A study of elastic-plastic buckling of cylindrical shells under torsion

The elastic-plastic buckling of cylindrical shells under torsion is analysed with a deep thick-shell model under various boundary conditions. The word ‘deep’ means that in the general equations of equilibrium the three non-linear terms that involve the torsional force are all retained for the buckling analysis. In the Donnell-type shallow-shell theory, however, only one of such terms is retained. The word ‘thick’ means that in calculating strains and stress resultants the factor (1+z/R) is retained. This factor results from the trapezoid-like shape of the cross-section and is usually neglected in the thin-shell theory. For boundary conditions, not only the conventional geometrical boundary conditions, which are in terms of displacements and rotations, but also the mechanical boundary conditions, which are in terms of forces and moments, are considered. The numerical results of examples assess the effect of the additional non-linear terms, the effect of the factor (1+z/R), and the effect of the mechanical boundary conditions.     

Understanding imperfection-sensitivity in the buckling of thin-walled shells

The buckling of thin-walled shell structures under load is still imperfectly understood, in spite of much research over the past 50 years. In this paper the author traces the history of the ideas which have been deployed in order to shed light on what is often referred to as ‘imperfection-sensitive’ buckling behaviour of shells. The ideas, which recur in various combinations, involve interaction of competing buckling modes, nonlinear behaviour, the growth of initial geometric imperfections under load and the alteration of the distribution of membrane stress as imperfections grow. The author claims that there are strong grounds for supposing that ‘locked in’ initial stresses on account of imperfect initial geometry and the static indeterminacy of boundary conditions of real shells have a pronounced effect on the buckling performance. This effect has been ignored in the past, and is the subject of a current experimental study.     

Aesthetics of thin-walled structures

Thin-walled structures are assumed to include all ‘surface-acting’ structures. This is a brief review of the factors generally considered to influence our emotional and intellectual response to built forms. General problems arising from the strong forms and large plain surfaces of thin-walled structures are considered. A number of specific types are then treated under the headings of ‘bridges’, ‘shells’, ‘silos’, ‘tents’ and ‘air-supported structures’.     

A computational intelligent fuzzy model approach for excavator cycle time simulation

The tracked hydraulic excavator is one of the most versatile and widely utilised piece of earthmoving equipment. In many instances, the ‘excavator’ represents the first choice of earthmoving plant for both construction managers and estimators, since when properly employed (i.e. with a competent operator and in an appropriate working environment), it offers high production rates at economical cost. Nonetheless, predicting machine production performance is difficult; given the typical multiple operational parameters (e.g. machine weight, machine configuration, ground conditions, operator ability) that can apply. Consequently, determination of accurate cost estimates and predicted contract durations are subject to considerable inaccuracy, especially where a significant amount of site work is needed.

To address this inadequacy, this paper presents a computational intelligent ‘fuzzy’ model with the ability to forecast excavator cycle time. In this context, a cycle is defined as one complete revolution, from ‘place empty bucket in dig material’ through ‘fill bucket’, ‘move charged bucket to target’, ‘empty charged bucket’ and ‘return bucket to dig material’. The developed model is based upon 70 separate cycle time observations obtained from four plant manufacturers. These data provide a representative spread of machine cycle times since they include a range on a continuum from optimum to adverse operational parameters. Tests on the derived model identified that its accuracy was acceptable; but the accuracy could be improved using larger samples and a more comprehensive and exhaustive range of variables to predict machine cycle time.     

Interrupted interface on the cybernetics of digital design process

From the viewpoint of Batesonian cybernetics, ‘conscious purpose’ and ‘artistic process’ are distinct ends of a spectrum of the functioning of ‘self’. The conceptual stage of the design process is essentially an artistic activity unencumbered with accuracy, dimensionality, scale, program or even tectonics, albeit all of these maybe tacitly present at the ‘back of a designer's mind’. Artistic activities involve broad mental processes that are beneath the stratum of consciousness. By definition, consciousness is selective awareness; it is linear in execution and limited in its capability to synthesize complex parameters. One of the central questions of this paper is “if artistic process requires one to abandon, or relinquish conscious purpose at the time of the generation of the work of art, and if the early stages of artistic process is a result of a vast number of ‘unconscious’ forces and impulses, then how can the computer, which demands (and thus propagates) geometric precision, focused operation and rational execution, be part of that process?” This paper will explore how, cybernetically, the computer can be ‘coupled’ with ‘self’ (via a visual interface) and the artistic process. Three specially devised conceptual design exercises—namely BlurrDesign, BlinDesign and BlitzDesign—and three corresponding “interruptive” computer interface modifications were deployed in an introduction to digital media course. The results of this study are now under consideration for their effectiveness in promoting conceptual design using the computer, and how the ‘self’ might form a cybernetic whole with the machine. The findings could have implications in design pedagogy, informatics and interface design.     

Coupled dynamics of tensioned buoyant platforms and mooring tethers

A mathematical model is presented of the coupled dynamics of a tensioned buoyant surface platform and the lateral dynamics of its taut mooring tethers. A finite element model of the lateral mooring tether dynamics is extended to take account of nonlinear square law fluid damping by a simple global ‘whole tether’ linearization and an alternative computationally more expensive ‘element-by-element’ linearization scheme. The resultant finite element model is combined with the surface platform dynamic analysis and the coupled dynamic model is used to investigate the effects of water depth, surface platform mass and tether mass per unit length on the tether displacements and bending stresses as well as the resultant surface platform displacements. The results show that the bending stresses induced in the tethers due to their dynamic motions are quite small and that the tether dynamics only affect the motion response of the platform appreciably if the tethers are long (1500 m or more), have a large mass per unit length and have a relatively low tension; or if the platform mass is relatively small. It was also found that the additional computing time required for the local ‘element-by-element’ equivalent linearization was not matched by the small improvement in accuracy obtained over the global ‘whole tether’ technique.     

Generic nonlinear modelling of a steel beam in a frame sub-assembly at elevated temperatures

This paper presents a virtual work formulation that enables the elevated temperature response of a steel beam with translational and rotational restraint, which forms a component of a compartment sub-assembly in a steel frame structure, to be determined in the pre-yielded range of structural response. The differential equations for the temperature-induced transverse and longitudinal deformations are derived, and in particular the static boundary conditions which can sometimes be counter-intuitive in analyses under thermal actions eventuate from the virtual work formulation. In lieu of most advanced finite element packages whose application generally does not permit an overall ‘feeling’ for the underlying structural mechanics, the technique proposed here is generic and leads to solutions in closed form for arbitrary regimes of temperature loading prior to attaining first yield. The technique can further be extended to quantify the post-yield response in a generic fashion. The application of the method is then illustrated with a simple numerical example.     

The reconstruction of the Cathedral of Noto

The Cathedral of Noto, one of the most important baroque monuments that has given notoriety to the town of Noto, collapsed in the 13th of March 1996, approximately 5 years after the earthquake of ‘Santa Lucia’ that took place in December 13th 1990. The collapse has pointed out serious deficiencies in constructive materials of pilasters, even in their capacity to support vertical loads. That is why the earthquake of 1990 has acted as ‘the last blow’, coming after a long period of fatigue. Instead, the geometry of the church results to be ‘good’, and for this reason the reconstruction design consists merely in new pilasters, well made with local material, the well known ‘pietra di Noto’ and the replacement is going to take place both on the right, in the place of the collapsed pilasters, and on the left, in substitution of the old pilasters, really not adaptive for restoration. In this way, and with few other interventions, the ‘new’ church should result capable to resist to earthquakes similar to the one that occurred in 1693.     

Analysis, synthesis and simulation of fire signals as a tool for the test of automatic fire detection systems

A simulation method for the test of one input fire detection algorithm is presented. A signal model is described, capable of synthesising time variant random signal sequences with similar statistical properties, as observed for signals of smoke- and temperature-measuring devices in reality. The time varying model parameters are derived from recorded signals in realistic environmental conditions. The necessary analysis method is discussed.

The simulation method is equally well suited for synthesising signal sequences in the ‘fire-’ and ‘nonfire’ case, which is necessary for the detectivity test of a detection algorithm. A fast transputer implementation yields acceptable calculation times even for estimates of the false alarm rate, corresponding to years observation time in reality.     

Loading arrangement and structures, as one system—temporary instability of elements and structure

The paper deals with some special questions of experimental research being important when frames are tested and the load carrying capacity of the structure is lost because of any instability phenomena. Computer simulation results are presented:

• -to illustrat the difference between the real and ‘virtual’ structural behaviour, which is dependent on the character of loading, and

• -to show that in certain conditions a structure, which became unstable because of the instability of one of its elements, is able to turn to a stable state and to carry increasing loads before complete loss of its load carrying capacity.

This paper is a modified (slightly extended) version of that which was presented during the ‘First International Conference on Coupled Instabilities in Metal Structures’. Reflections of participants initiated some modification.     

Wind turbine drives — Test results from the ‘falcon’

A wind turbine drive has been designed and built for full scale trials in order to assess the use of this type of propulsion for commercial craft. This wind turbine rig is fitted to the 5.6m long Blackwater sloop ‘Falcon’.

Test results from this boat are presented over a range of operating wind conditions up to Beaufort force 6 (11.5m/s) for two different wind turbine rotors. Maximum boatspeeds reached were 3.5 knots upwind (in 7.2m/s wind), 5.0 knots across the wind (9.8m/s wind) and 5.2 knots downwind (8m/s wind). The ‘hull speed’ of this boat is around 5.0 knots.     

Tangible virtuality—perceptions of computer-aided and physical modelling

Designing—giving form to new objects or environments—is largely a question of anticipating the workings of spatial and material environments, which can become ‘reality’ only by being built. Until ‘realized’, a design is essentially a figment of the designer's imagination, although his or her ideas may be laid down and conveyed to others via specialized design media. In this way, impressions of the design may be shared with clients, colleagues or other ‘actors’ in the design process.

Such products of the designer's imaging process can be relatively abstract or begin to approach future reality. Form & Media research can be ‘revealing’, stimulating insights concerning preferences, working processes and the effects of products of the designer's imagination. In the past 10 years, we have gained considerable practical experience with both virtual and tangible (scale) models. We have compared different techniques in conference workshops, within educational settings and in our Form & Media research laboratory. The research projects ranged from the development of practical techniques and working methods to protocol analyses of designing architects.

This contribution draws comparisons between different computer-aided modelling techniques, with an indication of their perspectives, making use of the experience gained from various experiments in an educational context, and will highlight the potentials for different combinations of digital and physical modelling techniques.     

Adaptive architecture: integrating low-energy technologies for climate control in the desert

The article describes a ‘climatically adaptive’ approach to intelligent building in which a variety of technologies are integrated in the architectural design to provide thermal comfort with a minimal expenditure of energy. This concept is illustrated by the design of the Blaustein International Center for Desert Studies, a multi-use complex completed recently at the Sede-Boker Campus of Ben-Gurion University of the Negev. In response to the local climate of this desert region, a number of strategies were developed by the authors to exploit natural energy for heating and cooling: earth berming of major parts of the building, ‘selective glazing’ for seasonal shading and energy collection, a down-draft ‘cool-tower’ for evaporative cooling and a hybrid mechanism for hot-air supply are several of the unique systems whose performance and feasibility are analyzed within the context of the overall building design.     

Use of meteorological/constructional statistics to estimate building heat needs

Building heat needs over a month or a year are often estimated on the basis of the relevant degree day value, in which some crude account is taken of solar and other casual gains in the choice of the base temperature. A more realistic assessment of building heat needs including the effects of passive gain (through a window or glazed wall) can be made using the ‘empirical’ approach of Davies. In this article a statistical approach is developed in which use is made of degree day type data, and measures of days which are ‘disadvantageous’ and ‘advantageous’ from the point of view of saving back-up heating, together with the first four moments of the distribution of advantageous days (7 statistics in all). From five of these statistics a curve can be fitted to the distribution. After suitable integration of the fitted curves another estimate of building heat needs is obtained (the ‘curve fitting’ value). The heat needs as obtained empirically and by curve fitting agree closely. The 7 statistics are easily handled by a mini computer. They thus provide a convenient and accurate extension to degree day methods when local meteorological information has been processed in conjunction with passive solar collector characteristics. Window area, or glazed wall resistance, and assumptions about ventilation rate are only needed in the final stage of computing heat needs.     

Hot wedge fusion welding of HDPE geomembranes

Double track fusion welding offers more control, produces more consistent welds and is easier to nondestructively test than other types of welding system. However, as with other types of welding, there are a number of parameters which need to be understood and controlled in order to make consistently good welds. Heat, pressure and speed must be properly set, taking into consideration the resin being welded, the thickness of the sheet and environmental conditions.

Speed, heat and pressure must be controlled and their effect on weld quality understood. The ‘welding window’ for these variables has distinct boundaries. A test weld which exhibits melting, ‘squeeze-out’ and peels without breaking the liner may not need more pressure or heat. Tests show that for some bonds which peel apart, reducing the heat and/or pressure may be the correct action in order to obtain a film tearing bond (FTB).     

Short vs. long column behavior of pultruded glass-fiber reinforced polymer composites

Analytical and experimental studies were conducted for the purpose of establishing a distinguishing criterion between short and long FRP composite column behaviors. The results of investigating 24 full-scale GFRP composite columns are presented. The experimental studies utilized specimens with commonly used ‘Universal’ and ‘Box’ cross-sections which were manufactured by the pultrusion process using E-glass fibers in various forms as reinforcement and polyester and vinylester as binding matrices. The effective slenderness ratios (L/r) of the investigated columns were 3.79, 32.7, 47.9, 63.1 and 75.4 for the ‘Universal’ section and 9.38, 53.9 and 78.9 for the ‘Box’ section. The specimen lengths were 18 inches (0.46 m), 8 foot (2.44 m), 12 foot (3.66 m), 16 foot (4.88 m) and 19.25 foot (5.87 m). All columns were tested in a vertical position and under compressive axial static loading and the fundamental pinned–pinned end-conditions. The columns’ compressive strains, buckling and crippling loads, lateral displacements, initial curvatures, and modes of failure were documented during the course of this investigation. Orthotropic mechanical properties of the composites were experimentally obtained utilizing 44 coupons cut from the column specimens. Analytically, Euler's formula was employed to obtain critical loads for the slender columns. For short columns, the classical plate theory was used to predict columns’ buckling loads. Based on experimental evaluations and analytical results, a slenderness ratio based criterion was established for distinguishing between short and long composite column behaviors. Further conclusions and design recommendations were made.     

Flexural capacity of thin wall fibre reinforced cement

A method of calculating the flexural strength (cracking) of fibre reinforced cement floor and wall systems is presented. The method for flexural strength is based on non-linear stress-strain relationship at failure tension and corresponding stress distribution in section at cracking and subsequent failure. The method is verified in tests of full-scale ‘V’ and ‘U’ shaped panels made of asbestos-cement which were tested as simply supported beams designed for the construction of low cost housing and wall facades. Additional tests were performed on small sized samples to verify the mechanical properties of material used.     

Risk based structural safety methods in context

Structural reliability calculations are now recognised by many as only one part of the assessment of structural safety. They provide partial evidence about the total problem which has to be integrated with other evidence to support decision making. It is proposed that a ‘holistic’ systems view based on a clear understanding of process is required. The model consists of interacting processes arranged in a hierarchy of differing levels of precision of definition and scope. The various attributes of each process are outlined under the generic headings of ‘who, what, why, where, when and how’. The ‘how’ attributes of a process which are important in the control of a process are hazard and risk. It is suggested that Interval Probability Theory is a suitable measure of the evidence that a process is being and will be managed to a successful conclusion. This can be combined with structural reliability predictions to give bounds on the evidence about the success of the total process which covers all foreseeable aspects of the uncertainty.