Numerical modeling of masonry-infilled RC frames subjected to seismic loads

The behavior of masonry-infilled reinforced concrete frames under cyclic lateral loading is complicated because a number of different failure mechanisms can be induced by the frame-infill interaction, including brittle shear failures of the concrete columns and damage of the infill walls. In this study, nonlinear finite element models have been used to simulate the behavior of these structures. Diffused cracking and crushing in concrete and masonry are described by a smeared-crack continuum model, while dominant cracks as well as masonry mortar joints are modeled with a cohesive crack interface model. The interface model adopts an elasto-plastic formulation to describe the mixed-mode fracture of concrete and masonry. The model accounts for cyclic crack opening and closing, reversible shear dilatation, and joint compaction due to damage. The constitutive models have been validated with experimental data and successfully applied to the dynamic analysis of a three-story, two-bay, masonry-infilled, non-ductile, reinforced concrete frame tested on a shake table. The results have demonstrated the capabilities of the finite element method in capturing the nonlinear cyclic load–displacement response and failure mechanisms of the structure, and indicated the important contribution of infill walls to the seismic resistance of a non-ductile reinforced concrete frame.     

Finite element simulation of the elastic behaviour of infilled frames with openings

A simple iterative finite element method of analysis of infilled frames is presented in this paper. The method can be extended to infilled frames with openings, with or without stiffeners around the openings, and takes into consideration separation, slip, and frictional loss at the interface of the infill and the frame. Also, the results of the effect of size of opening and the effect of different types of stiffeners on the lateral stiffness of the frame and on the stress distribution in the infill are reported. Some of the theoretical results are compared with those of experiments carried out on half-scale models.     

基于深度学习的视频语义分割综述

目前对视频语义分割的研究主要分为两方面,一是如何利用视频帧之间的时序信息提高图像分割的精度;二是如何利用视频帧之间的相似性确定关键帧,减少计算量,提升模型的运行速度.在提升分割精度方面一般设计新的模块,将新模块与现有的CNNs结合;在减少计算量方面,利用帧序列的低层特征相关性选择关键帧,同时减少操作时间.本文首先介绍视频语义分割的发展背景与操作数据集Cityscapes、CamVid;其次,介绍现有的视频语义分割方法;最后总结当前视频语义分割的发展情况,并对未来的发展给出一些展望和建议.     

Optimization of frame structures with flexible joints

In the traditional approach to analysing and optimizing frame structures, joints are assumed to be rigid. Research has shown, however, that flexibility in the joints of a structure has a great effect on its behaviour. In this paper we present a formal method for optimizing frame structures to account for joint flexibilities and their design parameter sensitivities. We also present a simplified method that accounts for joint flexibility in the initial configuration, but neglects sensitivities to the design parameters. In a case study on an automotive sub-frame structure we compare optimization results using the traditional rigid joint model and the flexible joint methods proposed here. Results suggest that the simplified flexible joint method is more accurate than the traditional rigid joint model, and provides a more conservative design than the full flexible joint method while saving physical and computational effort. Received January 18, 1999     

Temporal segmentation and assignment of successive actions in a long-term video

Temporal segmentation of successive actions in a long-term video sequence has been a long-standing problem in computer vision. In this paper, we exploit a novel learning-based framework. Given a video sequence, only a few characteristic frames are selected by the proposed selection algorithm, and then the likelihood to trained models is calculated in a pair-wise way, and finally segmentation is obtained as the optimal model sequence to realize the maximum likelihood. The average accuracy on IXMAS dataset reached to 80.5% at frame level, using only 16.5% of all frames in computation time of 1.57 s per video which has 1160 frames on the average.     

多关键帧特征交互的人脸篡改视频检测

目的深度伪造是新兴的一种使用深度学习手段对图像和视频进行篡改的技术,其中针对人脸视频进行的篡改对社会和个人有着巨大的威胁。目前,利用时序或多帧信息的检测方法仍处于初级研究阶段,同时现有工作往往忽视了从视频中提取帧的方式对检测的意义和效率的问题。针对人脸交换篡改视频提出了一个在多个关键帧中进行帧上特征提取与帧间交互的高效检测框架。方法从视频流直接提取一定数量的关键帧,避免了帧间解码的过程;使用卷积神经网络将样本中单帧人脸图像映射到统一的特征空间;利用多层基于自注意力机制的编码单元与线性和非线性的变换,使得每帧特征能够聚合其他帧的信息进行学习与更新,并提取篡改帧图像在特征空间中的异常信息;使用额外的指示器聚合全局信息,作出最终的检测判决。结果所提框架在FaceForensics++的3个人脸交换数据集上的检测准确率均达到96.79%以上;在Celeb-DF数据集的识别准确率达到了99.61%。在检测耗时上的对比实验也证实了使用关键帧作为样本对检测效率的提升以及本文所提检测框架的高效性。结论本文所提出的针对人脸交换篡改视频的检测框架通过提取关键帧减少视频级检测中的计算成本和时间消耗,使用卷积...     

Effects of spatial and temporal variation in environmental conditions on simulation of wildfire spread

Environmental conditions, such as fuel load and moisture levels, can influence the behaviour of wildfires. These factors are subject to natural small-scale variation which is usually spatially or temporally averaged for modelling fire propagation. The effect of including this variation in propagation models has not previously been fully examined or quantified. We investigate the effects of incorporating three types of variation on the shape and rate of propagation of a fire perimeter: variation in combustion conditions, wind direction and wind speed. We find that increasing the variation of combustion condition decreases the overall rate of propagation. An analytical model, based on the harmonic mean, is presented to explain this behaviour. Variation in wind direction is found to cause the development of rounded flanks due to cumulative chance of outward fluctuations at the sides of the perimeter. Our findings may be used to develop improved models for fire spread prediction.     

Nonlinear behaviour of non-integral infilled frames

The nonlinear behaviour of non-integral infilled frames (in which the infill and the frame are not bonded together) is studied both experimentally and analytically. In the theoretical study, finite element method is used and the nonlinearities of the material and the structural interface are taken into account. It is shown that the stress redistributions towards collapse are significant, and that the strength of non-integral infilled frames is very much dependent on the bending strength of the frame. The effects of initial lack of fit and friction at the interface are also studied theoretically. Furthermore, an empirical formulae for estimating the equivalent strut width is proposed.     

Finite element modelling of compartment masonry walls in fire

A finite element model called MasSET has been developed which is capable of predicting the structural behaviour of single leaf masonry walls subject to elevated temperatures. The analysis models a slice through the wall as a column strip in plane stress, and also includes material with geometric non-linearity. The model has been previously validated by comparison with experimental results [1] and is used in this paper to conduct a parametric study investigating the effects of slenderness ratio, load eccentricity and boundary conditions. The results of the investigation are presented by way of failure temperatures for each condition, and show conclusive findings to the effects of each parameter investigated.     

Bat inspired algorithm for discrete size optimization of steel frames

Bat inspired (BI) algorithm is a recently developed metaheuristic optimization technique inspired by echolocation behavior of bats. In this study, the BI algorithm is examined in the context of discrete size optimization of steel frames designed for minimum weight. In the optimum design problem frame members are selected from available set of steel sections for producing practically acceptable designs subject to strength and displacement provisions of American Institute of Steel Construction-Allowable Stress Design (AISC-ASD) specification. The performance of the technique is quantified using three real-size large steel frames under actual load and design considerations. The results obtained provide a sufficient evidence for successful performance of the BI algorithm in comparison to other metaheuristics employed in structural optimization.     

A comparison of simulated annealing and genetic algorithm for optimum design of nonlinear steel space frames

In this article, two algorithms are presented for the optimum design of geometrically nonlinear steel space frames that are based on simulated annealing and genetic algorithm. The design algorithms obtain minimum weight frames by selecting suitable sections from a standard set of steel sections such as the American Institute of Steel Construction (AISC) wide-flange shapes. Stress constraints of AISC Load and Resistance Factor Design (LRFD) and AISC Allowable Stress Design (ASD) specifications, maximum (lateral displacement) and interstorey drift constraints, and also size constraints for columns were imposed on frames. The algorithms were applied to the optimum design of three space frame structures, which have a very small amount of nonlinearity. The unconstrained form of objective function was applied in both optimum design algorithms, and constant penalty factors were used instead of gradually increasing ones. Although genetic algorithm took much less time to converge, the comparisons showed that the simulated annealing algorithm yielded better designs together with AISC-LRFD code specification.     

Temperature heterogeneity during travelling fire on experimental building

In order to follow modern trends in contemporary building architecture which is moving off the limits of current fire design models, assumption of homogeneous temperature conditions used for structural fire analysis needs to be validated. In this paper it is described, how temperature distribution in a medium-size fire compartment has been investigated experimentally by conducting fire test in two-storey experimental building in September 2011 in the Czech Republic. In the upper floor, a scenario of travelling fire was prepared. It has been observed that as flames were spreading across the compartment, considerable temperature gradients appeared. Numerical simulation of the travelling fire test conducted using FDS (Fire Dynamics Simulator) has been compared with simulation of compartment fire under uniform temperature conditions to highlight the potential impact of the gas temperature heterogeneity on structural behaviour. The temperature measurements from the fire test have been used for validation of the numerical simulation of travelling fire. The fire test has provided important data for design model of travelling fire and shown that its impact on structural behaviour is not in agreement with the assumption of homogenous temperature conditions.     

Post-buckling analysis of spring-hinged cantilever columns

The post-buckling behaviour of spring-hinged cantilever columns has been investigated in this paper. The governing nonlinear differential equation with variable coefficients has been solved using Galerkin finite element method. Three loading conditions, namely, a concentrated tip load, a uniformly distributed load and a linearly varying load, acting on the column are considered in the present analysis. Very accurate estimates of the axial load parameters have been presented for different end slopes and for different spring constants.     

Computer simulation of large frame structures

A weapon test canister frame provides a structural platform for a weapon device and associated diagnostic instrumentation. The underground environment in which a test canister is placed results in the canister being subjected to large static loads. Excessive canister deflection under load may result in misalignment of optical lines-of-sight, with resulting degradation of the quality of the experimental physics data which is obtained. Each weapon test costs tens of millions of dollars and takes months or years to design and construct, and thus it is essential that the canister structure maintains functionality and prevents excessive deformation under load. In the current paper, a discussion of computer modeling and model verification for canister structures is presented. Two different approaches for performing computer simulation of the response of canister structures are discussed. One approach consists of a brute force detailed finite element model in which a classical finite element technique is employed to model the structure in great detail. The second approach employs a reduced-order, equivalent continuum model to represent the canister structure. The continuum model is based on a special purpose methodology which allows representation of the global behavior of large canister structures with an efficient and economical model. Canister modeling results for both modeling approaches are presented in this paper and compared to experimental data from actual canister tests. Finally, the utility of numerical simulations in identifying an effective technique for canister frame stiffening is discussed.     

Load mapping for seamless interface between hydrodynamics and structural analyses

This paper addresses the problem of a seamless interface between hydrodynamics and structural analyses. A pressure distribution on a hydro model computed from seakeeping analysis needs to be transferred to a structural model for evaluating structural strength and its integrity. However, due to the differences in the computation and representation methods for both analyses, the load on the hydro model may not be correctly transferred to the structural model, leading to a different load distribution on the structural model and resulting in some unbalanced force and moment components. In this paper, a method is proposed to solve this problem. A pressure distribution on the hydro model is mapped on the structural model through projection, and force and moment imbalances on the structural model are eliminated through optimization of the nodal forces on the structural model. Moreover, a viscous force distribution along the center of each member of the hydro model is transferred to the nodal forces on the structural model based on the minimum distance measure with resolving any force and moment imbalance. Examples are presented to demonstrate the validity of the proposed method.     

Real-time finite element structural analysis in augmented reality

Conventional finite element analysis (FEA) is usually carried out in offsite and virtual environments, i.e., computer-generated graphics, which does not promote a user’s perception and interaction, and limits its applications. With the purpose of enhancing structural analysis with augmented reality (AR) technologies, the paper presents a system which integrates sensor measurement and real-time FEA simulation into an AR-based environment. By incorporating scientific visualization technologies, this system superimposes FEA results directly on real-world objects, and provides intuitive interfaces for enhanced data exploration. A wireless sensor network has been integrated into the system to acquire spatially distributed loads, and a method to register the sensors onsite has been developed. Real-time FEA methods are employed to generate fast solutions in response to load variations. As a case study, this system is applied to monitor the stresses of a step ladder under actual loading conditions. The relationships among accuracy, mesh resolution and frame rate are investigated.     

Large elastic deformations of clamped skewed plates by dynamic relaxation

This investigation is concerned with the nonlinear behaviour of clamped Isotropic skew plates of constant thickness subjected to a uniformly distributed transverse load. The recently developed numerical technique of dynamic relaxation has been adopted for the analysis. A detailed study of the large deflection behaviour of skew plates has been made by varying three parameters, viz. skew angle, load, and aspect ratio. Numerical results have been compared with the available solutions. Representative nondimensional solutions are presented in the form of graphs to elucidate the nonlinear effect due to large deflection at higher loads.     

A visual tracking system for the measurement of dynamic structural displacements

A transputer-based system has been developed for the measurement of dynamic displacements in real time by visually tracking moving objects from a video Input. Parallel processing techniques have been used to track multiple objects at frame rate, and various predictive tracking algorithms have been assessed and implemented on the transputer network. The system is primarily intended for applications such as the monitoring or testing of large physical structures where actual displacements may not easily be otherwise determined. Results from the monitoring of the behaviour of the Humber Bridge at the centre of its span are presented.