On the plate-like and layer-like response of slab foundations to ground-borne vibration

The increasing urban population is leading to the exploitation of building sites, close to surface or underground railways, with considerable levels of ground-borne vibration. An important design consideration regards the levels of perceptible vibration and/or re-radiated noise in the completed buildings. A fundamental question concerns to what extent the mass and stiffness of a building foundation influences these levels. This paper explores this question in relation to a concrete slab foundation. Previous research has explored the influence of the coupling between a thin, flexural plate and an elastic half-space on the free-surface displacements arising from surface Rayleigh waves. Here, a numerical, wave-based approach is used to model the slab foundation as an elastic layer of finite thickness, overlying the half-space. The latter is subjected to incident waves in the form of Rayleigh, P- and SV-waves. It is found that thin-plate theory alone is insufficient for modelling the slab over the full frequency range of interest, and that the assumed soil-slab boundary condition plays a significant role. Design plots are presented in order to summarise the influence of the salient dimensionless parameters, and to help guide the design of a slab foundation to achieve a specific reduction in ground vibration level.WSP Group Lt

Inertial interaction in pile-groups: A study of the influence of coupling via an iterative wave-scattering approach

The increasing urban population is leading to the exploitation of building sites close to sources of ground-borne vibration, such as railways and busy roads. Piled foundations can provide a significant vibration transmission path into a building, which can then cause disturbance to occupants and sensitive equipment. There is a strong need to develop numerical models that can capture the essential dynamics of a piled foundation, over the frequency range associated with ground-borne vibration, to help practising engineers decide on appropriate countermeasures. In this paper, a piled foundation is modelled as a pile-group embedded in a homogeneous half-space. Previous research has explored the dynamics of pile-groups to inertial loading at relatively low frequencies, over the seismic range. Here, an iterative approach is developed using a source-receiver boundary-element model to account for the wave-scattering effect that becomes more significant at higher frequencies. Predictions of the dynamic interaction factors, which describe the pile-soil-pile interaction, show very good agreement with a standard boundary-element model for a range of geometric and material parameters. The results show that using uncoupled source-receiver models can account effectively for the interaction between piles without resorting to fully coupled models, even at frequencies well above those of previously published results

High strain-rate effects from blast loads on laminated glass: An experimental investigation of the post-fracture bending moment capacity based on time–temperature mapping of interlayer yield stress

To enhance the resilience of buildings, laminated glass panels are increasingly used in glazed façades. These ductile panels provide a superior blast resistance to that provided by monolithic glass panels, due to the improved residual capacity offered by the polymer interlayer following the fracture of the glass layers. The complex interaction between the attached glass fragments and the interlayer is still only partially understood. To help address this, this paper investigates experimentally the post-fracture bending moment capacity of laminated glass. Three-point bending tests are performed at low temperature on specimens pre-fractured before testing, to ensure controlled and repeatable fracture patterns. The low temperature simulates the effects of the high strain-rates that result from short-duration blast loads by taking advantage of the time-temperature dependency of the viscoelastic interlayer. In these experiments, polyvinyl butyral is considered as the interlayer, this being the most common interlayer for laminated glass used in building facades. A new time-temperature mapping equation is derived from experimental results available in the literature, to relate the temperatures and strain-rates that result in the same interlayer yield stress. The results of the low-temperature tests demonstrate an enhancement of the ultimate load capacity of the fractured glass by two orders of magnitude, compared to that at room temperature. This suggests an improved post-fracture bending moment capacity associated with the now stiffer interlayer working in tension and the glass fragments working in compression. Due to the time-temperature dependency of the interlayer, a similar enhancement is therefore anticipated at the high strain-rates associated with typical blast loading. Finally, the assumed composite bending action is further supported by the results from additional specimens with thicker PVB and glass layers, which result in enhanced capacity consistent with the bending theory of existing analytical models.EPSRC Grant Reference No. EP/L016095/1 and ICE Research and Development Enabling Fun

Accounting for the influence of the free surface on the vibration response of underground railway tunnels: A new iterative method

This paper presents a new method for calculating the ground-borne vibration from shallow underground railways. The method is based on an iterative wave-scattering approach, which decouples the problem into two sub-systems: (1) a tunnel embedded in a full-space and (2) a half-space domain for the soil alone. The Pipe-in-Pipe (PiP) model is first used to find the soil response remote from the tunnel, in the absence of a free surface. The reflected wave-field approaching the soil-tunnel interface from the free surface is then computed using a Boundary-Element Method (BEM) model, before applying this as an external load on the tunnel wall to calculate the revised response. The process is repeated for multiple iterations until convergence is achieved. To the authors’ knowledge, this is the first time that an iterative approach has been applied to an elastodynamic problem. The results from this iterative PiP-BEM model are compared with those of a coupled Finite-Element-Boundary-Element Method (FEM-BEM) model and found to agree well over the frequency range typically associated with ground-borne vibration.Financial support provided by Qatar University under Qatar Rail Project No. QUEX-CENG-Rail 17/1

The effects of high strain-rate and in-plane restraint on quasi-statically loaded laminated glass: a theoretical study with applications to blast enhancement

AbstractLaminated glass panels are increasingly used to improve the blast resilience of glazed facades, as part of efforts to mitigate the threat posed to buildings and their occupants by terrorist attacks. The blast response of these ductile panels is still only partially understood, with an evident knowledge gap between fundamental behaviour at the material level and observations from full-scale blast tests. To enhance our understanding, and help bridge this gap, this paper adopts a ‘first principles’ approach to investigate the effects of high strain-rate, associated with blast loading, and the in-plane restraint offered by blast-resistant frames. These are studied by developing simplified analytical beam models, for all stages of deformation, that account for the enhanced properties of both the glass and the interlayer at high strain-rates. The increased shear modulus of the interlayer results in a composite bending response of the un-fractured laminated glass. This also enhances the residual post-fracture bending moment capacity, arising from the combined action of the glass fragments in compression and the interlayer in tension, which is considered negligible under low strain-rates. The post-fracture resistance is significantly improved by the introduction of in-plane restraint, due to the membrane action associated with panel stretching under large deflections. This is demonstrated by developing a yield condition that accounts for the relative contributions of bending and membrane action, and applying the upper bound theorem of plasticity, assuming a tearing failure of the interlayer. Future work aims to complete the theoretical framework by including the assessment of plate-action and inertia effects.</jats:p

On the performance of base-isolated buildings: a generic model

Ground-borne vibration has existed ever since the development of urban road and rail networks. Vibration generated by the moving traffic propagates through the ground and into buildings, resulting in unacceptable levels of internal noise and vibration. A common solution to this increasingly significant problem is the base-isolation of buildings by incorporating vibration isolation bearings between the buildings and their foundations. This technique has been employed for over forty years but the exact performance of base isolation remains uncertain. This dissertation is concerned with the development of a generic computational model; generic in that it accounts for the essential dynamic behaviour of a typical base-isolated building in order to make predictions of isolation performance. The model is a linear one, formulated in the frequency domain, and consists of a two-dimensional portal-frame model of a building coupled to a three-dimensional boundary-element model of a piled-foundation. Both components of the model achieve computational efficiency by assuming they are infinitely long and using periodic structure theory. The development of the model is described systematically, from the modelling of a building and its isolation bearings to that of its foundation. The majority of the work is concerned with the piled-foundation model, which is comprehensive in that it accounts for the vertical, horizontal and rotational motion of the pile heads due to both direct pile-head loading and interaction through wave propagation in the surrounding soil. It is shown that this level of detail is important in the prediction of base isolation efficiency. A key question facing designers is not only how but on what basis base isolation should be assessed, since fundamental problems exist with the existing measures of isolation performance. Power flow analysis is explored and the concept of power flow insertion gain, based on the total mean vibrational power flow entering a building, is introduced as a useful measure of isolation performance. This is shown to offer clear benefits by providing a single measure of performance that is suitable for design purposes. Finally, the development of a prototype force-sensitive vibration isolation bearing is described as a contribution to verifying base-isolation theory with experiments.This work was supported by the Engineering and Physical Sciences Research Council (PhD Studentship

Base-isolated buildings: Towards performancebased design

Base-isolated buildings, founded on steel springs or elastomeric bearings, have been employed since the 1960s in locations susceptible to ground-borne vibration. Examples exist across a wide range of buildings, from residential to commercial, and include specialist buildings such as concert halls and hospitals. In all cases, the objective is to reduce internal levels of perceptible vibration and re-radiated noise, with the most common sources of concern being nearby surface or underground railways. Despite the extensive use of base isolation, there is a significant lack of guidance on all aspects of design, from the selection of bearing type and their location within a building, to questions such as how performance should be evaluated, and the most fundamental question of all: is isolation necessary? This paper reviews current practice in base-isolation design, and highlights some of the challenges and future research efforts in moving towards a performance-based design approach for controlling ground-borne vibration. This is the author accepted manuscript. The final version is available from ICE Publishing via http://dx.doi.org/10.1680/jstbu.15.0005

The dynamic interaction effects of railway tunnels: Crossrail and the Grand Central Recording Studios

In cities around the world, underground railways offer an environmentally friendly solution to society’s increasing demand for mass transport. However, they are often constructed close to sensitive buildings, where the resulting ground-borne noise and vibration can cause disturbance to both the occupants and the equipment. Such a scenario occurred in central London, where the new twin tunnels of Crossrail were bored beneath the Grand Central Recording Studios, causing an immediate concern. As a result, vibration in the studios’ building was monitored throughout the Crossrail construction period. Since Crossrail is yet to operate, the resulting data provide a unique opportunity to investigate the effect of new tunnels, acting as passive buried structures, on the existing vibration environment. This paper presents the results of such an investigation, together with complementary results from a theoretical four-tunnel boundary-element model. The data analysis, presented in the first half of the paper, indicates that the construction of the second Crossrail tunnel has led to an overall reduction in the noise and vibration levels beneath the studios, due to the operation of the nearby Central line trains of London Underground. This is predominantly due to a reduction of approximately 6 dB in the 63 Hz band-limited levels but accompanied by a slight increase, of approximately 2 dB, in the 125 Hz band. Further analysis indicates that any seasonal variations in vibration levels over the measurement period are negligible, adding weight to the conclusion that the observed changes are a causal effect of the tunnel. The second half of the paper presents results from the model, which aims to simulate the dynamic interaction between the Central line tunnels and those of Crossrail. With nominal parameter values, the results demonstrate qualitative similarities with the measurement findings, thereby adding to the growing body of evidence that dynamic interaction between neighbouring tunnels can be significant. </jats:p

Rapid pretreatment of Miscanthus using the low-cost ionic liquid triethylammonium hydrogen sulfate at elevated temperatures

Deconstruction with low-cost ionic liquids (ionoSolv) is a promising method to pre-condition lignocellulosic biomass for the production of renewable fuels, materials and chemicals. This study investigated process intensification strategies for the ionoSolv pretreatment of Miscanthus X giganteus using the low-cost ionic liquid triethylammonium hydrogen sulfate ([TEA][HSO4]) in the presence of 20 wt% water, using high temperatures and a high solid to solvent loading of 1:5 g/g. The temperatures investigated were 150, 160, 170 and 180°C. We discuss the effect of pretreatment temperature on lignin and hemicellulose removal, cellulose degradation and enzymatic saccharification yields. We report that very good fractionation can be achieved across all investigated temperatures, including an enzymatic saccharification yield exceeding 75% of the theoretical maximum after only 15 min of treatment at 180°C. We further characterised the recovered lignins which established some tunability of the hydroxyl group content, subunit composition, connectivity and molecular weight distribution in the isolated lignin while maintaining maximum saccharification yield. This drastic reduction of pretreatment time at increased biomass loading without a yield penalty is promising for the development of a commercial ionoSolv pretreatment process