In-plane stiffness of traditional timber floors strengthened with CLT

Five full-scale timber floors were tested in order to analyze the in-plane behaviour of these structural systems. The main objective was to assess the effectiveness of the in-plane strengthening using cross laminated timber (CLT). For that, one unstrengthened specimen (original), one specimen strengthened with a second wood board, two specimens strengthened with 3 CLT panels and one specimen strengthened with 2 CLT panels were tested. Moreover, because of its importance in the composite behaviour, the first phase of the experimental program was composed by push-out tests on specimens representing the shear connection between the timber beams and the CLT panels. This paper describes the tests performed and the numerical modelling aimed to evaluate the composite behaviour of the strengthened timber floors

In-plane stiffness of timber floors strengthened with CLT

Five full-scale timber floors were tested in order to analyse the in-plane behaviour of these structural systems. The main objective was an assessment of the effectiveness of in-plane strengthening using cross-laminated timber (CLT). To this end, one unstrengthened specimen (original), one specimen strengthened with a second layer of floorboards, two specimens strengthened with three CLT panels, and one specimen strengthened with two CLT panels, were tested. A numerical analysis was then performed in order to analyse the composite behaviour of the timber floors in more detail. Due to its importance as regards composite behaviour, the first phase of the experimental programme was composed of push-out tests on specimens representing the shear connection between the timber beams and the CLT panels. This paper describes the tests performed and the numerical modelling applied to evaluate the composite behaviour of the strengthened timber floors. The use of CLT panels is revealed to be an effective way to increase the in-plane stiffness of timber floors, through which the behaviour of the composite structure can be significantly changed, depending on the connection applied, or modified as required.The present work is part of a research project supported by the Quadro de Referencia Estrategico Nacional (QREN) programme, Project Number 21635, from the Agencia de Inovacao (ADI). This research activity fits into RILEM TC 245 "Reinforcement of Timber Elements in Existing Structures". The authors would like to thank Binderholz and Rotho Blaas for all the support offered, particularly in the preparation and execution of the experimental programme

Seismic Resilient Steel Frames Equipped with Self-Centering Column Bases with Friction Devices

In the last two decades many researchers focused on the development of innovative building structures with the aim of achieving seismic resilience. Among others, steel Moment Resisting Frames (MRFs) equipped with friction devices in beam-to-column joints have emerged as an effective solution able to dissipate the seismic input energy while also ensuring the damagefree behaviour of the system. However, to date, little attention has been paid to their column bases, which represent fundamental components in order to achieve resilience. In fact, column bases designed by current conventional approaches lead to significant seismic damage and residual drifts leading to difficult-to-repair structures. This work assesses the seismic performance of steel MRFs equipped with an innovative damage-free, self-centering, rocking column base joints, developed in accordance with the aims of the European project FREEDAM. The proposed column base consists of a rocking splice joint where the seismic behaviour is controlled by a combination of friction devices, providing energy dissipation capacity, and pre-loaded threaded bars with disk springs, introducing restoring forces in the joint. The design procedure of the column base is presented, a numerical OpenSees model is developed to simulate the seismic response of a perimeter seismic-resistant frame, including the hysteretic behaviour of the connection. Non-linear dynamic analyses have been carried out to investigate the effectiveness of the column base in protecting the first storey columns from yielding and reducing the residual storey drifts. The results show that the damage-free behaviour of the column bases is a key requirement when self-centering of MRFs is a design objective

Crack growth around stress concentrations in pipes and tubes

Fatigue crack growth behaviour in pipes fundamentally differs from fatigue growth in shafts and flat plates. The aim of this paper is to give a better understanding of this phenomenon. In a first part of the paper, the general principles of the fracture mechanics are concisely described. The energy approach as well as the stress intensity factor (SIF) approach are explained. An analytical method, a numeric method as well as an experimental method to determine the SIF are discussed. Special attention is given to the experimental method. A theoretical model predicting the deflection of a pipe tested in a resonant bending test setup is evaluated and compared to experimental measured deflections. Several methods to measure the crack growth in a pipe during and after a fatigue bending test are discussed. In addition, an overview is given of results obtained by other authors in the field of fatigue crack growth behaviour of pipes

Tension-only ideal dissipative bracing for the seismic retrofit of precast industrial buildings

3siopenNew precast frame industrial structures are seismically designed according to reliable modern criteria. However, most of the existing built stock hosting many workers and both regular and strategic industrial activities was designed and detailed neglecting the earthquake load or according to outdated seismic design criteria and regulations. Its seismic retrofit is a main challenge for the Engineering Community and a critical objective for institutional and private bodies. Among the envisaged solutions, the introduction of dissipative braces appears to be promising, although mostly inapplicable for these buildings, due to the brace lengths required by their typical large dimensions and the related proportioning against buckling. In this paper, an innovative seismic retrofitting technique based on monolateral dissipative bracing is investigated. The device proposed in this paper, yet in phase of preliminary design and testing, dissipates energy through friction in tension only while freely deforming in compression, which makes the issue related to compressive buckling irrelevant. A numerical analysis is carried out to investigate the efficiency of the proposed device in seismic retrofitting of precast industrial frame buildings with the aim to explore its feasibility and to better orient the definition of the slip threshold load range and the future development of the physical device. The simplified Capacity Spectrum Method (CSM) is employed for the global framing of the structural behaviour of the highly nonlinear retrofitted structures under seismic actions. A numerical tool is set to automatically apply the CSM based on the definition of few main parameters governing the seismic response of precast frame structures. The efficacy of the CSM is critically analysed through the comparison with the results of a set of nonlinear dynamic analyses. A smart simplified design process aimed at framing the most efficient threshold slip/yield load of the device given an existing structural configuration is presented with the application of the CSM through the identification of the most efficient performance indicator related to either displacement, shear force, equivalent dissipation of energy or a combination of them.openDal Lago B.; Naveed M.; Lamperti Tornaghi M.Dal Lago, B.; Naveed, M.; Lamperti Tornaghi, M

Fatigue/fracture mechanics analysis of threaded tether connections

The use of threaded connections for joining tubes and pipes is widespread within the oil and gas drilling industry. Such connections have more recently been employed for the joining of tethering elements for a new generation of offshore platform, the Tension Leg Platform (TLP). The platform design depends totally on the integrity of the tethering system and the threaded connection between tether elements has been identified as a critical structural component. The hostile environment of the North Sea leads to severe cyclic loading on the tethering system and fatigue is the most likely in-service damage mechanism. This study involves an analysis of the fatigue behaviour of large threaded connections of the type proposed for tethering applications and considers the implications for subsequent in-service inspection and integrity assessment. A simplified model for the prediction of the non-uniform load distribution within the connection is proposed and this is validated using finite element (FE) modelling of a complete connection. A methodology for the use of this model, in conjunction with simple FE sub models, for the prediction of dynamic stresses in preloaded and unpreloaded connections is presented. Fatigue initiation and fracture mechanics based crack growth models are proposed for this application and large scale tests, to provide experimental data for validation of these models, have been conducted. An inspection system was developed to enable fatigue crack growth to be measured during the test. It is likely that this system will be suitable for integrity monitoring of large scale threaded connections removed from service. The requirements for integrity monitoring during service, based on a knowledge of the likely fatigue behaviour, are considered for a tethering system and a methodology for defining service inspection intervals as discussed

Variations in low-grade wood modification and stress lamination

Sitka spruce (Picea sitchensis) trees in UK forests are expected to yield nearly 30% more softwood than current levels in the coming decades. These are relatively fast-growing trees, yielding low-grade wood that is incompatible with standards for glue-based lamination. Alongside this forecasted increase, there is a major worldwide shift towards building taller and faster with massive laminated and engineered wood products, mainly as substitutes for steel and concrete. For making the best use of the expected increase in UK softwood and also to expand the scope of tall wood construction, alternative ways of working with low-grade wood are needed, along with developing new variations of existing techniques in processing and construction. This thesis examines two strategies: wood modification by impregnation and stress lamination. The former involves treating wood under pressure in a liquid solution. Once impregnated, the liquid is then solidified in-situ, grafting onto the wood to enhance its properties through direct molecular interactions in the cell wall. While wood treatments are usually done to increase durability, literature and preliminary small-scale testing suggest that impregnation can also lead to increases in stiffness and strength. Compared to wood modification, stress lamination is a relatively simple yet effective technique, and is useful for laminating wood without glue. Although widely used in timber bridges, stress lamination has found little to no application in buildings, primarily due to concerns regarding losses in prestress levels from creep and the moisture-related movement and shrinkage of wood. Scale models and testing emphasise the technique’s potential for both standard and bespoke structural elements for buildings. Full-scale detailing, construction, and testing of straight columns further establishes structural performance and feasibility. Test results from shearing five full-scale stress-laminated connections show performance beyond that of conventional mechanical fasteners. Twenty-five columns were also tested at full-scale, showing comparable buckling performance to Eurocode estimates for solid timber. Examples from literature and a new detail with overdried hardwood plates, tested during a six-month period, demonstrate that prestress losses can be mitigated to ensure long-term reliability in buildings. The full-scale testing performed in this thesis therefore highlights the usefulness, performance, and reliability of stress lamination with low-grade wood for multi-storey construction.UK Engineering and Physical Sciences Research Council (EPSRC) Cambridge Commonwealth Trust Ramboll Foundation Natural Sciences and Engineering Research Council of Canada (NSERC

Seismic Performance of Earthquake-Resilient RC Frames Made with HSTC Beams and Friction Damper Devices

Seismic behavior of RC frames with hybrid steel-trussed concrete beams is affected by panel zone damage due to a large amount of longitudinal reinforcement. Here the seismic efficiency of innovative frames characterized by friction damper devices (FDDs) at beam-to-column connections is compared against traditional frame. Three configurations are investigated: FDDs alone; FDDs with column-to-foundation connections having preloaded threaded bars and disk springs; FDDs with self-centering friction devices. Non-linear analyses show that FDDs alone prevent plastic hinge formation at beam ends and beam–column joint damage. FDDs with self-centering friction devices effectively limit both peak and residual drifts, avoiding column base plasticization

Behaviour of bolted connection system in pultruded GFRP structures

The pultruded GFRP hollow sections, in particular, have received growing interest from the engineering community due to better torsional rigidity, effective resistance of out-of-plane forces, high load transfer and improved strength and stiffness of the minor axis. However, one of the significant issues that hinders the widespread use of pultruded GFRP hollow sections is the inadequacy or unpredictability of its connection system. In this study, the behaviour of pultruded GFRP truss structure using through-bolt connection system was investigated based on the current industrial practice in Australia. The through-bolt connection system is incorporated with an FRP mechanical insert as a filled-type connection element and currently, there is no scientific research focusing on joint behaviour of pultruded GFRP hollow profiles with mechanical inserts. This has been the key motivation for this research whereby the suitability and joint strength adequacy of bolted connection with insert for pultruded FRP, in particular, tubular profiles were examined. Therefore, the study ultimately investigated this particular jointing technique on pultruded GFRP trusses and aimed to understand how the loads are resisted, transferred, and distributed to each FRP component. The experimental data and theoretical predictions developed in this study are critical to produce a safe, reliable and adequate connection system for pultruded GFRP hollow sections. This thesis is presented as a compilation of technical papers. In the first paper, effects of threaded bolt with varying end distance to bolt diameter, laminate thickness, clamping pressure and laminate orientations (longitudinal and transverse) on the joint strength behaviour, joint efficiency and mode of failure were evaluated using a double lap joint test set-up configuration. The test results obtained from the effects of using threaded bolt were compared to that of plain bolt in order to assess the differences in joint behaviour and possible reduction in joint capacity. In this experiment, the joint was designed to promote bearing failure as it is preferable in composite joint due to its progressive nature of failure. From this study, approximately 30-40% reduction in joint strength was observed for specimens with longitudinal laminate orientation caused by laminate tearing of the bolt. In addition, under scanning electron microscope (SEM) imaging, this damaging effect was further observed to better understand its mechanism and how it affects the resulting mode of failures. In the second paper, the joint behaviour of pultruded GFRP hollow sections with a single all-threaded bolt and mechanical insert connection system was investigated under elevated in-service temperature. A comparison of different bolted joint configurations of pultruded GFRP hollow sections, namely joint without mechanical insert (N), joint with mechanical insert with tight-fit attachment (I) and joint with mechanical insert bonded with epoxy adhesive (G) was conducted and the effects on the joint strength and failure mechanism were evaluated. The results of this experimental work have demonstrated that the bolted joint with adhesively bonded mechanical insert sustained the highest load-carrying capacity across the elevated temperatures compared to other configurations. Also, the proposed joint strength prediction equation, which incorporates the strength reduction and modification factors based on different joint configurations involving mechanical insert, produced reasonable outcomes against experimental failure load. These results suggest that, the use of mechanical inserts to strengthen bolted connections system can be adopted in pultruded GFRP hollow sections and the joint performance of this configuration at a structural level was discussed in the next paper. In the third paper, the joint behaviour of through-bolt connection with mechanical insert under eccentric loading was investigated. The joint configuration was adopted in pultruded GFRP T-joints using both single and double bottom chords, with the former imbalanced configuration intended to impart load eccentricity. This eccentric condition can be found in composite truss bridges. The experimental results showed that, the presence of mechanical inserts in both single and double bottom chords of the T-joints had improved the joint strength and fixture stiffness when compared to their insert-less counterparts. It was found that the mechanical insert has prevented bolt flexure and contributed to the improvement in bending resistance when subjected to a couple moment developed due to eccentricity. In the last paper, the structural behaviour of double-chorded pultruded GFRP trusses connected using through-bolt with mechanical inserts under different load cases were investigated. The structural performance of the trusses was described in terms of load-midspan deflection response, force distribution of internal members and mode of failure. The results of this study indicate that the adopted through-bolt with mechanical insert connection system possess high joint load-carrying capacity and demonstrated effective transmissitheoretical strength limits of pultruded GFRP truss members in tension, compression and flexural according to ASCE pre-standard were in close agreement with the experimental results. Meanwhile, the prediction equation proposed in the second paper was used here to evaluate the joint load-carrying capacity of the pultruded GFRP trusses. A two-dimensional numerical model to simulate the behaviour of the pultruded GFRP trusses was constructed using Strand7 finite element analysis software. Satisfactory comparisons against experimental results were achieved and this demonstrates the validity of the Strand7 simplified numerical model. From this overall research program, it can be concluded that the combination of through-bolt and mechanical insert is a promising connection system for pultruded GFRP in truss application. The proposed factors and theoretical joint strength equations developed in this research can be important tools for practitioners to perform strength analysis of through-bolt with mechanical insert connection system, encouraging its acceptance and utilisation, especially in truss application.on of internal forces to other truss members. Th

Seismic Response of a Steel Resilient Frame Equipped with Self-Centering Column Bases with Friction Devices

In the last two decades many researchers focused on the development of innovative building structures with the aim of achieving seismic resilience. Among others, steel Moment Resisting Frames (MRFs) equipped with friction devices in beam-to-column joints have emerged as an effective solution able to dissipate the seismic input energy while also ensuring the damage-free behaviour of the system. How-ever, to date, little attention has been paid to their column bases, which represent fundamental com-ponents in order to achieve resilience. In fact, column bases designed by current conventional ap-proaches lead to significant seismic damage and residual drifts leading to difficult-to-repair structures. The present paper evaluates the seismic performance of steel MRFs equipped with an innovative dam-age-free, self-centring, rocking column base joints. The proposed column base consists of a rocking splice joint where the seismic behaviour is controlled by a combination of friction devices, providing energy dissipation capacity, and pre-loaded threaded bars with disk springs, introducing restoring forces in the joint. The design procedure of the column base is presented, a numerical OpenSees model is developed to simulate the seismic response of a perimeter seismic-resistant frame, including the hysteretic behav-iour of the connection. Non-linear dynamic analyses have been carried out on a set of ground motions records to investigate the effectiveness of the column base in protecting the first storey columns from yielding and in reducing the residual storey drifts. Incremental Dynamic Analyses are used to investigate the influence of the record-to-record variability and to derive fragility curves for the whole structure and for several local engineering demand parameters of the frame and of the column base connection. The results show that the damage-free behaviour of the column bases is a key requirement when self-cen-tering of MRFs is a design objective