Collapse assessment of steel moment frames using endurance time method

In endurance time(ET) method structures are subjected to a set of predesigned intensifying excitations. These excitations are produced in a way that their response spectrum, while complying with a specifi ed spectrum, intensifi es with time so they can be used approximately to simulate the average effects of several ground motions scaled to different intensities. In this paper applicability of the ET method for evaluating collapse potential of buildings is investigated. A set of four steel moment frames is used for collapse assessment. The process of using ET method in collapse evaluation is explained and the results are compared with incremental dynamic analysis(IDA) results. It is shown that although the computational effort using the ET method is much less than the IDA analysis, the results of both methods are consistent. Finally collapse fragility curves using ET and IDA methods are produced and it is shown that the probabilities of collapse in different hazard levels are also consistent.

Efficient structural seismic performance evaluation method using improved endurance time analysis

Evaluation of structural performance under seismic excitations from low intensity to high intensity is essential to verify the seismic resistant capacity of a structure, and usually carried out by the incremental dynamic analysis (IDA) method or pushover method. The recently developed endurance time (ET) method is another method that uses dynamic pushover excitations, i.e., endurance time acceleration function, to obtain results similar to those obtained by IDA or pushover methods with low computational cost and acceptable accuracy. This study proposes an improvement on the ET method by considering more restrictions for both the elastic and inelastic response spectra in the generation procedure, and by specifying a target duration. Four reinforced concrete frame structures with 4, 8, 12, and 16 stories are adopted to verify the accuracy of the improved method. Comparison of the results obtained by the proposed method, the ET method and the IDA method shows that the improved method has a higher accuracy than the ET method. For evaluation of structural responses under specifi c ground motion intensity, which is typically required in seismic design codes, the results obtained by the proposed method are compared with fi ve commonly used ground motion selection methods, and shows the proposed method provides acceptable accuracy for engineering applications.

Seismic assessment of unanchored steel storage tanks by endurance time method

Liquid storage tanks are essential structures that are often located in residential and industrial areas; thus an assessment of their seismic performance is an important engineering issue. In this paper, the seismic response ofunanchored steel liquid storage tanks is investigated using the endurance time （ET） dynamic analysis procedure and compared to responses obtained for anchored tanks under actual ground motions and intensifying ET records. In most cases, the results from ground motions are properly obtained with negligible differences using ET records. It is observed that uplifting of the tank base, which is closely related to the tank aspect ratio, has the greatest significance in the responses of the tank and can be predicted with reasonable accuracy by using currently available ET records.

Probabilistic seismic loss estimation via endurance time method

Probabilistic Seismic Loss Estimation is a methodology used as a quantitative and explicit expression of the performance of buildings using terms that address the interests of both owners and insurance companies. Applying the ATC 58 approach for seismic loss assessment of buildings requires using Incremental Dynamic Analysis （IDA）, which needs hundreds of time-consuming analyses, which in turn hinders its wide application. The Endurance Time Method （ETM） is proposed herein as part of a demand propagation prediction procedure and is shown to be an economical alternative to IDA. Various scenarios were considered to achieve this purpose and their appropriateness has been evaluated using statistical methods. The most precise and efficient scenario was validated through comparison against IDA driven response predictions of 34 code conforming benchmark structures and was proven to be sufficiently precise while offering a great deal of efficiency. The loss values were estimated by replacing IDA with the proposed ETM-based procedure in the ATC 58 procedure and it was fotmd that these values suffer from varying inaccuracies, which were attributed to the discretized nature of damage and loss prediction functions provided by ATC 58.

Estimation of Concrete Arch Dams Responses in Linear Domain Using Endurance Time Analysis Method

In this paper, Endurance Time Analysis （ETA） method which is a new time-history based dynamic pushover procedure is introduced and its application in linear analysis of concrete arch dams is investigated. In this method the structure is subjected to gradually intensifying acceleration functions and its performance is evaluated based on the length of the time duration that can satisfy required performance criteria. For this purpose Dez arch dam is selected as case study, fluid-structure interaction is taken into account and F.E. model of the system is excited in three performance levels. ETA method gives an approximation of maximum response at the equivalent target time, resulted from analyzing the system based on natural records. Extracted results are displacement, velocity and acceleration of the crest at crown cantilever. Results show using of ETA method can reduce at least 50% in number of analyses and 70% in total time of analyses at the current case. Furthermore, it is found that although the results of the ETA are not exactly consistent with the results of time-history analyses using real ground motions, errors are reasonable and ETA can identify performance levels of the dam with acceptable accuracy.

Generation of endurance time excitation functions using spectral representation method

In this study,application of the spectral representation method for generation of endurance time excitation functions is introduced.Using this method,the intensifying acceleration time series is generated so that its acceleration response spectrum in any desired time duration is compatible with a time-scaled predefined acceleration response spectrum.For this purpose,simulated stationary acceleration time series is multiplied by the time dependent linear modulation function,then using a simple iterative scheme,it is forced to match a target acceleration response spectrum.It is shown that the generated samples have excellent conformity in low frequency,which is useful for nonlinear endurance time analysis.In the second part of this study,it is shown that this procedure can be extended to generate a set of spatially correlated endurance time excitation functions.This makes it possible to assess the performance of long structures under multi-support seismic excitation using endurance time analysis.

Nonlinear Seismic Response of Steel Concentrically Braced Frames Using Endurance Time Analysis Method

In present paper application of Endurance Time Analysis （ETA） method in nonlinear seismic assessment of steel concentrically braced frames is studied. In this method structure is subjected to a set of predesigned increasing acceleration functions and various responses of the structure are recorded during synthetic seismic excitation. The averages of maximum values for various responses in Nonlinear Time-History Analyses （NTHA） extracted from real ground motions are expected to be close to those obtained from ETA up to equivalent target time. For this purpose a set of 9, 11, 13 and 15 stories steel frames were designed and plastic hinge method was selected as source of nonlinearity in all cases. Responses were compared based on interstory drift ratio and story shear in two methods and results show satisfactory consistency between two methods. Finally, it was concluded that the ETA method can be used as alternative method in nonlinear seismic analysis of concentrically braced steel frames.

The effects of oral smokeless tobacco administration on endurance performance

Background： Smokeless tobacco is widely used by athletes to enhance performance. Nicotine is a central nervous system stimulant and acts on cardiocirculatory and metabolic systems, involving tissue blood flow and circulatory vasoreactivity. The aim of this study was to investigate the effects of the oral smokeless tobacco （Swedish snus （SS）） on the perception of fatigue and time to exhaustion （TTE） during moderate-intensity aerobic exercise. Methods： Fourteen healthy non-tobacco male users were recruited for a double-blind, controlled crossover design （SS vs. snus placebo （SP））. Subjects were tested for 3 sessions： experimental session 1 （Exp 1） consisted of an incremental test to determine the maximal aerobic power out-put （Wmax）, whereas Exp2 and Exp3 consisted of exercising at 65%Wmax until exhaustion in SS or SP conditions. During Exp2 and Exp3, muscle and cerebral oxygenation was assessed by means of near-infrared spectroscopy, and the rating of perceived exertion （RPE） was recorded. Results： Comparing SS with SP tests, significant differences （p 〈 0.05） were found in the values of cerebral （-3%） and muscular tissues oxygenation （-4%） in the first 30 min of exercise. The RPE values were not significantly different between the 2 conditions （SS vs. SP）. No significant difference was found in TTE （SS： 54.25 ± 21.84 min; SP： 50.01 ± 17.03 min）. Conclusion： This study showed that muscular and cerebral oxygenation increased significantly with snus administration during an endurance exercise until exhaustion, but this did not affect fatigue perception and TTE. The results showed that snus could not be considered an ergogenic substance in non-tobacco users.2018 Published by Elsevier B.V. on behalf of Shanghai University of Sport. This is an open access article under the CC BY-NC-ND license. （http：//creativecommons.org/licenses/by-nc-nd/4.0/）.

An analysis of the size and direction of the association between mental toughness and Olympic distance personal best triathlon times

Background: Athletes, coaches, sport psychology practitioners, and researchers suggest that mental toughness represents an important construct that is associated with athletic performance. Unfortunately, the absence of real-world performance as a dependent variable has limited our ability to substantiate this claim. The concern of a lack of ecologically valid measures of sports performance might be addressed by examining the relationship between mental toughness and sports performance using a standardized measure of personal best(PB) triathlon time and a validated unidimensional measure of mental toughness.Methods: Three hundred and sixteen triathletes completed the 8-item mental toughness index(MTI), reported their age, and provided a PB Olympic distance triathlon time and the total number of triathlons they had completed to date. Given that males are typically quicker than females, a standardized time was calculated by dividing the PB by the current Olympic record for gender; we also hypothesized that more experienced triathletes would report quicker PB times because of greater frequency and duration of training. Once we had controlled for gender and experience,we predicted that mental toughness would be negatively associated with triathlon time and the size of the relationship would be of a moderate magnitude.Results: Results revealed small-to moderate-sized negative relationships between both completed number of triathlons and mental toughness with standardized PB time.Conclusion: The hierarchical regression analysis showed that mental toughness provided a unique contribution to the variability in standardized Olympic triathlon PB after controlling for the total number of triathlons completed.

Susceptible time window and endurable duration of cotton fiber development to high temperature stress

The development of the cotton fiber is very sensitive to temperature variation, and high temperature stress often causes reduced fiber yield and fiber quality. Short-term high temperature stress often occurs during cotton production, but little is known about the specific timing and duration of stress that affects fiber development. To make this clear, pot experiments were carried in 2014 and 2015 in a climate chamber using cotton cultivars HY370WR（less sensitive variety） and Sumian 15（heat sensitive variety）, which present different temperature sensitivities. Changes of the most important fiber quality indices（i.e., fiber length, fiber strength and marcironaire） and three very important fiber development components（i.e., cellulose, sucrose and callose） were analyzed to define the time window and critical duration to the high temperature stress at 34°C（max38°C/min30°C）. When developing bolls were subjected to 5 days of high temperature stress at different days post-anthesis（DPA）, the changes（Δ%） of fiber length, strength and micronire, as a function of imposed time followed square polynomial eq. as y=a＋bx＋cx^2, and the time around 15 DPA was the most sensitive period for fiber quality development in response to heat stress. When 15 DPA bolls were heat-stressed for different durations（2, 3, 4, 5, 6, 7 days）, the changes（Δ%） of fiber length, strength and micronire, as a function of stress duration followed logistic equations y=A_1-A_2/1＋（x/x_0）~p＋A_2. Referred to that 5, 10 and 15% are usually used as criteria to decide whether techniques are effective or changes are significant in crop culture practice and reguard to the fiber quality indices change range, we suggested that 5% changes of the major fiber quality indices（fiber length, fiber strength and micronaire） and 10% changes of fiber development components（cellulose, sucrose and callose） could be taken as criteria to judge whether fiber development and fiber quality have been significantly affected by high temperature stress. The key time window for cotton fiber development in response to the high temperature stress was 13–19 DPA, and the critical duration was about 5 days.

基于耐震时程法的高拱坝地震易损性分析

基于耐震时程分析方法(ETA),建立了三维坝体-库水-地基动力相互作用模型,进行高拱坝抗震性能评价。以基于目标反应谱合成的10条耐震加速度时程曲线为地震输入,选取坝体损伤体积比、坝顶坝踵顺河向相对位移和拱冠梁横缝开度作为结构性能指标,通过对结构不同性能水准的定义,建立了极限状态与结构性能指标之间的关系,以此为基础绘制易损性曲线。结果表明:随着耐震时程输入时间的增加,拱坝的各项响应值随之增大,不同响应指标对地震强度的敏感性不同,且ETA法与增量动力分析法(IDA)得到的响应结果基本一致,在同一性能水平下采用不同评价指标时对应的超越概率也不相同。因此,在进行结构的抗震性能评价时应综合考虑不同结构响应指标,选取不同性能下超越概率最大的结构响应指标进行分析。

大学生耐力素质的动态分析——以娄底职业技术学院2021—2023级学生为例

大学生的耐力素质的高低直接影响到学习和工作,通过2021级-2023级新生的耐力素质情况和2021级大学3年的体育训练对学生耐力素质的影响情况进行了分析,分析的方法采用时间序列和t检验,发现新生耐力有所下降,2021级的学生通过大学的体育教育耐力素质有所提高。进而分析了影响耐力素质的原因,并提出了解决措施,为教学及相关管理部门提供参考。

基于机器学习的铁路桥墩地震损伤快速评估

在桥梁的震后抢通工作中,桥梁结构的快速损伤评估是恢复交通的关键环节。以具有代表性的铁路矩形桥墩为研究对象,通过4组拟静力试验验证有限元建模方法的合理性,并对1000组桥墩有限元模型分别按照纵桥向和横桥向进行耐震时程分析,通过搭建BP神经网络对地震动力响应的需求结果进行拟合,构建铁路矩形桥墩震损快速评估模型,最终通过一座三跨混凝土梁桥验证该模型的适用性。研究结果表明:配筋率、配箍率、剪跨比和轴压比是影响桥墩地震损伤的四种主要因素,长宽比、混凝土和钢筋强度是影响桥墩地震损伤的三项次要因素;当发生PGA为0.32 g的设计地震时,通过数值分析和神经网络模型快速评估这两种方法计算所得桥梁四个桥墩轻微损伤概率分别为96.7%、44.6%、49.1%、96.7%和95.6%、40.4%、60.9%、95.8%,中度损伤概率分别为40.1%、1.2%、1.6%、40.1%和37.4%、2.3%、6.0%、37.7%;BP神经网络算法能够有效建立构造参数与地震响应之间的联系,输出误差处于合理范围内,回归程度较好。基于BP神经网络的桥梁地震损伤评估模型具有较好的普适性,能替代部分数值仿真计算工作。

基于耐震时程法的大跨度钢桁拱桥易损性分析

为克服复杂桥梁结构在地震易损性评估时需要进行大量非线性时程分析的不足,提出一种基于耐震时程法(ETM)的易损性分析方法用于桥梁结构易损性的快速评估。首先,基于ETM合成了若干条耐震时程曲线(ETA),将其与概率地震需求模型相结合。随后,考虑桥梁模型的不确定性,借助OpenSEES建立一座大跨度上承式钢桁拱桥的有限元模型样本库,并分别使用ETM和增量动力分析(IDA)法对其中的桥梁模型进行时程分析并得到典型构件的易损性曲线,将二者结果进行对比以验证ETM的适用性。最后,利用验证后的方法分析该桥各构件纵向地震下的易损性。结果表明:ETM与IDA所得结果的相对误差基本控制在5%以内,说明基于耐震时程的易损性方法可有效兼顾分析效率和计算精度;在纵向地震作用下,上承式钢桁拱桥的引桥墩和交界墩在50%中等损伤概率下的地震动强度指标仅为0.597g和0.972g,远小于其他构件,说明混凝土桥墩为最易损构件;拱肋和立柱在地震作用下发生破坏的概率较小,但拱顶和拱脚相对薄弱;设置固定支座的短立柱在纵向地震作用下易发生损伤,可通过改变拱上立柱的支座布置形式进行优化。

咖啡因对耐力运动员运动表现的影响:效果及机制

咖啡因作为一种功能增进营养补剂,广泛应用于耐力运动员的训练和竞赛中。通过系统梳理探讨摄入咖啡因对耐力运动员计时赛、恒定负荷运动和递增负荷运动至力竭3种类型运动表现影响的相关研究,总结咖啡因对耐力运动员运动表现的影响规律,以期为我国教练员和运动员优化营养补剂方案提供理论依据。结果显示,自行车、中长跑及赛艇等耐力运动员多以胶囊形式摄入低中高剂量咖啡因(3~9 mg/kg)来提高耐力运动表现,但对于不同类型运动表现的有效剂量及作用机制存在差异。另外,咖啡因与其他营养补剂混合摄入的效果以及咖啡因不同摄入方式、性别差异、基因组别、安慰剂效应等对其功能增进效果的影响是咖啡因研究领域未来的发展方向。

耐震时程法在大跨连续刚构桥纵向抗震性能评估中的应用

采用简化方法对大跨连续刚构桥的抗震性能进行高效和准确的评估,在该类桥梁的设计和加固改造时是非常必要的。耐震时程法作为一种评估桥梁抗震性能的简化方法,只需要一次时程分析就可以实现桥梁从完好到倒塌的全过程分析。为研究耐震时程法在评估大跨连续刚构桥纵向抗震性能的适用性与精确性,本文以一座大跨波形钢腹板连续刚构桥为研究对象,基于中国公路桥梁抗震设计规范设计反应谱,生成3条耐震加速度时程曲线作为地震输入,同时选择16条天然地震动进行增量动力分析,对比研究了该连续刚构桥地震响应特征。研究结果表明:耐震时程分析结果处于增量动力分析结果的包络线之内,耐震时程法中位值与增量动力分析结果的均值曲线吻合较好,因此,耐震时程分析方法可以作为大跨连续刚构桥抗震性能评估的一种高效率方法。

间隔时间效应对同期训练改善女大学生肌肉力量和心肺耐力的影响研究

目的:探讨同期肌肉力量与心肺耐力训练中不同训练间隔的时间效应对同期训练改善女大学生心肺耐力与下肢肌肉力量的影响。方法:选取66名普通女大学生为研究对象,随机分为S组、E组、CT-8h组、CT-24h组;被试进行12周运动干预,每周2次;于训练前、训练后第6周、第12周检测被试的心肺耐力、下肢肌肉力量以及下肢肌肉iEMG值。结果:(1)最大摄氧量(VO 2max):运动干预后,CT-24h组、CT-8h组、E组的VO 2max值均存在显著性差异(P<0.05);S组的VO 2max值存在极显著性差异(P<0.01);E组、CT-8h组、CT-24h组被试间VO2max值均无显著性差异(P>0.05)。(2)深蹲1RM值:运动干预后,CT-24h组、CT-8h组、S组的深蹲1RM值均存在显著性差异(P<0.05);E组的深蹲1RM值存在极显著性差异(P<0.01);CT-24h组的深蹲1RM值无显著性差异(P>0.05);CT-8h组存在极显著性差异(P<0.01)。(3)下肢肌肉iEMG值:运动干预后,各组的下肢肌肉iEMG值均存在极显著性差异(P<0.01);CT-24h组的下肢肌肉iEMG值无显著性差异(P>0.05),CT-8h组存在极显著性差异(P<0.01)。结论:12周的肌肉力量和心肺耐力同期训练能够提高普通女大学生心肺耐力、下肢肌肉力量;间隔24小时同期训练对于提高普通女大学生心肺耐力、下肢肌肉力量效果更佳。

基于耐震时程法的地下结构序列地震作用抗震性能研究

针对当前地铁地下结构抗震性能研究大多仅考虑主震影响而忽略地震后可能伴随发生的多次余震作用问题,开展了地下结构在主余震序列作用下的动力响应规律研究,选取典型两层三跨地铁车站为原型,建立了土-地下结构相互作用的有限元数值模型,基于耐震时程法基本概念并结合我国相关抗震标准设计反应谱生成2条耐震时程曲线,构造了12条主余震序列输入,研究了地铁地下车站在主余震序列作用下的侧向变形特征以及损伤破坏规律。结果表明:耐震时程法可以作为主余震序列作用下地下结构抗震性能研究的一种新的高效方法;主震后带损伤结构的侧向变形应充分考虑主余震序列峰值强度比α影响,主震水平与破坏状态越高,则产生余震增量损伤的临界值强度比α越小,结构面临更严重破坏的风险也越高。研究成果为结构在序列型地震作用下的抗震分析与评估提供了新的思路。

手过头姿势下钻孔作业肌肉疲劳MET预测模型

为评价手过头姿势下钻孔作业过程中人员肌肉疲劳发展情况,先通过模拟手过头姿势下的钻孔试验,测量试验前肌肉的最大随意收缩(MVC)、试验后的最大剩余肌力(MRF)、力量输出衰减程度(ΔF)以及手腕、手肘和肩部的主观疲劳评分(RPE),记录最大耐受时间(MET);通过设定3种操作面(正面、侧面、底面)和3种操作高度的不同操作组合作为试验变量,比较不同操作方式对手过头姿势下钻孔作业的肌肉疲劳影响情况,随后分析3种操作面(正面、侧面、底面)和3种操作高度下对MET、MRF、ΔF以及手腕、手肘和肩部RPE的影响。研究结果表明:采用正面操作并降低手臂上举高度能有效减轻肌肉疲劳;不同操作面显著影响MET、MRF及ΔF,同时,显著影响手腕、手肘和肩部的RPE;不同的操作高度显著影响MET、MRF以及手肘的RPE;所建立的MET预测模型可反映手过头姿势下钻孔作业过程中人员肌肉疲劳状态。

基于易损性的藏式碉楼可保护性评估方法研究

为了对现存碉楼的现状进行合理评估,基于易损性构建了一套碉楼震后可保护性评估方法,以黑虎寨碉楼、丹巴碉楼和直波碉楼为参考,建立了不同外形、不同高度、不同墙体收分率的碉楼有限元模型,基于耐震时程地震分析进行了易损性与可保护性的评估,探明了现存碉楼的可保护情况。