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Most bridges are designed using a static analysis, adjusted by a dynamic amplification factor which is a function of the first flexural frequency. This method is imprecise but continues to be used, partly because of the apparently complex processes required to estimate accurately and to provide for the levels of vibration in a bridge. This paper investigates the natural frequencies and associated mode shapes of bridge superstructures. It compares field observations with theoretical idealizations and finds that, while a single beam idealization is accurate for straight, nonskewed bridges and for some continuous superstructures, many other bridges require an eigenvalue analysis of a finite beam element grillage. A simplified method for estimating the natural frequency of vibration is developed. An application of the Rayleigh method to a grillage model of the bridge, it is quick to apply and accurate to within 10%. The paper also compares the effects of using the static and dynamic moduli of elasticity of concrete in estimating the natural frequency of vibration, and concludes that the dynamic modulus is more appropriate. Finally, it debates the significance of certain types of support stiffness in estimating the fundamental frequency, and finds that their effect is negligible. 相似文献
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Tetraglycidyl-4,4′-diaminodiphenyl methane cured with 4,4′-diaminodiphenyl sulphone was irradiated with varying dosages of 0.5 MeV electrons and 1.17 and 1.33 MeV gamma rays and was studied using electron spin resonance. The radical concentration increases with increasing radiation dose and decays, at ambient temperature, in a way consistent with a model that assumes two simultaneous second-order reactions occurring in different zones. 相似文献
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Epoxy/graphite fiber, polyimide/graphite fiber, and polysulfone/graphite fiber composites were exposed to 1.33 MeV γ irradiation and 0.5 MeV electron bombardment for varying periods of time. The effects of the irradiation treatments on the breaking stress and Young's modulus were studied by a three point bending test. Effects were small; indeed, both electron radiation up to 5000 Mrad and γ radiation up to 350 Mrad resulted in slight increases in both stress and modulus. 相似文献
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Crosslinked epoxy resins, tetraglycidyl 4,4′-diamino diphenyl methane cured with 4,4′-diamino diphenyl sulfone, were soaked in water at either 25°C or 70°C for varying lengths of time. The infrared spectra and DSC thermograms were obtained for samples that were soaked, or soaked and dried. There was a monotonic decrease in exothermic reaction energy with water content. The glass transition was also lowered, although samples soaked at 70°C showed a leveling in the Tg around 115°C. When the soaked samples were dried, the exothermic reaction energy showed near reversibility for samples soaked at 25°C while the 70°C samples were highly irreversible. IR of the latter samples showed that the 70°C water soaking resulted in reaction of some of the unreacted epoxide groups that remained after the initial cure. 相似文献
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