Moment-rotation Characteristics Research Articles

RHS beam-to-column connection with web opening—experimental study and finite element modelling

Rectangular Hollow Sections (RHS) have superior structural performance compared to conventional steel sections. However, the application of RHS in structural steel framework is limited because suitable connection configurations have not been developed between such members. Also adequate information on the moment–rotation characteristics of connections between RHS members is not available for design. To overcome these problems, a new and efficient connection is proposed which is easy to fabricate and convenient for erection. The connection employs channel connectors welded to the column flange and bolted to the beam to transfer beam flange forces into the column webs thereby avoiding the need to provide internal diaphragms in the column. An opening in the web facilitates the installation of bolts and can be used to pass service lines. The bolts are loaded in shear, so as to obtain improved performance of the connection under cyclic loading. By choosing suitable dimensions for the channel connectors, the strength and stiffness of the connection can be varied. The behaviour of the connection is evaluated by cyclic tests and non-linear finite element analysis. Test results are presented in the form of hysteretic curves and failure modes. In the case of channel connectors of high strength, failure occurs at the beam net section away from the face of the column, similar to beams with Reduced Beam Sections (RBS). It is shown that the connection has adequate ductility and hysteretic energy dissipation capacity. The moment–rotation characteristics of the connection can be expressed in terms of the three-parameter power model, for semi-rigid frame analysis.

Modelling of unstiffened flush end-plate bolted connections in fire

The present paper studies the behaviour of flush end-plate bare-steel connections at elevated temperatures using the general purpose finite element software ABAQUS. The finite element model was used to establish the moment–rotation characteristics of the connections under the combined loading of a concentrated force and elevated temperature. The connection components were modelled using three-dimensional brick elements, while contact between the various components was modelled using Coulomb friction. Materials for steel members and connection components were considered to behave nonlinearly. Degradation of steel properties with increasing temperature was assumed in accordance with EC3 recommendations. Finite element results and experimental tests conducted on flush end-plate connection in fire conditions were compared, and these showed good agreement. The obtained simulated failure modes and moment–rotation–temperature characteristics of the connections compared well with the experimental data.

Combined flexure and web crippling strength of a low-ductility high strength steel decking: experiment and a finite element model

Profiled decking or sheeting of high strength low-ductility steel (ASTM611 Grade E) is a relatively new development in building construction industry. Due to inadequate information, the major international codes of design practices are yet to include this steel, and apply additional restrictions on their design and use. This type of decking shows high sensitivity to distortional as well as local buckling. Strength of such decking under combined flexure and web crippling as well as moment–rotation capacity are of principal concern if such decking is to be designed as a continuous structure to achieve better economy. This paper describes an experimental study of the behaviour of re-entrant decking of low-ductility steel under combined web crippling and flexure. Based on this experimental study, a nonlinear finite element model has been proposed which can predict the moment–rotation characteristics of such decking with reasonable accuracy.

Behaviour of composite haunched beam connection

This paper is concerned with the behaviour of steel–concrete composite haunch connections. Experiments were carried out to investigate the moment-rotation characteristics and ultimate moment capacity of these connections. Details of the experiments giving information on test specimens, instrumentation, test set-up and test procedures are described. Results obtained for connection moment capacity, rotation capacity and failure modes are presented. It is found that through proper design and detailing, these connections display the characteristics of a rigid connection.

Bending collapse of thin-walled beams with ultralight filler: numerical simulation and weight optimization

A study on the bending collapse of thin-walled beams filled with aluminum foam or aluminum honeycomb was carried out. The strengthening effects of ultralight metal fillers were quantified numerically and experimentally. The moment-rotation characteristics of filled sections were derived, and the results were then incorporated with structural optimization technique to develop a methodology for crashworthiness optimization of filled members. The proposed methodology requires relatively simple computations and is suitable for early stage design of crash members. Finally, the optimization problem of filled sections under combined compression/bending loading was formulated and solved. The optimization results showed potentials of significant weight saving and volume reduction by utilizing ultralight metal filler.

Experimental and numerical study on bending collapse of aluminum foam-filled hat profiles

Abstract The deep bending collapse of thin-walled empty and foam-filled hat profiles are studied experimentally and numerically. The deep bending tests involve two steps. First, the bending tests are run in a conventional three-point bending mode up to approximately 40° rotation. Then, the bent specimens are loaded vertically in compression, and are bent further up to 150°. These tests revealed salient features of empty and foam-filled hat members subjected to large bending rotations. The moment–rotation characteristics of the generalized plastic hinges with the rotation angle up to 150° are obtained experimentally, and they are found to be qualitatively similar to the case of axial folding of thin-walled members, with peaks and troughs on M–θ curves corresponding respectively to the initiation of foldings and jammings. It is found that foam-filled members provided a 30–40% increase, compared to traditional non-filled members, of the specific energy absorption. Numerical study using explicit nonlinear finite element method are also conducted. Quasi-static simulations of deep bending collapse of empty and foam-filled single-hat beams are carried out. Numerical results are compared with the experimental results showing good agreement.

Nonlinear finite element analysis of top- and seat-angle with double web-angle connections

Four finite element (FE) models are examined to find the one that best estimates moment-rotation characteristics of top- and seat-angle with double web-angle connections. To efficiently simulate the real behavior of connections, finite element analyses are performed with following considerations: 1) all components of connection (beam, column, angles and bolts) are discretized by eight-node solid elements; 2) shapes of bolt shank, head, and nut are precisely taken into account in modeling; and 3) contact surface algorithm is applied as boundary condition. To improve accuracy in predicting moment-rotation behavior of a connection, bolt pretension is introduced before the corresponding connection moment being surcharged. The experimental results are used to investigate the applicability of FE method and to check the performance of three-parameter power model by making comparison among their moment-rotation behaviors and by assessment of deformation and stress distribution patterns at the final stage of loading. This research exposes two important features: (1) the FE method has tremendous potential for connection modeling for both monotonic and cyclic loading; and (2) the power model is able to predict moment-rotation characteristics of semi-rigid connections with acceptable accuracy.

Probabilistic study of the behavior of steel frames with partially restrained connections

The reliability of steel frames with type PR (partially restrained) connections is dependent on the variability in the moment–rotation characteristics of the connections. In this paper, the contribution of this source of variability to the uncertainty in frame deformation is examined using stochastic finite element analysis to compare the effect of initial connection stiffness to that of bending stiffnesses of the beams and columns. The PR connections have top and seat angles connecting the beam and column flanges, and double web angles connecting the beam web to the column flange. A stochastic finite element analysis is used to develop system fragilities, which depict the probability of exceeding three limit states related to the maximum horizontal displacement, maximum interstory drift ratio and elastic bending moment capacity as a function of lateral force. The effect of the variability in initial connection stiffness on frame response is sufficiently important to require careful consideration in performance-based design.

Stiffness of restraint for steel struts with elastic end supports

The force induced in a restraint for steel struts is given by BS 5950, Part 1, 1990 as 1% of the applied axial load, and for struts forming the compression flange of lattice girders as 2·5%. These values are used in the ultimate strength design of restraints. The code also states that these restraints should have sufficient stiffness but it does not quantify this requirement. The available literature deals with the determination of restraint forces and their required stiffness for struts with pinned or fixed ends. In practice, it is rare to have such ideal end conditions, but somewhere in between. This is due to the fact that connections in structural steelwork possess resistance to moment ranging from nominal to full rigidity depending on the moment–rotation characteristics of the joint under consideration. This paper deals with the design of restraints considering strength as well as stiffness through a mathematical model based on the elastic behaviour of a strut elastically supported at its ends and restrained at a point located along its length by an elastic restraint. The results obtained show that, for a given induced restraint force, the restraint stiffness requirement decreases with the increase of rotational stiffness at the ends of the strut. Although this, to some extent, is expected from a qualitative point of view, it does deserve to be better known by designers, and the present work attempts to quantify the effect of the various parameters involved so that the results can be used in the relevant design cases.

Stiffness of restraint for steel struts with elastic end supports

The force induced in a restraint for steel struts is given by BS 5950, Part 1, 1990 as 1% of the applied axial load, and for struts forming the compression flange of lattice girders as 2·5%. These values are used in the ultimate strength design of restraints. The code also states that these restraints should have sufficient stiffness but it does not quantify this requirement. The available literature deals with the determination of restraint forces and their required stiffness for struts with pinned or fixed ends. In practice, it is rare to have such ideal end conditions, but somewhere in between. This is due to the fact that connections in structural steelwork possess resistance to moment ranging from nominal to full rigidity depending on the moment–rotation characteristics of the joint under consideration. This paper deals with the design of restraints considering strength as well as stiffness through a mathematical model based on the elastic behaviour of a strut elastically supported at its ends and restrained at a point located along its length by an elastic restraint. The results obtained show that, for a given induced restraint force, the restraint stiffness requirement decreases with the increase of rotational stiffness at the ends of the strut. Although this, to some extent, is expected from a qualitative point of view, it does deserve to be better known by designers, and the present work attempts to quantify the effect of the various parameters involved so that the results can be used in the relevant design cases.

Semi-rigid column–base connections in pultruded GRP frame structures

A review of moment–rotation tests on bolted connections between pultruded Glass Reinforced Plastic (GRP) Wide Flange (WF) sections highlights the lack of test data on column–base connections. Details of five bolted web and web and flange cleat connections and the test rig used to determine their moment–rotation characteristics are given. Moment–rotation responses are then presented for each connection. Simple models are described for predicting their initial rotational stiffness, ultimate moment and moment–rotation response. The models predict the response of bolted web and flange cleat column–base connections reasonably well and may be useful for design.

Full-scale field tests on directly embedded steel pole foundations

The results of eight full-scale tests on directly embedded steel pole foundations are presented. Fully instrumented poles were tested to measure the various design parameters. Different types of backfills such as sand, in situ gravelly sand, crushed stone, and flowable material were used. Various parameters were measured, including applied moment, ground line deflection-rotation, rotation of the pole below the ground level, soil pressures, and bending moments in the poles. The behaviour of these foundations was explained through ultimate capacity and moment-rotation characteristics. Based on these test results, it was found that the capacity of the directly embedded pole foundation depends primarily on the compaction levels of backfill and the embedment length of the pole. Flowable backfill material, which does not require any compaction, was found to be most effective and promising. Even when the backfill was loose, the lateral capacity significantly increased by the addition of a baseplate or by installing the pole with an additional embedment depth. Various theories developed for laterally loaded rigid piles were used to predict the moment-rotation behaviour and the ultimate capacity of the directly embedded pole foundation with different types of backfill material. Results from the analytical investigations were compared with those obtained from the full-scale load tests. Comparisons show that the ultimate capacities predicted by the models ranged from 0.30 to 2.20 times the measured capacities.Key words: backfill, compaction, full-scale tests, laterally loaded rigid piles, transmission steel poles, ultimate moment.

Moment-Rotation-Temperature Characteristics of Steel/Composite Connections

This paper presents a 3D mathematical model based on the finite-element method to simulate the structural response of steel and steel/concrete composite connections so that their moment-rotation-temperature characteristics can be predicted. The treatment of material nonlinearity at elevated temperatures is described. A special element has been developed to model the bolts, which may expand freely without creating unrealistic forces on the connected components. The composite concrete slab is modeled by 3D brick elements with a simple crack-crush capability. The numerical model was compared with the results from eight different series of connection fire tests, all incorporating tests with different loading levels. The type of connections included those that are normally assumed to be “pinned” and those that could be classified as “rigid.” The range of parameters included the end-plate thickness and the number and size of bolts. This theoretical model is shown to be capable of providing accurate moment-rotation characteristics of connections that can be incorporated in the analysis of global frame behavior in the event of fire. It can also enrich the database of connection behavior at elevated temperatures.

Collapse behaviour of sway frames with end-plate connections

A test program consisting of 10 frame specimens loaded to collapse at the National University of Singapore as part of the broad investigation on “analysis and design of semi-rigid frames” has been completed and briefly reported in Liew et al. (1997). This paper provided the details of three test specimens and describes the test observations of the sway frames under the combined actions of gravity and lateral loads. The load–deformation responses of the test frames and their connections under sequential and non-sequential loading are determined and compared with theoretical predictions. The moment–rotation characteristics of such connections are determined so that advanced analysis can be carried out and compared with the test results to determine if the ultimate load carrying capacity and load–deflection characteristics of such frames can be predicted by the proposed analysis method. This paper focuses the discussion on three of the frame specimens constructed using extended end plate connections. Comparison of test results with second-order refined plastic hinge analysis and Eurocode are made using the moment–rotation characteristics of individual beam-to-column connections established through tests.

Effective length factor of columns in flexibly jointed and braced frames

In order to rationally evaluate the stability of columns in flexibly jointed and braced frames, the governing equations for determining the effective length factor ( K-factor) of columns are derived considering the effects of the non-linear moment–rotation characteristics of beam-to-column connections. In this study, to derive the equations, the alignment chart approach is used and a recent study on the stability of flexibly jointed frames and the concept of the Shanley's inelastic column buckling theory are applied. With a proper evaluation of the tangent connection stiffness for semi-rigid beam-to-column connections at buckling state and with the introduction of the modified relative stiffness factors, the alignment chart in the present AISC-LRFD specification can be used to find the corresponding column K-factor in flexibly jointed frames. This is described in this paper. Furthermore, using two flexibly jointed and braced portal and one-bay two-story frame examples, the effects of the non-linear moment–rotation characteristics of connections on the K-factor are numerically studied and the K-factors obtained by two simplified methods are compared with the present method.

Finite element evaluation of pier moment-rotation characteristics in continuous-span steel I Girders

This paper summarizes the implementation and execution of a reasonably comprehensive set of finite element parametric studies to fill in knowledge gaps in the available experimental data pertaining to the hogging moment-plastic rotation behavior of steel and composite steel-concrete bridge girders. The paper highlights key requirements for proper finite element modeling of the behavior, outlines the design of the finite element parametric studies, and presents the important 'numerical test' results. Based on the finite element predictions, simple moment-rotation relationships are developed for use in design and rating of bridge structures.

Analysis and design problems for PR composite frames subjected to seismic loads

The design and analysis of partially restrained frames for seismic loads present some interesting and challenging issues to practicing engineers. The most important are how to determine the moment-rotation characteristics for the connections, the effect of floor slabs, and the influence of the partially-restrained connections on the dynamic characteristics of the frames. The use of databases for the determination of the appropriate moment-rotation curves is discussed and found lacking. A more rational, mechanistic approach to the problem is described and its merits discussed. The influence of floor slabs is shown to be important and simplified techniques to handle this effect are proposed. The behavior of FR frames under seismic loads is shown to be equivalent to that of PR frames once the dynamic characteristics are properly modeled. The design of PR frames will require both the development of new preliminary design approaches and advanced analysis tools because the conventional equivalent lateral load analysis cannot account for the dynamic effects properly.

Experimental analysis of semi-rigid boltless connectors

Boltless semi-rigid connections are used in the storage rack industry. The connectors consist of end plates welded to each end of a beam and an interlocking arrangement to engage with perforated columns, hence supporting the beams and restraining the columns. There are a variety of types and designs in use, characteristic of different rack manufacturers. In the absence of theoretical approaches, currently the only way to determine the properties of such joints is by testing. The aim of this paper is to determine the parameters governing an efficient beam end connector design. The paper includes an appraisal of commercially available beam end connectors and attempts to classify the connectors based on their special features. The paper presents the results of the tests carried out on a selected number of connectors and compares their moment-rotation characteristics, as well as their deformation modes using stress-sensitive lacquer. A theoretical investigation is carried out to estimate the influence of the flexibility of the beam and the column used in the tests on the stiffnesses determined experimentally. The effects of dimensional changes on moment-rotation characteristics are considered, and the main parameters controlling the behaviour of beam end connectors are discussed.

Application of artificial neural networks to the prediction of minor axis steel connections

In steel frames, it is usually the minor-axis beam-to-column connections that govern restraint to the columns against buckling. There is, however, no generally accepted method to predict the behaviour of such connections. To clarify the response, a series of tests has been performed, in which significant parameters have been systematically varied. The results have been used to train an artificial neural network (ANN) to predict bi-linear moment-rotation characteristics for minor-axis connections. The paper describes the test programme, the choice of ANN and the result for each connection, based on learning from 20 other connections. The results are found to provide approximations to the experimental response that are satisfactory for use in structural engineering design.

Moment-rotation behaviour of groups of press-joins in cold-formed steel structures

This paper describes the behaviour of groups of press joins in cold formed steel subjected to pure bending moment. The influence of material thickness and spacing of the joints have been studied. It appears that the thickness and spacing of the joints have a marked effect on the peak moment and rotational characteristics. A computer program has been developed that predicts the moment-rotation characteristics accurately.