无支撑钢框架在高温下的楼层稳定性

受高温影响的钢框架的侧向稳定性与其在环境温度下的稳定性不同。这是由于高温下钢框架的弹性模量的降级将导致柱的横向刚度也跟着减少。在楼层屈曲理论的基础上调查无支撑钢框架在高温下的横向稳定性。首先,模拟一个钢柱暴露于高温之下,提出一个分析模型来测试无支撑框架中的轴向载荷、高温以及受热边界限制的钢管的横向刚度所带来的影响。之后,提出一个评估无支撑钢框架在高温下的稳定承载力的方法。数值实例用于展示所提出的方法的评估过程,并且研究不同场景中框架构架暴露于高温时框架结构的稳定性。使用有限元分析对所提出方法进行了数值分析,验证了该方法的有效性。     

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。     

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.     

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。     

Experimental investigation of the P-Δ effect of low rise unbraced steel frames

In this study, the influences of the P-Δ effect on the behavior of low rise unbraced steel frames and the validity of factor B2, which is suggested by KBC 2008 (draft) to consider the P-Δ effect, were evaluated using the experimental approach. Seven specimens, one story-one bay unbraced steel frames, were tested under static loading conditions. The main parameters of the tests were the stiffness ratio of the column to the beam, and the axial load ratio. As revealed by the experimental research that was conducted, the axial load and the decrement of frame stiffness are the main factors that govern the P-Δ effect and that influence the P-Δ effect on the frame behavior. The story stiffness of the frame, which is related to factor B2, was dramatically decreased when the nonlinear behavior started. Factor B2 was compared with the moment amplification factor obtained in the test that had been conducted, and it was found that factor B2, which is based on the linear elastic theory, does not properly estimate the P-Δ effect of an unbraced frame when its assumptions are not satisfied. Thus, careful consideration is needed to apply factor B2 if the design is made using the inelastic part of the material.     

钢框架柱考虑侧移效应的弯矩放大系数实用公式

在现行钢结构设计规范的基础上,结合无支撑纯框架柱二阶近似弹性分析的二阶弯矩放大系数,提出了用于弱支撑框架柱考虑侧移效应的二阶近似弯矩放大系数实用公式。实用公式充分考虑了柱端约束条件、作用于柱上的荷载、柱刚度以及长细比等因素的影响,用于设计更加合理。     

多高层钢结构的分类及其稳定性设计

提出了与钢结构稳定性计算有关的多高层钢结构的分类 ,由此可以更好地理解钢结构稳定性的计算方法 ,合理选择钢结构稳定计算的图表或公式 ,判断某些钢结构规范稳定性计算条文的合理性。     

不同高跨比门式刚架平面内稳定性研究

应用ANSYS有限元程序,在均布荷载作用下,对不同高跨比的门式刚架进行了弹性几何非线性分析。研究表明,在均布荷载作用下,除了梁柱线刚度比,刚架高跨比及屋顶坡度对刚架的稳定性能影响很大。     

半刚性梁柱节点连接钢框架柱的计算长度取值初探

本文给出了具有半刚性梁柱节点连接框架梁的弯矩转角位移方程。按此方程导出了无侧移框架和有侧移框架柱的计算长度取值方法。该方法与我国规范规定的计算长度取值协调一致,不过考虑半刚性连接时要对横梁的约束刚度进行适当的修正。     

Analysis and behaviour of steel storage pallet racks

For steel storage pallet racks, lateral stiffness in down-aisle direction is usually provided by beam-to-column joints and base-plate connections, owing to the impracticability of using bracing systems in selected areas of pallet racks. Because of the high slenderness of members, second order effects influence remarkably the frame performance. The model of unbraced semi-continuous frame is hence adopted for design.

This paper presents the main results of a numerical study on the response of pallet racks commonly used in Europe. The influence of beam-to-column joint modelling on the overall frame response is singled out with reference to both service condition and ultimate limit states. The effect of an accurate definition of frame imperfections is considered. Finally, the influence of base-plate joints on the overall stability of the rack systems is discussed focussing attention on both stiffness and strength of the column bases.     

Behavior and design of reinforced concrete frames retrofitted with steel bracing against progressive collapse

Steel bracing is able to improve progressive collapse resistance of reinforced concrete (RC) frames, but the bracing design is typically based on seismic retrofitting or lateral stability. There is no approach for design of steel bracing against progressive collapse. To this end, a retrofitting approach with steel braces is proposed based on analysis of macro finite element (FE) models with fiber beam elements. The FE models were initially validated through the experimental results of a braced frame and then used to investigate the effects of pertinent parameters on the progressive collapse resistance of planar frames. The results suggest the braces should be placed at the top story. Thereafter, macro FE models are built to investigate the dynamic responses of the three‐dimensional prototype RC frames under different column removal scenarios (CRS) and show the necessity of retrofitting. Accordingly, the design approach of steel bracing is proposed with incremental dynamic analysis (IDA) and assuming independent contribution of braces and frames to resistance. Finally, the fragility analysis of the frames under a corner‐penultimate‐exterior CRS is conducted through IDA and Monte Carlo simulation, and the results confirm the validity of the proposed design approach for retrofitting RC frames.     

Column effective lengths considering inter-story and inter-column interactions in sway-permitted frames

The physical meaning of the traditional effective length coefficients (ELCs) in a sway permitted frame is first explained. It is found through an example that the critical loads of columns in a story have the same percentage increase or decrease after considering the inter-story interaction. It is also pointed out that the ELCs obtained by an overall buckling analysis are applicable only in the weakest story, for the other stories their ELCs simply implied the proportional loading assumption.With three assumptions of the conventional effective length approach discarded, this paper presents a mergence approach for buckling analysis of frames in which the column linear rigidities and axial loads are summed up, and the rotational restraints provided to columns by beams are summed at the merged beam-to-column joints. Effective lengths considering both inter-story and inter-column interactions are determined in unbraced two-story and three-story frames by simple algebraic operations. For frames with more than three stories, a three-story sub-assemblage, which includes beams at far ends of the columns above and below the story under consideration, is used to find the effective lengths of columns in the middle story.For frames with different bases, a equivalence procedure transforms the frames into those with the same type of bases before the mergence. For multi-span frames, the mergence approach overestimate the story lateral stiffness, a modification is proposed to eliminate this overestimation.Although overall buckling analysis of frames is easily carried out today, the findings and the proposed method of this paper will help to understand the inter-column and the inter-story interactions in frames.