Bending Failure Mode and Prediction Method of the Compressive Strain Capacity of A Submarine Pipeline with Dent Defects

A dent is a common type of defects for submarine pipeline.For submarine pipelines,high hydrostatic pressure and internal pressure are the main loads.Once pipelines bend due to complex subsea conditions,the compression strain capacity may be exceeded.Research into the local buckling failure and accurate prediction of the compressive strain capacity are important.A finite element model of a pipeline with a dent is established.Local buckling failure under a bending moment is investigated,and the compressive strain capacity is calculated.The effects of different parameters on pipeline local buckling are analyzed.The results show that the dent depth,external pressure and internal pressure lead to different local buckling failure modes of the pipeline.A higher internal pressure indicates a larger compressive strain capacity,and the opposite is true for external pressure.When the ratio of external pressure to collapse pressure of intact pipeline is greater than 0.1,the deeper the dent,the greater the compressive strain capacity of the pipeline.And as the ratio is less than 0.1,the opposite is true.On the basis of these results,a regression equation for predicting the compressive strain capacity of a dented submarine pipeline is proposed,which can be referred to during the integrity assessment of a submarine pipeline.

Reply to Discussion on “Empirical methods for determining shaft bearing capacity of semi-deep foundations socketed in rocks”

Semi-deep foundations socketed in rocks are considered to be a viable option for the foundations in the presence of heavy loads imposed by high-rise buildings and special structures, due to the low settlement and high bearing capacity. In this study, the unconfined compressive strength（UCS） and rock mass cuttability index（RMCI） have been applied to investigating the shaft bearing capacity. For this purpose, a comprehensive database of 178 full-scale load tests is compiled by adding a data set（n = 72）collected by Arioglu et al.（2007） to the data set（n = 106） presented in Rezazadeh and Eslami（2017）.Using the database, the applicability and accuracy of the existing empirical methods are evaluated and new relations are derived between the shaft bearing capacity and UCS/RMCI. Moreover, a general equation in case of unknown rock types is proposed and it is verified by another set of data（series 3 in Rezazadeh and Eslami（2017））. Since rock-socketed shafts are supported by rock mass（not intact rock）,a reduction factor for the compressive strength is suggested and verified in which the effect of discontinuities is considered using the modified UCS, based upon RMR and RQD to consider the effect of the rock mass properties.

Empirical methods for determining shaft bearing capacity of semi-deep foundations socketed in rocks

Semi-deep foundations socketed in rocks are considered to be a viable option for the foundations in the presence of heavy load imposed by high-rise structures, due to the low settlement and high bearing capacity. In the optimum design of semi-deep foundations, prediction of the shaft bearing capacity, rs, of foundations socketed in rocks is thus critically important. In this study, the unconfined compressive strength(UCS), qu, has been applied in order to investigate the shaft bearing capacity. For this, a database of 106 full-scale load tests is compiled with UCS values of surrounding rocks, in which 34 tests with rock quality designation(RQD), and 5 tests with rock mass rating(RMR). The bearing rocks for semi-deep foundations include limestone, mudstone, siltstone, shale, granite, tuff, granodiorite, claystone, sandstone, phyllite, schist, and greywacke. Using the database, the applicability and accuracy of the existing empirical methods are evaluated and new relations are derived between the shaft bearing capacity and UCS based on the types of rocks. Moreover, a general equation in case of unknown rock types is proposed and it is verified by another set of data. Since rock-socketed shafts are supported by rock mass(not intact rock), a reduction factor for the compressive strength is suggested and verified in which the effect of discontinuities is considered using the modified UCS, qu(modified), based upon RMR and RQD in order to take into account the effect of the rock mass properties.

Upper Bound Solution of the Resisting Moment Bearing Capacity of A Composite Bucket Shallow Foundation of An Offshore Wind Turbine in Sand

Bearing the large moment that is generated by the wind load that acts on the upper structure of offshore wind turbines is an important feature of their foundations that is different from other offshore structures.A composite bucket shallow foundation(CBSF)has been proposed by Tianjin University to address the soft geological conditions in the offshore regions of China for wind turbines.The CBSF is a new type of foundation and is effective against large moments.The soil deformation test of a CBSF and the numerical simulation study under the same working conditions are carried out to determine the failure mechanism of a CBSF under moment loading.The resisting soil compression rateηm is defined as a new empirical parameter that indicates the ability of the soil inside the bucket to resist moment loading.The upper limit of the resisting moment bearing capacity of the bucket foundation is derived through the upper bound theorem of classical plasticity theory based on the failure mechanism.The calculation method is validated by tests of bucket models with different height-diameter ratios in sand under moment loading.

Experimental Investigation on Low-Velocity Impact Response and Residual Compressive Bearing Capacity of Composite Stringers

Three types of composite stringers were impacted from two different directions.Relationships between impact energy and visible defect length were found.The critical impact energy corresponding to barely visible impact damage(BVID)of each stringer was determined.Specimens with BVID were then compressed to obtain the residual strength.Experimental results showed that for all types of stringers,the critical energy of in-plane impact is always much lower than out-plane ones.In-plane impact causes much more decrement of stringers'bearing capacity than outplane impact.For both impact directions,I-stringers own the highest defect detectability,T-stringers come second.Meanwhile,I-stringers own the better residual strength ratio than I-stringers and I-stringers.Synthetic considering impact defect detectability and residual bearing capacity after impact,T-stringers own the best compression-afterimpact(CAI)behaviors.

Bearing Capacity Analysis of Shallow Foundations Supported on an Artificial Fill(Substation at Kashari,Tirana)

The paper presents conclusions related to the design of shallow foundations,at the site where the Substation at Kashari,in Tirana is foreseen to be constructed.The project includes buildings with small to medium structures with 1-3 stories.The geological survey works include 6 boreholes of a depth 20.0 m,90 SPT(Standard Penetration Tests)tests,and laboratory works include 13 direct shear tests,27 unconfined and compressive strength tests,7 CU(consolidated undrained)tests and 9 UU(unconsolidated undrained)tests,etc.The foundation design must fulfill both,bearing and settlement criteria,but in this case the settlements of foundations are not possible to be calculated,since the laboratory works do not include any consolidation test.So,the local bearing capacity is expected to control the design in terms of bearing capacity and settlements.The local bearing capacity of shallow square foundations is evaluated by using Terzaghi’s formula,based on reduced shear strength parameters of soils below the bottom of foundations.The results are compared with the bearing capacity values calculated by using Burland and Burbdige(1984)method,based on the data of SPT tests.This method is used for checking the settlement(serviceability)criterion in the foundation design,when the direct settlement calculation is missing.The paper presents some conclusions related to local bearing capacity foundation-based design.

局部锈蚀圆钢管混凝土短柱轴压承载力试验研究

钢管混凝土(CFST)在服役环境中被腐蚀,导致其钢结构承载力降低,严重威胁到结构的服役性能和使用寿命。首先,采用机加工车铣方法制作模拟局部锈蚀的人工缺陷,然后以钢管外表面局部环向贯通锈蚀位置、锈蚀钢管体积损失率、锈蚀外表面面积损失率(简称锈蚀面积损失率,下同)为试验参数,对45根局部锈蚀圆CFST短柱试件进行轴压承载力试验;其次,分析锈蚀位置、锈蚀钢管体积损失率、面积损失率和壁厚损失率对锈蚀试件承载力、刚度和延性的影响,揭示锈蚀CFST试件破坏机理和承载力退化机制;最后,针对局部锈蚀圆CFST短柱构件轴压承载力提出一个简化实用计算公式。研究结果表明:各试件具有类似的破坏特征,主要呈明显的腰鼓状破坏,且发生在锈蚀区;随着锈蚀钢管体积损失率增大,锈蚀CFST柱的承载力、刚度和延性均出现不同程度的降低;在锈蚀钢管体积损失率和面积损失率相同的情况下,就局部锈蚀位置影响而言,中部影响最大;就锈蚀程度表征参数影响而言,锈蚀钢管体积损失率影响最大,面积损失率次之,壁厚损失率最小;本文提出的简化实用公式可为圆钢管混凝土构件全寿命设计提供参考依据。

RPC外包方钢管混凝土柱轴压受力性能研究

为了研究活性粉末混凝土(Reactive powder concrete,RPC)外包方钢管混凝土柱的轴压受力性能,以RPC厚度和长细比为参数,对6根构件开展轴压试验研究,探讨该类组合柱的破坏模式、受力机理、套箍效应和承载力。试验结果表明:外层RPC可为内部方钢管混凝土柱提供较强的约束力,在受力后期套箍效应凸显。随着RPC厚度的增加,组合柱的承载力逐渐提高;随着组合柱长细比的增大,虽然其弹性刚度和承载力逐渐降低,但延性改善。采用ABAQUS软件建立RPC外包方钢管混凝土柱的有限元模型,分析RPC强度、RPC厚度、钢管强度等级、含钢率、内部混凝土强度和长细比等参数对组合柱承载力的影响规律,可得方钢管混凝土柱发挥系数的变化幅值为0.78~1.01。在试验研究、有限元参数分析和理论探讨的基础上,借鉴钢管混凝土结构技术规范,提出了RPC外包方钢管混凝土轴压短柱和长柱的承载力简化计算方法,公式计算值与已有数据样本吻合良好。

十字形钢管束自密实混凝土短肢剪力墙轴压性能研究

为研究十字形钢管束自密实混凝土短肢剪力墙的轴压受力性能,通过模型试验研究和ABAQUS数值模拟相结合的方法,对不同腔体数十字形钢管束自密实混凝土短肢剪力墙的轴压性能进行分析。首先,根据轴压试验结果对剪力墙的破坏形态和轴向荷载-应变曲线等轴压受力性能进行分析,提出轴压承载力计算公式。其次,建立剪力墙轴压有限元计算模型,将试验结果和模型计算结果进行对比以验证有限元模型的准确性。最后,根据有限元参数化分析结果验证并修正剪力墙轴压承载力计算公式。研究发现,剪力墙的轴向荷载-应变曲线在不同腔体数下具有相似的特征,表明其变形能力良好。在破坏过程中,钢管束对核心混凝土的约束作用显著,提高了试件的延性和承载能力,其中钢管束的破坏主要表现为多波鼓曲和焊缝开裂,尤其是在钢管束中部的1/4区域。此外,随着腔体数增加,剪力墙极限承载力提升,但其延性系数有所下降。本研究提出的简化公式与有限元分析结果的误差控制在10%以内,该公式可为工程设计应用提供理论借鉴。

动力打桩桩周泥岩损伤特性与损伤模型

针对泥岩地基同施工场地条件下动力打入桩的承载力差异性大且部分打入桩承载力不足等异常现象,文章开展了桩周0.2m处打桩前后的桩周泥岩单轴抗压强度对比试验,和打桩后的桩端中风化泥岩三轴压缩试验,明确打桩后桩周泥岩的损伤特性,基于统计损伤理论建立考虑受打桩影响的桩周泥岩损伤本构模型,并对泥岩地基打入桩的承载性能进行探讨。研究表明:打桩后桩周0.2m处泥岩强度平均损伤28.2%,弹性模量平均损伤41.4%;桩端中风化泥岩损伤后抗剪强度参数平均值为c=217.2kPa、φ=21.6°,φ仅是同场地原状强风化和全风化泥岩的49.1%和51.4%;打桩后桩周泥岩应力-应变曲线的损伤模型计算值的与实测值吻合较好,验证模型的合理性;泥岩地基打入桩承载力异常特征为桩端受力小,但桩端沉降显著,静载破坏时桩端阻力的平均占比为5.25%;动力打桩对桩周泥岩的损伤是泥岩地基打入桩承载力异常的重要原因;泥岩地基打入桩的设计计算与数值模拟采用原状泥岩力学参数不合理,泥岩损伤后不易恢复,建议按损伤特性赋予参数。研究结果对泥岩地基打入桩的设计、施工、承载力评价有较强的参考价值。

钢筋锈蚀粘结削弱后轴心受压柱承载能力试验研究

通过去除钢筋周围粘结混凝土的方法来模拟钢筋锈蚀后粘结性能下降对柱抗压承载力的影响情况。研究发现:钢筋锈蚀程度对粘结力削弱的轴心受压柱的承载力影响较明显,在钢筋锈蚀不严重时,钢筋与混凝土的应变基本一致,柱破坏时两种材料强度能发挥出来;钢筋严重锈蚀时,混凝土应变滞后,柱破坏时混凝土的强度不能充分发挥出来,且钢筋锈蚀根数越多柱承载力下降越快;同时,随着粘结性能下降,柱破坏时裂缝的条数、长度及宽度都有所减少。根据试验研究结果,推导出粘结削弱轴心受压柱混凝土柱承载力计算方法。

间断焊接包管加固圆钢管构件轴压极限承载力研究

为研究间断焊接包管加固杆件的轴心受压性能,设计了13组试件进行单调加载静力试验。对试件极限承载力、破坏模式、荷载-位移曲线和截面应变发展规律进行分析,对比了负载加固与非负载加固对试件极限承载力的影响。结果表明:包管构件的破坏模式可分为整体失稳破坏和内管外伸段强度破坏;包管构件内外管协同工作、共同受力,内管和外管都在屈服后发生整体失稳破坏;负载加固与非负载加固试件的极限承载力基本一致,相差2.2%,非负载加固试件与未加固试件相比,极限承载力最大提高了479.9%。根据试验结果对包管构件的ABAQUS有限元模型进行修正,并用该模型分析了各主要影响因素对包管构件极限承载力的影响规律。提出了包管构件极限承载力经验计算公式,且公式计算结果与试验结果吻合较好。

圆中空夹层不锈钢管水泥砂浆短柱轴压性能研究

海洋工程平台塔架结构中,圆形截面中空双钢管是主要受力构件,工程界十分关注其受压性能和耐腐蚀性能。为研究圆中空夹层不锈钢管水泥砂浆短柱的轴压力学性能,以外不锈钢管壁厚、内碳素钢管直径和是否含夹层水泥砂浆为试验变量,共设计了12个短柱试件。分析各试件的破坏情况、荷载-位移曲线、承载力、变形能力及应变特征,研究不同变量对极限承载力的影响规律。结果表明:试件破坏形态为外钢管端部或中部局部屈曲破坏;随着不锈钢管壁厚的增大,试件的轴压承载力近似呈线性增长;夹层水泥砂浆对试件力学性能影响较大,可有效提高试件承载力,抑制外不锈钢管的变形,提高试件延性;对于含夹层水泥砂浆的试件,承载力随空心率增大而减小,不含夹层水泥砂浆的试件,承载力随空心率增大而增大。根据国内外规范以及相关学者提出的理论公式,计算了试件的极限承载力,并与试验承载力进行了对比分析,给出了设计建议。

外包薄壁夹心钢-混组合管柱层间作用对其受压承载力的影响

为了研究外包薄壁夹心钢混组合管柱层间作用下的力学性能,建立了组合管柱有限元分析模型,通过库伦摩擦定义层间作用力,改变影响参数值,研究了轴压、偏压作用下组合管柱的力学性能。结果表明:当摩擦系数由0.2增大至0.4时,轴心受压柱与偏心受压柱极限承载力分别提高了1.06%、13.35%,由0.4增大至0.6时,分别提高了0.06%、13.61%;随着内外钢管与混凝土之间外力的增加,层间作用力和机械咬合力也相应地增加,增强层间作用力可以明显提高外包薄钢混凝土偏心受压组合管柱的极限承载力。

锚杆钢-木组合托盘极限承载力学性能研究

为降低动压冲击对支护系统的破坏,适应大变形巷道的需要;基于球壳结构理论构建锚杆拱形钢托盘承载力学模型,给出钢托盘最大承载力计算式,分析钢托盘各参数尺寸对其承载力的影响关系,研究木托盘压缩实验特征;通过Abaqus软件对钢托盘与组合托盘开展有限元静态力学强度实验,对比分析钢托盘与组合托盘受压状态下的不同特征,分析让压距离对组合托盘承载性能的影响规律。结果表明:对钢托盘承载力的影响顺序为拱高>拱底圆半径>孔口半径,拱高与承载力服从对数增加关系,拱底圆半径、孔口半径与托盘承载力基本成线性增加关系;钢-木组合托盘中木托盘先受力,并经历了压密阶段、弹性变形阶段和塑性变形阶段,木托盘被压缩可降低钢托盘30.03%的承载受力,组合托盘的应力-位移曲线右移,组合托盘的极限承载力增加;钢托盘受压破坏从孔口处开始,基本成圆形向四周扩展,托盘底部四边中间区域与四角区域形成的高应力差是导致托盘四角破坏的主要原因。

大高宽比矩形钢管混凝土柱压弯性能研究

为了研究大高宽比矩形钢管混凝土柱的压弯性能,对大高宽比钢管混凝土短柱进行偏压试验并结合有限元模拟补充了4组试件进行对比,包括不同偏心率、高宽比、钢管壁厚度、截面尺寸情况下柱性能的对比分析。最后参考常用的4本国内外规范进行柱承载力计算,对比并判断现有规范是否适用。结果表明:试件破坏模式均为长边鼓曲,当荷载小于极限承载力的80%时,横截面高度上的应变分布服从平截面假定;随着偏心率增大,柱延性提升,在偏心率超过0.205后,柱延性开始下降;随着壁厚增加,柱延性提升,在壁厚达到8 mm后,柱延性开始下降;随着高宽比提升,柱延性降低;随着截面尺寸增大,在壁厚不变的情况下,柱延性降低,在壁厚等比例增大时,柱延性变化不大;随着高宽比、截面尺寸、钢管壁厚度的增大,柱承载力提升;随着偏心率升高,柱承载力降低;参考规范JGJ 138—2016对大高宽比钢管混凝土柱偏压承载力的计算更为准确。

装配式带夹芯保温槽型玻璃钢-钢板-槽钢-混凝土组合剪力墙轴压性能研究

提出一种装配式带夹芯保温槽型玻璃钢-钢板-槽钢-混凝土组合剪力墙,设计3个具有不同配筋形式和混凝土种类的墙板试件进行轴心受压试验,获得墙板在轴压作用下的破坏模式、荷载-位移曲线、荷载-应变曲线、承载力储备系数、延性系数、材料利用率等性能指标。通过ABAQUS有限元分析软件对墙板进行参数化分析,研究内嵌钢板强度和厚度对其轴压性能的影响。结果表明,墙板具有良好的承载力储备能力和延性性能;混凝土中掺入1%体积比的钢纤维可显著提高墙板内混凝土的抗拉性能,使混凝土强度得到更加充分的利用,布置钢筋可以显著提高墙板延性;墙板的承载力和延性随内嵌钢板厚度和强度的增加而增大,强度为Q355、厚度为4、6mm时,墙板材料利用率更高。提出适用于该类墙板的轴压承载力设计公式,考虑钢板屈曲和截面组合效应的墙板轴心受压承载力设计公式计算结果较精确。

火灾后高强钢管混凝土柱轴压的参数化分析

为研究不同参数对火灾后高强钢管混凝土柱受轴压时的性能影响,利用ABAQUS有限元软件模拟火灾后高强钢管混凝土柱轴压模型,对该构件以钢管屈服强度、受火时间、混凝土强度为主要参数进行承载力-位移曲线分析,并剖析火灾后高强钢管混凝土柱典型曲线。结果表明:随着钢管屈服强度和混凝土强度增大,构件极限承载力增大;随着受火时间增加构件极限承载力出现下降趋势;从整体上看,受火时间分别为60、90、120 min时,构件中的混凝土承担主要载荷。

内置碳钢不锈钢管海洋混凝土柱轴压试验及承载力计算

为研究不锈钢管海洋混凝土柱及内配碳钢后的轴压力学性能,以型钢类型、螺旋筋间距、纵筋直径为变化参数,完成了10个试件的轴心受压静力加载试验。观察了试件的受力破坏全过程及形态,获取了轴向荷载-位移曲线及各钢材应变曲线,基于试验实测数据,就各变化参数对试件的轴压承载力、延性、耗能能力和损伤发展的影响规律进行了分析。结果表明:不同试件的破坏形态相似,均表现为不锈钢管局部屈曲压皱,混凝土斜向剪切破坏;内置碳钢可以提高试件的极限承载力、轴压延性和耗能能力,抑制试件的损伤发展;在增加相同用钢量的条件下,增加圆钢管含钢量对试件轴压性能提升幅度最大;基于“复合约束叠加法”推导计算公式所得承载力计算值与试验值吻合良好。

圆钢管超高性能混凝土短柱偏压力学性能研究

为研究钢管超高性能混凝土短柱的偏压性能,以荷载偏心率和钢管径厚比为变化参数,设计了12个圆钢管UHPC短柱试件并对其进行偏心受压加载试验,分析了该类构件的破坏模式、荷载-挠度曲线、钢管应变和变形系数等,考察了主要因素对短柱偏压性能的影响规律。结果表明:试件的破坏特征为钢管屈服和核心混凝土压坏;荷载-挠度曲线有较明显的峰值点,偏心率越大和径厚比越小,曲线的下降段越平缓;达到60%峰值荷载时,钢管开始产生明显的约束作用,达到90%峰值荷载时,截面变形不再符合平截面假定;偏心率增大使试件的承载力和刚度下降,而径厚比减小可降低这种不利影响。在试验基础上,结合数值模拟对短柱的N-M曲线进行分析,建立了临界偏心率和套箍系数的关系表达式,并基于此提出短柱偏压承载力实用计算方法,理论与试验结果吻合较好。