Analysis of the post yield response of concrete structures subject to demolition using finite element analysisO'Callaghan, David January 2000 (has links)
This Engineering Doctorate thesis summarises the research conducted at the Applied Mechanics Division, UMIST and Reverse Engineering limited (REL), the sponsoring company, from 1994 to 1999 on the analysis of concrete structures. With the advent of numerical techniques such as finite element analysis being used to analyse complex structural behaviours, this research investigated the post yield behaviour of concrete structures subject to demolition using a Natural Draught Cooling Tower (NDCT). Both implicit and explicit integration schemes were used to assess the response of the structure to buckling and collapse respectively. A review of demolition techniques was carried out including collapse analysis of an eight storey composite concrete-steel building. The aim of the research was to investigate various engineering aspects, both experimental and numerical, and bring closer to industry recommendations for further investigation.
The development of computational methods for cracked bodies subjected to cyclic variable loads and temperaturesHabibullah, Mohamed Salahuddin Mohamed Mohidin January 2003 (has links)
This thesis is concerned with the development of computational procedures in the assessment of the structural integrity and lifetime of cracked bodies subjected to cyclic variable loads and temperatures. The foundation of these techniques is the Linear Matching Method (LMM), related to the methods of elastic compensation and Gloss r-node, used in design calculations for a number of years. It involves matching the behaviours of a non-linear material to that of a linear material, whereby sequences of linear solutions with spatially varying linear moduli are produced. The developed iterative programming algorithms, implemented within the finite element scheme, ABAQUS, would then generate a monotonically reducing sequence of upper bounds, ultimately converging to the least upper bound loads. In their applications, the significance of these programming methods is two-fold. The first is the investigations into the overall behaviour of cracked structures under the combined actions of mechanical and thermal loads. The numerical limit loads and ratchet limits so identified, which describe the onset of plastic collapse and the unlimited accumulation of plastic strains respectively, were found to be stable, with good converged solutions achieved within 40-60 iterations. The analyses also revealed the insensitivity of the ratchet boundaries to cyclic hardening, as the perfectly plastic and complete cyclic hardening limits yielded almost identical results. The other is the examination into the relationship between the near crack tip fields and the cyclic loading histories, in creep and plasticity conditions. It was established that the HRR field criterion is an appropriate representation of the behaviour of the mechanically and thermally induced crack tip fields. This enabled the crack tip fracture criterion to be evaluated in all conditions, with the observed phenomenon described by two distinct behaviours; strongly influenced by the effect of the elastic stress intensity factor and the reference stress respectively. The analyses conducted demonstrated the capability of the adopted numerical procedures in appraising the behaviour of cracked structures under cyclic loading histories, with the conservativeness of current solution procedures in R5 clearly evident in the results enclosed.
Corrosion of reinforcing steel is a major serviceability issue with reinforced concrete structures, often resulting in significant section and bond loss. However, current non-destructive diagnostic techniques do not allow corrosion to be reliably detected at the very early stages of the process, before damage to the concrete occurs. This research describes the development of an Acoustic Emission (AE) technique as a practical tool for the early detection of corrosion of reinforcing steel embedded in concrete. The study falls into three main areas: (i) determining the influential material parameters of reinforced concrete that affect the magnitude of the acoustic emissions; (ii) investigating the influence of diurnal and seasonal temperature variations on corrosion rate and thus the rate of acoustic emissions; and (iii) developing a testing and evaluation procedure that combines the findings of the first two stages with existing knowledge about corrosion and deterioration of concrete structures. In the first phase of the research material parameters such as cover thickness, compressive strength and rebar diameter were investigated to ascertain the influence of varying these factors on the magnitude of AE Energy obtained per gram of steel loss. The experimental results confirmed that early age corrosion, verified by internal visual inspection and mass loss measurements, can be detected by AE before any external signs of cracking. Furthermore results show that compressive strength was the primary influential parameter, indicating an exponential, empirical relationship between compressive strength and AE Energy. An increase in temperature usually induces an increase in corrosion activity, which should be measurable using the AE technique. Consequently the influences of seasonal and diurnal temperature variations were investigated to determine their impact on undertaking AE measurements. This phase of the research demonstrated that seasonal variations in temperature impart a negligible influence on measured AE Energy. However measurement of AE Energy per hour followed trends in the diurnal temperature and corrosion rate evolution, these being in a state of constant flux. Therefore AE measurements of corrosion in reinforced concrete are more responsive to a change in temperature, and so corrosion rate, as opposed to a specific and constant corrosion rate. In the final phase practical experience with AE from site trials and laboratory work were coupled with leading research and existing knowledge of corrosion in concrete and structural deterioration, to develop a testing and evaluation procedure for on-site application. This rigorous procedure enables reliable corrosion measurements to be undertaken on reinforced concrete structures using AE technology and enabling an assessment of the rate of corrosion induced damage to be made. As far as the author is aware this is the first site testing procedure for detecting corrosion in reinforced concrete using AE. Future research in this area might involve more site testing with a view to improving accuracy and analysis of on-site data, underpinning the developed procedure.
Dynamic impact testing and computer simulation of wheelchair tiedown and occupant restraint systems (WTORS)Gu, Jun January 1999 (has links)
Occupant Restraint Systems (ORS) have been widely used in Public Service Vehicles (PSVs). A Wheelchair Tiedown and Occupant Restraint System (WTORS) has been developed to provide effective occupant protection for disabled people who are seated in wheelchairs. An international laboratory study had been conducted to produce a compliance test protocol that included specification of the sled deceleration versus time history and the crash pulse corridor. Currently effort at the international level is being focused through the International Standards Organisation (ISO) to produce standards for WTORS and transportable wheelchairs. Dynamic sled testing of WTORS was conducted in Middlesex University Road Safety Engineering Laboratory (MURSEL) to develop a test protocol in a WTORS System. This research has been concerned with the effects to which the occupant of a wheelchair secured by a WTORS is subjected in a frontal impact. Both occupant Forward Facing Frontal (FFF) and Rearward Facing Frontal (RFF) impact configurations have been considered. A Surrogate wheelchair with a tiedown restraint System, a Surrogate occupant restraint System, and an Anthropomorphic Test Dummy (ATD) were used to facilitate highly controlled tests. Production wheelchairs were also crash tested to validate the response of the Surrogate System. A 48 km/h-20g crash pulse falling within the ISO standard crash pulse corridor was specified. The Crash Victim Simulation (CVS), one of the computer modelling methods, and Finite Element Analysis (FEA) models were designed to study the dynamic response of a restrained wheelchair and its occupant in a crash environment. Two CVS computer packages: MADYMO®, DYNAMAN® and one of FEA programs: PAFEC were used in WTORS models to predict the occupant response during impacts and hence provide data to optimise future system design. A modelling protocol for WTORS was developed based on the results of ninety (90) sled tests of WTORS Surrogate system and forty (40) dynamic tests of production wheelchairs. To illustrate the potential of these models the results of simulations were validated by sled tests. A random effects Statistical method was used to quantify the results. The load-time histories were also traced to qualify the test and model results. A literature review highlighted twenty years of wheelchair crash research. The correlation between computer model and experimental results was made more accurately. The modelling technique of interconnection of FEA models into CVS program was also introduced. The velocity profile and the natural frequency of WTORS analysis were used to explain why the wheelchair and dummy experienced acceleration amplifications relative to the sled. The shoulder belt load at floor-mounted configuration was found to be higher than that at B pillar configuration. Energy principles were also applied to show why more compliant wheelchair tiedown Systems subjected restraints to a less severe crash environment. A decomposition of forces using the computer model showed why quasi-static analysis is insufficient in WTORS design. It is concluded that the B pillar anchorage of the occupant diagonal strap is superior to the floor-mounted configuration.
Aldaikh, Hesham S. H.
The problem of Dynamic Structure-Soil-Structure Interaction (SSSI) refers to the mutual interaction of adjacent buildings in built-up high density areas through the underlying soil under earthquake excitation. Due to the complexity of the problem, past studies have mainly considered the use of intricate mathematical formulations or the computationally demanding numerical Finite Element and Boundary Element methods. In the present study, linear elastic two-dimensional formulations are proposed using simple discrete lumped parameter models for structures and soil for groups of two and three adjacent buildings systems. The formulation includes a rotational spring as a key buildings interaction mechanism. Inverse power laws are proposed for this rotational interaction and for soil/foundation springs stiffnesses which turn out to be functions of spacing between adjacent buildings. These relationships are obtained by equating energies from the low order discrete and high order Finite Element models.
Nichol, Eric Andrew
Buildings with non coincident centres of mass and stiffness respond in both translation and rotation during seismic ground excitations. This translational and rotational interaction (torsional coupling) can lead to excessive forces in some structural members. This could possibly lead to structural failure if the building is not properly designed to accommodate this response interaction. Previous elastic analytical studies have determined the structural parameters that govern the degree of torsional coupling. However, the parameters found influencing torsional coupling during inelastic response in previous analytical studies have been found to be both more numerous and contradictory than those associated with the elastic response. This study concentrates on the inelastic behaviour of a series of four storey models representing idealized buildings. These building models have been developed from a previous experimental study on the elastic behaviour of torsional coupling. In this inelastic study, hinge units have been designed and used to simulate the yielding of the column or beam members in the experimental model, while maintaining ease of repeatability between tests. The yielding moment in these hinge units can be adjusted to alter the effective strength of the columns or beams in the model. This, along with the ability to vary the floor mass distribution, column sizes (diameter and length), and stiffness distribution allows for a degree of control on the structural parameters deemed important in previous inelastic analytical studies. Results are presented which illustrate the effects that the various structural configurations have on the different measures of inelastic building response, and its vulnerability to damage. These include changes in the building frequencies, member displacement ductilities and vulnerability, hysteretic energy dissipation, and peak structural responses. The study presents a comprehensive investigation of the column-yielding building models. Additionally, select key cases of the column yielding configurations are compared to both the beam-yielding models, and a computational model.
No description available.
Natural methane hydrate soil sediments attract worldwide interest, as there is huge commercial potential in the immense global deposits of methane hydrate that lies under deep seabeds and permafrost regions. Methane hydrate develops and exists in the pores of soil sediments under the conditions of high pressure and low temperature. The methane hydrate-bearing sediment can be exploited to extract methane gas, as methane gas is the predominant element of natural gas. However, the sediment’s geomechanical behaviour is poorly understood, but it has impacts on geotechnical issues, such as the instability of the seabed sediment layers and wellbore collapse, and it may also cause various negative environmental effects, particularly in regards to the exploration and exploitation process. Hence, further scientific research is needed. Due to the limitations of in-situ and laboratory studies, in this PhD research, a numerical method Discrete Element Method (DEM) was employed to provide a unique particle-scale insight into the granular geomechanical behaviours of hydrate-bearing sediment. A comprehensive DEM research was performed in order to simulate two commonly used geomechanical investigation methods employed in hydrate-related studies: the triaxial compression test and seismic wave propagation. Accordingly, the six major contributions of this DEM research are: (1) two typical types of microscopic hydrate distribution patterns within soil pores were investigated via a consistent basic soil model: the pore-filling hydrate pattern and the cementation hydrate pattern; (2) The large-strain deformation and the critical state behaviours were explored; (3) a wave propagation study was performed using the DEM hydrate-bearing sediment samples; (4) the bonding strength effect in the cementation model was systematically discussed; (5) the effect of elongated soil particles on the geomechanical behaviours of sediments was studied; and most importantly (6) a comprehensive particle-scale microscopic analysis was conducted to assist the interpretation of the macro responses in the in-situ, laboratory and numerical studies.
Fibre reinforced polymer (FRP) structures have been increasingly applied in construction, including in road bridges due to their advantages of lightweight, high specific strength and corrosion resistance. However, the brittle nature of FRPs leads to the need for ductile joints to improve the safety of FRP structures. To that end, Glass FRP rebar-reinforced Polymer Concrete (GFRP-RPC) joints between cellular GFRP deck units, combined with an adequate external energy dissipation device (that functions as a replaceable fuse), may fulfil this need for such decks. This PhD study focuses on the load responses of GFRP-RPC joints between cellular GFRP deck units by full-scale anchorage and joint tests, along with simplified methods for predicting the failure loads of the joints. From these studies it is concluded that the GFRP-RPC joints failed by moment-induced rebar-to-polymer concrete anchorage failure. The joints were of high efficiency and shear-strength, and exhibited impressive recovery ability after unloading from large deflections after failure. Further, friction between the polymer concrete confined within the cells of the GFRP decking and the GFRP rebars can contribute to energy dissipation during load cycling. Using rebar-to-polymer concrete shear bond stresses deduced from anchorage testing, it is shown that anchorage failure can be monitored by the third of three phases of behaviour observed during testing. Simplified methods for predicting shear and bending moment capacities of GFRP-RPC joints generally showed good correspondence to experimental data. Future work can focus on cumulative residual shear bond stresses and irreversible slip observed in the anchorage tests, and also on shear failure behaviours of GFRP-RPC joints. Further, a half plastic hinge (HPH) joint (GFRP-RPC joint combined with an adequate external energy dissipation device) between FRP components has been under development by the author. It is expected that "a ductile FRP structure" can be achieved via these HPH joints.
The soil response under cyclic loading conditions is of interest for a number of geotechnical structure such as road pavements, tank foundations and offshore structures. When a geotechnical structure is subjected to cyclic loading, permanent settlements and rotations are accumulated affecting the serviceability of the structure. In the last years, a number of modelling strategies have been proposed to quantify the strain accumulation of soils under cyclic loading; however, most of the models are valid only for limited loading and drainage conditions, and they generally employ complex constitutive formulations. In this thesis, a new constitutive model, the Memory Surface Hardening model, which accounts for the effects of cyclic loading on the soil response, is proposed. The primary aim of this research is to develop a simple set of equations which can accurately predict the cyclic mechanical response of granular soils under generalised loading and density conditions. The modelling strategy is developed in an existing critical state - bounding surface - state parameter - elasto-plastic framework. A new surface, the memory surface, is introduced to track the experienced stress history. In the experiments available in the literature, it is observed that the soil response is highly affected by the experienced loading states. The memory surface evolution responds to two rules: the yield surface is always enclosed by the memory surface; the memory surface expands or contracts following the experienced plastic strains. The last rule is the key to reproduce the typical features observed experimentally for granular soils subjected to cyclic loading. Whenever the soil experiences contractive volumetric strains, the memory surface expands; on the contrary, when the soil experiences dilative plastic volumetric strains, the memory surface contracts. The plastic soil stiffness is affected by the size of the memory surface. The evolution of the memory surface can be interpreted as a representation of the evolution of the soil fabric when the soil is subjected to cyclic loading conditions. The model is developed by maintaining the same hardening rules for any loading conditions, minimising the number of implemented rules and employing a limited number of constitutive parameters. The model is proposed for both the triaxial and the multiaxial stress space. The model has been validated for different types of granular soils under different loading conditions, drained and undrained conditions. The evolution of model surfaces for different loading conditions is presented in the simulations and the occurring mechanisms are widely described.
Page generated in 0.0654 seconds