Recent years have seen more architects and clients asking for tall timber buildings. In response, an ambitious timber community has been proposing challenging plans and ideas for multi-storey commercial and residential timber buildings. While engineers have been intensively looking at gravity-load-carrying elements as well as walls, frames and cores to resist lateral loads, floor diaphragms have been largely neglected.
Complex floor geometries and long span floor diaphragms create stress concentrations, high force demand and potentially large deformations. There is a lack of guidance and regulation regarding the analysis and design of timber diaphragms so structural engineers need a practical alternative to simplistic equivalent deep beam analysis or costly finite element modelling.
This paper proposes an equivalent truss method capable of solving complex geometries for both light timber framing and massive timber diaphragms. Floor panels are discretized by equivalent diagonals, having the same stiffness as the panel including its fasteners. With this method the panel unit shear forces (shear flow) and therefore fastener demand, chord forces and reaction forces can be evaluated. Because panel stiffness is accounted for, diaphragm deflection, torsional effects and transfer forces can also be assessed.
This paper describes the structural design of low-rise multi-storey timber buildings using a new and exciting structural system. This system, originally developed for use with pre-cast concrete, combines un-bonded post-tensioning and additional energy dissipaters, providing a recentering capability after the earthquake, while greatly reducing the structural damage. This new structural system can be used in multi-storey buildings, with large structural timber members made from laminated veneer lumber (LVL) or glulam timber, with lateral loads resisted by prestressed timber frames or walls, separately or in combination. A case study of a six storey timber office building in a moderate seismic area is analysed and a virtual design is carried out, allowing investigation of different methods of structural analysis, and development of many construction and connection details for rapid construction. Total building cost is compared to equivalent steel and reinforced concrete options.
New Zealand Society for Earthquake Engineering Conference
April 10-12, 2015, Rotorua, New Zealand
This paper discusses the design of timber diaphragms, in response to the growing interest in multi-storey commercial timber structures, and the lack of guidance or regulations regarding the seismic design of timber diaphragms.
Proper performance of floor diaphragms is required to transfer all lateral loads to the vertical systems that resist them, but design for earthquake loads can be more complex than design for wind loads. This paper confirms that the seismic design of a diaphragm is intimately linked to the seismic design of the whole building. Diaphragm failure, even if restricted to a limited diaphragm portion, can compromise the behaviour of the whole building. It is therefore necessary to design and detail diaphragms for all possible load paths and to evaluate their influence on the load distribution within the rest of the structure. It is strongly recommended that timber diaphragms be designed as elastic elements, by applying dynamic amplification and overstrength factors derived from the lateral load resisting system.
This paper shows that some current design recommendations for plywood sheathing on light timber framing can be applied to massive wood diaphragms, but for more complex floor geometries an equivalent truss method is suggested. Diaphragm flexibility and displacement incompatibilities between the floor diaphragms and the lateral resisting systems also need to be accounted for.
This paper presents an experimental study on ductility and overstrength of dowelled connections. Connection ductility and overstrength derived from monotonic testing are often used in timber connection design in the context of seismic loading, based on the assumption that these properties are similar under monotonic and cyclic loading. This assumption could possibly lead to non-conservative connection design. Therefore, it is necessary to quantify ductility and overstrength for cyclic loading and contrast them with their monotonic performance. For this purpose, monotonic and quasi-static cyclic experimental tests were performed on dowelled LVL and CLT connections. The experimental results were also compared with strength predictions from state-of-the-art analytical models in literature that were verified for ductile and brittle failure under monotonic loading. This work also allowed investigation into a generally applicable overstrength factor for push-pull loaded dowelled connections.
The following paper describes the first stage of dynamic testing of a post-tensioned timber building to be performed in the structural laboratory of the University of Basilicata in Potenza, Italy as part of a series of experimental tests in collaboration with the University of Canterbury in Christchurch, New Zealand. During this stage of testing a 3-dimensional, 3-storey post-tensioned timber structure will be tested. The specimen is 2/3rd scale and made up of frames in both directions composed of post-tensioned timber. The specimen will be tested both with and without the addition of dissipative steel angles which are designed to yield at a certain level drift. These steel angles release energy through hysteresis during movement thus increasing damping. The following paper discusses the testing set-up and preliminary numerical predictions of the system performance. Focus will be placed on damping ratios, displacements and accelerations.
Second European Conference on Earthquake Engineering and Seismology
August 25-29, 2014, Istanbul, Turkey
Floor diaphragms have an important role in the seismic behaviour of structures, as inertia forces are generated by their masses and then transferred to the lateral load resisting system. Diaphragms also link all other structural elements together and provide general stability to the structure. As with most other structural components, there is concern about damage to floor diaphragms because of displacement incompatibilities. This paper describes two different experiments on engineered timber floors connected to post-tensioned timber frames subjected to horizontal loading.
First a full scale two-bay post-tensioned frame was loaded with lateral loads through a stressed-skin floor diaphragm. Different connection configurations between the floor units on either side of the central column were tested. Secondly a three dimensional, three storey post-tensioned frame building was tested on a shaking table. The diaphragm consisted of solid timber panels connected to the beams with inclined fully threaded screws. For all tested connections, the diaphragm behaviour was fully maintained throughout the testing and no damage was observed.
The test results showed that careful detailing of the floor panel connections near the beam-columnjoint and the flexibility of timber elements can avoid floor damage and still guarantee diaphragm action at high level of drifts in post-tensioned timber frame buildings.
March 29-31, 2012, Chicago, Illinois, United States
This paper describes initial experimental testing to investigate feasible sources of passive damping for the seismic design of post-tensioned glue laminated timber structures. These innovative high performance structural systems extend precast concrete PRESSS technology to engineered wood structures, combining the use of post-tensioning bars or cables with large post-tensioned timber members. The combination of these two elements provides elastic recentering to the structure while the addition of damping using a specialised energy dissipation system gives the desirable `flag shaped' hysteretic response under lateral loading. Testing has been performed on a full scale beam-column joint at the University of Basilicata in Italy in a collaborative project with the University of Canterbury, New Zealand. The experimental testing uses engineered wood products, extending the use of laminated veneer lumber (LVL) structures tested in New Zealand to testing of glue laminated timber (glulam) structures in Italy. Current testing is aimed at further improvement of the system through additional energy dissipation systems.
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.
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.