Post-tensioned Timber Research Articles

A Fully Coupled Numerical Model for Unbonded Post-tensioned Timber Structures

The paper presents a fully Lagrangian mesh-free solver to simulate the dynamic behavior of post-tensioned timber structures. Weakly Compressible Smoothed Particle Hydrodynamics (SPH) is employed to model both the timber and the tendon. An efficient and simple coupling method between the timber and the tendon is proposed by considering the numerical stability. Besides, the same coupling algorithm is used to model the interaction between column and beam elements. Although the column is treated as rigid in the simulations, the coupling algorithm accounts for the initial compression of the column resulting from post-tensioning. For the verification of the code for solids and material nonlinearity of timber, benchmark problems available in the literature are used. Finally, the solver's capability is demonstrated through dynamic analysis of post-tensioned timber structures. The solutions obtained for all the cases are in good agreement with the experimental and theoretical data, which indicates the applicability and accuracy of the solver.

Seismic vulnerability assessment of post-tensioned timber building fitted with dissipative bracing systems

A post-tensioned (PT) timber building coupled with dissipative bracing systems is a sustainable seismic-resistant structural system. This system combines desirable qualities of self-centering, energy dissipation, and lightweight timber, which attracts lesser seismic force and contributes to a better resilient system. Despite significant advancement in the provision of these sustainable seismic-resistant timber-based structural archetypes, more study is required to derive a fuller knowledge of the behaviour of these innovative structural systems under a range of ground motion intensities and limited state capacities. Direct displacement-based design, a performance-based design approach, is used for the design of this system, and performance assessment is carried out using nonlinear dynamic analysis. This study investigates the performance of the braced PT frame under different limit state considerations using ground motion records consistent with the NBC 2020 seismic hazard.

Moment response of post-tensioned timber beam-column connections in fire – Experimental behaviour of a simple connection

Post-Tensioned Timber (PTT) frames combine high-strength tendons with heavy timber elements to create seismic low-damage or longer span timber structures. Although many aspects of structural performance, including seismic and long-term performance, have been investigated the behaviour of beam-column connections in fire has not been studied. Since it is the ductile rocking beam-column connections that give this system its advantageous response, the behaviour of these connections is key to evaluating the full-structural performance in fire. This paper presents the results of an experiment of a simple PTT beam-column subassembly under an applied load and exposed to an ISO 834 Standard Fire. The connection decompressed after 3 min and the beam rotation increased exponentially until it the applied moment was unable to be resisted at 20 min. Temperatures in the timber connection away from and at the connection are presented. Although the tendon force decreased from the effects of heating, the primary reduction in tendon force was due to crushing of timber under the anchorage.

Experimental testing of post-tensioned steel-timber hybrid frames equipped with energy-dissipating braces

The weak properties of wood in perpendicular-to-grain direction have imposed significant design challenges for post-tensioned (PT) timber frame structures. To resolve the challenges, the authors of this study have introduced a novel solution, the steel-timber hybrid PT frame. To further enhance its lateral performance, additional damping and stiffness (ADAS) braces were integrated into the hybrid frame. In this study, the feasibility of the combination was experimentally examined at the system level. Cyclic tests were conducted to evaluate the performance of two 2/3 scaled frame specimens: a conventional PT timber frame and a PT steel-timber hybrid frame with ADAS braces. The combination of a hybrid frame with ADAS braces showed substantial enhancements in lateral strength, initial stiffness, system ductility, and energy dissipation compared to the conventional PT timber frame. The experimental results highlighted the significant role of ADAS braces in enhancing energy dissipation, while their contribution to stiffness was less dominant. Analytical analysis further revealed an inverse relationship between the increase in ADAS contribution to lateral stiffness and the self-centering competence of the frame. A design equation was proposed to assist practitioners in achieving an optimal balance between the self-centering capacity of the PT hybrid frame and the stiffness enhancement provided by ADAS braces.

Analytical Modeling of Posttensioned C-Shaped CLT Core Walls

Post-tensioned (PT) timber technology, also referred to as Pres-Lam (prestressed laminated timber) provides a low damage seismic design solution. So far PT timber research and practical implementation have focused on moment resisting frames, planar shear walls and coupled planar shear wall—or column-wall-column—systems and their analytical prediction models were adapted and extended from precast concrete to account for the unique characteristics of engineered timber. Following a recent experimental study on a PT cross-laminated timber (CLT) C-shaped core-wall system aiming to enhance lateral strength and stiffness, this paper presents an analytical framework/model to capture three unique kinematic rocking mechanisms for a PT C-shaped CLT core-wall system connected primarily with self-tapping screws. Depending on the relative stiffness of the screwed connections to the PT and the energy dissipaters, the model considers different kinematic responses, and that a staged kinematic response could occur at different imposed core-wall base connection rotations. It also accounts for the material inhomogeneity of CLT with nonedge glued lamella and implements a nonlinear spring model for the screwed connections calibrated from component testing and expected elastic core-wall deformations. The study showed that, for the given specimen configurations presented, the compressive flange wall could be neglected for a PT C-shaped CLT core wall. The analytical model was verified against three large-scale 8.6 m high PT C-shaped core-wall experimental tests and the model prediction error was within 10%. The analytical model was limited to capturing the envelope (push-over) curve of a four wall PT C-shaped CLT core-wall system.

Long-term performance assessment of post-tensioned timber connections under different climates

Timber structures are sensitive to the changes in climatic condition as wood absorbs or desorbs moisture from the ambient environment. The varying climatic condition can induce time-dependent deformation of timber that further leads to long-term variation of structural performance. Such issue needs to be carefully considered during the design process especially for structures in outdoor environment and uncontrolled indoor environment. This study assesses the long-term prestressing force loss of post-tensioned timber connections under different climatic conditions. First, the climatic conditions in ten Chinese and Canadian cities were identified. The statistics and yearly variations of relative humidity and temperature were analyzed and compared. Subsequently, a validated numerical model was used to predict the long-term variation of moisture content over 50 years in the ten cities. The effects of initial moisture content and member sizes were discussed. Finally, four long-term performance (prestressing force loss) levels were proposed based on the maximum lifetime prestressing force loss percentage. Various post-tensioned timber connections accounting for different climatic conditions, initial moisture content, member sizes and connection details were simulated and assessed by determining the long-term performance level. The results in this study demonstrated that climatic condition significantly affected the long-term performance level of post-tensioned timber connections. Furthermore, the percentage of extensive prestressing force loss during the long-term service was closely correlated with the initial moisture content, member sizes and connection details.

Post-tensioned rocking dissipative timber wall systems: Numerical prediction

Low-damage post-tensioned technologies were born with the aim to obtain structural solutions able to guarantee, in case of seismic events, not only the safeguarding of human life, but also the rapid re-use of the buildings without permanent damage. In this context, structural systems based on rocking dissipative timber walls have been introduced, in which rocking behavior is entrusted to post-tensioned bars, and the dissipative contribution is provided by steel dampers, easily replaceable after an intense seismic event. The paper presents a nonlinear numerical model to predict the response of different configurations of Post-Tensioned (PT)-timber wall systems, particularly useful to support the design phase. More in detail, the geometrical non-linearity due to the rocking behavior of the system and material non-linearities that characterize the dampers are taken explicitly into account. The proposed numerical modelling strategy has been validated on different experimental literature tests performed by non-linear static analyses on both single and double wall systems equipped with different damper configurations. The comparison between numerical and experimental results is performed to demonstrate the effectiveness of the proposed model in predicting the behavior of post-tensioned walls subjected to cyclic loads. Further, parametric analyses have been developed in order to investigate the effect of different parameters, such as wall dimensions, initial post-tensioned force in the bars, number of axial and shear dampers, on the global behaviour of post-tensioned timber systems.

Implications of Siesmic Detailing on the Fire Performance of Post-Tensioned Timber Frames

Post-Tensioned Timber (PTT) frames have significant advantages over traditional timber frame systems especially where a low damage design and fast construction are desired. New Zealand practitioners designing timber structures for fire are accustomed to applying ambient design methods to an element cross-section reduced by a char depth based on a duration of Standard Fire exposure following NZS 3603 or AS/NZS 1720.4. The behaviour of PTT frames in fire remains a concern because this approach does not account for the actual mechanics of PTT connection and frame response under natural fires that will occur in the structure. This paper examines the individual and interdependent response of PTT connection components (tendon, dissipater, fasteners, etc) to fire. It is shown that the ambient analysis tool for PTT connections, the Modified Monolithic Beam Analogy, cannot be applied to the fire case by only using char reduced cross-sections of timber elements. This approach of combining ambient methodologies with reduced cross-sections does not account for the specific connection detailing, which result in unique damage in fire that may govern the structural response. The responses of two seismically detailed PTT connections are predicted using this approach and compared to a first principles assessment of connection behaviour to demonstrate that failure will occur earlier than otherwise predicted. Numerical thermal analyses of these two connections also qualitatively corroborate the damage that occurs. This investigation establishes that additional studies are required to understand the complex behaviour of these connections when exposed to fire before a design methodology can be developed.

Experimental and Numerical Investigation of Long-Term Loss of Prestressing Force in Posttensioned Timber Joints with Different Structural Details

The long-term performance of posttensioned timber joints is closely related to the time-dependent prestressing force. Figuring out the variation of prestressing forces is vital for the long-term safety of posttensioned timber joints. In this study, posttensioned timber joints with different structural details were monitored over 750 days to investigate the prestressing force loss under varying environment. The time-dependent variations of temperature and relative humidity, moisture content, timber strain, and prestressing force were captured. The effects of key factors including prestressing ratios, timber deformation, environmental conditions, and reinforcements on the time-dependent prestressing force were carefully analyzed. Subsequently, a numerical model was established to simulate the prestressing force variation of posttensioned timber joints. Good agreements between experimental and simulated results validated the proposed numerical model. Lastly, the potential loss of prestressing force in posttensioned timber joints during a service life of 50 years was predicted by the validated numerical model. This study demonstrates that the prestressing ratios and environmental conditions have significant influence on the time-dependent prestressing force. As expected, the effect of deformation perpendicular to the grain on loss of prestressing force is much bigger than that of deformation parallel to the grain. Moreover, the steel reinforcement can effectively eliminate the loss of prestressing force.

Hybrid simulation of a post-tensioned timber frame and validation of numerical models for seismic design

The post-tensioned frame is one of the recently emerged structural systems for multi-story timber buildings. It is characterized by a high level of prefabrication and quick erection on the construction site. The post-tensioned frame developed at ETH Zurich is based on post-tensioned beam–column connections with hardwood reinforcement of the column in the connection region and column base connections with glued-in steel rods. Such a construction system is suitable for low- and mid-rise buildings that are located in regions characterized by low to moderate seismicity.This paper presents a series of hybrid simulations of the response of a two-story two-bay post-tensioned timber frame subjected to ground motion excitation. Nonlinear numerical models of both beam–column and column base connections to be used for design purposes are validated based on the experiments.

Experimental testing and analytical modelling of single and double post-tensioned CLT shear walls

Post-tensioned (PT) timber technology - also called Pres-Lam technology - can provide increased strength/stiffness for mass timber seismic load resisting systems while also providing energy dissipation and re-centering capabilities. Initial experimental tests and practical implementation on PT timber structures in the past 15 years primarily utilized laminated veneer lumber (LVL), with some glulam and cross-laminated timber (CLT) prototypes and real-case applications, and their analytical prediction models were extended and adapted from precast concrete to account for unique characteristics of engineered timber. More recently, CLT has emerged into a more dominant global mass timber product. This paper presents a large-scale experimental study on 8.6 m tall PT CLT single and double wall systems. The PT double walls utilized screwed connections at the in-plane joint and U-shaped flexural plates at the foundation to provide coupling effect and energy dissipation. With screwed connections, the PT double wall partial composite action of approximately 70% was achieved and the system stiffness was almost two times that of two PT single walls without partial composite action but of equivalent length. Analytical prediction models, accounting for the peculiar controlled rocking mechanism and originally developed for PT LVL systems were adopted for PT CLT wall systems, were found to have increased compressive toe strain variability due to the increased material inhomogeneity of CLT with non-edge glued lamella. The timber compressive strains and unique ‘end effect’ bearing phenomenon was investigated for the first time with Particle Tracking Technology (PTT). Extensions to the existing PT double wall analytical prediction model were proposed and validated to capture the unique kinematic rocking mechanism where wall uplift occurs due to the strong and stiff screwed in-plane connection between individual walls.

Prescriptive Seismic Design Procedure for Post-Tensioned Mass Timber Rocking Walls

Prescriptive Seismic Design Procedure for Post-Tensioned Mass Timber Rocking Walls

Performance-based seismic design and assessment of rocking timber buildings equipped with inerters

Over the last decades, performance-based design objectives have shifted towards damage control and continuity of operation after a design-level earthquake. In this context, the advantages of rocking have been applied to the development of a family of self-centring systems that can sustain large lateral deformations without noticeable damage. However, the bending moments and shear forces in uplifting structures can increase significantly due to the effects of higher modes, an issue to which timber structures are particularly prone. This paper assesses the seismic response of post-tensioned timber rocking walls combined with inerters as a means to control the rotation amplitude and suppress higher-mode effects on the system. To this end, a representative set of three post-tensioned rocking walled structures, comprising 3, 6 and 9 storeys, are designed following direct-displacement based design procedures. A simplified method to pre-dimension the inerter device is proposed and used to design a set of ball screw and gear inerters, with and without clutches. The performance of bare and protected structures with different levels of apparent mass ratios is assessed and compared considering a set of 7 records consistent with the displacement design spectrum. Special attention is paid to the resisting force developed in the inerter and the mechanism to transfer it safely to the structural diaphragm. Finally, a detailed performance-based assessment is conducted considering a database of 202 pulse-like ground motion records. It is concluded that the innovative combination of inerters and rocking is an efficient way to improve the seismic control of self-centring structures.

Numerical modelling and analysis of post-tensioned timber beam-column joint

As a part of sustainable development initiatives, post-tensioned timber structural systems have been developed in recent years with the intention of implementation in seismically active regions. Ductility is achieved through rocking connections between members and optional steel yielding elements. Numerical investigations on such systems have been mostly limited to macro-modeling to avoid complexities arising from the wood anisotropy. This paper examines the lateral cyclic behavior of post-tensioned Laminated Veneer Lumber (LVL) beam-column subassemblies using three-dimensional continuum finite element (FE) modeling. The results of a testing program on full-scale interior joints with unreinforced and steel reinforced configurations at the connection interface are presented. A description of the pre-processing and solution of the FE models, including material, geometric and contact nonlinearities, element mesh, and analysis is provided. The FE results are then validated against the experiments through a verification study. Focus is placed upon the anisotropy of the material as well as the difficulties emerging from performing the nonlinear analysis for such systems.

Experimental estimation of energy dissipated by multistorey post-tensioned timber framed buildings with anti-seismic dissipative devices

The need to satisfy high seismic performance of structures and to comply with the latest worldwide policies of environmental sustainability is leading engineers and researchers to higher interest in timber buildings. A post-tensioned timber frame specimen was tested at the structural laboratory of the University of Basilicata in Italy, in three different configurations: i) without dissipation (post-tensioning only-F configuration); ii) with dissipative angles (DF- dissipative rocking configuration) and iii) with dissipative bracing systems (BF - braced frame configuration). The shaking table tests were performed considering a set of spectra-compatible seismic inputs at different seismic intensities. This paper describes the experimental estimation of energy dissipated by multistorey post-tensioned timber prototype frame with different anti-seismic hysteretic dissipative devices used in the DF and BF testing configurations. The main experimental seismic key parameters have also been investigated in all testing configurations.

Dynamic Seismic Response of Nonlinear Displacement Dependent Devices versus Testing Required by Codes: Experimental Case Studies

Passive energy dissipation systems are one of the most resilient solutions to mitigate the seismic risk of structures. In case of strong motions, they can confine the eventual damages into easily replaceable anti-seismic devices. The performance characteristics of nonlinear displacement dependent devices (NLD) shall be defined by the force-displacement cyclic behavior, as well as the expected number of cycles related to both the duration of the earthquake and to the fundamental frequency of the structural systems. The aims of this paper are the comparison between the dynamic results of two different experimental campaigns performed on NLDs included in dissipative bracing systems and the assessment of the reliability of quasi-static testing procedures proposed by current seismic codes for type tests and factory production control tests. The number of cycles under the design earthquake of hysteretic dampers were experimentally evaluated through shaking table testing. Two experimental case studies of a two-story steel frame and of a three-story post-tensioned timber frame both with bracing systems including flexural steel dampers, hysteretic dampers (HDs), and U-shaped flexural plates (UFPs) respectively, were analyzed. Controlled-displacement tests of NLDs were performed considering quasi-static loading procedures specified by codes. Shaking table tests were carried out considering almost the same seismic sequence composed by a set of seven natural earthquakes at increasing peak ground acceleration (PGA) levels. More than one hundred inelastic cycles were experimentally recorded from dynamic tests before the failure of devices in both cases. In line with American standards testing requirements, the number of cycles at the design PGA level, estimated from shaking table tests and from non-linear dynamic analyses, shows a decreasing trend with the increase of ductility demand.

Experimental seismic response of a resilient 3-storey post-tensioned timber framed building with dissipative braces

With the increased number of multi-storey buildings in seismic areas, research efforts have been focused on developing earthquake resilient systems, such as low-damage techniques based on the combination of post-tensioning and dissipating devices. This paper describes the experimental study performed on a 3-storey post-tensioned timber framed (Pres-Lam) building equipped with energy dissipating systems. The testing project consisted of three phases adopting different configurations of the experimental model: (1) post-tensioning to beam-column joints only, (2) post-tensioning and dissipative rocking mechanisms and (3) post-tensioning and dissipative braces. The main objective of this paper is to experimentally investigate on the seismic response of a large-scale specimen with dissipative braces located in high seismic area, considering construction details similar to those adopted in practical applications. During the experimental campaign, the test frame was subjected to more than one hundred ground motions considering a set of seven spectra-compatible earthquakes at increasing intensity levels. The dissipating bracing system with external replaceable hysteretic dampers improves the seismic resilience of multi-storey Pres-Lam buildings, showing inter-storey drift comparable to those with rocking walls, with full recentring capability and without structural damages or post-tensioning losses through seismic tests.

Experimental and analytical investigations into the time-dependent performance in post-tensioned timber beam-column joints under sustained loads and varied environment

Experiments investigating the time-dependent performance of post-tensioned timber beam-column joints were conducted in an uncontrolled environment. The aim of these experiments is to study the deterioration of joint performance, especially the prestressing force losses and the rotational stiffness degradation. Two types of post-tensioned joint specimens were monitored: one was without external loads, and the other one was subjected to external loads. Environmental temperature and relative humidity were measured in combination with timber moisture content, prestressing force, joint rotation, beam deflection and timber strain. It was shown that the maximum loss percentage of the prestressing force was 11.4% and 19.1% for the unloaded and loaded joints, respectively. The variation of prestressing forces was dominated by the stress level and the relative humidity change. Meanwhile, the joint rotation increased by a factor of 6.56 in the first four months compared to the elastic joint rotation, and the deflection at the end of the beam was 2.55 times bigger compared to the elastic deflection. The stress concentration on the beam-column interface was found as a determinant factor causing the excessive joint rotation. An analytical model was adopted to simulate the prestressing force losses and achieved good agreement with the experimental results.

Fragility-based methodology for evaluating the time-dependent seismic performance of post-tensioned timber frames

Since 2010, the construction of post-tensioned wooden buildings (Pres-Lam) has been growing rapidly worldwide. Pres-Lam technology combines unbonded post-tensioning tendons and supplemental damping devices to provide moment capacity to beam–column, wall–foundation, or column–foundation connections. In low seismic areas, designers may choose not to provide additional damping, relying only on the post-tensioning contribution. However, post-tensioning decreases over time due to creep phenomena arising in compressed timber members. As a consequence, there is a reduction of the clamping forces between the elements. This reduction affects the seismic response of Pres-Lam buildings in the case of low- and high-intensity earthquakes. Therefore, understanding and accounting for the post-tensioning losses and their uncertainty are paramount for a robust assessment of the safety of Pres-Lam constructions. So far, however, there have been no comprehensive studies which tackle the overall seismic performance of such systems in the presence of time-varying post-tension losses and the associated uncertainty. This study tackles this research gap by introducing a comprehensive seismic evaluation of Pres-Lam systems based on time-dependent fragility curves. The proposed fragility analysis is specifically designed to account systematically for time-varying post-tension losses and the related uncertainty. The method is applied to two case studies, designed, respectively, with and without supplemental damping devices. In terms of structural performance, results show that the use of additional dissipaters mitigates the effect of post-tensioning loss for earthquakes of high intensity. Conversely, performance under low-intensity earthquakes is strongly dependent on the post-tensioning value, as the reduction of stiffness due to the anticipated rocking motion activation would lead to damage to non-structural elements.

Behaviour of Post-Tensioned Kempas Timber Beam with Two Tendon Types

The post-tensioning technology is commonly used for concrete structures especially bridges. This technology is applied for timber elements in many countries effectively. In Malaysia the timber post-tensioning still not known yet. This research aim to propose the post- tensioning technique for timber beams. The timber type is Malaysian Kempas and the device used to apply the post-tensioning force is hydraulic jack. Three types of specimens were tested using the bending test (four-point bending test) for a simple support beam. two type of specimens are post-tensioned timber beams with black threaded rod bar (PTBR) and post-tensioned timber beams with silver threaded rod bar (PTSR). All the specimens’ length is 1.2meter and the dimension of timber is 90mm x 40mm. The increment of moment capacity of post-tension timber beam compared to the timber beam the moment resistance increment is 1.42 times. The moment resistance of the post-tensioned timber beam had average increment is 27.4% - 31.8%. The other finding is the failure mode of specimens it is changed from brittle tension failure to ductile failure (compression failure) the cause of this behavior is the role of the post-tensioning rods.