Displacement‐based methods contained in recent standards for seismic safety assessment require the determination of the full nonlinear pushover curve for local failure mechanisms in historic masonry ...structures. This curve should reflect both the initial elastic behavior and the rigid body behavior after the activation of rocking. In this work, a rigid block model is proposed for the displacement‐based seismic assessment of local collapse mechanisms of these structures. Masonry is modeled as an assemblage of two‐dimensional rigid blocks in contact through frictional interfaces. Two types of contact models are formulated to capture, respectively, the pre and postpeak branches of the pushover curve: a unilateral elastic contact model, capturing the initial nonlinear behavior up to the force capacity of the structure, corresponding to the activation of the collapse mechanism, and a rigid contact model with finite friction and compressive strength, which describes the rigid‐body rocking behavior up to the attainment of the displacement capacity of the structure. Tension‐only elements are also implemented to model strengthening interventions with tie‐rods. The contact problems associated with the elastic and rigid contact models are formulated using mathematical programming. For both models, a sequential solution procedure is implemented to capture the variation of the load multiplier with the increasing deformation of the structure (P–Δ effect). The accuracy of the modeling approach in reproducing the pushover curve of masonry panels subjected to horizontal seismic loads is evaluated on selected case studies. The solution is first tested against hand calculations, existing analytical models, and distinct element simulations. Then, comparisons against experimental tests follow. As a final application, the failure mechanism and pushover curve of a triumphal masonry arch are predicted by the model and its seismic assessment is performed according to codified force‐ and displacement‐based methods, demonstrating the adequacy of the proposed tool for practice.
The meshless Petrov-Galerkin method (MLPG) is applied to plate bending analysis in ID orthorhombic quasicrystals (QCs) under static and transient dynamic loads. The Bak and elasto-hydrodynamic models ...are applied for phason governing equation in the elastodynamic case. The phason displacement for the orthorhombic QC in the first-order shear deformation plate theory depends only on the in-plane coordinates on the mean plate surface. Nodal points are randomly distributed over the mean surface of the considered plate. Each node is the center of a circle surrounding this node. The coupled governing partial differential equations are satisfied in a weak-form on small fictitious subdomains. The spatial variations of the phonon and phason displacements are approximated by the moving least-squares (MLS) scheme. After performing the spatial MLS approximation, a system of ordinary differential equations (ODEs) for nodal unknowns is obtained. The system of the ODEs of the second order is solved by the Houbolt finite-difference scheme. Our numerical examples demonstrate clearly the effect of the coupling parameter on both static and dynamic phonon/phason deflections.
Fluid invasion, displacement of one fluid by another in porous media, is important in a large number of industrial and natural processes. Of special interest is the trapping of gas and oil clusters. ...We study the impact of wettability on fluid pattern formation and capillary trapping in three‐dimensional glass beads packs (dmean = 1 mm) during fluid invasion at capillary numbers of 10−7 using μ‐CT. The invading fluid was water, and the defending fluid was air. The contact angle of the glass beads was altered between 5° and 115° using Piranha cleaning and silanization. We analyzed the front morphology of the invading fluid, the residual gas saturation, the fluid occupation frequency of pores, and the morphology and statistics of the trapped gas clusters. We found a sharp transition (crossover) at a critical contact angle θc = 96°. Below θc the morphology of the displacement front was flat and compact caused by the strong smoothing effect of cooperative filling. Above θc the morphology of the displacement front was fractal and ramified caused by single bursts (Haines jumps). Across this dynamical phase transition the trapping efficiency changes from no trapping to maximal trapping. For θ > θc the experimental results show that invasion percolation governs the fluid displacement. Strong indicators are the universal scaling behavior of the size distribution of large clusters (relative data error εdata < 1%) and their linear surface‐volume relationship (R2 = 0.99).
Key Points
Wettability‐controlled dynamical phase transition in trapping efficiency for 3‐D porous media (1‐mm glass beads)
Trapping efficiency changes from no trapping to maximal trapping: Above the critical contact angle θc smaller pores are occupied by gas
For θ > θc the results show that invasion percolation governs the fluid displacement: universal scaling for the trapped gas clusters
Abstract Background context The objective of the North American Spine Society's (NASS) Evidence-Based Clinical Guideline for the Diagnosis and Treatment of Lumbar Disc Herniation with Radiculopathy ...is to provide evidence-based recommendations to address key clinical questions surrounding the diagnosis and treatment of lumbar disc herniation with radiculopathy. The guideline is intended to reflect contemporary treatment concepts for symptomatic lumbar disc herniation with radiculopathy as reflected in the highest quality clinical literature available on this subject as of July 2011. The goals of the guideline recommendations are to assist in delivering optimum efficacious treatment and functional recovery from this spinal disorder. Purpose To provide an evidence-based educational tool to assist spine specialists in the diagnosis and treatment of lumbar disc herniation with radiculopathy. Study design Systematic review and evidence-based clinical guideline. Methods This guideline is a product of the Lumbar Disc Herniation with Radiculopathy Work Group of NASS' Evidence-Based Guideline Development Committee. The work group consisted of multidisciplinary spine care specialists trained in the principles of evidence-based analysis. A literature search addressing each question and using a specific search protocol was performed on English-language references found in Medline, Embase (Drugs and Pharmacology), and four additional evidence-based databases to identify articles. The relevant literature was then independently rated using the NASS-adopted standardized levels of evidence. An evidentiary table was created for each of the questions. Final recommendations to answer each clinical question were developed via work group discussion, and grades were assigned to the recommendations using standardized grades of recommendation. In the absence of Level I to IV evidence, work group consensus statements have been developed using a modified nominal group technique, and these statements are clearly identified as such in the guideline. Results Twenty-nine clinical questions were formulated and addressed, and the answers are summarized in this article. The respective recommendations were graded by strength of the supporting literature, which was stratified by levels of evidence. Conclusions The clinical guideline has been created using the techniques of evidence-based medicine and best available evidence to aid practitioners in the care of patients with symptomatic lumbar disc herniation with radiculopathy. The entire guideline document, including the evidentiary tables, suggestions for future research, and all the references, is available electronically on the NASS Web site at http://www.spine.org/Pages/PracticePolicy/ClinicalCare/ClinicalGuidlines/Default.aspx and will remain updated on a timely schedule.
The treatment of chronic sciatica caused by herniation of a lumbar disk has not been well studied in comparison with acute disk herniation. Data are needed on whether diskectomy or a conservative ...approach is better for sciatica that has persisted for several months.
In a single-center trial, we randomly assigned patients with sciatica that had lasted for 4 to 12 months and lumbar disk herniation at the L4-L5 or L5-S1 level in a 1:1 ratio to undergo microdiskectomy or to receive 6 months of standardized nonoperative care followed by surgery if needed. Surgery was performed by spine surgeons who used conventional microdiskectomy techniques. The primary outcome was the intensity of leg pain on a visual analogue scale (ranging from 0 to 10, with higher scores indicating more severe pain) at 6 months after enrollment. Secondary outcomes were the score on the Oswestry Disability Index, back and leg pain, and quality-of-life scores at 6 weeks, 3 months, 6 months, and 1 year.
From 2010 through 2016, a total of 790 patients were screened; of those patients, 128 were enrolled, with 64 in each group. Among the patients assigned to undergo surgery, the median time from randomization to surgery was 3.1 weeks; of the 64 patients in the nonsurgical group, 22 (34%) crossed over to undergo surgery at a median of 11 months after enrollment. At baseline, the mean score for leg-pain intensity was 7.7 in the surgical group and 8.0 in the nonsurgical group. The primary outcome of the leg-pain intensity score at 6 months was 2.8 in the surgical group and 5.2 in the nonsurgical group (adjusted mean difference, 2.4; 95% confidence interval, 1.4 to 3.4; P<0.001). Secondary outcomes including the score on the Owestry Disability Index and pain at 12 months were in the same direction as the primary outcome. Nine patients had adverse events associated with surgery, and one patient underwent repeat surgery for recurrent disk herniation.
In this single-center trial involving patients with sciatica lasting more than 4 months and caused by lumbar disk herniation, microdiskectomy was superior to nonsurgical care with respect to pain intensity at 6 months of follow-up. (Funded by Physicians' Services Incorporated Foundation; ClinicalTrials.gov number, NCT01335646.).
Lumbar disc herniation (LDH) is highly associated with inflammation in the context of low back pain. Currently, inflammation is associated with adverse symptoms related to the stimulation of nerve ...fibers that may lead to pain. However, inflammation has also been indicated as the main factor responsible for LDH regression. This apparent controversy places inflammation as a good prognostic indicator of spontaneous regression of LDH. This review addresses the molecular and cellular mechanisms involved in LDH regression, including matrix remodeling and neovascularization, in the scope of the clinical decision on conservative versus surgical intervention. Based on the evidence, a special focus on the inflammatory response in the LDH context is given, particularly in the monocyte/macrophage role. The phenomenon of spontaneous regression of LDH, extensively reported in the literature, is therefore analyzed here under the perspective of the modulatory role of inflammation.
Background
Intervertebral disc degeneration is a common condition with few inexpensive and effective modes of treatment, but current investigations seek to clarify the underlying process and offer ...new treatment options. It will be important for physicians to understand the molecular basis for the pathology and how it translates to developing clinical treatments for disc degeneration. In this review, we sought to summarize for clinicians what is known about the molecular processes that causes disc degeneration.
Results
A healthy disc requires maintenance of a homeostatic environment, and when disrupted, a catabolic cascade of events occurs on a molecular level resulting in upregulation of proinflammatory cytokines, increased degradative enzymes, and a loss of matrix proteins. This promotes degenerative changes and occasional neurovascular ingrowth potentially contributing to the development of pain. Research demonstrates the molecular changes underlying the harmful effects of aging, smoking, and obesity seen clinically while demonstrating the variable influence of exercise. Finally, oral medications, supplements, biologic treatments, gene therapy, and stem cells hold great promise but require cautious application until their safety profiles are better outlined.
Conclusions
Intervertebral disc degeneration occurs where there is a loss of homeostatic balance with a predominantly catabolic metabolic profile. A basic understanding of the molecular changes occurring in the degenerating disc is important for practicing clinicians because it may help them to inform patients to alter lifestyle choices, identify beneficial or harmful supplements, or offer new biologic, genetic, or stem cell therapies.
Purpose
The goal of this study was to determine the accuracy of displacement‐encoding with stimulated echoes (DENSE) MRI in a tissue motion phantom with displacements representative of those observed ...in human brain tissue.
Methods
The phantom was comprised of a plastic shaft rotated at a constant speed. The rotational motion was converted to a vertical displacement through a camshaft. The phantom generated repeatable cyclical displacement waveforms with a peak displacement ranging from 92 µm to 1.04 mm at 1‐Hz frequency. The surface displacement of the tissue was obtained using a laser Doppler vibrometer (LDV) before and after the DENSE MRI scans to check for repeatability. The accuracy of DENSE MRI displacement was assessed by comparing the laser Doppler vibrometer and DENSE MRI waveforms.
Results
Laser Doppler vibrometer measurements of the tissue motion demonstrated excellent cycle‐to‐cycle repeatability with a maximum root mean square error of 9 µm between the ensemble‐averaged displacement waveform and the individual waveforms over 180 cycles. The maximum difference between DENSE MRI and the laser Doppler vibrometer waveforms ranged from 15 to 50 µm. Additionally, the peak‐to‐peak difference between the 2 waveforms ranged from 1 to 18 µm.
Conclusion
Using a tissue phantom undergoing cyclical motion, we demonstrated the percent accuracy of DENSE MRI to measure displacement similar to that observed for in vivo cardiac‐induced brain tissue.
A two-dimensional displacement and alignment sensor is proposed based on two open-ended transmission lines, each loaded with a split ring resonator (SRR). In this arrangement, the depth of ...resonance-induced notches in the reflection coefficients can be used to sense a displacement of the loading SRRs in two orthogonal directions. Since the operation principle of the sensor is based on the symmetry properties of SRR-loaded transmission lines, the proposed sensor benefits from immunity to variations in ambient conditions. More importantly, it is shown that in contrast to previously published metamaterial-inspired two-dimensional displacement and alignment sensors, the proposed sensor can be operated at a single fixed frequency. The concept and simulation results are validated through measurement.
Landslide prediction is important for mitigating geohazards but is very challenging. In landslide evolution, displacement depends on the local geological conditions and variations in the controlling ...factors. Such factors have led to the “step-like” deformation of landslides in the Three Gorges Reservoir area of China. Based on displacement monitoring data and the deformation characteristics of the Baishuihe Landslide, an additive time series model was established for landslide displacement prediction. In the model, cumulative displacement was divided into three parts: trend, periodic, and random terms. These terms reflect internal factors (geological environmental, gravity, etc.), external factors (rainfall, reservoir water level, etc.), and random factors (uncertainties). After statistically analyzing the displacement data, a cubic polynomial model was proposed to predict the trend term of displacement. Then, multiple algorithms were used to determine the optimal support vector regression (SVR) model and train and predict the periodic term. The results showed that the landslide displacement values predicted based on data time series and the genetic algorithm (GA-SVR) model are better than those based on grid search (GS-SVR) and particle swarm optimization (PSO-SVR) models. Finally, the random term was accurately predicted by GA-SVR. Therefore, the coupled model based on temporal data series and GA-SVR can be used to predict landslide displacement. Additionally, the GA-SVR model has broad application potential in the prediction of landslide displacement with “step-like” behavior.
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