受焊缝缺陷影响的锥形薄壳屈曲性能研究

壳体表面的初始压力通常是在各种平板的轧制或焊接过程中产生的。提出与现行规范相关的基本设计规则非常重要。本文重点介绍壳体制作过程中由平板边缘的持续焊接造成的纵向缺陷。将14个试验试件分成2组,分别称之为SCC和DCC,并施加均匀静水压力。随着厚度由1t,2t增大到3t(t为薄壳厚度),试件出现1条或2条直线缺陷。该文得到的研究结果与国际上的规范及关于初始和整体屈曲及破坏的理论基本吻合。     

CFRP Effect on the Buckling Behavior of Dented Cylindrical Shells

Considering that the use of thin-walled shells is expanding every day, it is important to examine the problem of instability in this form of structure. Many steel structures such as high-water tanks, water and oil reservoirs, marine structures, and pressure vessels, including shell elements, are under stress tension. In addition, shell elements are subject to instability owing to the loads applied. Ten thin-walled cylindrical shell specimens in two groups with different dent depths of t_c and 2t_c, and the different dent number subject to uniform external pressure were tested in the present research (t_c is the thickness of cylindrical shell). The samples were modified to include either one or two dent line with amplitudes of h/3 in height (h the height of cylinder shell). Moreover, CFRP Strips on the dent depth was used in one of the groups. The results of testing under different theories and codes are compared.

     

Experimental study on damaged cylindrical shells under compression

Sensitivity to initial imperfections under compressive loading has been extensively studied in shell structures. However, due to the existence of a wide range of imperfections with various shapes and amplitudes, the real behavior of such structures needs to be further investigated when they face with a damaged area. This study presents an experimental program in which buckling and failure response of damaged shell specimens are analyzed. The results of this study can be generalized for many kinds of cylindrical shells to full scale of applications with similar D/t ratios.     

Tests and calculations for hollow structural steel (HSS) stub columns filled with self-consolidating concrete (SCC)

The behaviour of self-consolidating concrete (SCC) filled hollow structural steel (HSS) stub columns subjected to an axial load was investigated experimentally. A total of 50 specimens were tested. The main parameters varied in the tests are: (1) sectional types: circular and square; (2) steel yielding strength: from 282 to 404 MPa; and (3) tube diameter or width to wall thickness ratio (D/t or B/t): from 30 to 134.A mechanics model is developed in this paper for concrete-filled HSS stub columns. A unified theory is described whereby a confinement factor (ξ) is introduced to describe the composite action of the steel tube and the filled concrete. The predicted load versus deformation relationship was in good agreement with test results. The theoretical model was used to investigate the influence of important parameters that determine the ultimate strength of the composite columns. The parametric and experimental studies provide information for the development of formulae for the calculation of the ultimate strength and the axial load versus axial strain curves of the composite columns. Comparisons are made with predicted stub column strengths using the existing codes, such as ACI-1999, AISC-LRFD-1999, AIJ-1997, BS5400-1979 and EC4-1994.     

Photooxidation of benzene-structured compounds: influence of substituent type on degradation kinetic and sum water parameters

The combined influence of substituent type and UV/H₂O₂ process parameters on the degradation of four aromatic water pollutants was investigated using modified 3³ full factorial design and response surface methodology. Degradation kinetics was described by the quadratic polynomial model. According to the applied ANOVA, besides pH and [H₂O₂], model terms related with the pollutant structure are found to be significant. Different optimal operating conditions and values of observed degradation rate constants were determined for each of the pollutants indicating that the type of substituent influences the overall process effectiveness over structurally defined degradation pathway. Biodegradability (BOD₅/COD) and toxicity (TU) were evaluated prior to the treatment and at the reference time intervals t_1/2(P), t_3/4(P), t_1/4(OC) and t_1/2(OC) corresponding to the real time required to reduce the concentration of parent pollutant and organic content for 1/2, 3/4, 1/4 and 1/2 of initial amount. The observed differences are correlated to the structural differences of studied aromatics.     

Static and dynamic analysis of thin plates in bending— A novel finite element model

The static and dynamic behavior of thin flat plates in bending have been studied by means of a recently developed¹ doubly curved triangular shell element. The element's formulation is based on a modified mixed variational principle, wherein the primal variable σ (vector of shell stress resultants) and (boundary displacement vector) are required to satisfy a priori: (1) the complete shallow shell equilibrium equations, and (2) interelement C¹ displacement continuity. Several well-selected plate structures have been analyzed and the numerical results obtained indicate that the new element scheme competes most favorably with recently developed as well as with well-established elements included in commercial general-purpose finite element codes.     

Parametric study on buckling behaviour of dented short carbon steel cylindrical shell subjected to uniform axial compression

Generally, thin cylindrical shells are susceptible for geometrical imperfections like non-circularity, non-cylindricity, dents, swellings, etc. All these geometrical imperfections decrease the static buckling strength of thin cylindrical shells, but in this paper only effect of a dent on strength of a short (Lc/Rc∼1, Rc/t=117, 175, 280) cylindrical shell is considered for analysis. The dent is modeled on the FE surface of perfect cylindrical shell for different angles of inclination and sizes at half the height of cylindrical shell. The cylindrical shells with a dent are analyzed using non-linear static buckling analysis. From the results it is found that in case of shorter dents, size and angle of inclination of dents do not have much effect on static buckling strength of thin cylindrical shells, whereas in the case of long dents, size and angle of inclination of dents have significant effect. But both short and long dents reduce the static buckling strength drastically. It is also found that the reduction in buckling strength of thin cylindrical shell with a dent of same size and orientation increases with increase in shell thickness.     

A study of the influence of diameter and wall thickness of cylindrical tubes on their axial collapse

Axial compression experiments on aluminium cylindrical shells of diameter to thickness ratios (D/t) between 11.5 and 31.49 were conducted on a gravity drop hammer set up and Zwick machine. Typical histories of their deformation, variation of shell thickness along the fold length, inner and outer radii, folding parameter and size of fold, load–compression curves, energy absorbing capacity, initial peak load, and mean collapse loads obtained from the experiments are presented. Influence of the D/t values of the shell on their modes of collapse and energy absorption capacities are discussed. The shells are numerically simulated and analysed in detail by using the finite element code FORGE2. The material was modelled as rigid-viscoplastic. The experimental and computed results are compared. Typical contours of equivalent strain, equivalent strain rate, different stress components and velocity distribution are presented. The impact response of the shells is compared with their static response.     

Axial capacity of rectangular concrete-filled steel tube columns – DOE approach

This paper presents the effect of change in wall thickness of the steel tube (t), strength of in-filled concrete (f_cu), cross-sectional area of the steel tube (A) and length of the tube (L) on ultimate axial load and axial shortening at ultimate point of rectangular concrete-filled steel tubes (CFT). Taguchi’s approach with an L9 orthogonal array is used to reduce the number of experiments. With the help of initial experiments, linear regression models are developed to predict the ultimate axial load and the axial shortening at ultimate point. A total of 243 rectangular CFT samples are tested to verify the accuracy of these models at three factors with three levels. The experimental results are analyzed using Analysis Of Variance to investigate the most influencing factor on strength and axial shortening of CFT samples. Comparisons are made with predicted column strengths using the existing design codes, AISC–LRFD-1994 and EC4-1994.     

Multiple cardinality constraints and automatic member grouping in the optimal design of steel framed structures

For structural optimization problems, such as the weight minimization of steel framed structures, the sizing design variables are often defined as the cross-sectional areas of the members, which are to be chosen from commercially available tables such as those provided by the American Institute of Steel Construction. Alternatively, the cross-section dimensions, b_f, t_f, d and t_w (which may be discrete or continuous) can be defined independently for each profile. This paper discusses the structural optimization problem of framed structures involving sizing design variables where a special genetic algorithm encoding is proposed in order to establish a strategy to discover ideal member grouping of members. Advantages in fabrication, checking, assembling, and welding, which are usually not explicitly included in the cost function, are thus expected. The adaptive penalty method (APM) previously developed by the authors is applied to enforce all other mechanical constraints considered in the structural optimization problems discussed in this paper.     

Effects of lime addition on geotechnical properties of sedimentary soil in Curitiba,Brazil

The soil of the Guabirotuba geological formation(Paraná Basin, Brazil) has physico-mechanical properties which are not suitable for its utilization in pavement construction, in protection of hillsides and slopes, or as shallow foundation support. Treatment of this soil by lime addition would improve its usability. The present context intends to determine the ratio between the splitting tensile strength(q_t)and the unconfined compressive strength(q_u) of clayey soil in the metropolitan region of Curitiba City,which has been treated with different lime contents and curing times. The control parameters evaluated include lime content(L), curing time(t), moisture content(w), and ratio of porosity to volumetric lime content(η/L_v). It was observed that the q_t/q_u ratio is between 0.17 and 0.2 in relation to the curing time,and an exponential relation exists between them. Meanwhile, the unconfined compressive strength of lime-treated soil was found to be approximately four times the initial value.     

Axial strength of circular concrete-filled steel tube columns — DOE approach

This paper presents the effect of changes in diameter of the steel tube (D), wall thickness of the steel tube (t), strength of in-fill concrete (f_cu), and length of the tube (L) on ultimate axial load (P_ue) and axial shortening at the ultimate point (δ_ue) of circular Concrete Filled steel Tubes (CFT). Taguchi’s approach with an L9 orthogonal array is used to reduce the number of experiments. With the help of initial experiments, linear regression models are developed to predict the axial load and the axial shortening at the ultimate point. A total of 243 circular CFT samples are tested to verify the accuracy of these models at three factors with three levels. The experimental results are analyzed using Analysis Of Variance to investigate the most influencing factor on strength and axial shortening of CFT samples. Comparisons are made with predicted column strengths using the existing design codes, AISC-LRFD-2005 and EC4-1994.     

Behavior of thin-walled circular hollow section stub columns under axial compression

Thin-walled steel circular hollow section (CHS) is widely used in wind turbine towers, where the tube is subjected to axial load combined with bending moment. Understanding the behavior of axially loaded thin-walled tubes with large diameter-to-thickness (D/t) ratios is crucial for the design of such structures. To investigate the behavior of axially loaded thin-walled circular hollow section steel tubes, 16 stub columns were tested with the D/t ratio ranging from 75 to 300. The experimental results showed that the compressive strength decreased sharply with the increase of the D/t ratio. The experimental result indicated that the tubes with smaller D/t ratios failed with outward deformation without visible local buckling. With the increase of the D/t ratio, local buckling appeared at the peak load. The finite element method was used to model the behavior of axially loaded stub columns. The experimental results were used to validate the accuracy of the finite element results. Based on the finite element model, the influences of the D/t ratio, the initial geometric deformation and the initial residual stress were analyzed. Design guidelines in Eurocode3, AISC-LRFD, AS4100 and S16-9 on thin-walled steel members were used to compare with the experimental and calculated results. Among them, the results based on AS4100 were the closest to the experimental results.     

Plastic buckling of thin hemispherical shell subjected to concentrated load at the apex

This study presents the analytical, numerical, and experimental results of thin hemispherical metal shells into the plastic buckling range illustrating the importance of geometry changes on the buckling load. The hemispherical shell is rigidly supported around the base circumference against horizontal and vertical translation and the load is vertically applied by a rigid cylindrical boss at the apex. Kinematics stages of initial buckling and subsequent propagation of plastic deformation for rigid-perfectly plastic shells are formulated on the basis of Drucker–Shield's limited interaction yield condition. The effect of the radius of the boss, used to apply the loading, on the initial and subsequent collapse load is studied. In the numerical model, the material is assumed to be isotropic and linear elastic perfectly plastic without strain hardening obeying the Tresca or Von Mises yield criterion. Both axisymmmetric and 3D models are implemented in the numerical work to verify the presence of non-symmetric deformation modes in the case of thin shells. In the end, the results of the analytical solution are compared and verified with the numerical results using ABAQUS software and experimental findings. Good agreement is observed between the load–deflection curves obtained using three different approaches. A secondary bifurcation point is detected in thin shells in which the deformation degenerates from symmetric to non-symmetric behavior. The bifurcation point depends on the (R/t₀) ratio and the material parameters.     

A closed-form solution for elastic buckling of thin-walled unstiffened circular cylinders in pure flexure

To date, despite the significant development in the field of structural mechanics, there still remains a paradox in the solutions available for a classical shell buckling problem. The difference in strength between a cylindrical shell under uniform axial compression and that under pure bending is not quite well investigated. This lack of research is reflected in the wide variations in the elastic bending strength and the slenderness limits given in current international design standards. The discrepancies in the available classical solutions and hence the design rules have initiated the current research. The main aim of this paper is to present a closed-form solution for the elastic buckling strength of unstiffened circular cylinders under pure bending using a new simplified energy approach employing the well-known Ritz method. Two types of analyses are presented for cylinders with large (D/t>200) and medium (100<D/t<200) diameter-to-thickness ratios. A unique testing rig was used to experimentally verify the new theory using a Moiré fringe film. The theoretical results are compared against the available and present test results and the existing classical solutions. The current design rules for thin-cylinders in international steel specifications are also compared, and the newly derived design curve is proposed which was found in a good agreement with the available test results.     

Strength of biaxially loaded orthotropic plates

An approximate and simple method for rectangular orthotropic plates, loaded biaxially along two mutually perpendicular directions is presented. The theory is based on the energy method and it is free from difficult calculations such as those encountered when large deflection theory is used. Effects of initial imperfections are taken into account. The number and lengths of half-waves are assumed to be the same in the post-buckling stage as at incipient buckling. The finite buckled shape in the post-buckling stage is taken to be the same as that during incipient buckling. The Von Mises yield criterion is employed to determine the upper bound solution of the plate. Interaction curves for square plates subjected to longitudinal and transverse stresses, applicable to three different boundary conditions, four edges simply supported or four edges clamped or two opposite edges clamped with the other two edges simply supported, are presented in a non-dimensional form. Results for plates having different values of plate slenderness (b/t) and flexural rigidity ratios, D₂/D₁ are given.     

The in-plane behaviour and FE modeling of a T-joint connection of thin-walled square tubes

The behaviour of square hollow tubes with T-joints subjected to in-plane loading conditions is examined numerically. The stresses in the joint and its stiffness are determined from the finite element (FE) models using 2-D shell and 3-D solid elements. The parameters identifying the joint's stress concentration (ξ_m, ξ_b), and the bending stiffness reduction (α_K) are defined in terms of the ‘thin-wallness’ ratio (the side length to thickness) of the tube. These parameters permit modeling accurately the tubes and the T-connection by simple 1-D beam elements with certain properties modified. The FE models consisting of beam elements are significantly easier to prepare and analyze. The effectiveness of the approach is demonstrated by applying it to a structure with a T-connection and dimensions typical for the agricultural industry. It is shown that such beam models provide accurately all important information of the structural analysis (i.e., the stresses, displacements, reactions forces, and also the natural frequencies) at substantially reduced computational effort in comparison with the complicated FE models built of shell or solid elements.     

Fate of aniline and sulfanilic acid in UASB bioreactors under denitrifying conditions

Two upflow anaerobic sludge blanket (UASB) reactors were operated to investigate the fate of aromatic amines under denitrifying conditions. The feed consisted of synthetic wastewater containing aniline and/or sulfanilic acid and a mixture of volatile fatty acids (VFA) as the primary electron donors. Reactor 1 (R1) contained a stoichiometric concentration of nitrate and Reactor 2 (R2) a stoichiometric nitrate and nitrite mixture as terminal electron acceptors. The R1 results demonstrated that aniline could be degraded under denitrifying conditions while sulfanilic acid remains. The presence of nitrite in the influent of R2, caused a chemical reaction that led to immediate disappearance of both aromatic amines and the formation of an intense yellow coloured solution. HPLC analysis of the influent solution, revealed the emergence of three product peaks: the major one at retention time (R_t) 14.3 min and two minor at R_t 17.2 and 21.5 min. In the effluent, the intensity of the peaks at R_t 14.3 and 17.2 min was very low and of that at R_t 21.5 min increased (∼3-fold). Based on the mass spectrometry analysis, we propose the structures of some possible products, mainly azo compounds. Denitrification activity tests suggest that biomass needed to adapt to the new coloured compounds, but after a 3 days lag phase, activity is recovered and the final (N₂ + N₂O) is even higher than that of the control.     

Shear strength of horizontally curved steel I-girders — finite element analysis studies

This paper presents the results of finite element analysis (FEA) studies of four curved steel I-girder shear components tested experimentally in previous research, as well as parametric extensions of these tests. These studies focus on the influence of horizontal curvature on the maximum strength of transversely stiffened members with web slenderness D/t_w approximately equal to the largest value permitted in AASHTO [AASHTO LRFD bridge design specifications. 3rd ed. In: 2005 Interim Provisions, Washington (DC): American Association of State and Highway Transportation Officials; 2004], and with panel aspect ratios of d_o/D=1.5 and 3.0. These ratios are larger than previously considered in experimental tests of curved I-girders with similar or larger slenderness. The girders studied have subtended angles between their bracing locations of L_b/R=0.05 and 0.10, and web panel d_o/R values ranging from 0.03 to 0.10. The FEA models incorporate the measured material stress-strain relationships and section dimensions from the physical tests, detailed modeling of the test boundary conditions, residual stresses due to flame cutting and welding, and initial geometric imperfections in the form of buckling mode shapes. The load transfer mechanisms of the test girders are investigated via elastic buckling and full nonlinear analyses. The parametric studies are performed to investigate the effects of residual stresses and geometric imperfections, the behavior of equivalent straight girders, and the influence of reduced flange size on the peak shear capacity and moment-shear interaction.     

Experimental collapse of thin cylindrical shells submitted to internal pressure and pure bending

A thin-walled pressurised cylindrical shell is sensitive to buckling phenomena when it experiences locally a compressive stress. It is often considered that its behaviour under bending is rather similar to pure compression, but very few are the experimental investigations that precise the real behaviour of a thin pressurised cylinder submitted to a bending load. A large amount of experimental results is presented here, obtained on thin shells (550<R/t<1450) of moderate length (L/R≈2). The evolution of the cylinders' behaviour that has been recorded when internal pressure increases is outlined. It is shown that one must distinguish between local buckling and global collapse of the structure. A comparison of our experimental data to design recommendations given by two standards (NASA SP8007 and Eurocode 3) is finally achieved, putting in advance safety margins provided by these codes.