Effects of connection types and elevated temperature on the impact behaviour of restrained beam in portal steel frame

Based on the background of structural protection and Disaster Reduction Engineering, the dynamic behaviour and failure mechanism of restrained beams in portal steel frames in localised fire are investigated via experimental measurement and numerical simulation techniques. Comprehensive parametric studies are carried out to discuss the influence of end connection types, temperature, impact velocity,impact mass and span-to-depth ratio(SDR) on the dynamic response of the beams. The characteristics of deformation, internal force and energy distribution about the restrained beams and its joints are investigated. A temperature dependent criterion for evaluating the frame joint performance is proposed to measure the degree of performance degradation and impact resistance of the joint. The dynamic displacement amplification factor in different temperature environments are proposed for the different beam end constraint types and SDRs. Results of the experimental and numerical analysis show that the welded connection(WC) of three typical joint types is the strongest, and the extended endplate connection(EEC) is the weakest in terms of the impact resistance performance. With regard to the failure mode of the joint, the failure positions of the WC and the welded-bolted connection are located in the inner web of the column. Meanwhile, the EEC is located in the connection position between the beam and the endplate. Three different internal force stages and two obvious critical temperature boundaries of the restrained beams emerge with the increase in temperature, and they have significant characteristics in terms of deformation trend, internal force transfer and energy distribution. During the impact, a phenomenon known as “compression arch action” develops into “catenary action” with the increase in deflection in the frame beam mechanism.

Seismic Behavior of Diaphragm-Through Connections of Concrete-Filled Square Steel Tubular Columns and H-Shaped Steel Beams

Based on the introductions of a type of diaphragm-through connection between concrete-filled square steel tubular columns (CFSSTCs) and H-shaped steel beams,a finite element model of the connection is developed and used to investigate the seismic behavior of the connection.The results of the finite element model are validated by a set of cyclic loading tests.The cyclic loading tests and the finite element analyses indicate that the failure mode of the suggested connections is plastic hinge at the beam with inelastic rotation angle exceeding 0.04 rad.The suggested connections have sufficient strength,plastic deformation and energy dissipation capacity to be used in composite moment frames as beam-to-column rigid connections.

Experimental study on seismic behavior of semi-rigid connection between steel beam and concrete wall

Beam-column or beam-wall connections are an important problem in high-rise buildings. In this study, based on the analysis of an example structure, an analytical model for design of the semi-rigid connections between steel beams and RC walls in high-rise hybrid buildings is proposed. Also, the mechanical characteristics of these connections subjected to low-reversed cyclic loading are investigated through comparison of experimental results from three semi-rigid connections and two rigid connections. Moreover, some latent problems for design of these connections as well as the corresponding solutions are discussed. The results from the experiments and analyses indicate that semi-rigid connections exhibit satisfactory capacity and seismic performance, and the proposed design can be used in practice.

Monotonic and Cyclic Pushover Tests of Wood Beam-Column Connections:Ancient Chinese Tenon Joint vs Typical and Simplified US Connections

Wood beam-column frame is a popular structural system in United States and in ancient China. Chinese wood beam-column frame structures showed better seismic resistance properties than the US ones.The tenon joint is one of the reasons.This study performed monotonic and cyclic pushover tests to understand the behavior of Chinese tenon joints versus the behavior of the commonly used US wood beam-column connections. The test results indicate that the typical US wood beam-column connection is very strong under monotonic loads.The ancient Chinese tenon joint has the best behavior under cyclic loads.

Effects of temperature change on elastic behavior of steel beams with semi-rigid connections

Based on the nonlinear displacement-strain relationship,the virtual work principle method was used to establish the nonlinear equilibrium equations of steel beams with semi-rigid connections under vertical uniform loads and temperature change.Considering the non-uniform temperature distribution across the thickness of beams,the formulas for stresses and vertical displacements were presented.On the basis of a flowchart for analysis of the numerical example,the effect of temperature change on the elastic behavior of steel beams was investigated.It is found that the maximal stress is mainly influenced by axial temperature change,and the maximal vertical displacement is principally affected by temperature gradients.And the effect of temperature gradients on the maximal vertical displacement decreases with the increase of rotational stiffness of joints.Both the maximal stress and vertical displacement decrease with the increase of rotational stiffness of joints.It can be concluded that the effects of temperature changes and rotational stiffness of joints on the elastic behavior of steel beams are significant.However,the influence of rotational stiffness becomes smaller when the rotational stiffness is larger.

Seismic performance of interior precast concrete beam-column connections with T-section steel inserts under cyclic loading

An experimental investigation was conducted to study the performance of precast beam-column concrete connections using T-section steel inserts into the concrete beam and joint core,under reversed cyclic loading.Six 2/3-scale interior beam-column subassemblies,one monolithic concrete specimen and five precast concrete specimens were tested.One precast specimen was a simple connection for a gravity load resistant design.Other precast specimens were developed with different attributes to improve their seismic performance.The test results showed that the performance of the monolithic specimen M1 represented ductile seismic behavior.Failure of columns and joints could be prevented,and the failure of the frame occurred at the flexural plastic hinge formation at the beam ends,close to the column faces.For the precast specimens,the splitting crack along the longitudinal lapped splice was a major failure.The precast P5 specimen with double steel T-section inserts showed better seismic performance compared to the other precast models.However,the dowel bars connected to the steel inserts were too short to develop a bond.The design of the precast concrete beams with lap splice is needed for longer lap lengths and should be done at the beam mid span or at the low flexural stress region.

Influence of the column-to-beam flexural strength ratio on the failure mode of beam-column connections in RC frames

The column-to-beam flexural strength ratio(CBFSR)has been used in many seismic codes to achieve the strong column-weak beam(SCWB)failure mode in reinforced concrete(RC)frames,in which plastic hinges appear earlier in beams than in columns.However,seismic investigations show that the required limit of CBFSR in seismic codes usually cannot achieve the SCWB failure mode under strong earthquakes.This study investigates the failure modes of RC frames with different CBFSRs.Nine typical three-story RC frame models with different CBFSRs are designed in accordance with Chinese seismic codes.The seismic responses and failure modes of the frames are investigated through time-history analyses using 100 ground motion records.The results show that the required limit of the CBFSR that guarantees the SCWB failure mode depends on the beam-column connection type and the seismic intensity,and different types of beam-column connections exhibit different failure modes even though they are designed with the same CBFSR.Recommended CBFSRs are proposed for achieving the designed SCWB failure mode for different types of connections in RC frames under different seismic intensities.These results may provide some reference for further revisions of the SCWB design criterion in Chinese seismic codes.

Robust Broadband Beam-Forming Based on the Feature of Underwater Target Radiated Noise

To the problem of the unknown underwater target detection, according to the feature that the underwater target radiated noise contains the stable line spectrum, a weighted method based on the main-to-side lobe ratio （MSLR） is proposed for broadband beam-forming. This weighted method can be implemented by using the following steps. Firstly, optimize the spatial spectrum of each frequency unit by the second-order cone programming （SOCP）, and obtain the optimized spatial spectrum with lower side lobe. Secondly, construct weighting factors based on the MSLR of the optimized spatial spectrums to from weight factors. Lastly, cumulate the spatial spectrum of each frequency unit via the weight statistical method of this paper. This method can restrain the disturbance of background noise, enhance the output signal-to-noise ratio （SNR）, and overcome the difficulty of traditional four-dimensional display. The theoretical analysis and simulation results both verify that this method can well enhance the spatial spectrum of line spectrum units, restrain the spatial spectrum of background noise units, and improve the performance of the broadband beam-forming.

Numerical stress analysis of crack at welded beam-to-column connection

In the past, brittle fracture of steel structure was reported rarely under earthquake. However, recent earthquakes, especially Northridge Earthquake (USA) and Hyogoken Nanbu earthquake (Japan), astonished engineers in the field of construction. The experience from recent earthquakes of USA and Japan shows that brittle fracture of welded steel structure always starts from high stress zone with welded crack [1～5] . As backing bar for grooved weld on beam flange exists, artificial crack is formed because of lack of fusion at the root of flange weld. In this paper stress distribution of connection is computed with FEM, and stress concentration at the root of flange weld is also analyzed. Stress intensity factors (SIFs), K I, at the root of flange weld are computed in the method of fracture mechanics. The computation shows that stress intensity factor on bottom flange weld is obviously higher than that on top flange weld. It is proved by the fact that brittle fracture is liable to start at the root of bottom flange weld on actual earthquake [1,4] . Finally measures are brought forward to avoid fracture of weld structure against earthquake.

Feasibility of developing composite action between concrete and cold-formed steel beam

Cold-formed steel structures are steel structure products constructed from sheets or coils using cold rolling, press brake or bending brake method. These structures are extensively employed in building construction industry due to their light mass, ductility by economic cold forming operations, favorable strength-to-mass ratio and other factors. The utilization of cold formed steel sections with concrete as composite can hugely reduce the construction cost. However, the use of cold formed steel members in composite concrete beams has been very limited. A comprehensive review of developments in composite beam with cold formed steel sections was introduced. It was revealed that employing cold-formed steel channel section to replace reinforcement bars in conventional reinforced concrete beam results in a significant cost reduction without reducing strength capacity. The use of composite beam consisting of cold-formed steel open or close box and filled concrete could also reduce construction cost. Lighter composite girder for bridges with cold-formed steel of U section was introduced. Moreover, types of shear connectors to provide composite action between cold-formed steel beam and concrete slab were presented. However, further studies to investigate the effects of metal decking on the behavior of composite beam with cold-formed steel section and introduction of ductile shear connectors were recommended.

Numerical Modeling on Seismic Performance of Castellated Composite Beam⁃Reinforced Concrete Column Connection

The hybrid structure consisting of castellated composite beam and compound spiral hoop reinforced concrete column take full advantages of steel and concrete material.To popularize the structural form in real conditions,a beam⁃through⁃type beam⁃column connection is proposed.Two 1/2⁃scaled connection specimens were tested and three⁃dimensional finite element models of the beam⁃column connection were set up.The longitudinal reinforcements,concrete beam,and column were simulated by link and solid elements,respectively.The influences of the parameters such as expansion ratio,location of web opening,and original height of steel beam were studied.The results show that connections possessed high initial rigidity.The expansion ratio of steel beam showed more important influence on the connection’s ultimate bearing capacity.For the connection models with steel beam expansion ratio of 1.4,the maximum increment of the ultimate bearing capacity of the connection could reach 28%.In order to prevent the local buckling failure of steel beam from occurring near web opening,the expansion ratio of steel beam should not be greater than 1.3.

The Efficiency of Reduced Beam Section Connections for Reducing Residual Drifts in Moment Resisting Frames

In most framed structures anticipated deformations in accordance with current codes fall into acceptable limit states, whereas they go through substantial residual deformations in the aftermath of severe ground motions. These structures seem unsafe to occupants since static imminent instability in the immediate post-earthquake may be occurred. Moreover, rehabilitation costs of extensive residual deformations are not usually reasonable. Apparently, there is a lack of detailed knowledge related to reducing residual drift techniques when code-based seismic design is considered. In this paper, reduced beam section connections as a positive approach are taken action to mitigate the huge amount of residual drifts which are greatly amplified by P-Δ effects. To demonstrate the efficacy of RBS, a sixteen-story moment resisting frame is analyzed based on a suite of 8 single-component near field records which have been scaled according to the code provisions. The results are then processed to assess the effects of RBS detailing on drift profile, maximum drift, and residual drift. Besides, a special emphasis is given to estimate overall trend towards drift accumulation in each story in the presence of RBS assembly. A main conclusion is that using this connection predominantly alleviates the adverse effects of P-Δ on amplifying residual drifts.

Experimental study on full-scale steel beam-to-column moment connections

Ten full-scale steel beam-to-column moment connections used in moment-resisting frames （MRFs） were tested to study the failure process, failure mode, strength and plastic rotation capacity. The specimens include one traditional welded flange-bolted web connection, one traditional fully welded connection, four beam flange strengthened connections, three beam flange weakened connections, and one through-diaphragm connection. The test results show that the connections with flange cover plates or with partly cut beam flanges satisfy the beam plastic rotation demand for ductile MRFs. From the measured stress profiles along the beam flange and beam web depth, the mechanics of brittle fracture at the end of the beam is discussed. Design recommendations for steel beam-to-column moment connections are proposed.

SOME MULTIPLICITY RESULTS FOR AN ELASTIC BEAM EQUATION AT RESONANCE

This paper deals with multiplicity results for nonlinear elastic equations of the typewheregi[0,1] X R R satisfies Caratheodory condition L2[0,1]. The solvability of this problem has been studied by several authors, but there isn't any multiplicity result until now to the author's knowledge. By combining the Lyapunov-Schmidt procedure with the technique of connected set, we establish several multiplicity results under suitable condition.

SHEAR STRESS ANALYSIS OF TIMOSHENKO'S BEAM WITH MULTIPLY CONNECTED CROSS SECTION

In this paper, a finite element method is developed to numericallyevaluate the shear coefficient of Timoshenko's beam with multiplyconnected cross section. With focus on analyzing shear stressesdistributed at the neutral axis of the beam, an improved definitionof the shear coeffi- cient is presented. Based on this definition, aGalerkin-type finite element formulation is proposed to analyze theshear stresses and shear deflections. Numerical solutions of theexamples for some typical cross-sections are compared with thetheoretical results. The shear coefficient of tower sections of theTsing Ma Bridge is calculated by use of the proposed approach, sothat the finite element modeling of The bridge can be developed withthe accurate values of the sectional properties.

部分包覆钢-混凝土组合梁柱节点抗震性能试验研究

为研究不同连接构造的部分包覆钢-混凝土组合梁柱节点(PEC梁柱节点)的抗震性能,对2个PEC梁柱节点试件进行了拟静力加载试验,研究了低周往复荷载作用下PEC梁柱节点试件的破坏现象、滞回曲线、骨架曲线、延性、耗能能力和刚度退化等抗震性能。结果表明:强轴连接PEC梁柱节点的滞回曲线呈梭形和弓形,在达到极限承载力后仍能保持一定的延性和耗能能力;弱轴连接PEC梁柱节点牛腿与梁间的焊缝处发生破坏,未展现出预期的耗能能力,PEC梁仍在弹塑性状态,没有达到极限状态;PEC梁柱节点核心区混凝土替换为加劲肋板后,试件仍具有较好的承载力、延性和耗能能力,刚度退化规律无明显变化,且强轴连接节点与弱轴连接节点刚度变化规律基本一致;PEC柱牛腿设计过短会导致焊缝连接处断裂,试件延性和耗能能力得不到发挥,剩余刚度较大。

模块化钢结构梁柱子结构抗连续倒塌性能研究

模块化钢结构作为一种高度预制的装配式建筑,具有工业化程度高、绿色节能环保等优点。但是,由于模块化钢结构体系超静定次数较低,其在极端荷载工况下的抗连续倒塌性能应予以特别关注。模块化钢结构梁柱子结构作为主要的传力路径,具有多梁多柱等有别于传统框架梁柱子结构的特点,需对其抗连续倒塌性能开展研究工作。该文针对采用螺栓-角件连接节点的模块化钢结构梁柱子结构进行Pushdown试验及数值模拟,分析了在双柱失效、单柱失效工况下试件的破坏模式和抗连续倒塌机理。结果表明,梁柱子结构在两种工况下的受力机理不同:在双柱失效工况下,梁柱子结构抗力主要由梁机制提供,加载后期抗力提供方式体现出一定的悬链线机制,并随水平连接板厚度的增加而增强;在单柱失效工况下,子结构抗力主要由模块间水平连接抗剪能力提供,由梁机制及悬链线机制提供的抗力较小。

基于转动摩擦铰阻尼器的干式装配梁-柱节点抗震性能试验

基于转动摩擦铰阻尼器(RFHD),提出了转动摩擦耗能干式装配梁-柱节点(DRFDBJ)。为了验证DRFDBJ结构对于实现预期力学性能的可行性和合理性,以施加在摩擦片表面的螺栓预紧力(P_(c))为变量,开展了2个工况下的DRFDBJ试件低周往复拟静力试验研究。结果表明:DRFDBJ结构的力学性能主要由RFHD提供并控制,试验中节点呈现了稳定的承载力和理想的变形、耗能能力,并实现了预期的损伤集中;2个不同P_(c)水准下节点承载力的试验值与理论值误差不超过5%,通过调整P_(c)可实现节点承载力的调控,为DRFDBJ结构承载力的可调控提供了支撑。

钢管混凝土柱-钢梁外加强环螺栓连接节点抗震性能有限元分析

为深入研究钢管混凝土柱-钢梁外加强环螺栓连接节点的抗震性能,建立了螺栓连接节点的精细化三维实体有限元模型。在对已有试验成果进行验证分析的基础上,研究加劲肋、螺栓连接数量对节点塑性耗能分配机制的影响。结果表明:当钢材本构后期考虑韧性损伤后,有限元计算结果与已有拟静力试验结果更吻合;CFST-1节点(梁高300 mm,有螺栓垫板)耗能能力最优,但仅达到相应焊接节点的一半;与CFST-1节点相比,CFST-3节点(梁高300 mm,无螺栓垫板)及CFST-2节点(梁高250 mm,有螺栓垫板)的总耗能更低;各螺栓节点的塑性耗能分配机制主要依靠梁耗能,而相应的加强环焊接节点主要依靠梁与加强环耗能;当螺栓节点采用环板加劲肋、4排螺栓以及腹板加劲肋构造优化后,刚度、承载力、延性与相应焊接节点一致,总塑性耗能达到刚接节点的79.1%,基本达到刚接节点的抗震水平。

复合材料在汽车防撞梁上的应用研究进展

为促进复合材料在汽车构件中的应用,以汽车保险杠防撞梁为例,对复合材料防撞梁的材料、结构设计以及保险杠内部零件连接的研究进展进行综述。简述了碳纤维、玻璃纤维以及天然纤维等复合材料在汽车防撞梁上的研究进展,介绍了不同结构复合材料防撞梁对于耐撞性能的影响,分析了保险杠内部零件不同连接方式的优缺点。最后提出了汽车复合材料防撞梁的未来发展方向,应注重新型复合材料的开发、防撞梁结构的优化设计以及新型连接方式的研究。