D.A. Nethercot

Failure modes for castellated beams

Abstract Previous studies of the structural behaviour of castellated beams are reviewed and a number of different possible failure modes identified. Several of these do not occur with plain webbed beams since they are a direct result of the different way in which shear is transferred through the perforated web. Examples are a Vierendeel mechanism, web post buckling due to shear and web weld rupture. Failure by either the formation of a flexural mechanism or by lateral-torsional instability are essentially similar to the equivalent modes for solid web beams. Methods for predicting the loads at which each of these types of failure occurs are evaluated against the available experimental data and the limitations in a number of these analytical approaches is discussed. It is concluded that both lateral-torsional instability and the formation of a flexural mechanism may be handled by an adaptation of established methods for plain webbed beams, providing the cross-sectional properties are those corresponding to the centreline of a castellation. Currently available methods for the determination of collapse in the other modes, while rather less accurate, are adequate for design except in the case of web post buckling due to compression.

Modelling steel frame behaviour under fire conditions

Abstract A formulation for the noblinear analysis of two-dimensional steel frames under fire conditions using the finite element method is presented. Deterioration in material strength with increasing temperature is represented by a set of nonlinear stress-strain-temperature relationships using a Ramberg-Osgood equation in which creep effects are implicitly included. Structures subject to increasing loads or temperatures are analysed using an incremental Newton - Raphson iterative procedure. The analysis permits deformation history and either collapse load or critical temperature to be calculated at a specified temperature or load level respectively. It includes the effects of geometric nonlinearity, temperature dependent nonlinear material behaviour and variations in temperature distributions both along and across each member. The effects of thermal strains, residual stresses and thermal bowing are also considered and different material models may be used. Comparisons are made with fire test results on frames and columns that represent a wide range of problem parameters such as slenderness, end conditions, load levels and temperature distributions. In all cases, the agreement is very satisfactory. An example two-bay frame is analysed to illustrate the potential of the analysis and to show the influence of various forms of partial protection on the collapse temperature of sway frames.

Rotational stiffness characteristics of steel beam-to-column connections

Abstract A series of tests on a variety of beam-to-column connections suitable for rectangular frames using I-section members has been conducted. The principal objective was the provision of moment-rotation data so that a comparative assessment of the performance of the different types, in terms of connection stiffness and moment capacity, could be undertaken. Thus all tests employed similar beam and column sizes, test apparatus, instrumentation and test procedures. The connections studied were (in ascending order of stiffness and strength): web cleats, flange cleats, combined seating cleat and web cleats, flash end plate and extended end plate. Connections to the column flanges and the column web were included. Major sources of connection flexibility have been identified and the results prepared for subsequent use in assessments of semi-rigid joint action in steel frames.

Stiffness prediction for bolted moment-connections between cold-formed steel members

The authors have recently described a cold-formed steel portal framing system in which simple bolted moment-connections, formed through brackets, were used for the eaves and apex joints. Such connections, however, cannot be considered as rigid because of localised in-plane elongation of the bolt-holes caused by bearing against the bolt-shanks. To therefore predict the initial stiffness of such connections, it is necessary to know the initial bolt-hole elongation stiffness kb. In this paper, a finite element solid idealisation of a bolted lap-joint in shear will be described that can be used to determine kb; the results obtained are validated against experimental data. A beam idealisation of a cold-formed steel bolted moment-connection is then described, in which spring elements are used to idealise the rotational flexibility of the bolt-groups resulting from bolt-hole elongation. Using the value of kb in the beam idealisation, the deflections predicted are shown to be similar to those measured experimentally in laboratory tests conducted on the apex joint of a cold-formed steel portal frame.

Ultimate strength of bolted moment-connections between cold-formed steel members

The behaviour and design of bolted moment-connections between cold-formed steel members, formed by using brackets bolted to the webs of the section, is considered. The particular problem of the moment-capacity of such joints being lower than that of the cold-formed steel sections being connected because of web buckling, caused by the concentration of load transfer from the bolts, is addressed. In this paper, a combination of laboratory tests and finite element analyses is used to investigate this mode of failure. It is demonstrated that there is good agreement between the measured ultimate moment-capacity and that predicted by using the finite element method. A parametric study conducted using the finite element model shows that the moment-capacity of a practical size joint can be up to 20% lower than that of the cold-formed steel sections being connected. Web buckling so-caused must therefore be considered in the design of such connections.

Behaviour of flush end-plate composite connections with unbalanced moment and variable shear/moment ratios—I. Experimental behaviour

Abstract A series of seven end-plate beam-to-column connection tests is reported; these include one pure steel connection and six composite connections. The main variables investigated are the degrees of unbalanced moment and the shear/moment ratio. Comprehensive instrumentation has been used to monitor: beam strains, column strains, rebar strains and bolt forces as well as member and connection deformations. This comprehensive monitoring permits a full understanding of the behaviour of this kind of connection. In addition, the large number of recorded variables provides data for the detailed validation of numerical analysis methods.

Effect of semi-rigid connections on steel column strength

Abstract A review has been made of all available experimental data relating to the moment versus in-plane rotational behaviour of practical beam-to-column connection types as used in steel frames. This has clearly shown that all forms of connection possess some stiffness which will act as partial restraint to the columns ends. Moreover this restraint has been found to be a non-linear function of connection deformation. Methods of mathematically describing connection data have been reviewed and an improved representation based on the use of cubic B-splines proposed. This possesses the advantage that it will not give an (incorrect) negative connection stiffness. Using this technique to model connection behaviour a computer program for the maximum strength analysis of steel columns has been written. This is based on a Newton-Raphson incremental finite element approach and incorporates features such as initial lack of straightness and spread of yield through the cross-section including the effects of residual stresses. The results of some preliminary analyses, which demonstrate the increases in column strength and reduction in column deformation that result from a consideration of actual joint behaviour, are presented.

Prediction of initial stiffness and available rotation capacity of major axis composite flush endplate connections

Abstract Knowledge of the initial rotational stiffness of a connection is important for the global elastic analysis of frame structures. Based on a simple force transfer mechanism and consideration of the behaviour of individual components, a method has been developed to predict the initial stiffness of composite flush endplate connections. The approach is compatible with that proposed earlier to predict moment capacity for several types of composite connection. In order to apply plastic analysis to frame structures a knowledge of the available and the required rotation capacities is necessary. A simple technique to determine the available rotation capacity of composite flush endplates is described herein. Taken together, the two methods represent key steps in the development of an approach to predict the main measures of the behaviour of composite endplate connections. Used in conjunction with the moment capacity and failure mode prediction method developed earlier by the authors, they provide a complete representation of the design properties of composite flush endplate connections.

Required rotations and moment redistribution for composite frames and continuous beams

Abstract Using the theoretical method previously developed to determine the required rotations in the support regions of composite frames, several sets of numerical results have been obtained for variations in the key parameters known to influence behaviour. Using these numerical results, correlation analyses were conducted to determine the most influential factors governing the required rotations. By utilising the governing factors and regression analysis, empirical equations were obtained to directly link the rotation requirements to the degree of moment redistribution necessary to develop the full strength of composite beams. Based on these equations, some numerical results are presented which define achievable rotation requirements and thus of the degree of moment redistribution.

Determination of rotation capacity requirements for steel and composite beams

Abstract A method is presented for the calculation of the necessary joint rotations to permit the use of moment redistribution as the basis for the design of semi-continuous steel and composite frames. The method is based on the use of moment-curvature relationships obtained from consideration of the basic steel and concrete stress-strain curves. It simplifies the determination of the support rotations required for specific percentage redistributions of support moments to the mid-span cross-section as used in EC4 by considering these to be composed of an elastic and a plastic part. Several different load cases are considered, together with a wide range of beam properties. The resulting calculation method, which uses only formulae and graphs, is illustrated by an example.