The method of strengthening steel structure by welding while under load has now been widely used in numerous reinforcement projects.However, owning to various influence factors of technology itself and comprehensive complexity of process control, systematic research and results have not yet been accomplished at home and abroad.By comparison of influence factor analysis results, domestic and foreign test results and codes calculation results, a new suggested coefficient and formula for calculating welding residual deformation of steel members strengthened by welding under load were proposed.Simultaneously, constructive suggestions on out-plane stability capacity of steel beam-columns were put forward.

In order to study the seismic performance of the beam-to-column connections reinforced with cover-plate in steel frames and provide a reference for the new technical specification for strengthening steel structures, the FEM analysis under cycling loading for four beam-to-column connections with different initial loading was carried out to study the failure modes,limited resistance,hysteretic behavior,ductility, energy dissipation capacity,stiffness degradation of the connections, which were compared with unreinforced connections.The result showed that the reinforcement could effectively improve the limited resistance and ductility of the connections, which made plastic hinge shifts away a quarter of the beam height from the cover-plate end, thus avoiding the damage at the weld location and achieving the purpose of the reinforcement.

The compressive behavior of concrete exposed to-190℃ with different strength grades including C30,C40,C50,and C60 was investigated through experiments with 5 different kinds of mix proportion.The results showed the experimental macro-phenomena of concrete exposed to-190℃ differed greatly from those at normal temperature.The specimens tended to present an obvious brittle failure,and the failure modes of all specimens were alike.The compressive strength of all specimens exposed to-190℃ increased obviously,and the strength grade of concrete had a great influence on its compressive strength.The extent of the increase in concrete compressive strength depended greatly on the water content in the specimen.The compressive strength of concrete exposed to-190℃ with C30 and C40 was increased by 40％ and 65％,respectively,and they became similar to those for C50 and C60 and increased to above 190％ after their water content in concrete was increased.Based on the experiment results and the existing research results,a predictive model for the compressive strength of different strength grade concrete exposed to-190℃ was proposed and is expected to be applicable for reliable design and evaluation of concrete structures such as LNG tanks.

The static test of steel beam?columns was conducted to investigate the heat effect during process of welding strengthening while under initial load and the influence of different initial load on load?carrying behavior after welding reinforcement. The biaxial?symmetry I section columns were placed under boundary conditions that top of column was movable in plane while displacement constrained out plane and bottom of column was totally constrained. Therein 4 classes of initial loads were separately applied to the top of columns at a fixed in?plane eccentricity. Strengthening scheme was selected by symmetrically welding steel plates to outside surfaces of flanges, and grade of steel was carbon steel type Q345B. The displacement changes of specimen and welding stress and strain distribution of web were discussed during welding strengthening while under load, as well as the buckling mode and stability bearing capacity after strengthening. The results show that welding strengthening procedure decides the development mechanism of welding residual deformation and strain distribution. Initial load affects the magnitude of welding residual deformation while heat input and initial load coeffect the plateau length of load?displacement curve. Lower initial load has little influence on capacity while higher initial load obviously reduces capacity. Besides, the initial geometric imperfections also impact capacity after strengthening.

The seismic performance of masonry structure is low,and the existing large number of old masonry residences do not meet the requirements of the current seismic codes,therefore strengthening the masonry structures is necessary.The external shear wall reinforcement method is capable of providing an effective seismic reinforcement for masonry structures on the condition of improving the living conditions and it is extensively used in the seismic energy saving for old residences in Beijing.This method not only reduces the nuisance problems,but also has a good seismic strengthening effect.A seismic strengthening example is analyzed and the actual reinforcement effect of the external shear wall is illustrated.

In order to investigate the influence of reinforcement welding heat effect and different initial load on load-carrying behavior of I section steel beam-columns,three specimens strengthened by welding while under different load were simulated.Based on the thermal-structural coupling method considering heat effect,the analysis improved thermal input of heat source model and also considered initial geometric imperfection,initial residual stress and friction.Time history of displacement during welding,web stress-strain redistribution and load-displacement relationship were discussed,and meanwhile the effectiveness of the finite element analyses (FEA ) were verified by comparing the FEA results and test results. Furthermore,welding temperature field,stress-strain redistribution between flange and strengthening plate and flange margin yield capacity,which could not be measured by test,were acquired.And by comparison of bearing capacity results and code calculation results,the present design methods were discussed.The results showed that welding strengthening procedure decided the development of welding residual deformation,while thermal input and initial load effected the range of displacement change during welding under load and magnitude of welding residual deformation. Higher initial load propelled the development of stress-strain redistribution toward the eccentric loading direction,thus leading to lower bearing capacity.However,initial residual stress had little effect on ultimate capacity.FEA method considering heat effect showed certain practicability and overall security,and code design method remained space for improvement still.

Through experiments of 4 strength grades concretes cooled to -190℃and back to room temperature, the compressive strength was discussed.From the test results it could be shown that the effects caused by temperature change were different in 4 grades concretes.For high grade concrete, the strength reduction was little; instead, the strength reduction was more than 20% for low grade concrete.Heating modes made difference also.The reduction was larger through directly put in room temperature.Based on the experiment and the existing results, a relative formula for the compressive strength of concrete applied to the engineering design of concrete structures through-190 ℃and back to room temperature was also provided.Which could provide a reference for rational design of the concrete structures of LNG tank etc.

The purpose of this paper is to study the seismic performance of the beam-to-column connections rein-forced with cover-plate in steel frames and to provide a reference for the new technical specification for strengthening steel structures.The tests under cycling load for four beam-to-column connections with different initial loadswere carried out to study the failure modes,limited resistance,ductility,energy dissipation capacity,stiffness degrada-tion of the connections,and the results were compared with those with unreinforced connections.The result shows that the reinforcement can effectively improve the limited resistance and ductility of the connections,move plastic hinge away to a quarter of the beam height from the cover-plate end,effectively avoiding the damage at the weld location and achieving the purpose of reinforcement.

In order to study the finite element analysis method used to simulate the end plate connection nodes reinforced by welding under loads and its mechanical characteristics,mechanical properties of four end plate connection nodes beams reinforced by welding under different initial loads were analyzed by applying the general software ANSYS to establish the finite element models.The common method used only the technology of birth and death element without considering welding thermal process.The author had used the indirect thermal structure coupling method in finite element analysis considering the material properties of temperature-dependent stell and the welding thermal process and the finite element analysis method with the influence of welding process under loads taken into consideration was put forward.The effectiveness of the finite element model and the FEA method were verified by the comparison between the finite element analysis results and the test results.The finite element analysis results showed that the initial loads would greatly influence the ultimate bearing capacity of stell-structure end plate connection nodes that reinforced by welding under loads.

In order to investigate the mechanical behavior of compression‐bending member of steel structure strengthened by welding under load ,the finite element analysis method considering welding heat effect was used to analyze the influence factors , such as initial load , eccentric distance ,slenderness ratio ,class of weld heat input . The whole loading process simulation analyses of 72 members of unstrengthened and strengthened bending steel columns were carried out ,and the influence rules of the factors were acquired .The new second‐order design formula of nominal stress was verified as well .The results show that initial load level can be evaluated by the second‐order formula .The mode and amount of initial geometrical imperfections effect the direction of buckling and magnitude of residual deformation . Besides , the factor order of influencing on the ultimate capacity of strengthened specimen is eccentricity ,slenderness ratio , welding heat input and initial stress .While concerning the welding residual deformation ,the factor order turn out to be welding heat input ,initial stress and slenderness ratio .The conclusion can provide reference and basis to establish the design calculation method of compression‐bending member strengthened by welding under load in Code for Design of Strengthening Steel Struc‐ture .

Considering the slip between the steel beam and concrete slab,the governing motion equations of steel-concrete composite beams were deduced with the direct balanced method of the composite beam theory. By variable separation approach,the analytical solutions to natural frequencies and mode shapes of simply-sup-ported composite beams were obtained.Under the orthogonal conditions,the dynamic expression of composite beams under moving loads was obtained.The natural frequency and dynamic responses of the composite beam under moving loads were analyzed theoretically and then compared with the experimental results,consequently showing good agreement.The results show that due to the flexible shear connectors,the relative slip occurs between the steel beam and concrete slab of the composite beam,which makes the dynamic equations of the composite beam under moving loads more complex and produces different dynamic characteristics as compared with conventional beams.

The method of strengthening steel structure by welding while under load has been widely used in numerous reinforcement projects.However, despite some general tips, there are no specific provisions in present foreign technical standards.The few specific design provisions are only left within Chinese standards Technical Specification for Strengthening Steel Structures ( CECS 77∶96 ) and Technical Specification for Inspection, Assessment and Strengthening of Steel Structures ( YB 9257—96 ) .The provisions framework of domestic code and specification to solve the problem of strengthening by welding under load was combed.And the calculation examples and analyses related to design methods of steel columns under axial and eccentric load were given. The concept of joint reinforcement coefficient was also proposed to make unified definition for the unreasonable situation in two standards. By comparison, the discrepancies and inadequacies were pointed out to provide reference for the follow-up researches and code revision.

This paper is concerned with the dynamic analysis of simply-supported steel-concrete composite beams under moving loads. Considering the interface slip between steel girder and concrete slab, the governing motion equations are derived from the direct balanced method. By variable separation approach, the analytical solution of natural frequencies and mode shapes are obtained, as well as the orthogonal conditions. Then the dynamic responses of the composite beam under moving loads are analyzed, and compared with the experimental results. The analysis results show that the governing motion equations become more complicated when interface slip is taken into account, and the dynamic behaviors are significantly influenced by the shear connection stiffness. In the dynamic calculation of composite beams, the global stiffness should not be reduced as the same factor to all orders, but as different ones according to the dynamic stiffness reduction factor (DSRF), to which should be paid more attention in calculation, design and experiment, or else great deviation is inevitable.

通过常温及-30℃至-120℃间冻融循环作用混凝土的试验，得到混凝土超低温冻融循环下受压强度的变化规律。结果表明，经历冻融循环作用的混凝土破坏形态基本同常温，但冻融循环上限温度时不再呈对顶棱锥状；冻融循环作用导致的混凝土受压强度恶化主要集中于初期，随冻融循环次数继续增加则影响较小；冻融循环作用上下限温度时的混凝土受压强度变化规律相似，不同冻融循环工况下混凝土受压强度在冻融循环开始时相似，但随后有所不同。

经过20多年的不间断服役,北京市天宁寺二环主路桥的部分结构已出现一些病害,部分支座处的梁体与支座发生较大的相对偏移,甚至造成个别限位螺栓剪断.本文以加固后的天宁寺桥的结构健康监测为例,对基于复核计算指导的既有桥梁的健康监测方法及其应用进行研究,得到确定监测对象和监测指标的方法.研究结果表明:在实施监测项目之前,预先通过理论和数值分析对结构的整体状态、局部状态的静力和动力特性指标进行分析,找到影响监测项目的关键参数,进而选取合理的监测参数,可达到全面获取结构关键状态指标的目的.同时也可对监测过程中的参数变化规律、可能出现的测试结果以及相应的监测方案调整进行有针对性的预测,从而提高监测系统运行的效果和可靠性.

In order to study the effects of initial load applied to end-plate connection nodes on the mechanical proper-ties of welding reinforcement,static load test was completed. The test consisted of four different load levels,and the results were compared with those in Ref. [2],where the end-plate connection nodes were without reinforcement. Ma-terial types were all steel in grade Q345B,except that the high-strength friction-bolts were of M20 type with 10.9-magnitude. The mechanical properties of high-strength friction-bolts and the failure modes of node domain under dif-ferent load levels were studied. Moreover,the tension distribution of bolts,the shear deformation of node domain and the bending moment-rotation curve of nodes were given.

为进行城市桥梁养护与管理的深入研究,本文以北京市城市桥梁的现状和历年桥梁结构定期检测结果为研究背景,在定期检测的基础上,对北京市的城市桥梁养护与管理进行了剖析.分析结果表明:北京市近80％的桥梁为90年代建成,随着桥梁数量的进一步增加,今后一段时期内大部分桥梁的状态劣化衰退速度会呈现加快趋势,为应对旧桥养护高峰期的到来,必须尽快建立行之有效的养护和管理机制.本文探讨了基于结构定期检测的城市桥梁养护和管理机制,该思路、方法可供类似案例所参考.

泡沫玻璃作为液化天然气运输与储存设施的保温材料,至少应能承受其自重荷载.通过对泡沫玻璃砖常温和超低温下不同容重及制作精细程度的多组试件进行单轴受压性能试验,结合与常用建筑材料混凝土受力性能的对比,发现超低温下泡沫玻璃砖受力性能有所恶化且受压脆性明显,高容重与高制作精细程度有助于其强度提升.泡沫玻璃砖超低温受力性能明显不同于混凝土,实际工程中须考虑其受力特性的差异.