Elastoplastic solutions to predict tunnelling-induced load redistribution and deformation of surface structures

In this paper, an elastoplastic two-stage analysis method is proposed to model tunnelling-induced soil-structure interaction and incorporated into a computer program `ASRE'. This solution allows considering both vertical and horizontal greenfield ground movements, gap formation and slippage, continuous or isolated foundations, and a variety of structural configurations and loading conditions. After introducing the proposed formulation, the model predictions are first compared with previously published data for validation. Then, to isolate the effects of various structural characteristics (relative beam-column stiffness, presence of a ground level slab, column height, number of storeys) and foundation types (continuous versus isolated), several example structures are analysed. Results demonstrate the value of the proposed analysis method to study a broad range of building characteristics very quickly, and show how the soil-structure interaction occurring due to underground excavations is altered by both foundation and superstructure configurations. In particular, the difference in behaviour between equivalent simple beams and framed structures on separated footings is clarified.Engineering and Physical Sciences Research Counci

Sources of high temperature degradation of cement-based materials : nanoindentation and microporoelastic analysis

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2005.Includes bibliographical references (p. 208-213).The effects of high temperature exposure on cement-based materials have been under investigation for quite some time, but a fundamental understanding of the sources of high temperature degradation has been limited by measurement capabilities. Using recent developments in nanotechnology and microporoelastic modeling, this study identifies sources of high temperature degradation at the nanoscale for the first time. For reference and comparison with existing literature, the traditional methods of thermo-gravimetry, uniaxial compression, and resonant frequency are used to measure mass loss, compressive strength and elastic modulus, respectively. However, microscopic measurement of the elastic modulus and hardness is the primary experimental focus of this study. Microindentation is used to measure the properties of homogenized cement paste, whereas nanoindentation is used to measure the properties of the various phases which make up cement paste. All experimental methods are performed on cement paste subjected to specified investigation temperatures ranging from 250C to 700C. Using experimental results in combination with data in the literature, microporoelastic modeling is used to identify the sources of high temperature elasticity degradation which are inherent to each cement paste phase. Only through this unique combination of experimental and theoretical investigations are two primary sources of high temperature elasticity degradation separately identified at the nanoscale: 1) dehydration (loss of bound water) within the elementary building block of C-S-H, and 2) a decrease in packing density of both the low-density and high-density C-S-H phases above 3000C. Based on these identified sources of high temperature elasticity degradation, a model which predicts the elasticity of cement paste as a function of temperature (up to 700C) is developed.by Matthew J. DeJong.S.M

Seismic assessment strategies for masonry structures

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Architecture, 2009.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (p. 181-189).Masonry structures are vulnerable to earthquakes, but their seismic assessment remains a challenge. This dissertation develops and improves several strategies to better understand the behavior of masonry structures under seismic loading, and to determine their safety. The primary focus is on historic arched or vaulted structures, but more modern unreinforced masonry structures are also considered. Assessment strategies which employ simplified quasi-static loading to simulate seismic effects are initially addressed. New analysis methods which focus on stability or strength are presented, and the merits of these strategies are clarified. First, a new parametric graphical equilibrium method is developed which allows real-time analysis and illuminates the complex stability of vaulted masonry structures. Second, a finite element strategy for predicting brittle fracture of masonry structures is extended to incorporate non-proportional loading and shell elements. These extensions enable prediction of damage and collapse mechanisms in general, but are specifically used to predict the response of a full-scale masonry structure to quasi-static cyclic loading. Subsequently, assessment methods based on the dynamic response of masonry structures under earthquake loading are presented. First, rigid body dynamics and an experimental testing program are used to characterize the rocking response of the masonry arch for the first time.(cont.) An assessment criterion is developed which successfully predicts experimentally observed arch collapse under a variety of earthquake time histories. Second, the behavior of rocking structures is addressed in general, and clearly distinguished from typical dynamic oscillators. The rocking response is time dependent, evoking the development of a statistical method for predicting collapse. Finally, the ability of discrete element methods to predict the dynamics of masonry structures is evaluated through comparison with analytical and experimental results, and a rational method for assigning modeling parameters is proposed.by Matthew J. DeJong.Ph.D

Influence of building geometry on bending and shear deformations of buildings subject to tunnelling subsidence:centrifuge testing

Tunnelling-induced settlement damage to pre-existing buildings is a result of bending and shear deformations, which typically occur simultaneously. However, widely accepted methods to assess building damage caused by tunnelling subsidence focus only on the mode of deformation (i.e. shear or bending) that is assumed to govern the onset of building damage. Furthermore, building damage assessment methods typically relate the dominant mode of deformation to the length to height ratio, L/H, of the structure, while more recent research reported that facade openings significantly affect the dominant deformation mode. This paper presents a series of centrifuge tests that explore bending and shear effects on surface structures that are affected by a shallow tunnel excavation in sand. The tests were performed on3D printed building models with varying L/H ratio and facade openings. The response of these building models to the ground movements caused by the tunnelling operation is monitored using digital image correlation (DIC). Results show that bending deformations increase with the L/H ratio while shearing becomes dominant as the amount of facade openings increases. It is also shown that shearing and bending occur simultaneously and therefore should be combined in future damage assessment methods. The obtained experimental results provide essential benchmark data for computational modelling of tunnelling-induced settlement damage on surface structures, as presented in the companion paper

Influence of building geometry on bending and shear deformations of buildings subject to tunnelling subsidence:numerical modelling

Masonry buildings in urban areas can be damaged by differential ground movements caused by underground excavations. Existing procedures for the assessment of building damage due to excavation-induced settlements include the effect of the building on the settlement trough in terms of building stiffness relative to soil stiffness. In these procedures, the relative stiffness is calculated by considering either the bending stiffness or the shear stiffness of the building. In this paper, finite element modelling of buildings subjected to tunneling-induced settlements is used to determine the relative importance of shear and bending deformations in damage predictions. Computational modelling was first validated by simulating centrifuge tests on 3D printed small scale models of masonry buildings subjected to tunneling in sand. Using a similar modelling approach, a sensitivity study was then conducted on the governing effect of shear or bending deformations for different amounts of facade openings. Results indicate the need to include both shear and bending deformation in assessment procedures, and provide essential data towards this objective

Material and Seismic Assessment of the Great House at Casa Grande Ruins National Monument, Arizona

The authors characterized earthen wall materials and plasters in a mid-fourteenth-century Hohokam great house at Casa Grande Ruins National Monument (Arizona) and assessed the seismic susceptibility of its puddled earth walls. Characterization included determining the microstructure, microcomposition, porosity, aggregate mineralogy, and identification of phases in the binding matrix for each of 36 samples and reconstructing plaster technologies, including material selection, preparation, and application sequences. Findings support the ideas that earthen materials were manipulated to optimize their performance to suit the unique site conditions and needs of the ancient people using the structure and included finishes that were unusual in southwestern sites from this time period. By using a new set of tools that integrate the complicated geometry of individual wall segments as captured in light detection and ranging (LiDAR) scans (models were generated in Rhino version 5) with the dynamic analysis of rocking mechanisms (tools for this analysis were developed in Rhino), seismic collapse assessment was used to identify the most vulnerable parts of the building to earthquake loading and provided an initial evaluation of the seismic overturning capacity of these wall segments

A comparison of current analytical methods for predicting soil-structure interaction due to tunnelling

Current procedures for the assessment of buildings response to tunnelling take into account the effect of soil-structure interaction through the definition of the building stiffness relative to the soil stiffness. Limitations of these procedures are uncertainties in the evaluation of structural parameters and inconsistent results between different methods. In this paper, three existing formulations of the Relative Stiffness Method (RSM) have been critically evaluated by analysing the governing factors in the building stiffness calculation and their effect on the structural damage assessment. The results of a sensitivity study on building height, eccentricity, opening ratio, tunnel depth, soil and masonry stiffness, and trough width parameter quantified the effect of these factors on the considered RSMs. The application of different RSMs to a real masonry building adjacent to the Jubilee Line tunnel excavation underlined the significant effect of window openings, façade stiffness and neutral axis position on the building stiffness calculation and deformation prediction. These results highlight the need for a consistent and robust damage assessment procedure.</p

Fibre optic sensing of ageing railway infrastructure enhanced with statistical shape analysis

Funder: Aston UniversityAbstract: Developing early-warning sensor-based maintenance systems for ageing railway infrastructure, such as masonry arch bridges, can be a challenging task due to the difficulty of identifying degradation/damage as the source of small, gradual changes in sensor data, as opposed to other environmental and loading effects. This paper offers a new method of applying statistical modelling and machine learning to enhance the interpretation of fibre optic sensing data, and, therefore, improve deterioration monitoring of railway infrastructure. Dynamic strain and temperature monitoring data between 2016 and 2019 from a fibre Bragg grating (FBG) network installed in a Victorian railway viaduct are first presented. The statistical shape analysis adopted in this study is modified to track changes in the shape of FBG signals directly linked to train speed and dynamic strain amplitudes. The method is complemented by a support vector machine, which is trained to identify different classes of trains. After distinguishing train types, dynamic strain was found to be clearly correlated to temperature, verifying previous findings. No correlation with train speed was observed. The integrated system is then able to compensate for changes in the structural performance due to variations in train loading and ambient temperature, and identify changes in dynamic deformation caused by degradation, in an order comparable to the signal noise (± 2 micro-strain). As a result, the new procedure is shown to be capable of detecting small magnitudes of local degradation well before this degradation manifests itself in typical global measures of response