Strength reduction factors for seismic analyses of buildings exposed to near-fault ground motions

To estimate the near-fault inelastic response spectra, the accuracy of six existing strength reduction factors （R） proposed by different investigators were evaluated by using a suite of near-fault earthquake records with directivity-induced pulses. In the evaluation, the force-deformation relationship is modelled by elastic-perfectly plastic, bilinear and stiffness degrading models, and two site conditions, rock and soil, are considered. The R-value ratio （ratio of the R value obtained from the existing R-expressions （or the R-p-T relationships） to that from inelastic analyses） is used as a measurement parameter. Results show that the R-expressions proposed by Ordaz ＆ Perez-Rocha are the most suitable for near-fault ground motions, followed by the Newmark ＆ Hall and the Berrill et al. relationships. Based on an analysis using the near-fault ground motion dataset, new expressions of R that consider the effects of site conditions are presented and verified.

Assessment of compressive strength of jet grouting by machine learning

Jet grouting is one of the most popular soil improvement techniques,but its design usually involves great uncertainties that can lead to economic cost overruns in construction projects.The high dispersion in the properties of the improved material leads to designers assuming a conservative,arbitrary and unjustified strength,which is even sometimes subjected to the results of the test fields.The present paper presents an approach for prediction of the uniaxial compressive strength(UCS)of jet grouting columns based on the analysis of several machine learning algorithms on a database of 854 results mainly collected from different research papers.The selected machine learning model(extremely randomized trees)relates the soil type and various parameters of the technique to the value of the compressive strength.Despite the complex mechanism that surrounds the jet grouting process,evidenced by the high dispersion and low correlation of the variables studied,the trained model allows to optimally predict the values of compressive strength with a significant improvement with respect to the existing works.Consequently,this work proposes for the first time a reliable and easily applicable approach for estimation of the compressive strength of jet grouting columns.

Failure behavior and strength model of blocky rock mass with and without rockbolts

To better understand the failure behaviours and strength of bolt-reinforced blocky rocks,large scale extensive laboratory experiments are carried out on blocky rock-like specimens with and without rockbolt reinforcement.The results show that both shear failure and tensile failure along joint surfaces are observed but the shear failure is a main controlling factor for the peak strength of the rock mass with and without rockbolts.The rockbolts are necked and shear deformation simultaneously happens in bolt reinforced rock specimens.As the joint dip angle increases,the joint shear failure becomes more dominant.The number of rockbolts has a significant impact on the peak strain and uniaxial compressive strength(UCS),but little influence on the deformation modulus of the rock mass.Using the Winkler beam model to represent the rockbolt behaviours,an analytical model for the prediction of the strength of boltreinforced blocky rocks is proposed.Good agreement between the UCS values predicted by proposed model and obtained from experiments suggest an encouraging performance of the proposed model.In addition,the performance of the proposed model is further assessed using published results in the literature,indicating the proposed model can be used effectively in the prediction of UCS of bolt-reinforced blocky rocks.

Design and application of wireless signal strength measurement system on the near-ground

The wireless communication system's performance is greatly constrained by the wireless channel characteristics,especially in some specific environment.Therefore,signal transmission will be greatly impacted even if not in a complicated topography.Testing results show that it is hardly to characterize the radio propagation properties for the antenna installed on the ground.In order to ensure a successful communication,the radio frequency(RF)wireless signal intensity monitor system was designed.We can get the wireless link transmission loss through measuring signal strength from received node.The test shows that the near-ground wireless signal propagation characteristics still can be characterized by the log distance propagation loss model.These results will conduce to studying the transmission characteristic of Near-Earth wireless signals and will predict the coverage of the earth's surface wireless sensor network.

Fragility of disconnect switch subjected to random earthquake ground motions

A fragility calculation scheme is estabtished in this paper for porcelain-type equipments subjected to random earthquake ground motions. All steps of the method are illustrated by the seismic damage analysis of GW4-110 disconnect switch. The model of the equipment is built applying the finite element method with flexible joints, and the seismic response of the equipment is analyzed using elastic time history method. On the base, according to the strength damage index and Monte-Carlo Method, the seismic damage ratios are counted and the seismic fragility curves are presented. Then the seismic damage of GW4-110 disconnect switch can be predicted.

A case study of multi-seam coal mine entry stability analysis with strength reduction method

In this paper,the advantage of using numerical models with the strength reduction method(SRM) to evaluate entry stability in complex multiple-seam conditions is demonstrated.A coal mine under variable topography from the Central Appalachian region is used as a case study.At this mine,unexpected roof conditions were encountered during development below previously mined panels.Stress mapping and observation of ground conditions were used to quantify the success of entry support systems in three room-and-pillar panels.Numerical model analyses were initially conducted to estimate the stresses induced by the multiple-seam mining at the locations of the affected entries.The SRM was used to quantify the stability factor of the supported roof of the entries at selected locations.The SRM-calculated stability factors were compared with observations made during the site visits,and the results demonstrate that the SRM adequately identifies the unexpected roof conditions in this complex case.It is concluded that the SRM can be used to effectively evaluate the likely success of roof supports and the stability condition of entries in coal mines.

Study on strength reduction factors consid-ering the effect of classification of design earthquake

The strength reduction factors are not only the key factors in determining seismic action for force-based seismic design, but also the key parameters to derive the inelastic response spectra for performance-based seismic design. In this paper, with a high quality ground motion database that includes a reasonable-sized set of records from China, a statistical study on the strength reduction factors is conducted and a new expression of strength reduction factors involving classification of design earthquake, which is an important concept to determine design spectra in Chinese seismic design code, is proposed. The expression of strength reduction factors can reflect the ground motion characteristics of China to a certain extent and is particularly suitable for Chinese seismic design. Then, the influence effects of site condition, classification of design earthquake, period of vibration, ductility level, earthquake magnitude and distance to fault on strength reduction factors are investigated. It is concluded that the effect of site condition on the strength reduction factors cannot be neglected, especially for the short-period structures of higher ductility. The classification of design earthquake also has an important effect on strength reduction factors and it may be unsuitable to use the existing expressions of strength reduction factors to the design spectra of current Chinese seismic code. The earthquake magnitude has no practical effect on strength reduction factors and if the near-fault records with forward directivity effect are not taken into consideration, the effect of distance to fault on strength reduction factors can also be neglected.

Structural Reliability Analysis of Ship Hulls Accounting for Collision or Grounding Damage

Classical structural reliability analysis of intact ship hulls is extended to the case of ships with collision or grounding damages.Still water load distribution and residual bending moment capacity are included as random variables in the limit state equation.The probability density functions of these random variables are defined based on random damage parameters given by the Marine Environment Protection Committee of the International Maritime Organization,while the proposed reliability formulation is consistent with international recommendations and thus may be valuable in the development of rules for accidental limit states.The methodology is applied on an example of an Aframax oil tanker.The proposed approach captures in a rational way complex interaction of different pertinent variables influencing safety of damaged ship structure.

基于统一强度理论的扰损围岩巷道脆塑性解答

钻爆法施工常造成岩质巷道扰损区围岩性能劣化。针对扰损围岩、扰损外围岩的弹塑性状态下不同的扰损围岩巷道力学模型,采用统一强度理论、弹-脆-塑性模型和非关联流动法则,建立扰损围岩巷道有关应力、位移和塑性区半径的脆塑性解答,探讨巷道力学模型的转换路径及判定方法,并将结果进行对比、验证,获得各因素的影响规律。研究表明:提出的巷道脆塑性解答合理反映了中间主应力、围岩扰损范围和峰后强度、剪胀特性的综合影响,通过参数组合变换可退化为多种解答并得到正确性验证,具有重要的理论意义和应用前景;巷道力学模型判定与中间主应力、围岩扰损范围密切相关,显著影响巷道应力-位移分布和围岩特征曲线;支护压力和巷道稳定变形随着围岩峰后强度下降、剪胀特性参数增加而明显增大。

基于多水准的高强钢框架-D形偏心支撑结构层剪力分布研究

高强钢框架GD形偏心支撑结构中耗能梁段采用Q355钢,非耗能构件(即框架梁和柱)采用高强度钢材,形成一种新型结构体系.耗能梁段在结构遭遇罕遇地震时充分发挥塑性变形耗能,保护高强钢框架处于弹性受力状态,高强钢框架GD形偏心支撑结构由于在塑性状态下结构刚度发生改变,其层剪力分布模式不再符合基于强度的设计理论,因此,本文研究了高强钢框架GD形偏心支撑结构在多水准地震动状态下的层剪力分布模式.本文设计了四种不同层数(4层,8层,12层和16层)和三种不同耗能梁段长度(900mm,1000mm和1100mm)的高强钢框架GD形偏心支撑结构,输入40条近场脉冲地震和40条远场地震记录,得到结构在多遇地震、设防地震、罕遇地震以及极罕遇地震下的层剪力分布模式,并对罕遇地震下的层剪力分布模式进行参数标定,获取塑性状态下结构的弹塑性侧向力分布状态.

多元荷载作用下软基深水高桩码头结构承载特性研究

软基深水高桩码头结构受到恶劣外海环境荷载及复杂工作荷载等多元荷载的共同作用。研究表明,船舶撞击荷载是该结构水平向控制荷载,但波浪长期循环荷载作用会引起桩周土体软化,进而导致码头结构在其它荷载作用下承载特性的劣化。鉴于此,首先基于ABAQUS有限元软件建立了深水高桩码头结构-地基土体相互作用的三维弹塑性有限元模型;然后,借助USDFLD子程序实现了同时考虑土体强度弱化和刚度衰减的模拟,进而开展了未考虑土体软化、仅考虑土体强度弱化、仅考虑土体刚度衰减以及同时考虑土体强度弱化和刚度衰减对码头结构承载特性影响的对比分析。研究结果表明,与未考虑土体软化时撞击荷载作用下码头结构安全系数相比,仅考虑土体强度弱化时其值降低14.72%,仅考虑土体刚度衰减时其值降低15.28%,同时考虑土体强度弱化和刚度衰减时其值降低19.44%,且考虑土体软化后桩周土体的塑性应变范围明显增大,极限状态时桩身应力值减小,结构稳定性显著降低。

钢渣粉对高强混凝土力学性能和耐久性的影响

超细矿渣是制备高强混凝土的优质矿物掺合料,但是超细矿渣水化放热量较大、凝结较快,不利于高强混凝土的强度和耐久性发展。而钢渣粉具有延缓水泥水化、降低水化放热的作用,可以充当“缓凝剂”。通过在高强混凝土中复掺钢渣粉和超细矿渣,研究了钢渣粉对高强混凝土性能的影响。结果表明复掺15%钢渣粉时初凝时间和终凝时间分别比纯水泥组延长53.0%和35.9%。同时早期强度降低,但与超细矿渣复掺后28 d和90 d强度分别为77.3 MPa和84.6 MPa,比对照组高4.3%和8.6%。此外,钢渣粉有效降低了高强混凝土的绝热温升和自收缩率。单掺15%超细矿渣时7 d自收缩率为1189.63×10^(-6),约为纯水泥组的2.5倍;复掺15%钢渣粉时自收缩率为211.62×10^(-6),比纯水泥组降低了52%。复掺钢渣粉的高强混凝土与纯水泥制备的混凝土的抗氯离子渗透性等级均为“低”。

探地雷达+注浆技术在公路养护中的应用

沥青混凝土路面具有诸多优点,被广泛应用于各等级公路中。在沥青混凝土路面使用过程中,由于其材料的特点,比较容易发生因为干缩以及温缩引起的开裂病害。沥青混凝土路面一旦开裂,地表水会渗入基层和路基,若长期未得到有效的治理,有可能导致基层强度降低、路基被掏空等更为严重的病害。这类病害最简单的治理方法是开挖路基和路面结构层,并逐层填筑至路面,但这种方法耗时长,需中断交通。除此之外,还可以采用注浆的方法对路基和基层进行加固,再重新施做路面即可恢复通车运营,且施工期间可不中断交通,施工速度非常快。

人工冻结水泥改良软黏土冻融及强度特性研究

为探究人工冻结水泥改良软黏土冻胀融沉特性及冻融前后强度变化规律,通过室内冻胀融沉试验和单轴抗压强度试验,揭示不同冻融状态、不同影响因素等条件下水泥改良软黏土冻胀融沉及强度特性。研究结果表明,软黏土冻胀率和融沉系数随冻结冷端温度降低而减小,且融沉系数大于冻胀率;掺入水泥后融沉系数均小于冻胀率,且随水泥掺量增加而降低,当水泥掺量超过5%时,融沉系数小于2%。水泥土起始冻结温度随水泥掺量增加而降低,当水泥掺量为9%时,起始冻结温度最低达到-1.53℃;冻融过程沿试样不同高度温度场变化分为积极冻结、维护冻结和融化三个阶段;冻融后水泥改良软黏土强度降低,25组试样平均降低56%,冻融后强度折减最大达76%。研究指出,改良土强度分别随水泥掺量增加而增大,随含水率增大而减小,随龄期增长而增大。

石墨导电混凝土物理特性研究

石墨导电混凝土因其兼具良好的导电、耐腐蚀和力学等性能而广泛应用于接地模块、电磁屏蔽干扰、采暖和桥梁路面除冰防冻等工程领域。然而,目前针对石墨导电混凝土在电学性能、耐腐蚀性能和抗压强度方面仍缺乏相关研究,尤其是不同石墨配比对导电混凝土电阻特性、抗压强度等的影响规律尚不明确。本文以石墨作为导电材料添加,以水泥、砂等为基体制备了不同石墨配比的导电混凝土,并以石墨材料质量分数为10%时的导电混凝土特性开展研究,设计了稳定性试验、冷热循环试验、工频耐受试验和冲击耐受实验,并计算得到了对不同材料的表面平均年腐蚀率。为探究不同石墨配比下的导电混凝土特性变化规律,进一步分析了石墨配比分别为5%、10%、15%、20%、25%的导电混凝土电阻及抗压强度变化规律。相关研究为实现石墨导电混凝土的应用、保障电力系统运行安全提供了理论借鉴。

基于ERS强度分析的巴拿马型散货船事故抢险策略

为使搁浅船舶能够脱困和修理,需拟定整体起浮脱浅方案和后续的拖带方案。在通过前期水下探摸确定船体破损的情况下,利用船舶应急响应ERS的船体强度和稳性计算结果,对个别舱室不同封堵,排水和减载方案进行对比,为抢险和拖带方案提供最优化建议。最终船舶按照拟定的方案实施抢险,成功起浮,并安全拖带至船厂修理。通过实际现场工程案例,证明船舶应急响应ERS的船体强度和稳性计算能为海损事故船舶的抢险工作提供可靠的指导意见。

降低GIS母线绝缘子低电位气隙处电场强度值的方法研究

母线绝缘子是GIS的重要部件,在GIS设备中起到连接高电位部件与地电位外壳、支撑以及对地绝缘的作用,还可以将GIS设备分成若干个气室。绝缘子沿面电场强度的大小及均匀性极大地影响了母线整体的绝缘性能。绝缘子低电位与罐体法兰连接处有微小的气隙,经计算,该气隙处电场强度值较大,有较多绝缘子放电与该气隙处有关的现场运行案例。本文提出了降低该气隙处电场强度值的三种方法,即:增大气隙、增加接地屏蔽装置、改变罐体法兰形状。三种方法均可以有效地降低绝缘子低电位气隙电场强度值,产品设计时,可根据实际结构进行选择。本文为GIS产品结构设计提供了指导方向。

AGV小车接地柱脱落分析及控制措施

AGV小车中的接地柱具有防护触电,防静电,保障电气系统运行等作用,一旦脱落失效,将造成不可估量的损失,本文根据近期出现的接地柱在压接接地线缆时出现的脱落两种现象,利用鱼骨图进行分析,最终得出是由操作工人缺少焊接技能、接地柱抗拉能力与强度不足、以及装配力矩超出接地柱承受的力矩等原因所致,故提出螺柱填充焊的新型焊接方式、提高接地柱强度等级以及推导出接地柱的装配力矩范围等相应防护措施,制定测试方案对上述解决措施进行验证,证明了所提出的三种控制措施是可靠有效的,为同类产品或其他产品涉及接地柱脱落的预防性措施提供参考方案,具有重要的现实意义和工程设计借鉴作用。

花岗岩基岩凸起地层盾构施工新认识

以基岩凸起为代表的软硬不均地层对盾构施工有很大影响,特别是在市区施工受环境限制无法采用常规手段进行预破碎的情况下,盾构机能不能穿越,如何安全穿越基岩凸起成为困扰工程技术人员的重大难题。以厦门地铁2号线五缘湾南站—五缘湾站区间盾构通过长80m未预破碎的极高强度花岗岩基岩凸起为背景,分析基岩凸起的地质构成及其对盾构施工带来的危害,提出了针对性的解决措施并成功实践。

沙化盐碱地成库论证及防渗对策研究

昆都仑河水库整个库坝区覆盖层皆为沙化盐碱地层,透水性、腐蚀性和盐胀性皆较强,且厚度大而难以换填,导致其成库论证及防渗处置极其复杂,而两者成功与否是水库修建的决定性因素。本文基于地质调查、地质钻探和室内外试验成果,探索沙化盐碱地岩土体物理及化学特性,以求为成库论证及防渗对策研究奠定基础。结果表明:库坝区上部地层皆为强透水性,存在库区、坝肩和坝基渗漏,不具备成库蓄水天然地质条件。库坝区表层厚度约2.0m为沙化盐碱土,具有较强的腐蚀性和盐胀性,但皆随埋深增加而逐渐变弱。坝基全—强风化泥岩皆呈弱透水性,防渗性较好,为天然库底隔水层,其抗压强度及坝基承载力皆随埋深增加和风化程度减弱逐渐提高。推荐库坝区采用砼防渗面板铺盖方案,且砼必须具备抗硫酸盐、高密实性、高强度等特性,且严控施工和养护质量,可实现成库蓄水。