Effect of pre-tensioned rock bolts on stress redistribution around a roadway—insight from numerical modeling

The importance of the pre-tensioned force of rock bolts has been recognized by more and more researchers. To investi- gate the effect of pre-tensioned rock bolts on stress redistribution around roadways, a numerical analysis was carried out using FLAC3D and a special post-process methodology, using surfer, is proposed to process the numerical simulation results. The results indicate that pre-tensioned rock bolts have a significant effect on stress redistribution around a roadway. In the roof, pre-tensioned rock bolts greatly increase vertical stress; as a result, the strength of the rock mass increased significantly which results in a greater capacity of bearing a large horizontal stress. The horizontal stress decreases in the upper section of the roof, indicating that pre-tensioned rock bolts significantly reduce the coefficient and the size of the region concentration of horizontal stress. At the lat- eral side, pre-tensioned rock bolts greatly increase the horizontal stress; therefore, the rock mass strength significantly increases which results also in a greater capacity of bearing a large vertical stress. The greater the size of pre-tensioned force, the larger the region of stress redistribution around a roadway is affected and the higher the size of the stress on the roadway surface the more the rock mass strength increases.

Controlling deflection of long steel I-shaped girder bridge using two V-shaped pre-tensioning cables

Despite appropriate design of girder under bending and shear,the deflection of long steel girders usually exceeds the allowable range,and therefore the structural designers encounter challenges in this regard.Considering significant features of the cables,namely,low weight,small cross section,and high tensile strength,they are used in this research so as to control the deflection of long girder bridges,rather than increasing their heights.In this study,theoretical relations are developed to calculate the increase in pre-tensioning force of V-shaped steel cables under external loading as well as the deflection of steel girder bridges with V-shaped cables and different support conditions.To verify the theoretical relations,the steel girder bridge is modeled in the finite element ABAQUS software with different support conditions without cable and with V-shaped cables.The obtained results show that the theoretical relations can appropriately predict the deflection of girder bridge with V-shaped cables and different support conditions.In this study,the effects of the distance from support on the deflection of mid span are studied in both simply supported and fixed supported girder bridge so as to obtain the appropriate distance from support causing the minimum deflection.

Effect of Filament Pre-tension on Sirofil Yarn Properties

The mechanism of filament pre-tension is put forward. The effect of filament pre-tension on Sirofil composite yam properties, such as hairiness and structural stability, are theoretically and experimentally studied. The theoretical analysis agrees well with the experimental results. The present theory shows that suitable choice of filament pre-tension can enormously reduce hairiness and improve structural stability of Sirofil composite yarn.

Safety Limits for Pre-Tension of Membrane Roofs on Closed Structure in Air Flow

The stiffness required for the normal operation of membrane roof comes from the application of pre-tension. When the pre-tension is too small, it is easy to cause instability under the action of wind load, which leads to excessive deformation of the roof and local or overall damage. In order to ensure that the membrane roof is always in normal use state in the airflow field, this paper takes the membrane pretension as the control parameter to study the value of safety pretension of closed membrane roof. According to the theory of large deflection of membrane and Galerkin method, the nonlinear vibration differential equation of membrane roof under static wind is established, and the critical state of safe working of membrane roof is determined by judging the stability of the solution of the equation, and the expression of critical wind speed is obtained. By establishing the inequality relationship between local design wind speed and critical wind speed, the safety pretension limit of membrane roof under specific site can be obtained. The research shows that the safety pretension limits of closed membrane roofs are different in different areas under different design return periods. In addition, the value of safety pretension is related to the film geometry.

Suppressing abnormal grain growth and switching precipitation behaviors in ZK60 Mg profile by inducing pre-tension

The pre-tension was induced to suppress the abnormal grain growth(AGG)of ZK60 Mg profile during solution treatment.The effects of pre-tension on the microstructure evolution and mechanical properties were studied,and the suppressing mechanism was discussed.If the pre-tension strain was larger than 10%,AGG during solution could be effectively inhibited,resulting in a sharp decrease in the grain size.The suppression effects were realized by restricting the orientation dependent of AGG and promoting static recrystallization.The pre-tension reset the distributions of stored energy and sizes of the grains with〈11–20〉and〈10–10〉orientations,and thus retarded the orientation dependent of AGG.Moreover,the pre-tension introduced a mass of dislocations,twins,and stacking faults,all of which promoted the occurrence of static recrystallization,and the grain structure was further refined.The pre-tension accelerated the precipitation kinetics during aging,resulting in fine and dense precipitates.With the increase of pre-tension strain,the strength of ZK60 Mg profile monotonically increased.

Tailoring Texture to Highly Strengthen AZ31 Alloy Plate in the Thickness Direction via Pre-tension and Rolling-Annealing

Conventional wrought Mg alloys,such as AZ31 and ZK60 rolled plates,usually exhibit significantly low tensile yield strength in the thickness direction.This can be attributed to the high activity of{10-12}tension twinning due to the strong basal texture(<0001>//ND,normal direction).In this work,the tensile yield strength in the ND of the as-rolled(AR)AZ31 plate increased from 50 to 150 MPa(increased by 200%)via simple processing,i.e.,pre-tension and rolling-annealing(PTRA)treatment.The strong basal texture(<0001>//ND)of the AR plate was changed into a weakened fiber texture(<0001>⊥ND).The evolution of microstructures during PTRA treatment and the activated deformation modes during uniaxial tension were studied quantitatively and statistically by the means of intergranular misorientation(IM)and in-grain misorientation axes(IGMA)analysis.The results indicate that various twin variants,as well as{10-12}-{10-12}secondary twins,were activated during pre-tension and rolling,and most residual matrix was consumed by twins after annealing.The dominated deformation modes in tension changed from{10-12}tension twinning(the AR sample)to prismatic slip(the PTRA sample)in the early tensile deformation.The underlying formation mechanism of the fiber texture and corresponding strengthening mechanism were discussed.

Influence of pre-tension on ballistic impact performance of multi-layer Kevlar 49 woven fabrics for gas turbine engine containment systems

In this paper, ballistic impact tests on wrapped multi-layer Kevlar 49 woven fabric systems were carried out with a flat blade projectile to investigate the impact response during a fan blade out event. The influences of the number of Kevlar layers and pre-tension were discussed particularly. Test results were used to analyze failure modes and energy absorption characteristics of multi-ply Kevlar fabrics. Results show that there are two kinds of impact damage for fabrics： global deformation mainly involving stretching of yarns in the impact region and fabric wrinkle from both sides to the impact zone, and local damage characterized by yarn fracture, yarn pull-out, and yarn unraveling. The energy absorption capability of Kevlar 49 woven fabrics improves with the number of fabric layers. The energy absorbed by multi-layer fabrics increases slightly at the beginning and then decreases substantially with pre-tension. The work in this paper can provide guidance for designing light-weight multi-layer fabrics containment systems.

Free vibration of pre-tensioned nanobeams based on nonlocal stress theory

The transverse free vibration of nanobeams subjected to an initial axial tension based on nonlocal stress theory is presented. It considers the effects of nonlocal stress field on the natural frequencies and vibration modes. The effects of a small scale parameter at molecular level unavailable in classical macro-beams are investigated for three different types of boundary conditions:simple supports,clamped supports and elastically-constrained supports. Analytical solutions for transverse deforma-tion and vibration modes are derived. Through numerical examples,effects of the dimensionless nanoscale parameter and pre-tension on natural frequencies are presented and discussed.

Nonlinear analysis of pre-tensioned glass wall facade by stability function with initial imperfection

Pre-tensioned high strength trusses using alloy steel bar are widely used as glass wall supporting systems because of the high degree of transparency.The breakage of glass panes in this type of system occurs occasionally,likely to be due to error in design and analysis in addition to other factors like glass impurity and stress concentration around opening in a spider system.Most design does not consider the flexibility of supports from finite stiffness of supporting steel or reinforced concrete beams.The resistance of lateral wind pressure of the system makes use of high tension force coupled with the large deflection effect,both of which are affected by many parameters not generally considered in conventional structures.In the design,one must therefore give a careful consideration on various effects,such as support settlement due to live loads and material creep,temperature change,pre-tension force,and wind pressure.It is not uncommon to see many similar glass wall systems fail in the wind load test chambers under a design wind speed.This paper presents a rigorous analysis and design of this type of structural systems used in a project in Hong Kong,China.The stability function with initial curvature is used in place of the cubic function,which is only accurate for linear analysis.The considerations and analysis techniques are believed to be of value to engineers involved in the design of the structural systems behaving nonlinearly.

圆形预应力钢丝缠绕正交预紧机架的缠绕分析计算

在预应力钢丝缠绕正交预紧机架结构的基础上提出了一种全新的机架型式——圆形预应力钢丝缠绕正交预紧机架。对该型机架的缠绕进行理论分析和计算,进而求得缠绕施工张力表,为钢丝缠绕施工提供重要理论依据。该型机架的研发是超重型承载机架结构的重要技术突破。

往复式压缩机中间接筒与曲轴箱的连接螺栓断裂失效分析

针对某石化公司重整单元新氢往复压缩机2023年所发生的中间接筒与曲轴箱连接螺栓断裂问题,根据其外观形貌、螺栓材质、螺栓受力情况分析其可能的断裂原因,并从工艺操作、设备安装要求、螺栓紧固方式方面,提出解决方案,保障装置的安稳长满优运行。

万吨级张力拉伸机螺栓组预紧顺序工艺分析

为改善万吨级张力拉伸机力学性能,提高整机的可靠性,利用Ansys分析了7种不同的预紧顺序对拉伸机螺栓组连接性能的影响,并通过螺栓剩余预紧力、横梁的变形和螺栓结合面平均压应力3个指标对不同预紧顺序进行评价。结果表明对于螺栓剩余预紧力,最佳序列为序列3,其次为序列5;对于横梁的变形,最佳序列为序列1,其余序列的标准差和极差基本一致;对于螺栓结合面平均压应力,最佳序列为序列3,其次为序列1。对三个指标进行加权平均分析,得出预紧序列3的加权平均分为9.958分,为最佳序列,并且两边向中间预紧明显优于中间向两边预紧的次序。此结论对拉伸机的装配具有重要的工程价值。

桥梁施工中钢抱箍滑移原因与防滑措施研究

为研究桥梁施工中钢抱箍滑移原因并提出针对性的应对措施,对钢抱箍受力进行理论分析。针对钢-混界面进水及螺栓施拧不足造成的滑移,进行钢-混界面摩擦系数和螺栓拧紧力矩系数测试及定量分析,根据测试结果并结合现场经验提出钢抱箍防滑措施。结果表明:实测干燥状态下钢-混界面的静摩擦系数为0.62,大于常用设计值(0.3~0.4),钢抱箍偏安全;界面润湿后,静摩擦系数有所降低,但仍高于常用设计值,说明钢-混界面进水会降低钢抱箍承载能力,但降低程度还不足以导致钢抱箍滑移。螺栓拧紧力矩系数取值不当是钢抱箍滑移的主要原因,拧紧力矩系数常用设计值仅为测试值的1/1.7~1/3.6,偏不安全;拧紧力矩系数取值跟螺栓轴线与承压板法线间的夹角有关,建议现场对其进行实测,以保证螺栓预紧力与设计相符。钢抱箍设计时,摩擦系数可偏安全地取0.45,螺栓拧紧力矩系数可在参考范围内取大值,并结合1.5~2.0的安全系数确定螺栓数量,以避免钢抱箍滑移。

新型密柱支撑张弦铝合金结构设计与施工关键技术研究

世博温室项目屋盖为任意不规则曲线组成的无曲率平面,采用异形多边形网格加大透光率,基于这些特点,选型最终定为张弦铝合金网格结构。本项目挤压出新型“日”字型挤压铝合金构件以更好地承受屋盖面内弯矩,保证杆件与整体结构平面内有效传力。设计了新型刚接板式节点并通过足尺试验和精细化有限元分析研究了节点刚度与极限承载力,试验及有限元对比分析结果表明,该类节点有较高的抗剪、平面内和平面外承载力。阐述了弯扭构件数字化加工工艺及不规则铝合金特殊单元式的装配式预拼装技术,引入立面幕墙与主体结构一体化设计技术,其中立面幕墙面板设计首次通过整体协同分析考虑了主体结构变形的影响,计算结果表明设计构造满足要求。重点说明了双向正交布置拉索的施工张拉模拟计算方法,验证了索力分级张拉时结构处于安全状态,张拉完毕结构变形满足要求。

基于等效短路试验的变压器用层压垫块累积效应

层压垫块是油浸式变压器线圈轴向支撑结构的关键材料之一,多次短路条件下变压器层压垫块的累积效应规律尚不清晰。该文提出适用层压垫块的多次短路等效试验方法,可模拟暂态分量为50 Hz的动态短路力,突破材料尺度上静态试验的局限;试验获得层压垫块在不同短路力大小、老化程度、短路类型和短路次数等条件下塑性形变的变化规律。结果表明:塑性形变与短路力大小有正相关关系并随着短路次数增加呈饱和特性;塑性形变与间隔类型的相互关系并不明显;根据老化15年层压垫块塑性形变规律,将累积效应划分为形变上升、稳定和快速累积3个阶段,老化和力联合作用下层压垫块纤维状态是发生不同累积特点的直接原因;层压垫块弹性模量与短路力呈非线性相互关系。进一步提出变压器轴向预紧力损失计算方法,实例计算表明,多次短路后各线圈轴向预紧力发生下降或再分配现象。研究结论为深入掌握多次短路累积效应对变压器的影响提供一定的技术支撑。

强构造应力巷道高预紧力锚杆锚索联合支护效果研究

本文以新疆潞新二矿强构造应力巷道为背景,针对E4205下顺槽巷道围岩稳定性问题,对高预紧力锚杆锚索联合支护技术的应用效果进行了深入研究。首先,通过水压致裂地应力测试,测得最大水平主应力σ^(H)为13.33 MPa,最小水平主应力σ^(h)为7.05 MPa,垂直主应力σv为7.62 MPa,最大水平主应力方向北偏西47.0°;其次,采用FLAC^(3D)数值模拟,研究了两种预紧力等级下锚杆对巷道围岩应力分布的影响规律,揭示了高预紧力锚杆支护产生的应力值更大,形成的有效压应力区范围更广;最后,针对强构造应力地层巷道,提出了“高预紧力锚杆锚索联合支护”方式,通过监测,对比分析了采用高预紧力锚杆锚索联合支护技术应用前后巷道围岩的变形,发现高预紧力锚杆锚索联合支护方式能有效控制该类巷道的变形,保证工作面的安全生产。

基于MATLAB的悬式机械臂提升动力学特性分析

提出一种悬式机械臂在预张紧和提升阶段的动力学模型。该模型考虑柔性梁、电机以及圆柱形有效载荷之间的动态耦合。采用Rayleigh-Ritz法,将能量代入拉格朗日方程,从而得到系统微分方程,使用MATLAB内置的四阶Runge-Kutta法对微分方程数值求解,并得到部分重要的参数曲线。数值解和曲线与其实际工作情况显示良好一致。

大跨暗挖地铁车站喷锚式主动支护体系应用效果分析

文中以青岛地铁6号线一期工程石山路站暗挖主体为例,通过调研发现,岩质地层中锚杆作用被严重弱化,导致初支拱架设置过密、喷层过厚、锚杆无法与围岩有效承载等问题突出。文中针对该工程特点,在Ⅲ级围岩段,采用以预应力锚杆+网喷混凝土为主的锚喷式主动支护结构形式,根据数值分析,拱部最大沉降为4.7 mm,边墙最大水平位移为0.2 mm,拱部锚杆轴力约115~126 kN,边墙锚杆轴力约104~109 kN。经现场监测数据反馈,拱顶最大沉降3.2 mm,边墙收敛最大变形约2.4 mm,拱部及边墙锚杆轴力稳定值约100 kN。在硬岩地层中,对预应力锚杆施加预加力,及时恢复围岩三向受力状态,能有效发挥围岩自承能力,控制开挖轮廓变形,保证隧道施工安全。

先张法预应力FRP筋张拉锚具试验研究

通过对粘结型锚具的分析,总结出粘结型锚具更适用于FRP筋的锚固。鉴于此,提出了一种新型先张法夹具,开展了预应力BFRP筋夹具锚固性能研究,发现高强灌浆料-半灌浆套筒粘接式锚具能够对预应力FRP筋实现较为快速、稳定的锚固。

CFRP布对先张法预应力Bulb-T梁抗弯性能影响研究

为研究碳纤维增强复合材料(CFRP)布对先张法预应力Bulb-T梁抗弯性能的影响,采用有限元软件ABAQUS建立了三维实体模型,并进行了四点弯曲试验数值模拟,分析了在不同CFRP布加固方式下Bulb-T梁损伤过程及受力性能。结果表明:先张法预应力Bulb-T梁模拟值与试验结果误差较小,建立的模型可以较准确地模拟足尺预应力钢混梁的破坏形态、预拱度、荷载-位移曲线;CFRP加固能够显著提升梁体的抗弯刚度及开裂荷载,延缓开裂时间、抑制裂缝的扩展,其中,下翼缘板底部粘贴CFRP布可在失效前对跨中裂缝密集区起到有效控制,弯剪区包裹加固减小了梁体加载点及垫块之间“八”字形裂缝的展开;下翼缘板底部粘贴CFRP布对抗弯承载力的提升效果更加显著,且极限承载力和刚度随着加固层数的增加而增大。