A.R. Kemp

Required and available rotations in continuous composite beams with semi-rigid connections

The need to match the rotations required for redistribution of elastically calculated support moments in composite frames with the available inelastic rotation in those regions is highlighted. By defining the former purely in terms of the loading arrangement and structural configuration and the latter in terms of the properties of the various structural elements (beams, joints and columns), the contributions of items other than joint rotation is clarified. Relatively simple theory is used to quantify both aspects, with the approach being validated against appropriate test results.

Factors influencing the strength of continuous composite beams in negative bending

Abstract Continuous composite beams are widely used in multi-storey buildings and bridges. For the purpose of flexural strength calculations, such beams may be divided into two distinct regions, namely regions of positive bending and negative bending. The flexural strength of composite beams in positive bending is governed by the strength of the concrete and the steel beam and simple calculations based upon fully plastic stress blocks provide acceptable accuracy in predicting ultimate bending moment capacities. In regions of negative bending moments, however, local and lateral instabilities of the steel section and cracking of the concrete influence not only the ultimate bending strength of the composite section, but also the ability of the section to redistribute bending moment by absorbing inelastic rotations. The factors influencing the behaviour of composite beams in negative bending and their relationship to the section geometry and span lay-outs are discussed in this paper. A theoretical model is presented which provides a basis for assessing the influence of the distortional restraint of the slab on the steel beam. The theoretical models are compared to experimental results obtained from tests on composite beams. The model is then used as a basis for an interactive design approach that also considers local flange and web buckling.

Differences in inelastic properties of steel and composite beams

Abstract The inelastic behaviour of continuous steel and composite beams in which strain-weakening occurs due to local and lateral buckling is compared both theoretically and experimentally. While the required rotation capacity of composite beams is larger than that of equivalent steel sections, other factors offset this greater ductility requirement. The available rotation capacity is shown to be particularly sensitive to the proportion of the depth of the steel section that is in compression and the lateral slenderness ratio in the negative moment region. This behaviour is explained theoretically and illustrated in seven tests on steel and composite beams. The substantial benefits to be gained in available ductility by using semi-rigid end connections in continuous composite beams are also demonstrated.

Yield-line elements: for elastic bending of plates and slabs

Abstract A new four-node rectangular element is proposed for the elastic analysis of plates in bending. This element has been formulated in view of the complex nature of existing nonlinear theory. The elastic theory developed in this paper has been derived in a form that allows a natural progression to plastic theory. The proposed method is mixed in the sense that moments and out of plane deflections are chosen as the unknowns. The solution matrix is developed from flexibility and equilibrium equations. Flexibility equations (expressing the relationship between the rotations and nodal moments) assume a partial quadratic moment field and are equated to obtain interelement compatibility. Equilibrium equations are formulated from virtual work expressions used in yield-line theory that are modified to include the twisting moment. For one-way plate action (single curvature), the solution is exact irrespective of the aspect ratio or the number of elements. For two-way action (double curvature), acceptable accuracy is achieved. Comparisons with exact solutions and the displacement method are included to indicate the accuracy of the proposed element.

Compatibility relationships in a mixed flexibility-based method of frame analysis

A description is provided of an alternative mixed method of analysis of framed structures in which the unknowns are element end moments and sway deflections. The advantages of this approach in plastic and instability analysis are identified and illustrated. Derivations are provided for identifying the independent modes of sway and relating them to the global joint translations, which are then used to define corresponding sway equilibrium equations. The method is shown to provide an efficient and accurate solution to general nonlinear analysis of frames which can simply incorporate both ductility analysis and idealized nonlinear behaviour of semirigid joints.

A mixed flexibility approach for simplifying elastic and inelastic structural analysis of frames

Abstract A mixed-flexibility method of analysis of framed structures is described in which the unknowns are element end moments and independent modes of sway deflection. The advantages of this approach in plastic and instability analysis are identified and illustrated. The method is shown to provide an efficient and accurate solution to general non-linear analysis of steel frames which can incorporate yielding and strain hardening of the elements and non-linear behaviour of semi-rigid joints. Examples of non-linear analysis are provided in which the number and location of unknowns are similar to a simple elastic analysis.

Strength of pipes continuous over a series of saddle supports

Abstract An experimental investigation is described of the ultimate strength of pipe specimens representing the support region of continuous pipes on saddle supports. Variables that are considered include diameter-to-thickness ratios in the range from 28 to 100, span-to-diameter ratios from 16 to 98, and two different saddle arrangements. Ultimate-moment capacities are compared with code recommendations, and the influence of the saddle support is described.

Simplified amplification factors representing material and geometric inelasticity in frame instability

An approach for assessing the inelastic, in-plane, collapse load of framed structures using amplification factors is extended to a broader range of material inelasticity. This manual method is based on the amplification factor approach for elastic, rectangular frames in the Canadian steel design code, but allows for material and geometric nonlinearity of both rectangular and non-rectangular frames on a consistent basis. Improved accuracy is achieved with a linear variation of the incremental amplification factor between a lower limit at the onset of inelastic behaviour and an upper limit at which plastic collapse occurs. A method is demonstrated for notionally separating the effects of material and geometric nonlinearity in experimental measurements or advanced finite element analyses. This is used for validating the proposed inelastic models for a steel frame and a reinforced concrete frame.

Slenderness limits normal to the plane of bending for beam-columns in plastic design

Abstract This paper describes a theoretical approach for identifying the spacing of lateral restraints to the compression flange for beams, and beam-columns with limited axial force, in regions of moment gradient, in order to achieve an identified rotation capacity of the plastic hinge within this length. The predictions obtained from this approach are compared with the results of tests conducted by Smith and Adams. It is acknowledged that the analytical technique which is used is rather simplistic and could be expanded into a more sophisticated model, although the non-linear nature of the solution may make this cumbersome. Nevertheless a favourable comparison is obtained and it is possible to recommend appropriate formulae for design purposes.

Effect of ionising (gamma) radiation on female Anopheles arabiensis

Background In a mosquito sterile insect technique programme the ideal scenario is to release male mosquitoes only. However, because there are currently no sex separation strategies which guarantee total female elimination, this study investigated the effect of irradiation on physiological and reproductive fitness of females of an Anopheles arabiensis genetic sexing strain. Methods Female pupae were irradiated at 70 Gy and the effects of irradiation on adult emergence, longevity, blood-feeding capability, mating ability, fecundity and fertility were assessed. Results and conclusion Irradiation reduced adult emergence and fecundity but did not affect adult survivorship, mating and blood feeding ability, which suggests that irradiated female mosquitoes can transmit disease pathogens.