A Time-variant Reliability Analysis On Structural Members Under Fire

Steel and reinforced concrete structure occupies a very large proportion in modern buildings, Under high temperature in the fire, the building elements of the material properties deteriorate seriously, which will severely weakened the structural bearing capacity, significantly increase structural deformation and even endanger the structural safety. Especially for steel structure, high strength, light weight, good seismic performance and reliability are its advantages. When the temperature reaches about 400℃, the yielding strength will be reduced to half of that at room temperature. When the temperature reaches about 600℃, the steel will almost lose all the strength and stiffness. Thus, structural fire resistance research has been a hot spot in recent years, in which fire resistance of building structures is necessary and urgent. The traditional fire resistance of building has usually been analyzed by the deterministic mechanical models. For the inherent uncertainty of engineering design, construction and use, the traditional methods can not give a scientific explanation on safety and the value of safety factor was adjusted by the accident rate, which would inevitably result in excessive material waste and the potential cost of loss. In this paper, based on practical engineering problems, the effects of fire on the structure have been analyzed through structural reliability design. Based on previous research, the following aspects are given in this paper.Firstly,under the impact of fire, the influencing factor on reinforced concrete structure reliability has been discussed. The fire bearing capacity of reinforced concrete and the resistance force have been linked together and the laws of resistance force changing with time have been given. By the certainty function of resistance force changing with time, the performance function has been built for steel concrete components under fire. With the use of standard first order second moment method recommended by the norm, the structural reliability factor change under fire has been described and analyzed the influence of protective layer thickness, reinforcement ratio, concrete grade, and other major factors on the fire resistance reliability of reinforced concrete structures. Meanwhile, with fully consideration of the existence of cracks in the beam components of reinforced concrete, the random variables concerning the influence of cracks on the resistance force has been introduced and the corresponding limit state equation has been given.Firstly, based on the theory of conical flow field, with shock angle, semi-spanwise angle, expansion angle, length-to-span ratio and function coefficient as design control parameters, cone-derived waverider configuration is generated under conditions of mach number 6 and 30km high atmosphere. On the basis of the analysis of the effect of the design control parameters on the performance of the lift-to-drag ratio of the waverider configuration, volume efficiency and etc., complex method is introduced to optimize and process the matching of lift to weight and top surface isentropic expansion of the generated waverider and the optimized shape of waverider is obtained under the design condition. Secondly, the surviving chance of reinforced concrete structures under fire in a certain time have been obtained by the all-random process model. With the calculation of difficulties on reliability index computation, which is inconvenient in engineering, the safety factor method has been introduced and the the approximate analysis has been used in calculating the reliability of reinforced concrete structures under fire by the safety factor, which maintains the simplicity and convenience of the safety factor method for designers and takes into account the variation of stress and strength, as well as structural design reliability goals.Thirdly, the influence of protective layer thickness of steel components, thermal conductivity, and the load ratio on the reliability of steel girder has been analyzed. Through the time variant reliability analysis, the fire resistance of steel girder under ISO834 standard temperature rising curves has been given.Finally, Through the mathematical derivation on reliability and with the consideration of high temperature on steel structure strength and in-plane and out-plane stability, the limit state equation of steel columns under high temperature has been established and the reliability index of steel columns has been given when considering basic load combination.In conclusion, reliabilities according to all kinds of structural building components under fire have been studied systematically. Reliability analysis methods have been obtained for building components under fire, which hopefully would provide useful reference to practical engineering problems.