Special Issue on Performance of Timber and Hybrid Structures

A change in timber building height limit, from short to midrise story, has been introduced in the British Columbia, Canada, building design code. Various research activities culminating from Network on Innovative Wood Products and Building Systems (NEWBuilds) (newbuildscanada.ca), funded through Natural Sciences and Engineering Research Council of Canada (NSERC), shows that through the use of hybrid structures, the building height can indeed be increased further. Hybrid (composite or mixed) systems utilize different materials in the design of structures (e.g., wood-concrete, wood-steel, steel-concrete systems). The different materials can be integrated at the component levels (hybrid slab/diaphragms, hybrid beams, hybrid columns, hybrid diagonals, hybrid shear walls) and/or at the building system levels (hybrid shear wall system, tube system, vertical mixed system) (Taranath 2005). The utility of using hybrid systems is for aesthetic purpose, and optimal use of different material properties.

Though there are no clear design guidelines, hybrid structures are already used based on conservative design assumptions. For example, with the introduction of new hybrid systems for force-based seismic design, strength and ductility reduction factors, along with estimation of fundamental period, are of importance. To increase the adoption and to show utility of using hybrid systems, concerted research in connections (e.g., Schneider, Karacabeyli, Popovski, Stiemer and Tesfamariam, in this special issue), design parameters (e.g., Nishiyama et al. 2004), modeling capability (e.g., Mehanny and Deierlein 2001; Noguchi and Uchida 2004) is needed. Moore (2000) reported the application of a hybrid 12-story building where a concentrically braced frame is used for lateral load resistance with a glulam timber floor slab. The use of glulam floor slab led to a substantially reduced self-weight, compared with the reinforced concrete slab option. The lighter structure has made wind the governing design load rather than earthquake requirements. Thus, for design adoption, multihazard risk assessment (with consideration of fire, wind, earthquake) and compatibility of the different materials (e.g., steel and timber) should be considered.

The problem is further compounded in regions of high seismicity, where there is an exposure to recurrent earthquake hazard. Furthermore, the 2011 Tohoku earthquake in Japan and the 2011 Christchurch earthquake in New Zealand have highlighted that consideration of mainshock and aftershock earthquakes in the risk and loss assessment is important (Goda 2012).

Recognizing the complexities and challenges faced by the industry, a number of authors with varying expertise and research areas were invited to contribute to this special issue that explores the vast area identified earlier. The different topics can be categorized into system and component responses, connections, and utilization of innovative slab/deck designs. The contributions provide further insight to the academic and practicing engineering communities.

Dickof, Stiemer, Bezabeh, and Tesfamariam develop a new CLT-steel hybrid structure, where cross-laminated timber (CLT) is used as in-fill in a steel moment-resisting frame. An initial parametric study is carried out with a single-bay, single-story model using pushover analysis to investigate effect of CLT panel thickness, crushing strength, and confinement gap. Furthermore, using static pushover analysis, the authors have quantified the ductility and overstrength factors in congruence with the Canadian Seismic Design Code (NRC 2010). Their research findings highlight that the CLT does indeed significantly contribute to the seismic resistance of the steel frame, and should be considered in the design.

Hafeez, Mustafa, Doudak, and McClure, through ambient vibration measurements, predict the fundamental period of light-frame wood buildings, and compare it with the National Building Code of Canada formulation. The authors highlight that the fundamental period calculation using the code is conservative. Finally, the authors have developed a building stiffness evaluated using the current Canadian Standard CSA O86 Engineering Design (CSA 2009) in wood deflection equation for wood shear-wall buildings. Finally, a simple analytical shear-building model is developed to study the effect of multistory sway deformations.

Yang, Li, and Leelataviwat propose a performance-based design and optimization of a buckling-restrained knee brace truss moment frame (BRKBTMF). BRKBTMF is a novel steel structural system which combines steel trusses with buckling-restrained braces to form an alternative seismic-force resisting system. The authors, through analytical and experimental work, highlight the utility of using BRKBTMF: efficient design, with economical and designated energy dissipation devices. Finally, a performance-based plastic design procedure is derived to determine the member sizes of the BRKBTMF.

In the fourth paper, Chen, Chui, Ni, Doudak, and Mohammad present a model to capture load distribution in timber structures consisting of multiple lateral load-resisting elements with different stiffnesses. The developed model utilizes multiple springs, whereby the translational springs are used to model the diaphragm stiffness and the stiffness of the lateral load-resisting element. The model is validated with tests and finite-element results of the Network for Earthquake Engineering Simulation Wood (NEESWood) benchmark building.

Zhou, Ni, Chui, and Chen investigate increase in the story limit of wood frame buildings in British Columbia (BC), Canada, and the possibility of greater flexibility in these buildings. The authors then propose the consideration of a hybrid building consisting of a light wood frame and a reinforced masonry core. To highlight the impact of the stiffer core, the authors design a six-story light wood frame building with the consideration of 25, 50, and 100% shear resistance of the wood subsystem, and carry out a seismic performance assessment. Finally, the authors have conclude that seismic design of the wood subsystem in a hybrid building can be based on the use of the same seismic force modification factor for light wood frame system as published in the National Building Code of Canada.

Yang, Tung, and Xu propose new concrete-filled crisscross steel tubular columns (CCSTCs). Three half-scaled specimens are constructed and tested under cyclic displacement loading history. The experimental results are used to calibrate numerical models of the components and to examine the seismic behavior of a three-story prototype office building located in Los Angeles, California. The authors show that the proposed CCSTC has excellent energy dissipation and postearthquake performance.

Salami and Goda have analytically quantify seismic loss estimation of residential wood-frame buildings in southwestern British Columbia (BC) considering mainshock-aftershock sequences. The seismic loss estimation is conducted for four types of wood-frame house types prevalent in BC and having different seismic capacities. The authors show that moderate effects of aftershocks (5–20%) on maximum structural response and damage extent, and sensitivity of estimated seismic loss on the prevalent wood-frame house types.

Schneider, Karacabeyli, Popovski, Stiemer, and Tesfamariam conduct extensive monotonic and cyclic experimental work on different connectors and nails used in cross-laminated timber (CLT). Subsequently, the Kraetzig model, which is an energy-based cumulative damage assessment model, is calibrated with the experimental work, and damage scales for different performance limits are provided. The connection details provided in this paper are used as a basis for the hybrid structural system described in the first paper (by Dickof, Stiemer, Bezabeh, and Tesfamariam).

Kasal, Guindos, Polocoser, Heiduschke, Urushadze, and Pospisil present a full-scale test of seismic performance of a heavy laminated timber frame with rigid three-dimensional beam-to-column connections. The connections are made with self-tapping screws and hardwood blocks to support the beams. The authors show that the system is capable of meeting serviceability and collapse limit states, and also minimizing residual strain.

Meena, Schollmayer, and Tannert describe the fire resistance of a novel timber-concrete-composite (TCC) deck through experimental and numerical investigations. TCC systems, with timber at the bottom and concrete at the top, are efficient solutions for floors. The advantages highlighted are better fire resistance and easier connection to the surrounding structure and to mechanical installations. The concrete side of one test specimen is subjected to a fire according to the ISO-834 standard temperature-time curve, and the temperature behavior is numerically modeled. The temperature profiles for the TCC system is accurately modeled, allowing for the validated model to be used for future optimizations and predicting the fire-resistance ratings of the systems.

Hehl, Tannert, Meena, and Vallee report, through experimental and numerical investigations, groove connections for the innovative TCC system (as reported in the tenth paper). From the experimental work, the authors report that the grooves cut into the timber beams transmit the shear forces, and sufficiently connect the concrete deck vertically to the timber. The authors have numerically modeled and reproduce the experimental work as well.

Boccadoro and Frangi show experimental analysis on the structural behavior of timber-concrete composite slabs made of beech-laminated veneer lumber (LVL). The authors show the connection between the two materials (concrete and LVL) is accomplished with notches in the timber plate, instead of mechanical steel fasteners. Finally, the structural performance and adequacy of this system is evaluated with a series of bending tests.

Weckendorf, Hafeez, Doudak, and Smith deal with the floor vibration serviceability problem in wood light-frame buildings. To gain fundamental understanding of the nature of how floors in actual buildings behave as parts of complete building superstructure systems, the authors undertake field investigations on residential units of two multioccupancy, multistory buildings having distinctly different architectural and construction features. The authors conclude that architectural and construction detailing decisions play a major role in determining whether or not a particular building is likely to exhibit floor vibration serviceability problems.

Asiz and Smith explore control of building sway and force flows using ultralightweight slabs, such as cross-laminated timber (CLT). The authors show that such substitution reduces the gravitational mass. The authors present design implications for fire, wind, and earthquake for buildings of varying heights.

Erdle, Weckendorf, Asiz, and Smith discuss the effectiveness of distributed mass damper (DMD) systems for lightweight superstructures. The authors present the use of different DMD systems as a method of suppressing lateral motions of superstructures under wind and earthquake loads. The authors discuss the concept of incorporating DMD arrays as parts of multimaterial ultralightweight floor slabs (e.g., cross-laminated timber) for high-rise buildings.

The vast area of research presented in this special issue indeed shows that considerable progress is being made in the use of hybrid structures. The papers presented further highlight the research potential in this area and the need for concerted effort to bring the knowledge into design guidelines.

Dr. Solomon Tesfamariam (Fig. 1) received his Ph.D. degree from the Department of Civil Engineering of the University of Ottawa in 2008. He worked at the National Research Council Canada (NRC-IRC) for eight years in the Urban Infrastructure group. He is currently an Associate Professor in Civil Engineering in the School of Engineering at the University of British Columbia. For the past 14 years, his research activities have focused on developing novel frameworks for seismic risk assessment and management of civil infrastructure systems and risk-based aging infrastructure management (e.g., buildings, buried pipes, roads, bridges). He recently coedited the Handbook of Seismic Risk Analysis and Management of Civil Infrastructure Systems (Tesfamariam and Goda 2013). The last five years, as part of the NEWBuilds project, he has co-developed innovative steel CLT hybrid structures. Dr. Tesfamariam has published over 65 peer-reviewed journal articles, over 40 peer-reviewed conference papers, and various client reports.

Dr. Siegfried F. Stiemer (Fig. 2) has been teaching and researching at UBC for 31 years and has accumulated a number of influential achievements impacting Canadian steel design. Most notable have been early developments in electronic teaching methods (WELD_IT), computer design aids (first version of GDF2 on VAX, and CHEOPS), and the continuous development of formatted spreadsheet design modules, which were made regularly available to colleagues across Canada for evaluation and free use. His expertise in experimental structural research is second to none for all common civil engineering materials. He is coinventor of a timber construction patent owned by FPInnovation. Lately, his involvement in timber has been intensified and he is currently the group leader of hybrid building systems—structural performance in the NSERC Strategic Network, referred to as NEWBuildS.