不同应力比往复荷载作用后锈蚀钢筋混凝土梁承载力分析

研究锈蚀钢筋混凝土构件在不同应力比往复荷载作用后的实际承载力变化规律对于进一步研究钢筋锈蚀与往复荷载耦合破坏作用及确保结构安全使用方面具有重要意义。本文运用ABAQUS有限元软件,通过修改钢筋截面面积及钢筋强度等参数来模拟往复荷载应力比及钢筋锈蚀的影响,对不同应力比往复荷载作用后锈蚀钢筋混凝土梁的承载力进行静力非线性数值模拟分析,得出以下结论:往复荷载的应力比会显著影响钢筋混凝土简支梁的屈服荷载,且应力比对屈服荷载的影响程度要大于钢筋锈蚀率对屈服荷载的影响;当往复荷载的应力比较小时,应力比的变化对钢筋混凝土梁的极限挠度影响显著,当应力比较大时,应力比的变化对极限挠度的影响变小,而且,应力比与锈蚀率对极限挠度的影响程度基本一致;锈蚀率的增加对屈服荷载的影响程度不随应力比的变化而变化,但锈蚀率的变化对极限挠度的影响程度与应力比相关。     

预应力混凝土空心板受火后力学性能试验研究

为比较不同受火时间后预应力混凝土空心板剩余承载力、跨中挠度和破坏形态的异同,制作了7块预应力混凝土空心板试件,其中3块为未受火对比试件,4块为底面分别受火15 min,30 min,45 min和60 min试件,对其开展受火后力学性能的对比研究。研究结果表明：对比试件和受火试件均发生弯曲破坏;升温过程中预应力混凝土空心板跨中挠度随受火时间增加而显著增加,熄火自然冷却后跨中挠度大部分可恢复,跨中残余挠度接近20 mm;受火时间大于15min时,受火后预应力混凝土空心板开裂荷载和破坏荷载均有所降低;当受火时间达到60 min时,开裂荷载和破坏荷载均急剧下降;受火试件初始弯曲刚度较对比试件明显降低,降低幅度基本与受火时间呈线性关系;相同荷载作用下受火试件受拉边缘拉应变和受压边缘压应变均明显大于对比试件;采用等效截面法计算受火后预应力混凝土空心板的承载力基本满足工程精度要求。     

剪跨比对受火后RC梁抗剪和变形性能影响研究

进行了两个未受火和两个受火钢筋混凝土梁的抗剪性能静载试验研究,通过分析梁的抗剪承载力、跨中挠度、斜裂缝及箍筋应变等参数,重点研究了剪跨比和受火条件对钢筋混凝土梁抗剪和变形性能的影响.研究表明:随着剪跨比的增加,混凝土梁的抗剪承载力降低.达到极限荷载时,梁的跨中挠度随着剪跨比增加而增大.对于相同剪跨比的混凝土梁,受火后梁...     

火灾后中空夹层钢管混凝土柱偏压力学性能研究

为了深入研究火灾后中空夹层钢管混凝土在偏心荷载作用下的力学性能,利用有限元软件建立模型进行模拟分析,并与试验结果进行比较,二者结果吻合良好。对偏压构件的温度场进行计算,并以受火时间、偏心距、名义含钢率、外钢管屈服强度、混凝土抗压强度为主要变化参数,深入剖析了圆中空夹层钢管混凝土柱的偏压工作机理,并对其典型的荷载-跨中挠度关系曲线进行分析。结果表明:随着受火时间增加、偏心距增加、名义含钢率减小、外钢管屈服强度降低以及混凝土抗压强度的降低,构件的承载力均呈现出降低的趋势;构件的刚度随着受火时间的增加、偏心距的增加以及名义含钢率的降低而减小;构件的延性随着受火时间的增加以及偏心距的增加而变好。     

装配式混凝土梁受火后抗剪承载力可靠度分析

为了研究装配式混凝土梁受火后抗剪承载力可靠度变化规律,提出基于JC法(当量正态法)的可靠度计算方法。采用有限元分析软件ABAQUS对ISO-834标准升温曲线下的装配式混凝土梁进行温度场数值模拟,计算受火后混凝土、钢筋强度,得到折减后梁抗剪强度,应用数值分析软件MATLAB进行可靠度数值分析。考虑受火时间、荷载效应比等因素的影响,得到受火后装配式混凝土梁抗剪承载力可靠度。研究结果表明,随着荷载效应比和受火时间的增加,装配式混凝土梁的抗剪承载力可靠度指标降低,且受火时间的影响更为明显;当受火时间为120 min时,梁抗剪承载力可靠度大大降低。分析结果可以为受火后装配式混凝土结构性能研究提供理论参考。     

钢筋混凝土框架结构的力学性能研究

基于非线性梁柱单元模型,对柱顶作用垂直集中荷载和水平荷载的钢筋混凝土强柱弱梁门式框架进行了数值分析,研究了梁、柱的变形情况,框架的一阶、二阶非线性效应,柱顶垂直集中荷载对框架承载力、侧移的影响。得出：框架柱的侧移比梁挠度大很多,框架变形以柱侧移为主;垂直集中荷载对框架的水平极限承载力、侧移有明显影响;当有垂直集中荷载作用时,二阶非线性与一阶非线性分析结果比较,二者差异明显,表明几何非线性影响明显。     

高温下组合钢框架应力响应非线性分析

火灾高温对结构钢的材料性能有显著的影响,尤其是对钢结构材料的力学性能影响更加明显,因此而导致结构整体承载能力的显著变化。利用有限元软件ANSYS建立一榀两层两跨钢-混凝土组合梁和钢柱构成的组合钢框架有限元模型,对其在火灾高温下的温度分布和应力响应进行分析。结果表明,受火梁、柱截面温度非均匀分布,非线性变化;梁柱节点是组合钢框架的薄弱环节;受火组合梁内全过程为压应力,未达到屈服应力。     

钢桁-混凝土组合结构桥梁耐火性能研究

为研究钢桁-混凝土组合结构桥梁的耐火性能，选取公路交通简支下承式钢桁-混凝土组合结构桥梁为研究对象，建立了经试验验证的钢桁-混凝土组合结构桥梁三维实体非线性分析模型，研究其在桥下(下部)和桥面(上部)遭遇碳氢火灾时的温度空间分布特征，计算得到桥梁在不同受火长度和受火时间(下部受火)、不同受火位置(上部受火)影响下的挠度、承载能力的变化规律，给出不同火灾场景下的耐火极限。研究结果表明：受火长度和受火位置显著影响钢桁-混凝土组合结构桥梁的耐火性能；当桥下受火时，受火长度从L/20(L为计算跨径)增至L/4,钢桁-混凝土组合结构桥梁的耐火时间从大于60 min缩短至27 min,且挠度变形更为显著；当桥下20 m范围内受火45 min后，其承载力和结构刚度分别降低至常温下的72%和32%;桥面全车道和边车道受火均会使钢桁-混凝土组合结构桥梁先上挠后下挠，其耐火时间分别为25 min和31 min;边车道受火会使桥梁出现高温扭转变形；当全车道受火23 min后，其承载力和结构刚度分别降低至常温下的25%和10%;增大主桁高度可有效提高钢桁-混凝土组合结构桥梁的耐火性能，用箱形截面斜杆替换工字形...     

轴向约束杆件高温屈曲后强度对网架结构高温极限承载力的影响

局部火灾下,网架结构升温具有明显的非均匀性,结构中温度高的杆件热膨胀受到温度较低杆件的约束,使得网架结构的部分杆件具有显著的约束杆件高温力学特征。基于局部火灾下网架结构力学反应全过程数值追踪技术,考察结构升温非均匀性、结构荷载比、火源位置和结构跨度影响因素下,网架结构杆件高温屈曲后强度对整体结构抗火极限承载力的贡献。研究表明:当结构温度分布非均匀系数η在0.2≤η<0.6区间取值,且结构荷载比R在0.2～0.7范围内,杆件高温屈曲后强度对网架结构高温极限承载力有较大贡献;随着火源位置从网架几何中心偏移,杆件高温屈曲后强度对结构抗火极限承载力的贡献增大;随着网架跨度的增加,当结构荷载比R≤0.5时,杆件高温屈曲后强度对结构抗火极限承载力的贡献逐渐增大,当荷载比R>0.5时,杆件高温屈曲后强度对结构抗火极限承载力的贡献先逐渐减小然后转变为逐渐增大。上述研究结论可为建立网架结构第2阶段失效准则提供理论依据。     

约束高强钢柱受火后轴压剩余承载力影响参数研究

利用经试验验证的ABAQUS有限元模型,对轴心压力作用下的轴向约束高强钢柱受火后的剩余承载性能进行了参数分析,考虑了过火温度、荷载比、轴向约束刚度比、长细比以及高强钢材强度等级等参数的影响。参数分析结果表明:当过火温度小于屈曲临界温度时,受火过程对轴向约束钢柱的轴压承载力没有影响;当过火温度大于屈曲临界温度时,钢柱的轴压剩余承载力会明显减小,且随着过火温度的升高,剩余承载力逐渐降低;轴向约束刚度比和长细比对约束钢柱的过火温度为屈曲临界温度时对应的轴压剩余承载力影响显著;荷载比对约束钢柱的过火温度为破坏临界温度时对应的轴压剩余承载力影响明显;高强钢材强度等级对约束钢柱受火后轴压承载力影响较小。根据参数分析所得数据及规律,提出了超500 MPa高强钢轴向约束柱受火后轴压剩余承载力的简化计算方法,将计算结果与有限元分析结果对比,验证了该简化计算方法的可靠性。     

Prediction to nonlinear behavior of steel frames subjected to fire

A new approach is presented for analysis of nonlinear behavior of steel frames subjected to fire. The material and geometrical nonlinearity as well as the non-uniform profile of temperature across section of frame members are taken into account. Thermal forces induced by temperature variation are also considered. The elasto-plastic stiffness equation of elements is established by using the generalized Clough model. The analysis of strutures subjected to fire is converted to analysis of structures subjected to unbalanced forces induced by temperature increment. Then a procedure to calculate the nonlinear response and ultimate load of steel frames exposed to fire is proposed.

For verifying the validity of the approach proposed, tests on two large-scale models of single-story two-bay steel frames are conducted in gas furnace to simulate fire. The temperature elevation and deformation response of the models due to fire are measured. Good agreements between the results measured and calculated are demonstrated on the behavior of the models exposed to fire.     

Robustness assessment of planar steel frames caused by failure of a side column under localized fire

Progressive collapse of a building structure under fire is a disaster that may cause heavy casualties and serious economic loss. However, there is a lack of codified method to assess fire‐induced progressive collapse of building structures. A global–local analysis method (GLAM) has recently been proposed by the authors and their colleagues to assess progressive collapse of steel buildings under localized fire, and its application on fire scenarios that causes one inner column to fail has been verified. This paper extends the application of GLAM to fire scenarios that causes a side column to fail in a planar steel frame. The predictions of the GLAM were validated against the results obtained from nonlinear dynamic analysis of the whole frame model. Besides, effects of location of the heated column at different storeys and load level of the frame were also studied. The results show that GLAM gives the same collapse predictions to the case studies with detailed nonlinear dynamic analysis. The differences between the critical load obtained from GLAM and that provided by the nonlinear dynamic analysis is within 7%. Therefore, GLAM has good applicability on robustness assessment of planar steel frames caused by failure of a side column under localized fire.     

Survivability of steel frame structures subject to blast and fire

The lesson learned from the terrorist attacks on buildings is the need to assure structures’ ability to sustain local damage without total collapse. Some of the terrorist attacks take the form of blast followed by fire which may cause catastrophic failure of the structure. This paper presents a numerical model for analyzing steel frame structures subject to localized damage caused by blast load and subsequently investigating their survivability under fire attack. The proposed numerical method adopts a mixed-element approach for modeling large-scale framework and it is proven to be sufficiently accurate for capturing the detailed behaviour of member and frame instability associated with the effects of high-strain rate and fire temperature. Design implications related to the use of various numerical models for separate assessment of blast and fire resistance of steel structures and their components are discussed. Fire–blast interaction diagrams are generated to determine the fire resistance of columns considering the initial damage caused by the blast loads. A multi-storey steel building frame is analyzed so that the complex interaction effects of blast and fire can be understood and quantified. The frame is found to be vulnerable, as it possesses little fire resistance due to the deformation of key structural elements caused by the high blast load.     

钢框架结构抗火性能的试验研究

本文介绍四次不同水平荷载作用下,单层单跨钢框架结构的抗火试验,试件由特制的煤气炉进行加热升温,试验结果和理论分析结果吻合良好,文章最后提出了几点结论,供钢结构抗火设计参考.     

Nonlinear analysis of reinforced concrete cross-sections exposed to fire

A nonlinear structural analysis of cross-sections of three-dimensional reinforced concrete frames exposed to fire is presented. The analysis includes two steps: the first step is the calculation of the transient temperature field in cross-sections exposed to fire and the second step is the determination of the mechanical response due to the effect of thermal and mechanical load. A nonlinear finite-element procedure is proposed to predict the temperature field history. In this thermal analysis, the effect of moisture has been taken into account by introducing a water vapor fraction function to define the variation of enthalpy. A mechanical nonlinear analysis of the cross-sections is performed for each temperature distribution and for the applied exterior load using an algorithm of arc-length control. The mechanical and thermal properties of concrete and steel are taken according to the European Standard ENV 1991-1-2 [ENV. Eurocode 2, design of concrete structures, part 1–2: general rules—structural fire design. ENV 1992-1-2, 1995]. In order to validate the proposed thermal and mechanical models, comparisons between numerical and experimental results have been performed. The agreement found is in both cases, fairly good. In addition, a numerical example of the structural analysis of several cross-sections of a reinforced concrete waffle slab under external load and fire is shown.     

基于纤维梁模型的火灾下多层混凝土框架非线性分析

为分析和模拟多层混凝土框架结构在火灾下的反应规律及其破坏过程,基于建筑结构分析中常用的纤维梁单元,建立了钢筋混凝土梁、柱构件的火灾破坏数值模型。模型将构件截面划分成多个纤维,可以考虑构件截面的不均匀温度场分布以及材料非线性和几何非线性问题。对单层单跨混凝土框架进行火灾反应分析,并与试验结果进行比较,验证了此数值模型的准确性。通过对多层框架进行火灾反应模拟,比较不同火灾场景的模拟结果,分析其反应规律以及破坏过程。结果表明,纤维梁单元模型可以较好地模拟钢筋混凝土结构的受火破坏过程,并且火灾发生的位置不同,结构的破坏机制也不同,一定条件下蔓延的火灾比不蔓延的火灾对多层混凝土框架结构的破坏性更大。分析结果可以为实际结构的防火设计提供参考。     

爆炸荷载作用下钢框架结构连续倒塌分析

传统数值方法模拟建筑结构在爆炸荷载作用下的结构响应和连续倒塌时,具有计算模型复杂、计算量大的特点,实际应用价值不大。基于将爆炸荷载作用下结构响应分析分两步进行的数值模拟方法,利用非线性显式动力分析软件AUTODYN的Remap技术模拟爆炸波在空气中的传播过程,利用压强测点记录结构构件表面的爆炸压强时程曲线;建立结构精细化有限元模型,并将上一步记录的爆炸压强时程曲线施加于结构构件,利用LS-DYNA显式求解器分析结构在爆炸荷载作用下的动态响应和倒塌过程。将该方法应用于某钢框架结构在爆炸荷载作用下的动态响应和连续倒塌分析。结果表明：钢框架结构具有较好的抗爆性能,在发生1000kg TNT当量及以下规模的室外爆炸时,主体结构能够保证安全;在发生1500kgTNT当量及以上大规模爆炸时,发生次梁塌落等局部破坏,亦可能发生结构连续倒塌。     

Nonlinear behavior of steel structures considering the cooling phase of a fire

The nonlinear behavior of steel structures considering the heating and the cooling phases of a fire is investigated by the Vector Form Intrinsic Finite Element (VFIFE) method. The temperature dependent constitutive relations of steel which include strain reversal effects are adopted. The numerical model is first verified by comparing the results with the published analytical and experimental results for steel structures in the cooling phase. Several numerical examples are then fully studied to investigate the cooling behavior of steel structures. The proposed numerical model can effectively predict the nonlinear behavior of the steel structure in both the heating and cooling phases.     

型钢混凝土框架结构耐火性能有限元分析

为了研究型钢混凝土框架整体结构的耐火性能,为其抗火设计提供参考,采用受火楼层建立精细化有限元计算模型、非受火楼层建立梁单元计算模型的方法,建立了型钢混凝土框架整体结构的耐火性能计算模型。考虑火灾位置、荷载分布形式、柱轴压比等参数的影响,对火灾下型钢混凝土框架整体结构的变形规律、承载机制、破坏形态以及耐火极限进行参数分析。分析结果表明：火灾下框架结构出现了整体破坏和局部破坏两种典型的破坏形态,受火构件受到的约束作用对其耐火性能有较大的影响;在局部破坏形态中,由于受热膨胀,火灾下框架梁首先出现了较大的轴压力,受火框架梁处于压弯受力状态;之后,框架梁出现了悬链线效应,轴力对框架梁的受力状态有较大影响;在整体破坏形态中,根据轴压比及荷载分布形式的不同,框架出现了中柱破坏和边柱破坏两种典型破坏形态,同时,随楼层受火位置的升高,柱的轴压比减小,框架结构的耐火极限增加。     

受火后钢筋混凝土吊车梁加固方法的试验研究

为研究受火后钢筋混凝土吊车梁的维修加固方法,开展了粘贴CFRP布和粘贴钢板加固受火后足尺吊车梁受力性能的试验研究、理论分析和有限元分析。结果表明：未受火对比试件发生受拉区钢筋屈服、受压区混凝土压碎破坏;粘贴CFRP布加固试件的主要破坏模式为受拉区钢筋屈服和CFRP布拉断;粘贴钢板加固试件的主要破坏模式为受拉区钢筋和加固钢板受拉屈服以及受压区混凝土压碎。对于ISO 834等效受火60min后钢筋混凝土吊车梁,粘贴钢板加固效果优于粘贴CFRP布;ISO 834等效受火60min和90min后的钢筋混凝土吊车梁采用CFRP布加固后,极限荷载接近。加固后钢筋混凝土吊车梁试件跨中截面混凝土应变发展规律在混凝土腹板垂直裂缝完全发展前基本符合平截面假定。受火后加固试件的位移延性系数较未加固对比试件有所降低,粘贴钢板加固试件降低稍大。加固的受火后钢筋混凝土吊车梁极限荷载计算结果和有限元结果与试验结果误差绝对值为0.4%~7.8%,满足工程精度要求。