非均匀升温下无支撑钢框架稳定性分析

目前对钢框架稳定的研究主要基于钢柱均匀升温的假定。然而,实际火灾下,由于热浮力效应,建筑物内部的温度分布可能不均匀。该文基于楼层屈曲和两区域火灾模型概念,研究柱纵向非均匀升温作用下无支撑钢框架的弹性稳定性。首先,为模拟非均匀升温下的钢柱,给出一个可检验轴向载荷、非均匀升温分布、热边界条件对无支撑框架钢柱侧向刚度影响的分析模型。基于欧拉-伯努利梁理论推导出柱模型的侧向刚度方程。然后,采用该模型评价非均匀升温作用下无支撑钢框架的稳定性。采用数值案例分析非均匀升温作用下不同钢框架的稳定性,有限元分析方法验证了所给方法分析结果的有效性。     

柔性支撑钢框架结构扭转性能研究

相比于传统框架结构形式,柔性支撑钢框架的方钢管柱可采用较小的截面,抗扭刚度主要由柔性支撑提供。考虑到柔性支撑的特殊性,有必要对该体系的抗扭性能进行研究。以单层1跨柔性支撑钢框架为基础模型,分析了柔性支撑钢框架的刚度特性,发现柔性支撑钢框架不同的支撑松紧程度对应不同的抗侧刚度,抗侧刚度具有时变性,且在某些特殊的支撑松紧状态下,刚度具有跳跃点,而且不同的支撑初始松紧状态可能引起柔性支撑钢框架初始的刚心、质心偏离,从而在地震作用下发生扭转。采用OPENSEES有限元软件,对比了7组典型支撑初始松紧状态下的柔性支撑钢框架的静力及动力响应。结果表明:支撑初始松紧程度的不同使柔性支撑钢框架具备一定的控扭能力,但当扭转变形超过支撑初始松紧状态的调控范围后,控扭能力便不再显著。另外,当所有支撑进入塑性后,支撑初始的不同松紧状态所带来的影响随之消失。最后,对柔性支撑钢框架结构形式提出了建议,柔性支撑初始均须预紧,可通过对支撑不松不紧状态的柔性支撑钢框架考虑偶然偏心后的结果进行适当修正,来消除各支撑预紧程度不同的影响。     

支撑布置对钢框架结构抗侧刚度的影响

纯钢框架建筑结构体系刚度较小,层间位移起主要控制作用,设置支撑体系可有效地提高结构的整体抗侧刚度,减少结构用钢量。以广泛应用的多高层钢框架支撑结构为对象,通过计算不同支撑类型及支撑布置方式下钢框架支撑结构的顶点位移,研究了支撑形式对多、高层钢框架结构抗侧刚度的影响。研究结果表明,对于不同的支撑类型,支撑沿竖向集中布置于中间跨的钢框架结构抗侧移刚度好于将支撑布置在边跨以及其他跨上。对于相同的支撑布置方式,不论是中心支撑,还是偏心支撑,在抗侧刚度和经济性方面,人字形支撑框架均好于单斜杆支撑框架。     

不同火灾工况下无支撑钢框架的稳定性研究

正加拿大研究人员开展了不同火灾工况下无支撑钢框架的稳定性研究,分析了导致楼层不稳定的重大火灾场景。提出了一种新的全局优化方法来确定最高、最低温度和局部、分散的火灾场景造成的无支撑钢框架结构的不稳定情况。分析假设框架柱具有防火保护,而框架梁没有防火保护,可将提出的分析方法应用于其他构造和框架材料的     

半刚性连接无支撑组合框架的简单分析方法

为了考虑半刚性梁-柱连接的影响,对用于计算传统的无支撑的纯钢框架横向力分析的Muto方法进行了修正。运用第一定理,采用修正的Muto方法建立了这种钢框架层间位移的计算公式。通过比较精密的有限元程序计算出的结果可以看出,该方法给出了有侧移的半刚性连接纯钢框架比较精确的内力和层间位移的估计值。为了进行无支撑组合钢框架的手工分析,对修正后的Muto方法做了进一步改进,即用改良的等价的梁刚度来计算弯曲和下沉较大区域的梁刚度的变化。通过对半刚性连接无支撑组合框架严格的有限元分析,验证了这种简化分析方法的精确性。最后进行了参数研究,量化了半刚性连接对无支撑组合框架在侧向荷载作用下层间位移的影响。     

Y形偏心支撑钢框架耗能段长度研究

为研究耗能段长度对Y形偏心支撑钢框架滞回性能的影响,应用ABAQUS 13.0建立了8个不同耗能段长度的Y形偏心支撑钢框架模型并对其进行了数值分析,研究了耗能段长度变化对Y形偏心支撑钢框架承载力、刚度、耗能能力和延性的影响。结果表明:Y形偏心支撑钢框架具有良好的滞回性能,耗能段长度变化对Y形偏心支撑钢框架的承载力、刚度、耗能能力及延性均有较大影响;随着耗能段长度的增大,Y形偏心支撑钢框的承载力及刚度呈降低趋势,功比指数和延性系数先增大后降低。此外,根据有限元分析结果确定了Y形偏心支撑钢框架耗能段长度的合理取值。     

单斜杆式偏心支撑钢框架耗能段长度研究

为研究耗能段长度变化对单斜杆式偏心支撑钢框架滞回性能的影响,应用有限元软件ABAQUS建立6个不同耗能段长度的单斜杆式偏心支撑钢框架模型并对其进行非线性数值分析,研究耗能段长度变化对单斜杆式偏心支撑钢框架承载力、刚度、耗能能力和延性的影响。研究表明:单斜杆式偏心支撑钢框架具有良好的滞回性能,耗能段长度变化对单斜杆式偏心支撑钢框架的承载力、刚度、耗能能力及延性均有较大影响;随着耗能段长度的增加,单斜杆式偏心支撑钢框的承载力及刚度呈降低趋势,耗能能力和延性系数先增大后降低。最后根据有限元分析结果给出了单斜杆式偏心支撑钢框架耗能段长度的合理取值范围。     

高强钢组合K型偏心支撑框架抗震性能数值分析

对一个单层单跨剪切屈服型高强钢组合K型偏心支撑框架试件的滞回性能进行了低周往复循环试验研究,并且建立了多个层数不同的高强钢组合K型偏心支撑框架和Q345钢K型偏心支撑框架有限元模型,对其滞回性能进行了非线性数值分析,对两种结构的承载力、强度退化、刚度退化、延性、耗能能力以及用钢量进行了对比。研究表明:在满足抗震要求的前提下,高强钢组合K型偏心支撑框架的抗震性能良好,略差于相同设计条件下Q345钢K型偏心支撑框架的,但是构件截面较小,可以节省钢材、降低造价,具有较高的经济效益。     

测定钢货架竖版的横向抗剪刚度

通过螺栓连接的冷弯薄壁型钢竖直框架确保钢货架在多横向走道方向的稳定性。对二阶效应很敏感并能精确测定框架的抗剪刚度是抗震设计中必不可少的;由于外部的钢框架必须能够承受在多横向走道上风力所带来的水平位移,这也是确保货架整体稳定性,特别是高钢架和钢架支承建筑围护结构中至关重要的。钢架制造协会使用了Timosh-enko理论和Gere理论来进行保守的计算并推荐欧洲标准EN15512对其进行测试,然而并不能确定根据所推荐的测试程序得到的抗剪刚度是否正确。介绍的新修正后的澳大利亚标准AS4084适用于欧盟,同时也介绍了另一种测定直立框架横向弯矩和抗剪刚度共同作用的测试方法。文章评论并分析了影响冷弯薄壁型钢的螺栓竖直框架的剪力变形,介绍了一种在澳大利亚标准修正版中采用的检测装置。使用两种不同的测试方法对36个竖直框架进行测试,对试验结果进行讨论并与有限元分析得出的结果相比较。同时对在设计中如何运用这些测试结果提出了一些建议。基于这些建议,文章中认为这两种测试方法是不等效的,并且竖直框架的横向抗剪刚度的结果是不同的。     

基于均匀层间位移的多层梁贯通式支撑钢框架简化抗震设计方法

为实现均匀层间位移的目标,提出了基于均匀层间位移的梁贯通式支撑钢框架简化抗震设计方法,设计了梁贯通式支撑钢框架。在地震作用下,作为耗能构件的只拉不压中心支撑抵抗大部分水平作用,而主体钢框架则因其可忽略的抗侧刚度而保持弹性。只拉不压支撑单元的滞回耗能作用可等效为结构的黏滞阻尼,同时支撑和框架的抗侧刚度比对结构的地震响应影响较大。基于均匀层间位移响应提出设计方法的3个性能目标,并合理考虑刚度比和等效阻尼比,给出具体设计流程。分别设计了4层和6层平面支撑钢框架模型,通过Pushover分析和弹塑性时程分析验证了该方法的合理性。分析结果表明:支撑与框架抗侧刚度比最优取值为4~6。     

Storey-based stability of unbraced steel frames at elevated temperature

The evaluation of the lateral stability of steel frames subject to elevated temperatures is different from that at ambient temperature. This is because the degradation of the Young's Modulus of steel associated with elevated temperature will lead to the loss of column lateral stiffness. In this study, the lateral stability of unbraced steel frames subjected to elevated temperature is investigated based on the concept of storey based buckling. First, to simulate a steel column exposed to the elevated temperature, an analytical model was proposed to examine the effects of axial loading, elevated temperature, and thermal boundary restraints on the lateral stiffness of steel columns in unbraced frames. Then, a method of evaluating the stability capacity of unbraced steel frames at elevated temperature is proposed. Numerical examples are presented to demonstrate the evaluation procedure of the proposed method and investigate the frame stability subjected to different scenarios of frame members exposed to the elevated temperature. The validity of the proposed method is verified by the numerical analysis with the use of finite element analysis.     

A simplified analytical method for unbraced composite frames with semi-rigid connections

The original Muto’s method for the lateral force analysis of conventional unbraced bare steel frames is first modified to incorporate the effects of semi-rigid beam-to-column connections. From which, equations for the calculation of the inter-storey drifts of these frames are formulated using first principles. By comparing the frame analytical results calculated from a rigorous finite element programme, it is shown that the proposed method gives reasonably accurate internal forces and inter-storey drifts estimations of a sway bare steel frame with semi-rigid connections. The proposed modified Muto’s method is then further developed for the manual analysis of unbraced composite frame systems by introducing an improved equivalent beam stiffness to account for the variation of the beam stiffness in the hogging and sagging moment regions. The accuracy of this simplified analytical method is verified by the rigorous finite element analysis of an unbraced composite frame with semi-rigid connections. Last, a parametric study is conducted to quantify the effects of semi-rigid connections on the inter-storey drifts of unbraced composite frames under lateral loads.     

Stability of multi-storey unbraced steel frames subjected to variable loading

Proposed in this paper is an approach of evaluating the elastic buckling loads for multi-storey unbraced steel frames subjected to variable loading or non-proportional loading. In the case of variable loading, the conventional assumption of proportional loading is abandoned, and different load patterns may cause the frame to buckle at different levels of critical loads. In light of the use of the storey-based buckling concept to characterize the lateral sway buckling of unbraced framed structures, the problems of determining the lower and upper bounds among all of the frame buckling loads associated with different load patterns are presented as a pair of minimization and maximization problems subjected to elastic stability constraints. The problems take into account the semi-rigid behaviour of beam-to-column connections and the lateral stiffness reduction of columns due to the presence of an axial compressive load. The minimization and maximization problems are then solved by a linear programming method; thus, the lower and upper bounds of the frame buckling loads subjected to variable loading are obtained. Parametrical studies on the influence of the connection rigidity to the lower and upper bounds of critical loads and the comparisons to the conventional proportional loading are also presented in this paper.     

Elastic buckling load of multi-story frames consisting of Timoshenko members

The objective of this paper is to propose a method for the evaluation of the elastic critical buckling load of columns in frames consisting of members susceptible to non-negligible shear deformations, such as built-up members in steel frames, based on Engesser's approach. To that effect, a stability matrix is proposed and three general stability equations are derived for the cases of unbraced, partially braced and braced frames. Indicative graphic interpretation of the solutions for the stability equations of the braced and unbraced cases is shown. Slope-deflection equations for shear-weak members with semi-rigid connections are also derived and used for the presentation of a complete set of rotational stiffness coefficients, which are then used for the replacement of members converging at the bottom and top ends of the column in question by equivalent springs. All possible rotational and translational boundary conditions at the far end of these members, as well as the eventual presence of axial force, are considered. Five examples are presented, dealing with braced, unbraced and partially braced frames, with rigid and semi-rigid beam to column connections, loaded with concentrated or uniformly distributed loads, in a symmetrical or non-symmetrical pattern. In all cases the proposed approach is in excellent agreement with finite element results.     

Analytical investigation of the P-Δ effect of middle-rise unbraced steel frames

In this study, the influences of the P-Δ effect on the behavior of middle-rise unbraced steel frames and the validity of factor B2 were investigated using the analytical approach to suggest the basic design information on the P-Δ effect to the structural engineers. The refined plastic hinge method with an arc length algorithm was applied to the two-dimensional second-order inelastic analysis which can cover the post-maximum load behavior. Four types of middle-rise unbraced steel frames were selected as the subjects of the analysis. The main parameters of the analysis were the shape and the scale of the frame, the stiffness of the beam and the first-floor column, and the axial load ratio of the first-floor column. As revealed by the analytical research that was conducted, the P-Δ effect affects the behavior of middle-rise unbraced steel frames and the axial load, and the stiffness of the first-floor column are the main factors which govern the P-Δ effect. In addition, factor B2 is not applicable to the frame that has not clear story concept and it does not properly estimate the P-Δ effect of an unbraced frame when nonlinear-inelastic behavior is induced. Thus, reasonable guidelines are needed for the use of factor B2.     

柔性连接钢框架稳定分析的计算长度

根据钢框架柱稳定设计的计算长度概念,以部分框架为计算单元,用变刚度的螺旋弹簧模拟梁柱节点连接的柔性,通过引入柔性连接刚度修正系数,建立了无侧移和有侧移柔性连接钢框架柱计算长度系数修正公式,其形式与我国钢结构设计规范(GB17—88)规定的计算长度稳定方程完全相同,可供工程设计人员使用.     

复合主轴端板连接的结构特性

设计中,节点最关键的结构性能是其抗弯能力、刚度和转动能力。这些都取决于连接梁的截面高度。对梁截面高度达530mm的复合端板连接进行一系列试验。试验结果显示楼板的断裂通常导致结构失效,但是这一般都发生在该处的转动变形已经充分发展之后。     

Stability response of flexibly connected cold-formed steel space frames

Results for the elastic (bifurcation) stability analysis of cubic space frames composed of bisymmetrical cold-formed steel sections are presented. The influences of flexible connections and partial warping rigidity are considered. Calculation of the critical load is based on a finite element eigenvalue algorithm which combines a secant iteration, determinant search routine with a ‘shifted’ inverse iteration strategy. A parametric study on the effect of connection and warping rigidity for the frames subjected to gravity and lateral twist loading is presented. Effects of column orientation and member stiffness on frame stability are discussed.     

无支撑半刚性连接钢框架的简化分析

用变刚度的螺旋弹簧模拟梁柱节点半刚性连接的弯曲特性 ,建立了半刚性连接钢框架梁单元的刚度矩阵。通过分别引入横梁修正线刚度和修正转动刚度 ,将等效代替框架法和无剪力分配法推广应用于无支撑半刚性连接钢框架的结构分析 ,其计算十分简便     

支撑形式对钢框架结构抗震性能影响

钢结构框架类型,按其抗侧力体系可以分为:纯框架(UBF)、中心支撑框架(CBF)和偏心支撑框架(EBF)。纯框架具有较好的延性,刚度较小,往往满足了抗震变形要求;支撑框架刚度较大,具有两道纯抗震防线,适应于抗震设防较高的地区。通过有限元软件ANSYS对6层框架进行动力分析,比较这三种框架的抗震性能。由分析可知,支撑框架能较好地控制结构变形。