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).
This project has developed technologies for prefabricated structural systems constructed from engineered wood products for floors and building frames, suitable for buildings up to eight stories in height. The project included the design of a virtual multi-storey timber building, a review of commercial flooring systems, and the development of interim design procedures for timber concrete composite (TCC) floors. Compared with either solid concrete or timber floors, TCC floors provide an excellent balance between increased stiffness, reduced weight, better acoustic separation and good thermal mass.
Outcomes from the project have confirmed TCC floors as a viable alternative to conventional flooring systems. The life cycle analysis of the virtual timber building has highlighted the potential advantages of timber-based building systems for commercial applications. The project also resulted in the formation of the Structural Timber Innovation Company, a research company that will continue to develop timber building systems in non-residential buildings in Australia and New Zealand.
This paper describes numerical modelling to predict the fire resistance of engineered timber floor systems. The floor systems under investigation are timber composite floors (various timber joist and box floor cross sections), and timber-concrete composite floors. The paper describes 3D numerical modelling of the floor systems using finite element software, carried out as a sequential thermo-mechanical analysis. Experimental testing of these floor assemblies is also being undertaken to calibrate and validate the models, with a number of full scale tests to determine the failure mechanisms for each floor type and assess fire damage to the respective system components. The final outcome of this research will be simplified design methods for calculating the fire resistance of a wide range of engineered timber floor systems.
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.
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.
This article presents recent research on the seismic resistance of coupled post-tensioned timber walls for use in multi-story buildings. The walls are constructed from laminated veneer lumber (LVL), post-tensioned with unbonded vertical tendons, and coupled together with mild steel U-shaped flexural plates (UFPs) as energy dissipating elements. The timber wall design follows the same principles as used for post-tensioned precast concrete wall systems, using U-shaped plates to obtain a “hybrid” system, where energy is dissipated through yielding of the plates, while the vertical post-tensioning provides the restoring force. In this project, the same principles are applied to timber coupled walls. A series of quasi-static cyclic and pseudo-dynamic tests have been carried out to verify the applicability of the concept and the feasibility of the construction technology in timber buildings. The U-shaped plates showed stable energy dissipation characteristics and, in combination with the post-tensioning, desirable re-centering hysteretic behavior typically referred to as “flag-shape”. Because of the simplicity of these elements and the low cost of implementation, they have good prospects for practical application.
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.
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.
New Zealand Society for Earthquake Engineering Conference
April 26-28, 2013, Wellington, New Zealand
This paper describes options for seismic design of pre-fabricated timber core-wall systems, used as stairwells and lift shafts for lateral load resistance in multi-storey timber buildings. The use of Cross-Laminated Timber (CLT) panels for multi-storey timber buildings is gaining popularity throughout the world, especially for residential construction. This paper describes the possible use of CLT core-walls for seismic resistance in open-plan commercial office buildings in New Zealand. Previous experimental testing at the University of Canterbury has been done on the in-plane behaviour of single and coupled Pres-Lam post-tensioned timber walls. However there has been very little research done on the behaviour of timber walls that are orthogonal to each other and no research into CLT walls in the post-tensioned Pres-Lam system. This paper describes the proposed test regime and design detailing of two half-scale two-storey CLT stairwells to be tested under a bi-directional quasi-static loading. The test specimens will include a half-flight stair case with landings within the stairwell. The “High seismic option” consists of post-tensioned CLT walls coupled with energy dissipating U-shaped Flexural Plates (UFP) attached between wall panels and square hollow section steel columns at the corner junctions. An alternative “Low seismic option” uses the same post-tensioned CLT panels, with no corner columns or UFPs. The panels will be connected by screws to provide a semi-rigid connection, allowing relative movement between the panels producing some level of energy dissipation.
Seismic damage to floor diaphragms because of displacement incompatibilities are a point of concern in many structures. This paper studies the behaviour of timber diaphragms subjected to frame elongation and rocking of walls in post-tensioned timber buildings. Experimental tests with special connection details between floor panels and between the diaphragm and the lateral load resisting system show that floor damage in severe earthquakes can be avoided by designing for flexibility and proper connection detailing.