Behaviour of concrete frame structures under localised fire scenarios

This thesis encompasses an investigation of the behaviour of concrete frame structure under localised fire scenarios by implementing a constitutive model using finite-element computer program. The investigation phase included properties of material at elevated temperature, description of computer program, thermal and structural analyses. Transient thermal properties of material have been employed in this study to achieve reasonable results. The finite-element computer package of ANSYS is utilized in the present analyses to examine the effect of fire on the concrete frame under five various fire scenarios. In addition, a report of full-scale BRE Cardington concrete building designed to Eurocode2 and BS8110 subjected to realistic compartment fire is also presented. The transient analyses of present model included additional specific heat to the base value of dry concrete at temperature 100°C and 200°C. The combined convective-radiation heat transfer coefficient and transient thermal expansion have also been considered in the analyses. For the analyses with the transient strains included, the constitutive model based on empirical formula in a full thermal strain-stress model proposed by Li and Purkiss (2005) is employed. Comparisons between the models with and without transient strains included are also discussed. Results of present study indicate that the behaviour of complete structure is significantly different from the behaviour of individual isolated members based on current design methods. Although the current tabulated design procedures are conservative when the entire building performance is considered, it should be noted that the beneficial and detrimental effects of thermal expansion in complete structures should be taken into account. Therefore, developing new fire engineering methods from the study of complete structures rather than from individual isolated member behaviour is essential.

Short-circuit and ground fault analyses and location in VSC-based DC network cables

The application of high-power voltage-source converters (VSCs) to multiterminal dc networks is attracting research interest. The development of VSC-based dc networks is constrained by the lack of operational experience, the immaturity of appropriate protective devices, and the lack of appropriate fault analysis techniques. VSCs are vulnerable to dc-cable short-circuit and ground faults due to the high discharge current from the dc-link capacitance. However, faults occurring along the interconnecting dc cables are most likely to threaten system operation. In this paper, cable faults in VSC-based dc networks are analyzed in detail with the identification and definition of the most serious stages of the fault that need to be avoided. A fault location method is proposed because this is a prerequisite for an effective design of a fault protection scheme. It is demonstrated that it is relatively easy to evaluate the distance to a short-circuit fault using voltage reference comparison. For the more difficult challenge of locating ground faults, a method of estimating both the ground resistance and the distance to the fault is proposed by analyzing the initial stage of the fault transient. Analysis of the proposed method is provided and is based on simulation results, with a range of fault resistances, distances, and operational conditions considered.

Refractive error, cognitive demand and nearwork-induced transient myopia

Purpose. Whereas many previous studies have identified the association between sustained near work and myopia, few have assessed the influence of concomitant levels of cognitive effort. This study investigates the effect of cognitive effort on near-work induced transient myopia (NITM). Methods. Subjects comprised of six early onset myopes (EOM; mean age 23.7 yrs; mean onset 10.8 yrs), six late-onset myopes (LOM; mean age 23.2 yrs; mean onset 20.0 yrs) and six emmetropes (EMM; mean age 23.8 yrs). Dynamic, monocular, ocular accommodation was measured with the Shin-Nippon SRW-5000 autorefractor. Subjects engaged passively or actively in a 5 minute arithmetic sum checking task presented monocularly on an LCD monitor via a Badal optical system. In all conditions the task was initially located at near (4.50 D) and immediately following the task instantaneously changed to far (0.00 D) for a further 5 minutes. The combinations of active (A) and passive (P) cognition were randomly allocated as P:P; A:P; A:A; P:A. Results. For the initial near task, LOMs were shown to have a significantly less accurate accommodative response than either EOMs or EMMs (p < 0.001). For the far task, post hoc analyses for refraction identified EOMs as demonstrating significant NITM compared to LOMs (p < 0.05), who in turn showed greater NITM than EMMs (p < 0.001). The data show that for EOMs the level of cognitive activity operating during the near and far tasks determines the persistence of NITM; persistence being maximal when active cognition at near is followed by passive cognition at far. Conclusions. Compared with EMMs, EOMs and LOMs are particularly susceptible to NITM such that sustained near vision reduces subsequent accommodative accuracy for far vision. It is speculated that the marked NITM found in EOM may be a consequence of the crystalline lens thinning shown to be a developmental feature of EOM. Whereas the role of small amounts of retinal defocus in myopigenesis remains equivocal, the results show that account needs to be taken of cognitive demand in assessing phenomena such as NITM.