System reliability evaluation of in-service cable-stayed bridges subjected to cable degradation

Abstract

The cables in a cable-stayed bridge are critical components supporting the long-span girders and ensuring their functionality. However, cables are prone to fatigue damage and atmospheric corrosion, which directly affect the bridge safety. This study presents a framework for system reliability evaluation of in-service cable-stayed bridges subjected to cable degradation. The effect of cable strength degradation on the system reliability is demonstrated through simulation on a parallel-series system representation. Learning machines are utilised to approximate the non-linear and dynamic response functions of critical components due to cable rupture, and the system reliability is finally evaluated from the event tree established by the β-unzipping method. Both short-span and long-span cable-stayed bridges are selected as prototypes to investigate the influence of cable degradation on the system reliability. On this basis it is revealed that cable degradation can significantly influence the collapse mechanism of a cable-stayed bridge and thereby lead to a significant reduction in the system reliability. This phenomenon is associated with cable spacing, where a spare cable system seems more sensitive to cable gradation. It is demonstrated that the consideration of cable corrosion and correlation is essential for lifetime safety evaluation of in-service cable-stayed bridges.     

旋挖成孔灌注桩施工质量控制技术探讨

根据重庆地区旋挖桩施工现状,分析了松散高填方建筑场地旋挖桩塌孔及孔底沉渣过厚等技术问题,并在工程调研和实践的基础上,提出了解决问题的措施,供同行参考。     

On repairable earthquake resisting archetypes with a view to resiliency objectives and assisted self‐centering

Performance‐based seismic design, as an alternative to conventional methods of approach, has served engineers and the public rather well during the last two decades. Neither approach guaranties catastrophic collapse prevention nor post‐earthquake realignment and repairs (PERR) due to major seismic events. As a result, most code‐compliant buildings can be regarded as relatively safe but practically disposable. The paper presents a new philosophy that leads to sustainable design of new structures and the upgrading of existing earthquake resisting moment frames. Repairability‐based design (RBD) relies on softening and control rather than strength and resistance to elevate seismic performance to economically viable, physical collapse prevention, damage control, and post‐earthquake realignment and repairs. The new approach was inspired by design led analysis (DLA), performance control (PC), and recent developments in rocking core‐moment frame design. DLA is a displacement based method of analysis with built‐in results. PC is the ability to design a structure in such a way as to expect predetermined modes of response at certain stages of loading, extents of damage, and drift ratios. This paper advocates higher performance objectives than current codes of practice do. Several demonstrative examples have been provided.     

Horizontal bracing to enhance progressive collapse resistance of steel moment frames

In this paper, the effect of horizontal bracing on enhancing the resistance of steel moment frames against progressive collapse is investigated. Previously designed 6 bay by 3 bay 18‐story steel frame prototype building with 6 m bay span (namely, unbraced frame), which was susceptible to progressive collapse, is retrofitted by four types of horizontal bracing systems on the perimeter of the topmost story and analyzed using 3D nonlinear dynamic method. Six different cross‐sections for each bracing system type are considered, and the capacity curves for each model are obtained. Three column removal circumstances, namely, Edge Short Column, First Edge Long Column, and Edge Long Column are considered in this paper. The results imply that horizontal bracing would increase the resistance of moment frames against progressive collapse. However, one of the bracing types in which axial compressive force is created in braces is not appropriate for retrofitting.     

Collapse safety margin-oriented seismic retrofit strategy for corroded reinforced concrete frames using fibre reinforced plastics

Considering residual service life, this paper presents a collapse safety margin-oriented seismic retrofit strategy for corroded reinforced concrete (RC) frames using fibre reinforced plastics (FRP). With the assumed uniform corrosion model, corrosion-induced initial damage combined with subsequent earthquake-induced damage is identified by the multi-mode-based global damage model developed previously. The collapse-level earthquake intensity determined by incremental dynamic analyses (IDA) with the damage model and the maximum considered earthquake (MCE)-level intensity considering residual service life are combined to generate the time-variant collapse safety margin assessment of corroded RC structures. Based on this assessment, the collapse safety margin-oriented FRP seismic retrofit strategy is proposed and demonstrated on a 4-storey frame. The damage model originally developed for earthquake scenarios has also exhibited its rationality for characterising corrosion-induced initial damage and its influence on coupled damage development with subsequent earthquake excitation. Seismic retrofitting with FRP composites should consider the effects of the corrosion development stage, target collapse safety margin and residual service life. FRP retrofits can cause decreases in the MCE-level collapse probability and increases in the collapse resistance of corroded RC structures. Retrofitting carried out at different times achieves different efficiencies and different time-variant collapse safety margins within the residual service life.     

逐级加载下浅埋软岩隧道动力破坏振动台试验

浅埋隧道围岩的质量普遍较低,整体稳定性差,隧道震害表明强震作用下浅埋隧道极易发生震动破坏。通过开展V级围岩条件下浅埋隧道在小震下的震动响应和逐级加载下的震动垮塌振动台试验,研究了小震作用下围岩加速度沿地层的分布、衬砌结构的内力变化和围岩内部的水平位移变化规律,强震作用下衬砌结构裂缝开展和围岩震动垮塌。结果表明:围岩加速度随距地表距离的减小而增加,地表加速度约为拱顶处加速度的1.63倍,相同高度平面内靠近隧道的围岩振动具有一定的加强;隧道拱顶围岩内部的水平位移大约是拱腰围岩内部的1.23倍,围岩内部位移随着远离隧道而逐渐减小,随着震动烈度的增加而不断增加;隧道拱顶上方垮塌区形状近似漏斗,震动引起隧道衬砌结构拱脚处的轴力和弯矩变化最大,且拱肩和拱脚处裂缝分布最多,应加强拱肩和拱脚处结构的抗震性能。     

卸荷条件下黄土湿陷系数的计算方法及验证

自重湿陷性黄土场地桩间土在浸水作用下存在卸荷与湿陷的伴生过程,考虑卸荷作用的黄土湿陷变形还难以计算。首先探讨了卸荷湿陷与传统湿陷的差异性,分析了湿陷完成比、卸荷应力比等因素对黄土卸荷湿陷变形的影响,然后考虑各影响因素进行了卸荷湿陷试验,采用湿陷完成比、卸荷湿陷比和卸荷应力比来描述原状黄土的卸荷湿陷过程。由于逐级多次卸荷试验更符合工程实践中的连续卸荷工况,重点进行了多次卸荷试验。基于单次卸荷与多次卸荷试验结果,建立了考虑卸荷作用的湿陷系数计算方法。进一步采用卸荷湿陷系数,得到了考虑卸荷作用的自重湿陷性黄土场地桩间土自重湿陷量的计算方法,并通过室内验证试验与工程实例计算,论证了卸荷条件下黄土湿陷系数计算方法的合理性。     

方钢管混凝土柱-组合梁框架抗连续倒塌性能试验研究

为了研究组合梁对方钢管混凝土柱-组合梁框架抗连续倒塌性能的影响,设计了带有混凝土现浇楼板方钢管混凝土柱-组合梁框架(CFSTCBF)及不带楼板的方钢管混凝土柱-钢梁框架(CFSTSBF),采用拆除构件法对两个试件的抗连续倒塌性能进行竖向加载试验研究,对其在中柱失效工况下的破坏模式、受力机理以及主要的抗力机制进行分析,同时对现浇混凝土楼板和节点在连续倒塌过程中的性能进行探讨。研究表明：两个试件的破坏均是由正弯矩区钢梁破坏引起,结构的倒塌阶段可分为弹塑性阶段、塑性阶段、过渡阶段和悬链线阶段,压拱机制和悬链线机制均能改善结构的抗倒塌性能;混凝土楼板可以有效增强结构的承载能力,提高结构的初始转动刚度,但在改善结构变形能力方面的作用较弱。     

盲谷水库防渗处理——云南五里冲水库工程简介

云南五里冲水库是我国在岩溶地区利用盲谷采用以灌浆为主,结合堵、截进行防渗处理,首次建成的水深超百米的无坝中型水库.帷幕灌浆工程量21.6×104m,单层帷幕深160m,地下溶洞内建造了高100.4m,长50m,厚2.0～2.5m的钢筋混凝土防渗墙;在岩溶塌陷体内用高压灌浆建造帷幕.     

广东大兴煤矿特大突水事故机理分析

通过实地调查、测量以及地质及地球物理勘探，分析认为大兴煤矿“8·7”突水事故的突水机理为：防水煤柱受断层破碎带的影响，在上部水压、采动矿压、煤层自重、地下水的联合动力作用下，破碎的岩体沿垂向固有构造裂隙面向上塌落形成椭圆形柱体，待形成自然平衡拱后塌陷垮落停止，上覆岩层得到暂时稳定；随着采煤工作面的不断扩大，自然平衡拱被再一次打破，椭圆形柱体内的垮落继续向上发展，直至最后的关键层破坏或破碎达到临界突水系数而发生突水.