Monotonic Moment Research Articles

Influence of Shallow Foundations on the Response of Steel Wind Towers

The objective of this study, concerning the soil–structure interaction of shallow reinforced concrete foundations of wind towers with circular cross-sections, was determination in a closed form of the monotonic moment–rotation curve of the soil–foundation complex. This study was based on elastic and plastic analyses of shallow rigid foundations assuming a Winkler soil type including the flexibility of the foundation in the elastic range and the nature of the soil (cohesive and non-cohesive types) through corrective factors of the constant of the Winkler model. The flexibility of the foundations influences the moment–rotation response through the initial rotational stiffness with a coefficient between 1 and 0.7 for a width-to-span ratio between 5 and 2. The nature of the soil is considered through corrective factors of 0.75 and 1.3 of the Winkler constant for cohesive and non-cohesive soil, respectively. Analyses carried out stressed that a possible design valued to be adopted in a steel wind tower with shallow foundations is a diameter of the steel tube 1/15 of the height of the tower, a diameter of foundation 0.75 of the length, and a depth of foundation 1/10 of the diameter and thickness of steel tower ratio diameter equal to 1/10. In this range it was observed that the effects of the soil-to-foundation interaction in the elastic range influences the critical length in the stability of the steel wind tower, with values between 2.5 and 2 (column fixed at the base) in a range of Winkler constant between 0.1 and 1 daN/cm3. Finally, an experimental validation of the proposed model was carried out with the data available from the literature.

A Numerical Investigation on Behavior of Column Base Plates with Different Configurations

Base plates are one of the most important types of connections in structures. Due to complicated steel-concrete interaction, simple assumptions of the stress distributions are usually employed for designing the connection. Simple assumptions of compressive stress distribution in concrete may accelerate the design procedure, but they may lead to overdesign results. In this study, six different types of base plates with different configuration were studied numerically using a commercial Finite Element (FE) software and the numerical model was calibrated with an experimental test. The models were subjected to a constant axial load and then a monotonic moment loading was applied. To investigate the effects of the axial load, several axial load level were considered for each configuration. As a result, moment-rotation curves of these base plates, including their rotational stiffness, in the absence and presence of the axial loads, were compared. Moreover, the stress distribution in the concrete was studied in the FE models. For all cases, the stress distribution in the concrete was semi-triangular with the maximum stress between the column flange and the edge of the plate. Based on numerical results, some concepts of simplified assumptions were proposed to find the stress distribution of the base plates. These assumptions are more realistic than current assumptions in structural specifications.

ULTIMATE STRENGTH OF CONCRETE FILLED SQUARE STEEL TUBULAR BEAM-COLUMNS

The objective of this study is to examine the ultimate strength of CFT columns. The range of the axial load ratio and the slenderness ratio in which CFT beam-columns reach the full plastic moment are examined on the basis of the strength formulas specified by AIJ Recommendation for Limit State Design of Steel Structures. The CFT columns are subjected to the constant axial compressive force and the monotonic moment at the one end, as the analytical parameters the axial load ratio and slenderness ratio are selected. The analysis is carried out by the shooting method. Bending moment-rotational angle relationships are calculated by the shooting method and the maximum strengths of CFT columns are obtained. When the value obtained by multiplying the axial load ratio and the second power of the slenderness ratio is 0.05, the maximum strength reach 95% of the full plastic moment under the condition that the axial load ratio value is less than or equal to 0.75. When the value obtained by multiplying the axial load ratio and the second power of the slenderness ratio is 0.1, the maximum strength reach 95% of the full plastic moment under the condition that the axial load ratio value is less than or equal to 0.5.

Behaviour of Top and Seat Angle Semi-rigid Connections

Traditionally connections are classified as pinned or rigid, but in actual practice most of the connections fall under semi-rigid category. The design procedures for semi-rigid connections are more tedious but prove to be more economical. The top and seat angle connection which have been traditionally considered as a pinned connection for simplifying frame analysis is found to exhibit semi-rigid behaviour. In the present paper, an all-bolted top and seat angle connection is investigated in detail using the software ANSYS 11. The effect of geometric parameters on the monotonic moment–rotation relationship is discussed.

Creep Stress Analyses Affected by Load Properties on P91 Pipe with Local Wall Thinning under High Temperature

The local wall thinning defect is very normal on pipes in power plants, which may result in stress redistribution of the pipes during the service process at elevated temperature. For the purpose of understanding the stress redistribution and strain accumulation of pipes with local wall thinning affected by load properties under creep condition, three groups of models were calculated, using three-dimensional elastic-plastic finite element analyses (FEA) based on FEA codes ABAQUS. In this study, the pipes has an identical defect of local wall thinning, the load properties and values are changed. Three groups of load properties, considering here, were monotonic internal pressure, monotonic moment and both internal pressure and moment, respectively. The numerical simulation conducted on P91 full-scale steel pipes at 625°C, with local wall thinning located at the inner surface. Then, von Mises stress and creep strain of pipes after 100,000h could be obtained corresponding to different models. Based on the analysis, the figures of creep stress and strain varying with load properties were plotted. Then, the stress and strain of pipes with local wall thinning affected by load properties were discussed. The results indicate that creep stress and creep strain increase with load properties. The variation laws have been summarized. The research results can provide the possibility on safety assessment and structure integrity analysis of the pipe with local wall thinning at high temperature effectively.

Simplified monotonic moment–curvature relation considering fixed-end rotation and axial force effect

A simple analytical procedure to analyze the reinforced concrete (RC) beams with a cracked section is proposed on the basis of the simplified moment–curvature relations of RC sections. Unlike previous analytical models which result in the overestimation of stiffness and underestimation of structural deformations induced from assuming perfect bond condition between steel and concrete, the proposed analytical procedure considers fixed-end rotation caused by anchorage. Furthermore, the proposed analytical procedure, compared with previous numerical models, promotes the effectiveness of analysis by reflecting several factors which can influence the nonlinearity of an RC structure into the simplified moment–curvature relation. Finally, correlation studies between analytical and experimental results are conducted to establish the applicability of the proposed analytical procedure to the nonlinear analysis of RC structures.

Stress block parameters for concrete flexural members reinforced with superelastic shape memory alloys

The unique properties of superelastic shape memory alloys (SMAs) have motivated researchers to explore their use as reinforcing bars. The capacity of a steel reinforced concrete (RC) section is calculated by assuming a maximum concrete strain ec-max and utilizing stress block parameters, α1 and β1, to simplify the non-linear stress–strain curve of concrete. Recommended values for ec-max, α1, and β1 are given in different design standards. However, these values are expected to be different for SMA RC sections. In this paper, the suitability of using sectional analysis to evaluate the monotonic moment–curvature relationship for SMA RC sections is investigated. A parametric study is then conducted to identify the characteristics of this relationship for steel and SMA RC sections. Specific mechanical properties are assumed for both steel and SMA. Results were used to evaluate ec-max, α1, and β1 values given in the Canadian standards and to propose equations to estimate their recommended values for steel and SMA RC sections.

Experimental and analytical studies on the cyclic behavior of end-plate joints of composite structural elements

The subject of this paper is the analysis of end-plate joints under cyclic and monotonic loading conditions by experimental and analytical studies. The experimental programs are performed on bolted end-plate type joints of composite members under cyclic loading conditions with the purpose to study the seismic response of the considered connection type. The performed experimental research is the second and third steps of an international research project started in 1999 between the Budapest University of Technology and Economics (BME), Hungary and the Technical University of Lisbon (IST), Portugal. The monotonic behavior of the tested joints is followed by the Eurocode standard design method to evaluate the moment resistance and rotational stiffness of the joint. The comparison of the design and the experimental results are performed by the envelope moment–rotation relationships of the hysteretic curves and the design moment–rotation diagram. On the basis of the comparison the modification of the design model is proposed. The monotonic moment–rotation diagram is extended to large rotation regions with the purpose of covering the whole cyclic diagrams until the final failure of the specimen. A semi-empirical method is proposed to approximate the cyclic hysteretic behavior of the studied joints, based on the knowledge of the monotonic moment–rotation curve. This prediction method is based on all the available test results for each behavior mode type (6 tests on steel and 12 tests on composite specimens). The calculated hysteretic curve follows the cycles by polygonal lines taking into consideration the experimental observations. The proposed method establishes the absorbed energy of the consecutive cycles in the case of the studied joint arrangement using standard loading history. The proposed method is applied and verified in the case of each observed failure mode type. By these experimental and analytical investigations the favorable seismic behavior can be derived for the studied joint type.

Slenderness effects on the simulated response of longitudinal reinforcement in monotonic compression

The influence of reinforcement buckling on the flexural response of reinforced concrete members is studied. The stress-strain response of compression reinforcement is determined computationally using a large-strain finite element model for bars of varied diameter, length, and initial eccentricity, and a mathematical expression is fitted to the simulation results. This relationship is used to represent the response of bars in compression in a moment-curvature analysis of a reinforced concrete cross section. The compression bar may carry more or less force than a tension bar at a corresponding strain, depending on the relative influence of Poisson effects and bar slenderness. Several cross-section analyses indicate that, for the distances between stirrups prescribed in modern concrete codes, the influence of inelastic buckling of the longitudinal reinforcement on the monotonic moment capacity is very small and can be neglected in many circumstances.

Cyclic moment?curvature relation of an RC beam

A hysteretic moment–curvature relation to simulate the behaviour of a reinforced concrete (RC) beam under cyclic loading is introduced in this paper, and the non-linear behaviour of RC beams subjected to flexural cyclic loading is analysed by means of the introduced model. Unlike previous moment–curvature models and the layered section approach, the proposed model takes into consideration the bond-slip effect by using the monotonic moment–curvature relation introduced in the companion paper. This model was constructed on the basis of the bond-slip relation and corresponding equilibrium equation at each nodal point. In addition, unlike linearised hysteretic curves, the use of curved unloading and reloading branches inferred from the stress–strain relation of steel considering the Bauschinger effect is employed. This gives more exact structural responses, following a correct assessment of the energy-absorbing capacity of a structure at the large deformation stage. The advantages of the proposed model, compared to the layered section approach, may be on the reduction in calculation time and memory space in application to large-frame structures with many degrees of freedom. The modifications of the moment–curvature relation to reflect fixed-end rotation, axial force and pinching effect are also introduced. Finally, correlation studies between analytical results and experimental studies are conducted to establish the validity of the proposed model.

Monotonic moment?curvature relation of an RC beam

A monotonic moment–curvature relation to simulate the non-linear behaviour of reinforced concrete (RC) beams is introduced, and material non-linear analyses of RC beams are carried out on the basis of the introduced model. Instead of the sophisticated layered section approach, which has some limitations in application to large structures with many degrees of freedom, we use the moment–curvature relations of RC sections previously constructed through section analysis. To reduce numerical instability according to the finite-element mesh size used, a relation simulating the tension-softening branch is taken into consideration. With the purpose of removing the imprecision in calculation of ultimate resisting capacity, we include the plastic hinge length in finite-element modelling. In addition, the governing equation describing the bond-slip behaviour in beams is derived. Unlike the present bond elements using double nodes, the proposed model uses beam elements representing the structural response by two nodes at both ends simplifies the finite-element modelling and analytical process, along with effectively describing the bond-slip behaviour. Moreover, the developed algorithm is reflected in the moment–curvature relation of the RC section. Finally, correlation studies between analytical and experimental results are conducted with the objective of establishing the validity of the proposed algorithms.

Cyclic moment–curvature relation of an RC beam

A hysteretic moment–curvature relation to simulate the behaviour of a reinforced concrete (RC) beam under cyclic loading is introduced in this paper, and the non-linear behaviour of RC beams subjected to flexural cyclic loading is analysed by means of the introduced model. Unlike previous moment–curvature models and the layered section approach, the proposed model takes into consideration the bond-slip effect by using the monotonic moment–curvature relation introduced in the companion paper. This model was constructed on the basis of the bond-slip relation and corresponding equilibrium equation at each nodal point. In addition, unlike linearised hysteretic curves, the use of curved unloading and reloading branches inferred from the stress–strain relation of steel considering the Bauschinger effect is employed. This gives more exact structural responses, following a correct assessment of the energy-absorbing capacity of a structure at the large deformation stage. The advantages of the proposed model, compared to the layered section approach, may be on the reduction in calculation time and memory space in application to large-frame structures with many degrees of freedom. The modifications of the moment–curvature relation to reflect fixed-end rotation, axial force and pinching effect are also introduced. Finally, correlation studies between analytical results and experimental studies are conducted to establish the validity of the proposed model.

Monotonic moment–curvature relation of an RC beam

A monotonic moment–curvature relation to simulate the non-linear behaviour of reinforced concrete (RC) beams is introduced, and material non-linear analyses of RC beams are carried out on the basis of the introduced model. Instead of the sophisticated layered section approach, which has some limitations in application to large structures with many degrees of freedom, we use the moment–curvature relations of RC sections previously constructed through section analysis. To reduce numerical instability according to the finite-element mesh size used, a relation simulating the tension-softening branch is taken into consideration. With the purpose of removing the imprecision in calculation of ultimate resisting capacity, we include the plastic hinge length in finite-element modelling. In addition, the governing equation describing the bond-slip behaviour in beams is derived. Unlike the present bond elements using double nodes, the proposed model uses beam elements representing the structural response by two nodes at both ends simplifies the finite-element modelling and analytical process, along with effectively describing the bond-slip behaviour. Moreover, the developed algorithm is reflected in the moment–curvature relation of the RC section. Finally, correlation studies between analytical and experimental results are conducted with the objective of establishing the validity of the proposed algorithms.

Strength and ductility of HPS flexural members

An experimental and analytical research program was recently completed that examined in detail the parameters affecting the strength and ductility of high-performance steel (HPS) flexural members. HPS is a term used to describe a new class of steels being produced under strictly controlled conditions that have high strength, usually greater than 448 MPa (65 ksi) and exceptional toughness and weldability. The mechanical characteristics of these steels are different from conventional steels, leading to concerns over their use in some structural applications. Under earthquake loading, flexural members are expected to deform inelastically, so members fabricated with HPS steels must possess adequate ductility. This paper discusses the inelastic behavior of welded, I-shaped flexural members fabricated from an HPS steel, HSLA-80, having a nominal yield stress of 550 MPa (80 ksi) and an ultimate strength between 610–690 MPa (90–100 ksi) and compares the results to similar flexural members fabricated from conventional A36 steel. The effects of material properties: yield stress, strain-hardening modulus, yield stress-to-ultimate strength ratio, and strain at ultimate stress; cross-section geometry: flange slenderness, web slenderness, and lateral slenderness; and loading condition: monotonic moment gradient, monotonic uniform moment, and cyclic moment gradient are described from the results of experimental testing and analytical modeling. The results are evaluated against the existing design criteria established in the AISC-LRFD specifications and recommendations are made for revising the specifications.

Transient response of moment-resistant steel frames with flexible and hysteretic joints

A robust method for geometrically nonlinear analysis of flexibly connected steel frames with hysteretic connection subjected to dynamic loads is presented. The proposed method revises the available monotonic moment vs rotation curves for typical connections to include the hysteretic loop characteristics for dynamic analysis of semi-rigid frames. Using this simple and efficient computer method of analysis, the influence of the nonlinear hysteretic loop for connection over the structural response in a large deflection range of a steel frame is studied. The extent of such an effect is quantified. It was found that neither the rigid nor the linear semi-rigid connection can predict accurately the response of moment frames where damping at connections is significant. Also, the use of several common connection models will generate similar load-deflection curves, indicating that the choice of these models may not be essential.