Critical Design Case Research Articles

Design Aspects of a Deployable Tensegrity-Hollow-rope Footbridge

Tensegrity structures are composed of cables and struts in a prestressed self-equilibrium. Although tensegrity first appeared in the 1950s, it is seldom used in civil engineering. This paper focuses on the design aspects of a deployable tensegrity-hollow-rope footbridge. Deployment is usually not a critical design case for traditional deployable structures. However, for tensegrity systems deployment may be critical due to the actuation required. In this paper, deployment is investigated in a general design framework. The influence of clustered (continuous) cables and spring elements in statics and dynamics is studied. Finally, actuation schemes are explored to identify cases where deployment becomes a critical design case. For this configuration, deployment is a critical design case when the structure has spring elements and continuous cables.

Internal pressures – The dominant windward opening case – A review

A building with a dominant opening in a wall may experience internal pressures equal to, or greater than, the external wind pressures in a severe windstorm. This is a critical design case in severe wind events, during which accidental openings may be produced by windborne debris or by direct wind loading. This paper reviews previous work on the prediction of fluctuating and peak internal pressures arising from a dominant opening on a windward wall, and summarizes proposed, or current, code formulations in a unified non-dimensional format. Experimental data from a number of sources are plotted together, and are found to fall into two groups, depending on the turbulence intensity in the approach flow. It is shown that the proposed relationships for the ratio of internal to external pressure fluctuations differ considerably, depending on the assumptions made. An intermediate relationship, which agrees well with experimental data in approach flow with turbulence intensities representative of atmospheric wind flow in developed boundary layers, is presented, and a design example of its application is given.

Experimental Investigation of Turbulent Boundary Layer Flashback Limits for Premixed Hydrogen-Air Flames Confined in Ducts

The design of flashback-resistant premixed burners for hydrogen-rich fuels is strongly dependent on reliable turbulent boundary layer flashback limits, since this process can be the dominant failure type for mixtures with high burning velocities. So far, the flashback data published in literature is based on tube burner experiments with unconfined flames. However, this flame configuration may not be representative for the most critical design case, which is a flame being already present inside the duct geometry. In order to shed light on this potential misconception, boundary layer flashback limits have been measured for unconfined and confined flames in fully premixed hydrogen-air mixtures at atmospheric conditions. Two duct geometries were considered, a tube burner and a quasi-2D turbulent channel flow. Furthermore, two confined flame holding configurations were realized, a small backward-facing step inside the duct and a ceramic tile at high temperature, which was mounted flush with the duct wall. While the measured flashback limits for unconfined tube burner flames compare well with literature results, a confinement of the stable flame leads to a shift of the flashback limits towards higher critical velocity gradients, which are in good agreement between the tube burner and the quasi-2D channel setup. The underestimation of flashback propensity resulting from unconfined tube burner experiments emerges from the physical situation at the burner rim. Heat loss from the flame to the wall results in a quenching gap, which causes a radial leakage flow of fresh gases. This flow in turn tends to increase the quenching distance, since it constitutes an additional convective heat loss. On the one hand, the quenching gap reduces the local adverse pressure gradient on the boundary layer. On the other hand, the flame base is pushed outward, which deters the flame from entering the boundary layer region inside the duct. The flashback limits of confined flames stabilized at backward-facing steps followed this interpretation, and experiments with a flush ceramic flame holder constituted the upper limit of flashback propensity. It is concluded that the distribution of the flame backpressure and the flame position itself are key parameters for the determination of meaningful turbulent boundary layer flashback limits. For a conservative design path, the present results obtained from confined flames should be considered instead of unconfined tube burner values.

Forced Hydraulic Jumps below Abrupt Expansions

A new approach for studying forced hydraulic jumps below abrupt symmetric and asymmetric expansions is introduced. Intensive experiments for two expansion ratios, one asymmetry ratio, and different solid sill heights and locations were conducted specifically for the spatial jump case as a critical design case. A new parameter correlating sill height and sill location is presented and found to play an important role in the basin’s design, scour, and flow patterns. Using this parameter gives distinguished relationships for predicting sequent depth, energy dissipation, and basin length for the preliminary or operating design in expected or unexpected flow conditions. Empirical equations and experimental regression curves are introduced to predict the sequent depth and the basin length at any sill height and sill location. Scour and flow patterns are intensively studied experimentally and qualitatively described for the symmetric and asymmetric basins with or without sills for the spatial jump. A numerical ex...

Full-scale study of wind loads on roof tiles and felt underlay and comparisons with design data

Wind pressure data have been collected on the tiled roof of a full-scale test house at Silsoe in the UK. The tiled roof was of conventional UK construction with a batten-space and bitumen-felt underlay beneath the interlocking concrete tiles. Pressures were monitored on the outer surface of selected tiles, at several locations within the batten-space, and beneath the underlay. Data were collected both with and without ventilator tiles installed on the roof. Little information appears to exist on the share of wind load between tiles and underlays which creates uncertainty in the design of both components. The present study has found that for the critical design case of maximum uplifts it would be appropriate to assign 85% of the net roof load to the tiles and 15% to the underlay when an internal pressure coefficient of -0.3 is used, and to assign 60% to the tiles and 50% to the underlay when an internal pressure coefficient of +0.2 is assumed (an element of design conservatism is inherent in the apparent 110% net loading indicated by the latter pair of percentage values). These findings indicate that compared with loads implied by BS 6399-2, UK design loads for underlay are currently conservative by 25% whilst tile loads are unconservative by around 20% in ridge and general regions and by around 45% in edge regions on average over roof slopes of 15o-60o.

Design criteria for conceptual sizing of primary flight controls

AbstractThe European Supersonic Commercial Transport’s control surface configuration is based largely on Concorde’s and has been scaled to provide comparable un-augmented stability and manoeuvre performance. Hence, optimising the surface size could provide significant performance benefits in terms of reduced drag, noise, structural and actuator power requirements. Adequate control power will be required to meet current civil aviation regulations whose primary aim is to ensure the aircraft can be flown safely during both normal and emergency operation. Additional design criteria, combined with the optimum longitudinal control laws, are required to ensure desirable handling qualities with minimum pilot workload. Two critical low-speed flight conditions, normal and emergency, together with associated aircraft configurations for control surface sizing have been identified. The rudder must provide sufficient control power to achieve positive heading changes subsequent to a double asymmetric engine failure during normal operation. The fin should be sized to satisfy Dutch roll stability criteria with the un-augmented aircraft in its emergency configuration. The dual functionality of the elevons require that they are sized using both pitch and roll performance and handling quality criteria. The bank angle capture requirement provides the most critical elevon design case, the satisfaction of which also ensures adequate pitch control power. Validation using ‘pilot-in-the-loop’ simulation will be required whilst more explicit control surface size optimisation would require the definition of limiting airspeeds and operating conditions applicable to the European Supersonic Commercial Transport. Additional studies of control power requirements during transonic and supersonic cruise may also be required.

On the efficiency of crossing rate prediction methods used to determine extreme motions of moored offshore structures

Accurate prediction of motion extremes for moored floating offshore structures remains a subject of considerable interest to the offshore industry. The difficulty of predicting extremes lies in the method used to combine motions at the incident wave and slow-drift frequency scales. Since vessel excursions are dependent on both intensity and frequency composition of the incident wave field, the critical design case is unclear and hence accurate, efficient frequency domain analysis tools are becoming essential to preliminary design. Frequency domain extreme prediction methods may be broadly categorised as either empirical combination, such as those used in the current design codes and rules of recommended practice, or rigorous statistical methods which are based on predicting crossing rates and applying a ‘correction’ factor for the potential clumping of wave peaks. For the latter class of method, rapid prediction of the mean up-crossing rates is essential. This article compares various published methods for predicting crossing rates using the Faltinsen and Løken floating cylinder in irregular beam seas data. By comparison with methods of high computational intensity, it is shown that lower order methods are able to efficiently and accurately predict crossing rates and the associated (uncorrected) motion extremes.

Design of flat slabs for punching – European and North American practices

Punching is a critical design case for reinforced concrete flat slabs but the provisions for punching shear design and detailing of the shear reinforcement differ considerably among the various European and American design codes. Therefore the thickness of the slab or the amount or distribution of shear reinforcement may vary between different countries. The punching shear capacity of concrete, the punching shear resistance with shear reinforcement and the relevant detailing provisions of four European, two American codes and the CEB-FIP Model Code will be compared. The provisions have been compared by analysing flat slabs with typical dimensions and reinforcement ratios. The possibilities and limitations of each code and the consequences in practice will be demonstrated, using the flat slab of an office building as an example. With regard to practical aspects, the importance of shear reinforcement that can easily be installed will be emphasised.