Nicholas A. Alexander

Nicholas A. Alexander
University of Bristol | UB · Department of Civil Engineering

BSc(Eng), PhD, CMath, MIMA, CSci, GMICE

About

170
Publications
63,492
Reads
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2,870
Citations
Citations since 2017
58 Research Items
2135 Citations
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Introduction
My main research areas are Earthquake Engineering and Nonlinear Dynamics. I hold a first-class undergraduate degree in Civil Engineering at University College London (UCL) and was awarded the Institution of Civil Engineers (ICE) prize. I am a member of the Institute of Mathematics, a Chartered Mathematician (UK) and a Chartered Scientist (UK). I have supervised, to date, 17 Ph.D. students. My h-index is 25 (Google Scholar). Email: Nick.Alexander@bristol.ac.uk
Additional affiliations
March 2003 - March 2021
University of Bristol
Position
  • Professor (Associate)
September 2001 - May 2003
University College London
Position
  • VIsiting Lecturer
Description
  • Structural Dynamics (MSc unit)
Education
September 1986 - September 1989
University College London
Field of study
  • Nonlinear Dynamics
September 1983 - July 1986
University College London
Field of study
  • Civil Engineering

Publications

Publications (170)
Article
Full-text available
We explore the performance of a nonlinear tuned mass damper (NTMD), which is modeled as a two degree of freedom system with a cubic nonlinearity. This nonlinearity is physically derived from a geometric configuration of two pairs of springs. The springs in one pair rotate as they extend, which results in a hardening spring stiffness. The other pair...
Article
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This paper aims to experimentally verify the theoretical effects of energy pumping especially with external excitation. Energy pumping is irreversible transfer of energy from a linear or linearized structure to a nonlinear energy sink (NES) with relatively small mass. This NES can be used as a nonlinear absorber. This phenomenon is analyzed for dif...
Article
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In this paper, we introduce a novel algorithm for morphing any accelerogram into a spectrum matching one. First, the seed time series is re-expressed as a discrete Volterra series. The first-order Volterra kernel is estimated by a multilevel wavelet decomposition using the stationary wavelet transform. Second, the higher-order Volterra kernels are...
Conference Paper
Full-text available
An analytical dynamic 2-D structure-soil-structure interaction formulation using simple discrete models for structures and soil, and including a rotational spring as a key buildings interaction mechanism is proposed. Following a series of simplifying assumptions, the dynamic problem is conveniently reduced to a meaningful set of four parameters. In...
Conference Paper
Full-text available
Here a numerical model is presented that enables simulation of the nonlinear flexural response of RC components with corroded reinforcement. The model employs the force-based beam-column element and the fibre-type section model that are available in OpenSees. A new constitutive model for corroded reinforcing steel is used. This model simulates the...
Article
Previously, the authors presented a new theoretical energy dissipating technology termed a Frictional Sliding on a Sprung Slope (FSSS) device. Its purpose was to provide significant energy dissipation to post‐tensioned (PT) frames structures. The FSSS device has a triangular‐shaped hysteretic curve, which has a zero‐force activation threshold. This...
Article
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An application of Volterra series in nonlinear system identification is presented in this paper. This novel approach makes use of accelerometer data alone. The aim is to develop an algorithm that can rapidly detect damage, caused by earthquakes, in infrastructural systems without the need for a parallel transducer system that also monitors relative...
Article
During an earthquake, buildings are analysed dynamically without any consideration of their neighbouring structures and usually this also neglects the influence of surficial rotational ground motion. Population growth and increased land costs have increased the spatial density of buildings in urban areas. This closer proximity of buildings to one a...
Article
An innovative physical realisation of the previously reported conceptual frictional sliding on a sprung slope (FSSS) device is proposed in this paper. The device uses the end stiffness of cantilevers as the “sprung-slope”, which generate amplitude varying normal and frictional forces along the sliding plane. This novel configuration is here termed...
Article
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In this paper we explore the performance of RC bridge piers to seismic ground motion sequences, using both experimental and numerical models. Four RC columns were tested on the University of Bristol's shake table. These columns contained both well-confined and poor-confined cases. Spectrally matched by near-field without pulse (NFWP), near-field pu...
Article
Full-text available
Large sympathetic, resonance-like, structural behaviour to earthquake excitations with analogous frequency content often plays a critical role in determining its maximum seismic response. Earthquake excitation typically contains a broad spectrum of non-stationary frequency content, wave packets, which are difficult to observe from the recorded time...
Chapter
This work presents the results of a large-scale experimental study aiming to investigate the seismic performance of a three-storey reinforced concrete school building supported on a novel, low-cost, easy-to-implement seismic base-isolation system entitled PVC ‘sand-wich’ (PVC-s). With a modest additional cost and training of builders, the PVC-s sys...
Article
Dynamic characterization of offshore wind turbines (OWTs) is an important design consideration dictating two Limit States: Serviceability Limit State (SLS), and the Fatigue Limit State (FLS). Hence determining the natural frequency of the system considering Soil-Structure-Interaction, is an important design parameter and is also necessary in prelim...
Article
Full-text available
This paper focuses on the large-scale experimental investigation of a novel, highly efficient, low-cost and easily constructible seismic isolation system, defined as PVC-Rollers Sandwich (PVC-RS) seismic isolation. The proposed system is based on the placement of spherical roller bearings between two PVC surfaces in a sandwich configuration. This c...
Article
A conceptual novel frictional sliding on a sprung slope (FSSS) device is proposed for application with post-tensioned (PT) frames (which have a nonlinear elastic characteristic). This FSSS device has an amplitude-dependent hysteretic resistance nature. The mechanical schematization and design implementation are presented, followed by its analytical...
Article
Full-text available
Offshore wind turbines are a complex, dynamically sensitive structure due to their irregular mass and stiffness distribution, and complexity of the loading conditions they need to withstand. There are other challenges in particular locations such as typhoons, hurricanes, earthquakes, sea-bed currents, and tsunami. Because offshore wind turbines hav...
Article
Full-text available
This study presents a large-scale shaking table investigation aimed at the development of a dual, low-cost hybrid design approach for the reduction of seismic damage in developing countries. The introduction of a novel seismic isolation strategy, defined as 'PVC sand-wich' seismic isolation lies in the core of the proposed hybrid approach. The latt...
Article
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This correction is published in order to inform that typesetter overlooked author corrections regarding older figures & captions in the original publication.
Article
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This study uses a generalized and parametrized reduced-order model in the frequency-domain to evaluate the effects of asynchronous excitation on the lateral response of bridge structures. Bridge geometry parametric regions, corresponding conceptually to valley profile shapes, are explored. Both modal and bounding analyses, that are dependent on bri...
Chapter
The use of physical modeling in civil engineering design in general and geotechnical engineering, in particular, is not only well established but also highly valued in the engineering community. In geotechnical engineering, physical modeling is even more important as the main material under consideration is soil. Soil is a natural material that for...
Article
This paper explores the influence of Structure-Soil-Structure Interaction (SSSI) between multiple buildings in a three-dimensional arrangement under seismic excitation. A numerically simplified reduced-order model is proposed to significantly shorten computational run-times for the case of very large clusters of buildings. This formulation has been...
Conference Paper
Full-text available
The ultra-high-speed (UHS) Hyperloop is the next-generation mode of passen-ger/freight transportation, and is composed of a tube or a system of tubes through which a pod travels free of friction. The entire system must be supported by piers (multi-span viaducts), where the tubes act as the bridge deck. The UHS moving Hyperloops can exert large dyna...
Conference Paper
Full-text available
Stone Masonry in Mud Mortar is a widely used typology for construction of buildings for any use including schools in the Hill and Mountain range of the Himalayan region. It is the only local construction material abundant in this region. A major land portion of Nepal lies in the high mountains and rolling hills that accounts for about 83% of the to...
Article
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This paper aims to investigate the effect of the deformability of a sliding layer on the sliding displacement and acceleration response of structures subjected to harmonic and pulse ground motion excitations. A two-degree-of-freedom model is used for the simulation of the dynamic response of structures founded on sliding layers of varying deformabi...
Article
This paper presents a generalized reduced-order nonlinear model subjected to multi-support seismic excitation. The hysteretic, nonlinear, relationship of piers is phenomenologically captured by a calibrated Bouc-Wen model. This generalized reduced-order model is benchmarked against legacy physical experimental tests performed at the University of B...
Article
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A generalized reduced-order model of a multi-span continuous bridge, on flexible discrete supports, that is subjected to multi-support seismic excitation is presented. This model highlights the key non-dimensional system parameters. Real spatiotemporal ground motion time-series (from the SMART-1 array, Taiwan) are used, as an alternative to employi...
Article
This study presents a large-scale experimental investigation on the seismic performance of an innovative, low-cost seismic isolation system for developing countries. It is based on the beneficial effect of the encapsulation of sand grains between two PVC surfaces on the initiation of sliding and the dissipation of seismic energy between the surface...
Article
Full-text available
The role of signal-based nonlinear system identification methods for the rapid post-earthquake damage assessment of reinforced concrete (RC) bridge piers is explored. Experimental data from the shaking table tests of six RC columns with and without corrosion damage are used as benchmark data. The specimens are excited under three different ground m...
Article
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A set of benchmark, medium scale, shaking table tests on corroded reinforced concrete (RC) columns is conducted with the aim of investigating the effects of corrosion damage on the nonlinear dynamic behaviour of RC bridge piers. The experimental programme consists of an uncorroded control specimen and two corroded RC column specimens, with identica...
Article
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This paper explores the nonlinear dynamics of a multidegree of freedom (MDoF) structure impacting a rigid stop. The contact mechanics is simplified by continuous sigmoid function idealisation of a lossless spring. By introducing a smooth nonlinear formulation, we avoid the computational expense of event-driven, piecewise, nonsmooth dynamics. A larg...
Article
This paper explores the influence of Structure-Soil-Structure Interaction (SSSI) between a pair of buildings, one of which is unsymmetrical in plan and hence subject to seismically induced coupled torsion/sway motions. A reduced-order parametric model of this system is proposed and validated using finite element analysis. A database of 15 strong gr...
Article
Full-text available
This paper presents experimental investigations on the feasibility of using a sand-rubber deformable granular layer as a low-cost seismic isolation strategy for developing countries. The mechanical characteristics of a potential failure mechanism inside the sand-rubber layer are investigated. Direct shear testing is performed to quantify the angle...
Article
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The next generation of ultra-high-speed (UHS) trains, known as Hyperloop and TransPod, are aerospace type vehicles designed to carry passengers. The UHS employs a vehicle capsule within a protected vacuum tube deck, supported by reinforced concrete piers (i.e. multi-span viaduct). The tube environment allows multiple UHS vehicles to run in parallel...
Article
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This paper reports the results of a set of benchmark medium-scale shaking table tests to investigate the significance of the non-stationary characteristics of ground-motion on nonlinear dynamic responses and the structural damage of reinforced concrete (RC) columns. To examine the influence of ground-motion characteristics, four RC columns are test...
Article
Full-text available
To support large wind turbines in deeper waters (30-60 m) jacket structures are currently being considered. As offshore wind turbines (OWT's) are effectively a slender tower carrying a heavy rotating mass subjected to cyclic/dynamic loads, dynamic performance plays an important role in the overall design of the system. Dynamic performance dictates...
Article
The characteristics of the rotational components of strong earthquake ground motion and their effects on structural response are rarely considered. A reduced order parametric model is used to investigate the effects of the rotational ground motions upon buildings, which includes the influence of Soil-Structure Interaction. An indirect method of ext...
Article
Full-text available
This paper explores a proof of concept self-centring spinal column concept experimentally. The idea of the system is inspired by the mechanical interaction of the vertebral bones and intervertebral discs in human spine. Experimental tests are undertaken to explore whether a similar bridge pier system could be constructed to withstand seismic dynami...
Article
This paper evaluates the effect of Structure-Soil-Structure Interaction (SSSI) between two buildings under seismic excitation given different parameters of the buildings, inter-building spacing, and soil type. An extended simplified reduced-order model, that enables higher mode interaction between structures, is proposed. This enables the explorati...
Conference Paper
Full-text available
A summary of shaking table test campaign of three RC columns with different corrosion levels are reported. Accelerated corrosion procedure is employed to produce the desired corrosion level. This work aims to investigate the effects of chloride-induced corrosion on seismic behaviour of RC bridge piers located in a marine environment. The performanc...
Article
Full-text available
Rocking post-tensioned steel frames capitalise on the use of rocking joints, and unbonded post-tensioning strands to provide self-centring action. Investigations on the complex and unconventional nonlinear dynamics of tied rocking steel frames, exclusive of supplemental damping methods, are presently limited. Increasing levels of energy-dissipation...
Conference Paper
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Usually, buildings in urban areas are designed by considering the response of structures in isolation i.e. a single structure, with no neighbouring structures. However, the existence of a high density of buildings in large cities inevitably results in the possibility of seismic interaction of adjacent buildings through the underlying soil that can...
Conference Paper
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p>Modern code-conforming buildings have a high probability of surviving major seismic events without collapse, hence minimizing the number of casualties. Nevertheless, the risk of substantial post-earthquake economic losses remains high, as a consequence of inadequate damage prevention guidelines in current earthquake design codes. Rocking post-ten...
Conference Paper
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In seismically active regions, buried pipelines can be subjected to severe ground deformations due to faulting, landslides, lateral spreading and seismic settlement. The permanent ground deformations (PGD) may induce large strains in the pipeline. Prediction of those pipe strains is required to evaluate pipeline performance and to design earthquake...
Conference Paper
Full-text available
This study reports a set of benchmark shaking table tests of four large-scale RC columns. Three of columns have identical design dimensions and details. They are subjected to three difference ground motions: (i) a near-field without pulse record, (ii) a near-field pulse-like record and (iii) a far-field record. All three records are spectrally matc...
Conference Paper
Full-text available
Conventional seismic design standards are rooted in the notion of collapse-prevention to ensure life-safety during major seismic events. To this end, modern code-conforming buildings are designed to accept a certain level of damage during earthquakes. Nevertheless, this design philosophy does not explicitly address damage mitigation, which leads to...
Conference Paper
This study reports a set of benchmark shaking table tests of four large-scale RC columns. Three of columns have identical design dimensions and details. They are subjected to three difference ground motions: (i) a near-field without pulse record, (ii) a near-field pulse-like record and (iii) a far-field record. All three records are spectrally matc...
Article
Full-text available
Fault rupture is one of the main hazards for continuous buried pipelines and the problem is often investigated experimentally and numerically. While experimental data exists for pipeline crossing strike-slip and normal fault, limited experimental work is available for pipeline crossing reverse faults. This paper presents results from a series of te...
Article
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In this paper the dynamics of a set of ultra-high-speed (UHS) moving masses/loads traversing a continuous beam are explored. The proposed model is intended to simulate the dynamic response of continues bridges under the new Hyperloop/Transpod trains, which are proposed to travel at up to 1200km/h. This speed introduces a range of dynamic responses...
Article
This paper presents closed-form expressions for rocking spring stiffnesses and coupling interaction rotational spring stiffnesses for a set of closely spaced footings. Substructuring is used to derive analytically the exact reduced order spring models of the system. The stiffness coefficients of this reduced order model are determined by using (1)...
Article
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Cantilever grandstands are susceptible to vertical vibrations due to jumping or bobbing (vertical motion without loss of contact) of human occupants. Several major sports stadia have had vibration problems as a result. For bobbing, a simple mass-spring-damper-actuator model of the human (or group of humans) has been proposed and has been incorporat...
Article
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This study develops tsunami evacuation plans in Padang, Indonesia, using a stochastic tsunami simulation method. The stochastic results are based on multiple earthquake scenarios for different magnitudes (Mw 8.5, 8.75, and 9.0) that reflect asperity characteristics of the 1797 historical event in the same region. The generation of the earthquake sc...
Conference Paper
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Pipelines are generally referred to as lifelines since they play a pivotal role for running a nation's industries, services and economy. Past earthquake-related damage demonstrates the vulnerability of pipelines to earthquakes. Faulting is one of the main seismic hazards and significantly influences performance of pipelines. Particularly, reverse f...
Article
A novel device, called Vibrating Barrier (ViBa), that aims to reduce the vibrations of adjacent structures subjected to ground motion waves has been recently proposed. The ViBa is a structure buried in the soil and detached from surrounding buildings that is able to absorb a significant portion of the dynamic energy arising from the ground motion....
Article
Full-text available
This paper quantifies the impact of the non-stationary content (time-varying parameters that are not captured by power spectral content alone) of different ground-motion types (near/far field, with/without pulses time-series) on the nonlinear dynamic response of reinforced concrete bridge piers, taking into account the material cyclic degradation....
Article
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An advanced fibre-based modelling technique is developed to characterise the nonlinear flexural behaviour of rectangular reinforced concrete (RC) columns by accounting for the influence of inelastic buckling and low-cycle fatigue degradation of vertical reinforcement. The proposed uniaxial material model of reinforcing steel is calibrated using 22...
Article
Full-text available
A new numerical model is developed that enables simulation of the non-linear flexural response of reinforced concrete (RC) components and sections with corroded reinforcement. The numerical model employs a displacement-based beam–column element using the classical Hermitian shape function. Material non-linearity is accounted for by updating element...
Article
Full-text available
An automatic framework for tuning plastic constitutive models is proposed. It is based on multistart global optimisation method, where the objective function is provided by the results of multiple elastoplastic finite element analyses, executed concurrently. Wrapper scripts were developed for fully automatic preprocessing, including model and mesh...
Conference Paper
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An earthquake with a moment magnitude reported as 6.0 from INGV (Istituto Nazionale di Geofisica e Vulcanologia); occurred at 03:36 AM (local time) on 24 August 2016 in the central part of Italy. The epicenter was located at the borders of the Lazio, Abruzzi, Marche and Umbria regions, about 2.5 km northeast of the village of Accumoli and about 100...
Article
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This study assesses the tsunami hazard potential in Padang, Indonesia probabilistically using a novel stochastic tsunami simulation method. The stochastic tsunami simulation is conducted by generating multiple earthquake source models for a given earthquake scenario, which are used as input to run Monte Carlo tsunami simulation. Multiple earthquake...
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