High Load Situations Research Articles

Analysis of biomechanical and ergonomic aspects of the cervical spine in F-16 flight situations

With the help of a biomechanical neck model, several normal postures of an F-16 pilot were analysed. Measurements of accelerations and head positions were obtained during four flights, including simulated air combat. With the help of a model, muscle forces and joint reaction forces in the neck were estimated. Although at the present stage of research results of calculations must be interpreted carefully, conclusions can be drawn with respect to sitting posture, head position and helmet devices. The backward inclined back rest of the F-16 chair decreases the lordosis of the spine, resulting in reduced calculated forces in the lower cervical spine. In high load situations, calculated maximal forces are of the same order of magnitude as failure loads of vertebrae and estimations of maximum muscle forces. The calculated neck load is increased substantially by the helmet and helmet-mounted devices. This load can be reduced by lightening the helmet or shifting the centre of mass of the helmet backwards.

Motor temperature estimation incorporating dynamic rotor impedance

The estimation of induction motor temperature at different system, motor, and load conditions is investigated. Motor temperature is estimated using comprehensive interactive electrical, thermal, and mechanical models. The electrical model accounts for the skin effect in the rotor bars and predicts the dynamic motor impedance as motor speed changes. The thermal model accounts for the heating and cooling process in the motor. The interactive model is used to study motor behavior during balanced system condition, unbalanced system condition, high inertia load, locked rotor, and overload situations. Motor temperature during these states is predicted. It is shown that under certain conditions motor starting time may exceed the locked rotor time. Rotor thermal limit curves during an unbalanced voltage condition are presented. >

Analysis and design of single-pole transmission structures

The unguyed single-pole transmission structure is widely used to support transmission lines of up to 345 kV capacity. Most such structures are wood poles, although prestressed concrete poles and tubular steel poles are sometimes used in high decay or high load situations. A number of features combine to make this type of structure an interesting structural analysis and design problem. Whether naturally grown or manufactured, the pole structure is almost always nonprismatic. Wind, ice and snow loads acting alone or in combination are variable and occasionally severe in nature. Poles are slender and the combination of transverse wind load with vertical dead and ice loads often leads to behavior which is geometrically highly nonlinear. The continuous exposure to weather, fungi, accidents, etc. results in structures with capacities that often deteriorate very significantly with time in service. The material properties (strength, stiffness), especially of wood, are highly variable, as are the soil conditions. The configurations of conductors, davit and cross arms, and attachments that are in use vary widely. This large variety of structural shapes plus the necessity of changing the configurations of in situ structures to meet new conductor and clearance requirements prevents the use of only a few designs. Thus rapid ways to assess existing structures and to design new structures are required. Since there are more than 130,000,000 poles currently in use in single-pole and H-frame transmission structures and several million new poles are put into service each year, the design effort related to poles is large. As part of an eight-year effort to develop reliability-based design methods for transmission structures, a correct procedure for analyzing and designing single-pole transmission structures was established. Implemented in program POLEDA (POLE Design and Analysis), this procedure enables unguyed planar single-pole structures of virtually any material and configuration to be rapidly analyzed or designed. The analysis is based on the Newmark numerical technique which, it is shown, in the limit provides an exact solution to the differential equation for large deflections of transversely loaded beams. The four design options provided in POLEDA use the load and resistance factor design (LRFD) format. This permits the user to use current, deterministic, design procedures or to use load and resistance factors based on reliability studies. The program provides the necessary high-quality analysis capability which forms an integral part of reliability-based design. It is being widely adopted by the electric utility industry.

Reliability analysis of pole-type transmission structures

The unguyed single-pole transmission structure is widely used in the U.S. to support transmission lines of up to 345 kV capacity. Most poles are naturally grown wood, but some poles of prestressed concrete and some of steel are used, especially in high decay or high load situations. The resistance of each structure is a random variable which can be significantly influenced by time and location. Material strength and stiffness properties, especially of wood, vary from structure to structure, even for nominally identical poles. The life expectancy of poles varies with geographical location with poles in hot, humid climates having lower life times than those in hot, dry areas. The individual load causing phenomena of dead weight, wind velocity, and accretion of ice and snow on the structure are also random variables. Additionally, wind velocity, terrain conditions, temperature and structure shape interact to affect ice and snow accretion. Design of a structure to provide a known level of reliability requires that a reliability-based design methodology be established. To accomplish this the variable nature of the resistances and load causing phenomena must first be defined. Next, a correct method of analysis which properly considers all important response characteristics such as the highly nonlinear behavior of pole structures must be developed. The function of the analysis is to convert loads to load effects directly comparable to resistance. Finally, a procedure which allows the designer to size structures so they will provide the desired level of reliability must be created. A major part of producing such a practical design procedure is developing a method for performing the reliability analysis of transmission structures. As part of an eight-year effort to develop reliability-based design methods for transmission structures, several methods for performing reliability analyses of single-pole transmission structures were established. Implemented in program POLDAR (POLe Design, Analysis and Reliability), these procedures enable the user to perform a reliability assessment of unguyed planar single-pole structures of any material and configuration for the major loads normally considered in design. It is shown that the load and resistance factor design (LRFD) method developed based on reliability analysis results obtained with POLDAR produces structures having reliability levels very close to the target levels assumed by the designer. Thus a reliability-based design method is now available for use in the design of new structures and the assessment and upgrading of existing structures. Since over 130,000,000 wood poles are now in place, and several million additional poles are put into service each year at an in-place cost of $1000 to $2000 each, it is expected that improved design procedures will have a significant financial impact.

Analysis of Leg Loading in Snow Skiing

The complete excitation between the boot and ski was measured during skiing. Data from strain-gage force transducers mounted inside the test ski were FM transmitted to a receiving station about 3 km distant. Three skiing maneuvers (snowplow, stem christiana, parallel christiana) were tested and data from authentic falls were also recorded. Data were plotted in real-time and analyzed by digital computer. The loading histories reveal that the nonstationary loading is related to maneuver type. Torsion and bending component amplitudes, measured at the boot sole, are approximately equal to the static tibia ultimate strength in bending and torsion during both elementary skiing maneuvers and falls. In these high amplitude loading situations the skier was not injured. Data were analyzed for spectral content by the Fast Fourier Transform (FFT). Results show a 3.5 Hz cutoff frequency common to all load components and maneuvers. 3.5 Hz is a reasonable corner frequency for the human controller. Results suggest that below 3.5 Hz the skiing excitation is primarily muscle induced while above 3.5 Hz the excitation is due to ski-snow interaction.