excellent strength to weight ratio. With platform construction, stories are fixed together in a way that each floor bears into load bearing walls, therewith creating a platform for the next level. The latest research findings have shown that CLT platform buildings constructed with traditional fasteners can experience a high level of damage especially in those cases where the walls have adopted hold-down brackets and shear connectors with nails, rivets or screws. Thus, the current construction method for platform CLT structures is less than ideal in terms of damage avoidance. The main objective of this study is to develop a low damage platform timber panelised structural system using a new configuration of slip friction devices in lieu of traditional connectors. A numerical model of such a system is developed for a low rise CLT building and then is subjected to reversed cyclic load simulations in order to investigate its seismic performance. The result of these quasi-static simulations demonstrated that the system maintained the strength through numerous cycles of loading and unloading. In addition to this, the system is capable of absorbing significant amount of energy. The findings of this study demonstrate the proposed concept has the potential to be developed as a low damage seismic solution for CLT platform buildings.
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