Critical Load Cases Research Articles

Robust design optimisation of advance hybrid (fiber–metal) composite structures

Hybrid Composite Structures (HCSs) are consisting of alternating layers of Fiber-Reinforced Polymer and metal sheets. Mechanical properties and responses for off-design conditions of HCSs can be improved using an innovative methodology coupling Multi-Objective Genetic Algorithm and robust design method. The concept of robust design approach ensures that a structure will be tolerant to unexpected loading and operating conditions. In this paper, two applications are considered; the first is to maximise the stiffness of the HCS while minimising its total weight through a Multi-Objective Design Optimisation. The second application considers a Robust Multi-Objective Design Optimisation (RMDO) to minimise total weight of HCS and to minimise both, the normalised mean displacement and the standard deviations of displacement, considering critical load cases. For the optimisation process, a distributed/parallel Multi-Objective Genetic Algorithm in robust multi-objective optimisation platform is used and it is coupled to a Finite Element Analysis based composite structure analysis tool to find the optimal combination of laminates sequences for HCSs. Numerical results show the advantages in mechanical properties of HCS over the metal structures, and also the use of RMDO methodology to obtain higher characteristics of HCS in terms of mechanical properties and its stability at the variability of load cases.

Mechanical Characteristics of Degraded Silicone Bonded Point Supports

Abstract Tensile loading of bonded point supports is considered as the critical load case for adhesive material, e.g., silicone applied to such kinds of fixtures. From a durability point of view, the following defects of two-component adhesives are expected to have a significant impact on the response to this loading regime: the wrong mixing ratios of the adhesive components, inhomogeneous mixing due to insufficient or poor mechanical mixing procedures, load degradation of the adhesive bonding material, local degradations of the bonding, e.g., bubbles or poor adhesion. In order to investigate these aspects, different experimental approaches have been pursued. Regarding the mixing ratio aspect, material tests were performed with systematically varied mixing ratios for the analysis of the elastic and strength properties while for the other topics, tensile tests of bonded point supports were evaluated in detail. Cyclic tests of point supports in the tensile load regime were set up, differing in amplitudes and maximum peaks of the cycles, in order to analyze the impact of load histories on the mechanical characteristics of the specimens. For point supports subjected to monotonically increasing loads the qualitative assessment of the fracture surfaces revealed the existence and the potential impact of disturbances inside the bonding, such as bubbles or locally lacking adhesion, on the damage behavior and on related inferior mechanical performance. The main motivation for this kind of research is to improve confidence in the durability of bonded designs. Thus, our activities were focused on aspects which might affect durability from an application point of view, not from a purely academic one. Especially in Germany, the confidence of the authorities in this joining technique has to be strengthened by demonstrating a high degree of robustness in the application, and in view of the risk associated with potential in-field problems.

Optimal trends in Manoeuvre Load Control at subsonic and supersonic flight points for tailless delta wing aircraft

Manoeuvre Load Alleviation (MLA) is the concept of redistribution of forces and moments on airframe through optimal actuation of control surfaces. The effect is to minimise the stresses near wing root and improve fatigue life of airframe components, such as, the frequently studied wing root fitting connecting wing to the fuselage. The configuration is a typical tailless delta wing aircraft having inboard and outboard Elevons (a combination of Elevator and Aileron) placed symmetrically on starboard and port side. The problem is formulated as minimisation of independent conflicting multiple objectives of wing root bending and twisting moments, with stability equations as equality constraints and actuator hinge moments as inequality constraints. Hybrid method is used as optimisation technique, which is evolved from the combination of heuristic based Elitist non-dominated Sorting Genetic Algorithm (NSGA-II) and calculus based Goal Programming methods. The primary objective of this paper is to bring out the structural benefits attainable for some of the combined critical manoeuvre load cases at subsonic and supersonic flight points. The combined critical manoeuvre load cases demarcate the limit to which airframe is loaded. The minimisation achieved is better for flight points at supersonic as compared to subsonic Mach numbers.

Servo-hydraulic System for Controlled Velocity Water Impact of Marine Sandwich Panels

Slamming, the impact between a marine craft’s hull and the water surface is a critical load case for structural design of marine vessels. The importance of hull slamming has led to a significant body of work to understand, predict and model these impacts. There is however, a lack of experimental data for validation, particularly for deformable panels and sandwich structures. This paper describes a high-velocity panel slamming test system that enables the generation of comprehensive and reliable experimental data on slamming impacts for both rigid and flexible panel structures. The pressure magnitudes, time-histories and spatial distributions resulting from testing of a nominally rigid panel have been compared with previous analytical, semi-empirical and experimental studies. Slamming impacts of a deformable sandwich panel are shown to cause different pressures to those from a rigid panel impact, resulting in increased transverse shear loading at the panel edge.

Wind loads on heliostats and photovoltaic trackers of various aspect ratios

Abstract For the layout of solar trackers the wind loads on the structure have to be known. They can be calculated by using wind load coefficients given in literature. But so far these values are only valid for aspect ratios of the panel (width to height) of about 1.0. Therefore the wind load coefficients for heliostats of aspect ratios between 0.5 and 3.0 were determined to close this gap. As solar trackers are exposed to the turbulent atmospheric boundary layer the turbulence of the approaching flow has to be modeled. As a reliable method at reasonable cost wind tunnel measurements were chosen. Solar trackers of 30 m2 panel size were investigated at a model scale of 1:20. Wind direction and elevation angle of the panel were varied to investigate especially the constellations at which the highest wind loads are expected (critical load cases). By spires and roughness elements a wind profile and a turbulence intensity of the modeled wind according to typical sites for solar trackers were achieved. The loads were measured by a high frequency force balance placed underneath the models. Additionally measurements of the pressure distribution on a panel with aspect ratio of 1.2 were performed to better understand the effects that lead to the peak values of the wind load coefficients. A significant impact of the aspect ratio was measured. For the critical load cases the aspect ratio dependencies of the accordant peak wind load components were determined. By these the peak wind loads on solar trackers of varies aspect ratios can be calculated. Regarding the single solar tracker components the main results are: Higher aspect ratios are advantageous for the dimensioning of the foundation, the pylon and the elevation drive but disadvantageous for the azimuth drive.

Two Coupled Queues with Vastly Different Arrival Rates: Critical Loading Case

We consider two coupled queues with a generalized processor sharing service discipline. The second queue has a much smaller Poisson arrival rate than the first queue, while the customer service times are of comparable magnitude. The processor sharing server devotes most of its resources to the first queue, except when it is empty. The fraction of resources devoted to the second queue is small, of the same order as the ratio of the arrival rates. We assume that the primary queue is heavily loaded and that the secondary queue is critically loaded. If we let the small arrival rate to the secondary queue beO(ε), where0≤ε≪1, then in this asymptotic limit the number of customers in the first queue will be large, of orderO(ε-1), while that in the second queue will be somewhat smaller, of orderO(ε-1/2). We obtain a two-dimensional diffusion approximation for this model and explicitly solve for the joint steady state probability distribution of the numbers of customers in the two queues. This work complements that in (Morrison, 2010), which the second queue was assumed to be heavily or lightly loaded, leading to mean queue lengths that wereO(ε-1)orO(1), respectively.

Design of Dynamically Wind-Loaded Helical Piers for Small Wind Turbines

The expansion of alternative energy has created a demand for sustainable alternatives for wind turbine foundation design. This study investigates the proposed application of helical piers as foundations for guyed cables of small (1–10-kW) wind towers. Before the foundation system can be implemented, pier response to typical working loads and extreme environmental conditions must be determined. Field conditions were determined by equipping an existing wind tower with accelerometers and load cells while monitoring wind speed to determine tower response. A full-scale testing program was conducted, which simulated dynamic loading conditions based on the tower response. The testing program varied between typical working conditions and extreme load events to determine the critical loading case and creep potential from long-term loading. This paper discusses the effects of dynamic loading on helical pier performance and compares the results to that of uplift prediction methods typically used in helical pier design.

Probabilistic Analysis and Optimization of Roof Trusses

The influence of gap between wooden members and displaced nail-plates in roof trusses is investigated by means of probabilistic calculations and structural optimization. The employed mechanical model, using the finite element method, captures the non-linear behaviour of the nail-plate joints. The joint stiffness involves the size and location of the nail-plate, and the presence of contact or gap respectively between the wooden members. The analysis shows that even small initial gaps considerably increase the deflection of a roof truss. Misplaced nail-plates also effect the deflection negatively, but it is also shown that the deflection can be significantly decreased, and the utilization of nail-plates improved, by optimizing the nail-plate positions with respect to critical load cases and requirements.

Influence of pre-existing vibrations on the dynamic response of medium span bridges

Critical static bridge loading scenarios are often expressed in terms of the number of vehicles that are present on the bridge at the time of occurrence of maximum lifetime load effect. For example, 1-truck, 2-truck, 3-truck, or 4-truck events usually govern the critical static loading cases in short and medium span bridges. However, the dynamic increment of load effect associated with these maximum static events may be assessed inaccurately if it is calculated in isolation of the rest of the traffic flow. In other words, a heavy vehicle preceding a critical loading case causes the bridge initial conditions of displacement and acceleration to be nonzero when the critical combination of traffic arrives on the bridge. Failure to consider these pre-existing vibrations will result in inaccurate estimation of dynamic amplification. This paper explores these dynamic effects and, using statistical analyses, outlines the relative importance of pre-existing vibrations in the assessment of total traffic load effects.

Response and Modeling of Cantilever Retaining Walls Subjected to Seismic Motions

Abstract: A series of nonlinear, explicit finite difference analyses were performed to determine the dynamic response of a cantilever retaining wall subjected to earthquake motions. This article outlines the calibration and validation of the numerical model used in the analyses and comparisons are presented between the results from the finite difference analyses and results from simplified techniques for computing dynamic earth pressures and permanent wall displacement (i.e., Mononobe‐Okabe and Newmark sliding block methods). It was found that at very low levels of acceleration, the induced pressures were in general agreement with those predicted by the Mononobe‐Okabe method. However, as the accelerations increased to those expected in regions of moderate seismicity, the induced pressures are larger than those predicted by the Mononobe‐Okabe method. This deviation is attributed to the flexibility of the retaining wall system and to the observation that the driving soil wedge does not respond monolithically, but rather responds as several wedges. Additionally, it was found that the critical load case for the structural design of the wall differed from that for the global stability of the wall, contrary to the common assumption made in practice that the two load cases are the same.

Conceptual Design on Carbon-Epoxy Composite Wing of a Small Scale WIG Vehicle

Conceptual structural design of the main wing for the 20 seats WIG(Wing in Ground Effect)flight vehicle, which will be a high speed maritime transportation system for the next generation in Rep. of Korea, was performed[1,2]. The high stiffness and strength Carbon-Epoxy material was used for the major structure and the skin-spar with a foam sandwich structural type was adopted for improvement of lightness and structural stability. As a design procedure for this study, firstly the design load was estimated through the critical flight load case study, and then flanges of the front and the rear spar from major bending loads and the skin structure and the webs of the spars from shear loads were preliminarily sized using the netting rule and the rule of mixture[4,5]. In order to investigate the structural safety and stability, stress analysis was performed by commercial Finite Element code such as NASTRAN/PATRAN. From the stress analysis results, it was confirmed that the upper skin structure between the front spar and rear spar was weak for the buckling. Therefore in order to solve this problem, a middle spar and the foam sandwich structure at the upper skin and the web were added. After design modification, even thought the designed wing weight was a little bit heavier than the target wing weight, the structural safety and stability of the final design feature was confirmed. In addition to this, the insert bolt type structure with six high strength bolts to fix the wing structure at the fuselage was adopted for easy assembly and removal. As well as consideration of the fatigue limit load for more than 20 years fatigue life.

Structural collapse of a freestyle ski-jump

During the morning completion works on a ski-jump and before the re-opening of an annual sporting event on the second day, attended then by thousands of visitors, the jump’s landing ramp collapsed. Fortunately, nobody was injured; however, the event had to be cancelled with the concomitant financial losses. Due to the high risk potential of the accident, an analysis of the failure was ordered by the public prosecutor. The key issue was not only to find the cause of the accident, but also to define precautions for future events. The landing ramp with a 20 m span was built up from framework units assembled by means of plug-and-socket connections. Based on the macroscopic appearance of the damage, the most probable primary fracture was determined. Analytical calculations showed that the primary broken connection’s lug was located in the most highly loaded area of the ramp. During the snow preparation, the static load in this area was more than twice the intended final load because of the unequal distribution of the snow. This critical load case had not been considered during the design of the ramp. Nevertheless, the collapse still could not be explained based on this fault, as the ultimate strength measured on undamaged specimens would still have been sufficient. Consequently, a reduced load capacity at the primary fracture had to be assumed. This hypothesis was supported by the fact that the crack proceeded through the welded seams rather than trough the parent material. By means of fractographical and EDX analysis, zinc was detected at the edge of the fracture surface. The metallographic findings precluded brittle fractures during the hot-dip galvanising as a reason for this anomaly. However, impermissible welding imperfections and insufficient corrosion protection were also found. The estimated load capacity of the thus reduced cross-section corresponded well with the assumed load at the time of the collapse. Based on the conclusions of this study, recommendations were derived regarding the design of the ramp (e.g. redundancy) and the welded construction, as well as the corrosion protection of the framework units.

Evaluation of Shear Capacity of a Prestressed Concrete Box Girder Bridge using Non-Linear FEM

In this paper it is demonstrated how higher load carrying capacity with respect to combined shear and torsion can be determined by non-linear finite element (FE) analyses of bridges. A prestressed concrete box girder bridge, the Kallosund Bridge, was evaluated. The Kallosund Bridge,on the west coast of Sweden, is a 45-year-old, cast in situ concrete bridge with four spans, constructed by the free cantilevering method. The case study presented indicates a substantially higher load carrying capacity than for bridge assessment by conventional design methods. In the non-linear analyses of the Kallosund Bridge, about 40% higher design bogie load than in the initial structural assessment was reached (B = 250 kN), together with the other design loads in the ultimate limit state (ULS), before a shear failure developed for the critical load case studied. By improving the model and with "deformation controlled" loading, an even higher failure load is a likely result.

Modelling, testing, and construction of the first Ductal® canopy in the world

Two series of nine architectural shell structures — constructed of a 20 mm thick layer of a new ultra-high performance cement (UPHC) composite with organic fibres (Ductal®) — were proposed to form the roofs of a new station for the light rail transit system in Calgary. Each section of the roofs consists of three panels each 6 m × 4.5 m connected to provide an 18 m continuous system. The City of Calgary required testing of a full-scale prototype panel for both static and dynamic responses. Finite element analysis (FEA) was used to determine the boundary conditions to be imposed on one edge of the prototype panel during the tests to simulate the presence of the other two in the actual structure. The FEA was also used to determine the critical (independent) snow and wind loading cases. The analyses and the testing procedures for the full-scale instrumented panel are described. The panel withstood both full factored loads without damage. The methods for manufacturing and assembling the canopy and roofs are also described.Key words: UHPC, shell structure, finite element analysis, static test, dynamic test.

GLOBAL ANALYSIS OF THE AIRCRAFT STRUCTURE AND ITS APPLICATION TO THE PRELIMINARY DESIGN STAGE

Modern aircraft safety depends on sufficient strength and rigidity of the structure. This must sustain with lightest possible weight, because any excess mass has not only detrimental effect upon the performance but also is significant economic factor. The most rational way to achieve the proper structure seems to be global analysis commenced in the preliminary design stage already. The analysis outcomes provide base for local analysis of the details led parallel. Any revisions more or less relevant can be made in the numerical model with very expensive prototype changes avoiding. The paper illustrates efficiency of the airframe structure global analysis. As examples the aircrafts still in service but designed without computer application were chosen. The finite elements numerical model of each was created and some critical in-flight load cases were simulated.

Repair and strengthening of RC flat slab bridges using CFRPs

This paper reports on experimental investigations made of the performance of two separate CFRP based repair/strengthening schemes (a laminate strip scheme and a fabric system) adopted on two 40% scale model flat slab bridges. The models contained typical features of a large class of multi-span RC flat slab bridges with cantilever ends commonly found in the State of Victoria, Australia, that are now over 60 years old. The models had been previously tested to incipient collapse conditions under the critical design load case so were severely damaged prior to repair/strengthening. The performance was gauged using dynamic testing (experimental modal analysis) and static testing, again to incipient collapse. Both of the repair/strengthening systems were found to perform satisfactorily, suggesting that either could be used as a viable remedial strengthening strategy for this class of bridge structure.

Polling systems in the critical regime

We study polling systems with Poisson arrival streams and exhaustive service discipline in the critical load case ρ=1. We show that the system may exhibit null-recurrence or transient behavior and give the corresponding conditions in terms of the first two moments of the distribution of service and switching times. For models with two nodes we establish existence of moments of small orders for the return time to the origin of the corresponding embedded Markov chain in the case of null-recurrence.

Probabilistic wind loadings on transmission line structures in India

An evaluation of probabilistic wind loadings on transmission line structures in India according to the recommendations of International Electrotechnical Commission/Technical Committee 11 (IEC/TC 11) is presented in order to promote international unification. The wind loads thus obtained are compared with relevant Indian, German and other European specifications. Typical towers are then analysed for these loads considering critical load cases using the developed software, TLEXPERT. There are considerable differences in certain details when Indian Standard (IS): 802–1977, Indian Standard (IS): 802–1989 (proposed) and German Standard DIN VDE 0210 are compared. As a whole, however, the design according to IS and VDE lies within the bandwidth which is outlined by the IEC recommendations. The example discussed illustrates the difficulties encountered during the practical application of probabilistic design and provides a pointer towards desirable improvements.

Some aspects of the structural design of a mat foundation for various soil conditions

There are several important structural design parameters in the analysis of a mat type structure for a jackup mobile offshore drilling unit. These consist of dimensions for the mat, soil foundation types, and structural loads. Methods for determining the proper dimension of the mat structure, modeling the structure and its loading cases, modeling the soil conditions and a method for identifying critical load cases for each element and load case is presented. By using a database approach for the finite element output for all of the various loading cases, a rational approach is presented which solves the problem of sorting and analyzing all of the elements and load cases, ensuring that all are included and none of the critical cases are missed.

Post office microwave tower foundations : Anderson, C K Proc Symposium on Piled Foundations for Engineering Structures, Hamilton, 5–6 September 1986p19.1–19.6. Publ Wellington: IPENZ, 1986

The design of microwave tower foundations is discussed in this paper. The microwave towers are generally sited on hilltops where the design windspeed is higher than normal, therefore wind loads are the critical load case.