Post-tensioned rocking structures are known to perform well under seismic action, but as with most other structural systems, there is concern about possible damage to floor diaphragms. This is due to displacement incompatibilities, especially if frame elongation occurs due to gap opening at the beam-column-joints. This paper describes the experimental behaviour of an engineered timber floor connected to a post-tensioned timber frame subjected to horizontal seismic loading.
A full scale two-bay post-tensioned frame was loaded with lateral loads, which were applied through a strip of floor diaphragm spanning perpendicular to the beams. Several different connection configurations between the floor portions on either side of the central column were tested. The diaphragm deformation demand adjacent to the beam-columnjoint gap opening was accommodated through two mechanisms: a concentrated floor gap opening at the column or a combination of panel elongation and small gap openings over a number of floor elements. In all the tests, only elastic deformations were observed and the diaphragm behaviour of the floor elements was fully maintained throughout the testing.
The results showed that design to allow flexibility of timber elements combined with proper connection detailing can prevent damage at high level of drift to the floor diaphragms in post-tensioned timber frame buildings.
New, innovative timber structural components and systems now offer alternative building-solutions to traditional concrete and steel structural systems. A detailed design study on a proposed new building in Christchurch demonstrates the relative merits of each material in a multi-storey commercial setting. Defined criteria give rise to an Optimal ‘Hybrid’ Design – using ‘the right material for the right application’ – where timber components and systems are selected for many structural elements and compares this to an All Timber design and a ductile structural system. The study demonstrates the financial implications on the overall construction cost of selecting different materials for different purposes and the cost premium of a ‘damage-resistant’ building over conventional Ductility 3 code requirements.
The study shows that in a commercial context, there is no single structural material – either timber, concrete or steel - that is appropriate in all circumstances and material selection is often subjective. The study was carried out by the University of Canterbury and Irving Smith Jack Architects Ltd. on behalf of the New Zealand Government’s Ministry of Primary Industries (MPI).
The NMIT Arts & Media Building is the first in a new generation of multi-storey timber structures. It employs an advanced damage avoidance earthquake design that is a world first for a timber building. Aurecon structural engineers are the first to use this revolutionary Pres-Lam technology developed at the University of Canterbury. This technology marks a fundamental change in design philosophy. Conventional seismic design of multi-storey structures typically depends on member ductility and the acceptance of a certain amount of damage to beams, columns and walls. The NMIT seismic system relies on pairs of coupled LVL shear walls that incorporate high strength steel tendons post-tensioned through a central duct. The walls are centrally fixed allowing them to rock during a seismic event. A series of U-shaped steel plates placed between the walls form a coupling mechanism and act as dissipators to absorb seismic energy. The design allows the primary structure to remain essentially undamaged while readily replaceable connections act as plastic fuses. In this era where sustainability is becoming a key focus, the extensive use of timber and engineered-wood products such as LVL make use of a natural resource all grown and manufactured within a 100km radius of Nelson. This project demonstrates that there are now cost effective, sustainable and innovative solutions for multi-story timber buildings with potential applications for building owners in seismic areas around the world.
This paper describes the results of experimental tests on post-tensioned Cross-Laminated Timber (CLT) corewalls tested under bi-directional quasi-static seismic loading. The half-scale two-storey test specimens included a stair with half-flight landings.
The use of CLT panels for multi-storey timber buildings is gaining popularity throughout the world, especially for residential construction. Post-tensioned timber core-walls for lift-shafts (elevator shafts) or stairwells can be used as tubular structures for resistance to seismic loads and wind loads in open-plan commercial office buildings.
Previous experimental testing has been done on the in-plane behaviour of single and coupled timber walls at the University of Canterbury and elsewhere. However, there has been very little research done on the 3D behaviour of timber walls that are orthogonal to each other, and no research to date into single post-tensioned CLT walls or CLT tubular structures.
This paper describes a “High Seismic option” consisting of full height post-tensioned CLT walls coupled with energy dissipating U-shaped Flexural Plates (UFPs) attached at the vertical joints between coupled wall panels and between wall panels and steel corner columns. An alternative “Low Seismic option” consists of posttensioned CLT panels connected by screws, to provide a semi-rigid connection, allowing relative movement between the panels, producing some level of frictional energy dissipation. The Low Seismic option is suitable for wind loading in non-(or low-) seismic regions.
Structural systems made of prefabricated laminated timber members connected by unbonded post-tensioning and additional mild steel reinforcement have recently been proposed for multi-storey timber buildings. The benefits of the use of post-tensioning to assemble prefabricated timber elements are rapid erection, simple connections, and high seismic resistance. It has been shown that prefabricated post-tensioned timber members can be designed to have excellent seismic resistance, with the post-tensioning providing re-centering capacity after major earthquakes, while energy is dissipated through yielding of replaceable steel elements. Both post-tensioning and energy dissipating elements contribute to the stiffness and strength of the overall system. Investigation into the seismic response of twin post-tensioned timber walls, uncoupled and coupled, with and without energy dissipaters has been performed as part of a larger research programme on timber structures at the University of Canterbury. The walls were fabricated from laminated veneer lumber (LVL). A number of special fuses all made of mild steel were used as energy dissipating devices. The energy dissipaters are attached externally so that they can be removed and replaced easily after a major earthquake. Under gravity or low-seismic loading they would be able to provide, as per standard mild steel reinforcement, substantial stiffness and strength. As additional option, plywood sheets have been used to couple the LVL walls in which case the nails dissipated energy through yielding during rocking motion of the walls. This paper discusses the experimental tests and numerical validation of the response of posttensioned timber wall systems. The results show excellent seismic behaviour with very little residual damage. This research also demonstrates the practical feasibility of post-tensioned timber walls for multi-storey timber buildings as well as their versatility of design and use.
This article summarizes the design procedures for ensuring fire safety in cross laminated timber (CLT) buildings in Australia and New Zealand, with reference to the Building Codes in both countries. New Zealand and Australia are located close together geographically and have similarities in some areas of building control, but prescriptive code requirements are often very different. There is a small but growing local CLT industry, but CLT materials for large projects are imported from Europe.
New Zealand Society for Earthquake Engineering Conference
April 27-29, 2017, Wellington, New Zealand
With the increasing acceptance and popularity of multi-storey timber buildings up to 10 storeys and beyond, the influence of higher mode effects and diaphragm stiffness cannot be overlooked in design. Due to the lower stiffness of timber lateral load resisting systems compared with traditional construction materials, the effect of higher modes on the global dynamic behaviour can be more critical. The presence of flexible timber diaphragms creates additional vibration modes, which have the potential to interact with each other, increasing the seismic demand on the whole structure. This paper uses a parametric non-linear time-history analysis on a series of timber frame and wall structures with varying diaphragm flexibility to study their dynamic behaviour and to determine diaphragm forces. The analyses results showed that although higher mode effects play a significant role in the structural dynamic response, this increased demand can be successfully predicted with methods available in literature. The parametric analyses showed that the diaphragm flexibility did not significantly increase the shear and moment demand; however, stiff wall structures with flexible diaphragms experienced large inter-storey drifts measured at diaphragm midspan compared with the drift of the wall alone. As expected, the diaphragm forces observed from the time-history analyses were significantly higher than the forces derived from an equivalent static analysis, leading to a potentially unsafe design. The paper presents a simplified approach for evaluating these amplified peak inertial diaphragm forces.
New Zealand Society for Earthquake Engineering Conference
April 27-29, 2017, Wellington, New Zealand
With global urbanization trends, the demands for tall residential and mixeduse buildings in the range of 8~20 stories are increasing. One new structural system in this height range are tall wood buildings which have been built in select locations around the world using a relatively new heavy timber structural material known as cross laminated timber (CLT). With its relatively light weight, there is consensus amongst the global wood seismic research and practitioner community that tall wood buildings have a substantial potential to become a key solution to building future seismically resilient cities. This paper introduces the NHERI Tallwood Project recentely funded by the U.S. National Science Fundation to develop and validate a seismic design methodology for tall wood buildings that incorporates high-performance structural and nonstructural systems and can quantitatively account for building resilience. This will be accomplished through a series of research tasks planned over a 4-year period. These tasks will include mechanistic modeling of tall wood buildings with several variants of post-tensioned rocking CLT wall systems, fragility modeling of structural and non-structural building components that affect resilience, fullscale biaxial testing of building sub-assembly systems, development of a resilience-based seismic design (RBSD) methodology, and finally a series of full-scale shaking table tests of a 10-story CLT building specimen to validate the proposed design. The project will deliver a new tall building type capable of transforming the urban building landscape by addressing urbanization demand while enhancing resilience and sustainability.
Laminated veneer lumber (LVL) structural members have recently been proposed for multi-storey timber buildings based on ongoing research at University of Canterbury, New Zealand. The members are designed with unbonded post-tensioning for recentering and energy dissipation through the ductile connections. This paper describes the experimental and numerical investigation of post-tensioned LVL walls coupled with plywood sheets, under quasistatic cyclic testing protocols. It is observed that energy is dissipated mostly through yielding of the nails, and the LVL walls return close to their initial position while remaining virtually undamaged. The same specimen has been tested under repeated cyclic loading to investigate the performance of the arrangement under more than one seismic event (a major earthquake followed by a significant aftershock). Different nail spacing and arrangements have been tested to compare their energy dissipation characteristics.
The results indicate good seismic performance, characterized by negligible damage of the structural members and very small residual deformations. The only component significantly damaged is the nailed connection between the plywood sheet and the LVL walls. Although the nails yield and there is a reduction in stiffness the system exhibits a stable performance without any major degradation throughout the loading regime. The plywood can be easily removed and replaced with new sheets after an earthquake, which are reasonably cheap and easy to install, allowing for major reduction in downtime. With these additional benefits the concept has potential for consideration as an alternative solution for multi-storey timber buildings.
The NMIT Arts & Media Building in Nelson, New Zealand is the first in a new generation of multi-storey timber structures. It employs a number of innovative timber technologies including an advanced damage avoidance earthquake design that is a world first for a timber building. Aurecon structural engineers are the first to use this revolutionary Pres-Lam technology developed at the University of Canterbury.
This technology marks a fundamental change in design philosophy. Conventional seismic design of multi-storey structures typically depends on member ductility and the acceptance of a certain amount of damage to beams, columns or walls. The NMIT seismic system relies on pairs of coupled timber shear walls that incorporate high strength steel tendons post-tensioned through a central duct. The walls are centrally fixed allowing them to rock during a seismic event. A series of U-shaped steel plates placed between the walls form a coupling mechanism, and act as dissipaters to absorb seismic energy. The design allows the primary structure to remain essentially undamaged in a major earthquake while readily replaceable connections act as plastic fuses.
With a key focus on sustainability the extensive use of timber and engineered-wood products such as laminated veneer lumber (LVL) makes use of a local natural resource, all grown and manufactured within an 80 km radius of Nelson.
This IstructE award winning project demonstrates that there are now cost effective, sustainable and innovative solutions for multi-storey timber buildings with potential applications for building owners in seismic areas around the world.