‘Mass timber’ engineered wood products in general, and cross-laminated timber in particular, are gaining popularity in residential, non-residential, as well as mid- and high-rise structural applications. These applications include lateral force-resisting systems, such as shear walls. The prospect of building larger and taller timber buildings creates structural design challenges; one of them being that lateral forces from wind and earthquakes are larger and create higher demands on the ‘hold-downs’ in shear wall buildings. These demands are multiple: strength to resist loads, lateral stiffness to minimize deflections and damage, as well as deformation compatibility to accommodate the desired system rocking behaviour during an earthquake. In this paper, contemporary and novel hold-down solutions for mass timber shear walls are presented and discussed, including recent research on internal-perforated steel plates fastened with self-drilling dowels, hyperelastic rubber pads with steel rods, and high-strength hold-downs with self-tapping screws.
The effects of long duration ground motions on the seismic performance of a newly constructed two-storey balloon-type cross-laminated timber (CLT) building located in Vancouver, Canada, was studied. A three-dimensional numerical model of the building was developed in OpenSees. The connection and shear wall models were validated with test data. Twenty-four pairs of long and short duration records with approximately the same amplitude, frequency content, and rate of energy build-up were used for nonlinear dynamic analyses. Fragility curves were developed based on the results of incremental dynamic analysis to assess the building’s collapse capacity. At design intensity level, ground motion duration was shown not to be a critical factor as the difference in inter-storey drift ratio between the two sets of records was negligible. However, due to the larger number of inelastic cycles, the long duration motions increased the median probability of collapse by 9% when compared with the short duration motions. Further research is required to evaluate the duration effects on taller and platform-type CLT buildings.
Multi-storey buildings require mitigation of consequences of unexpected or accidental events, to prevent disproportionate collapse after an initial damage. Cross-laminated timber (CLT) in platform-type construction is increasingly used for multi-storey buildings, however, the collapse behaviour and alternative load paths (ALPs) are not fully understood. A 3D non-linear component-based finite element model was developed for a platform-type CLT floor system to study the ALPs after an internal wall loss, in a pushdown analysis. The model, which accounted for connection failure, timber crushing and large displacements, was calibrated to experimental results and then adapted for boundary conditions corresponding to typical residential and office buildings. Subsequently, five parameters (floor span, connection type, vertical location of the floor, tying level, horizontal wall stiffness) were varied, to study their effects on the ALPs in 80 models. The results showed that three ALPs occurred, of which catenary action was the most dominant. Collapse resistance was mainly affected by the floor span, followed by the axial strength, stiffness and ductility of the floor-to-floor connection, the weight of the level above and the floor panel thickness. This study provides an approach to model ALPs in a platform-type CLT floor system to design disproportionate collapse resistant multi-storey CLT buildings.
Proceedings of the Canadian Society of Civil Engineering Annual Conference 2021
Experimental investigations on full-scale Timber Concrete Composite (TCC) floor systems with various composite connectors are presented in this paper. The stiffness, strength, and failure modes of were evaluated. The 9.2 m long and 2.4 m wide TCC floor segments were comprised of 245 mm thick, 7-ply Cross-laminated Timber (CLT) panels with 150 mm concrete topping connected with three types of shear connectors: (i) self-tapping screws, (ii) steel kerf plates, and (iii) glued-in Holz-Beton-Verbund (HBV) plates. Six TCC floor segments were tested to failure under symmetric four-point bending and three TCC floor segments were tested under torsional bending by applying eccentric loading near the edge. The floor deformations at nine locations and connector slips at CLT-concrete interfaces at eight locations along the length of the floor were measured. The full-scale tests showed that the steel kerf plates—for the selected connector configurations- exhibited the highest capacity and stiffness.
Cross-laminated timber (CLT) constitutes a promising solution for numerous structural applications, including for large and tall residential and commercial buildings. The prospect of building larger timber structures creates some structural challenges, amongst them being that lateral forces created by high winds and strong earthquakes are higher and create higher demands of “holddowns”. The Canadian Standard for Engineering Design in Wood CSA-O86 does not (yet) provide any specific procedures to estimate the resistance of mass-timber Lateral Load Resisting Systems (LLRS) nor how to facilitate the targeted kinematic mode, especially for multi-panel walls where the LLRS behaviour is a function of connection behaviour.
The project investigated the viability of internal-perforated-steel-plates (ISP) with self-drilling dowels as high-performance connections for CLT LLRS. The project objective was to contribute towards the development of reliable design guidance for ISP connections. To achieve this objective, first at the material level, the properties of the used steel-plates and dowels were verified. Then, at the component level, the performance of shear connections and hold-downs were investigated by performing quasi-static monotonic and reversed cyclic tests.
The most significant finding of the component level tests was the proof that it is possible to control the strength, stiffness, and ductility only through the IPSP and avoid bending of the SDD or crushing of the wood. Furthermore, the length of the steel perforations had a large impact on the performance with the steel-plates with the long slots (Type-D and Type-E) exhibiting lower strength and stiffness. For the hold-down tests, the same perforation geometry as for the shear-connection tests was chosen. As already determined in the shear-connection tests, the hold-down specimens with the short perforation slots resulted in the strongest and stiffest connection.
The results from this project will be used to design and test CLT shear walls with ISP connections.
There is much potential for engineered wood products to be used beyond low-rise residential construction when incorporating the notion of hybrid systems like timber-concrete-composites (TCC). TCC systems are comprised of a timber element connected to a concrete slab through a shear connection. By combining the complimentary properties of timber and concrete, the performance of timber floors can be enhanced, including bending stiffness, load-bearing capacity, dynamic response, airborne sound transmission, structural fire rating, and thermal mass. A large number of T-beam TCC systems existed for decades; however, the growing availability of panel-type products in North America offers designers greater versatility in terms of structural and building physics performance. While stiffness and strength design of TCC systems is straight-forward, there is little design guidance available in terms of vibration and long-term performance. The bending, vibration and long-term performance for a range of TCC systems in several EWPs were validated on small-scale shear tests, floor panels subjected to serviceability loads for 2.5 years, and subsequent full-size bending tests. The tests confirmed that calculations according to the Gamma-method can predict the basic stiffness and dynamic properties of TCC floors within a reasonable degree of accuracy.
Modular and Offsite Construction (MOC) Summit Proceedings
Cross-laminated timber (CLT) is becoming a viable option for mid-rise buildings in North America. CLT walls are very effective in resisting lateral forces resulting from wind and seismic loads, yet no standard provisions are available to estimate the resistance of CLT shear walls under lateral loading. The present research investigated CLT shear wall’s performance by evaluating the preferred kinematic rocking behaviour. An analytical procedure was proposed to estimate the resistance of CLT shear walls in a platform type construction. Finite element models of CLT shear with various brackets and hold-downs connections were developed. The models were validated against experimental results. Furthermore, a parametric study on CLT shear walls with the variation of type and number of connectors was conducted. The resistance estimated from parametric study and against analytical were compared. The proposed formulas can be useful tool for the design of CLT platform-type buildings, however, require further experimental validation.
The UBC Brock Commons building in Vancouver, which comprises of 18 stories and stands 53 m in height, was at the time of completion in 2016 the world’s tallest hybrid wood-based building. The building’s 17 stories of mass-timber superstructure, carrying all gravity loads, rest on a concrete podium with two concrete cores that act as both the wind and seismic lateral load-resisting systems. Whereas the construction of the concrete cores took fourteen weeks in time, the mass-timber superstructure took only ten weeks from initiation to completion. A substantial reduction in the project timeline could have been achieved if mass-timber had been used for the cores, leading to a further reduction of the building’s environmental footprint and potential cost savings. The objective of this research was to evaluate the possibility of designing the UBC Brock Commons building using mass-timber cores. The results from a validated numerical structural model indicate that applying a series of structural adjustments, that is, configuration and thickness of cores, solutions with mass-timber cores can meet the seismic and wind performance criteria as per the current National Building Code of Canada. Specifically, the findings suggest the adoption of laminated-veneer lumber cores with supplementary ‘C-shaped’ walls to reduce torsion and optimize section’s mechanical properties. Furthermore, a life cycle analysis showed the environmental benefit of these all-wood solutions.
International Association for Bridge and Structural Engineering Symposium
September 19-21, 2018, Nantes, France
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.
The innovation in tall mass-timber buildings is illustrated by the Brock Commons student residence at the University of British Columbia also known as the UBC Tall Wood Building. It is amongst the world’s tallest timber hybrid building with 18 stories and 53 meters’ height. The building has 17 stories of mass-timber superstructure resting on a concrete podium with two concrete cores that act as a lateral force resisting system for earthquake and wind forces. Construction of the mass-timber superstructure took ten weeks whereas the concrete cores were built in fourteen weeks. There could have been a substantial reduction in the project timeline leading to cost savings, as well as a further reduction of environmental footprint if mass-timber had been used for the cores. The objective of this work was to evaluate the possibility to design the UBC Tall Wood Building using mass-timber cores. A validated numerical model was used to study the feasibility of replacing the concrete cores by cores made of Cross Laminated Timber (CLT). The results presented herein show that, with adjustments in the configuration, the structure can meet the seismic performance criteria as per the Canadian code with CLT cores.