The majority of research on high strain-rate effects in timber structures has been limited to the study of the load-bearing members in isolation. Limited work has been conducted on timber connections and full-scale timber assemblies under blast loading, and these have generally been constrained to qualitative observations. In North America, the increasing prevalence of mid- and high-rise timber structures makes them susceptible to blast effects. In addition, questions remain on how to design and optimize these timber assemblies, including the connections, against blast loads, due in part to the limitations on comprehensive design provisions.
The effects of far-field blast explosions were simulated using the University of Ottawa shock tube. A total of fifty-eight dynamic tests were conducted on connection-level and full-scale specimens. The research program aimed to investigate the behaviour of heavy-timber connections when subjected to simulated blast loads. The experimental results showed that connections with a main failure mechanism consisting of wood crushing experienced significant increases in dynamic peak load when compared to the static peak load. In contrast, connections where steel yielding and rupturing occurred experienced no statistically significant increase in dynamic peak load. Full-scale glulam specimens with bolted connections designed to yield via wood crushing and bolt bending performed better than those with overdesigned connections. Bolted connections which failed in splitting led to premature failure of the glulam assembly. Reinforcement with self-tapping screws allowed these bolted joints to fail in a combination of bolt yielding and wood crushing, and provided more ductility when compared to unreinforced specimens. Specially designed energy-absorbing connections significantly increased the energy dissipation capabilities of the timber assemblies. The basis of these connections was to allow for connection yielding while delaying failure of the wood member. This was achieved via elastoplastic connection behaviour, which effectively limited the load imparted onto the wood member.
Based on the experimental results, limitations in the current Canadian blast provisions were highlighted and discussed. A two-degree-of-freedom blast analysis software was developed and validated using full-scale and connection-level experimental results and was found to adequately capture the system response with reasonable accuracy. Sensitivity analyses regarding the applicability of using single-degree-of-freedom analysis were presented and discussed.
Wind-induced dynamic excitation is becoming a governing design action determining size and shape of modern Tall Timber Buildings (TTBs). The wind actions generate dynamic loading, causing discomfort or annoyance for occupants due to the perceived horizontal sway – i.e. vibration serviceability failure. Although some TTBs have been instrumented and measured to estimate their key dynamic properties (natural frequencies and damping), no systematic evaluation of dynamic performance pertinent to wind loading has been performed for the new and evolving construction technology used in TTBs. The DynaTTB project, funded by the Forest Value research program, mixes on site measurements on existing buildings excited by heavy shakers, for identification of the structural system, with laboratory identification of building elements mechanical features coupled with numerical modelling of timber structures. The goal is to identify and quantify the causes of vibration energy dissipation in modern TTBs and provide key elements to FE modelers. The first building, from a list of 8, was modelled and tested at full scale in December 2019. Some results are presented in this paper. Four other buildings will be modelled and tested in spring 2021.
Cross Laminated Timber (CLT) is increasingly used as a structural material for tall buildings, due to its structural properties and low carbon footprint. CLT is a mass timber product, which is made by crosswise gluing layers of timber lamellae. Recent architectural trends include having visible CLT surfaces, which, in the event of a fire, can become involved in the fire and act as fuel to the fire. A study by the Fire Protection Research Foundation (FPRF; USA), National Fire Protection Agency (NFPA; USA), National Research Council Canada (NRC-CNRC; Canada), Research Institutes of Sweden (RISE; Sweden) and National Institute of Standards and Technology (NIST; USA) has focused on the contribution of exposed CLT to compartment fires. The study included a review of previous compartment fire tests, full-scale fire tests of compartments with and without exposed CLT structures, the development of design methods for engineers and intermediate scale fire tests to identify high-temperature resistant adhesives for CLT. The full-scale compartment tests showed the undesirable consequences of CLT delamination during a fire (i.e. fall-off of exposed lamellas), which occurred due to weakening of the CLT adhesive. These consequences included fire regrowth after a period of decay or a continuation of a fully developed fire. This can make self-extinction of a compartment fire not possible, implicating that the fire will lead to collapse if the fire is not manually extinguished or extinguished by sprinklers. In order to achieve self-extinction of flaming combustion in compartments with exposed CLT it is important to avoid fire-induced delamination. It was shown that fire-induced-delamination can be avoided using high-temperature-resistant adhesives. A test method was developed to identify adhesives that are not prone to fire-induced-delamination under relevant fire conditions. A summary of the test methodology, evaluation and results is discussed in this article.
This article presents a test method that was developed to screen adhesive formulations for finger-jointed lumber. The goal was to develop a small-scale test that could be used to predict whether an adhesive would pass a full-scale ASTM E119 wall assembly test. The method involved loading a 38-mm square finger-jointed sample in a four-point bending test inside of an oven with a target sample temperature of 204°C. The deformation (creep) was examined as a function of time. It was found that samples fingerjointed with melamine formaldehyde and phenol resorcinol formaldehyde adhesives had the same creep behavior as solid wood. One-component polyurethane and polyvinyl acetate adhesives could not maintain the load at the target temperature measured middepth of the sample, and several different types of creep behavior were observed before failure. This method showed that the creep performance of the onecomponent adhesives may be quite different than the performance from short-term load deformation curves collected at high temperatures. The importance of creep performance of adhesives in the fire resistance of engineered wood is discussed.
International Association for Bridge and Structural Engineering Symposium
Research Status
Complete
Notes
September 19-21, 2018, Nantes, France
Summary
Contemporary structures are required to be earthquake-resistant, sustainable and flexible to changing occupancy needs over time. Hybrid wood-based construction systems are promising solutions for modern buildings and research for cost-efficient systems is underway to compete with more traditional and widely spread non-wood building systems. This paper presents an innovative modular and prefabricated wood-based hybrid construction technology. It is a dry solution obtained by fastening on-site steel frames and composite CLT-steel members using only bolts and screws. The main results obtained from a comprehensive experimental programme with focus on the in-plane and out-of-plane behaviour of floors are reviewed. The influence of connections on the response of floors is discussed. The findings are of practical relevance with direct impacts on other applications.
Through collaboration with the NHERI TallWood Project funded by the National Science Foundation,an alternative non-prestressed cross-laminated timber rocking wall system with replaceable fuse components was developed by Katerra engineers and tested at the outdoor shake table at the University of California San Diego. The objective of this specific design and testing is to prove a concept for a new high performance seismic lateral system that is easy to modularize and install, and can be rapidly repaired after major earthquakes. This paper presents the results from a total of thirteen tests conducted on the proposed system, including several repairs after major shaking. The test results showed that the structural system was damage-free under service level ground motions, and experienced repairable damage at designated connection locations for design basis earthquakes and maximum considered earthquakes. Overall the system was able to limit residual drift to an acceptable level and provide a high load displacement capacity for the building system.
Lag screw bolt (LSB) has been used widely for composing glulam moment resisting column-leg as well as beam-column joints for constructing semi-rigid wooden frame structures. A serious problem on the existing LSB joint, however, was its brittle failure mode. In order to avoid this characteristic, Slotted Bolted Connection (SBC) systems, which is a kind of the friction damper for steel truss structure, was introduced to the existing glulam LSB joint system serially. Experiments on full-scale column-leg joint and beam-column joint, which were intended to be used in a three storey glulam school building, showed satisfactory performance on the requirements for the stiffness, yielding and ultimate performance. By this innovative investigation, a glulam semi-rigid portal frame, which has high initial stiffness, clear yielding capacity, rich ductility, and free from glulam brittle fractures, might be possible to be realized.
With the increased usage of Cross Laminated Timber (CLT) in the United States, research efforts have been focused on demonstrating CLT as an effective Seismic Force Resisting System (SFRS). Presented in this paper are the findings of full-scale shake table tests of a two-story 223 m2 (2400 ft2) building with two sets of CLT shear walls on the first and second story. The testing consisted of three phases, each with a unique wall configuration, but only the first phase is presented herein, which consisted of a shear wall with 4:1 aspect ratio CLT panels. The structure was subjected to ground motions scaled to intensities that correspond to a Service Level Earthquake (SLE), Design Base Earthquake (DBE), and Maximum Considered Earthquake (MCE) respectively. In all phases and motions the structure performed well and was in accordance with FEMA collapse prevention requirements for each motion intensity.
An experimental study of four full-scale cruciform sub-assemblages of beam-to-column steel-timber composite joints with extended end plates was conducted to simulate the behaviour of an internal joint in a semi-rigid steel-timber frame. In this system, the Cross-Laminated Timber (CLT) panels were attached compositely to the steel beam using coach screws to achieve the shear connection and the steel-CLT composite beams were connected to the steel columns by bolted extended end plates. In addition, one specimen without a CLT slab was constructed and tested as a control with which to assess the influence of the CLT panels on the performance of the joint. The structural behaviour of this type of joint which requires the connection of the two juxtaposed CLT panels subjected to tension near the column was explored. The test results show that these novel composite joints have credible rotation and moment capacities and provide a viable alternative to their steel-concrete counterparts within a paradigm of reduced carbonemissions in the construction sector.
There is a current trend towards mid- and high-rise mass timber buildings. With this trend, there is a research need to develop a comparison between mass timber compartment fires and non-combustible compartment fires. In an effort to address the knowledge gaps in the fire performance of cross-laminated timber compartments, a full-scale fire test series was developed. The fire test series included five tests with varying levels of exposed cross-laminated timber on a two story cross-laminated timber structure. Here we present a detailed summary of the fire test series, instrumentation plan, and an overview of the results.
