Research On Fire Resistance And Fire Safety Design Method Of Prestressed Concrete Beam And Slab

Fire can severely destruct building structures and even cause it collapse. In recent years, prestressed concrete (PC) structures grow rapidly, which are used more and more widely. Structural fire design should ensure that the structure is safe during fire and after fire. Although fire design method of PC structure has been preliminarily mentioned in Eurocode 2-1-2: structural fire design (EC2-1-2), the mechaincal performance of PC beam and slab during/after fire need to be researched further. 5 parts of research work are accomplished in this dissertation.(1) The mechanical property of prestressing steel wire and non-prestressed steel bar after fire is the key factor in damage evaluation and repair after fire for prestressed structures. At present, the stress in the prestressing steel wire and non-prestressed steel bar at elevated temperature is not considered in their mechanical property after fire. In order to investigate the mechanical properties of prestressing steel wire and non-prestressed steel bar after fire, 38 prestressed concrete slabs and beams after fire are smashed and the prestressed steel wire and non-prestressed steel bar are taken out. A tensile test of 481 prestressing steel wires and 489 non-prestressing steel bars after fire is completed. Based on the experiment data, the relative strength, elastic modulus, and specific elongation of prestressing steel wire and non-prestressed steel bar after fire are obtained respectively. The strain-stress relationship of prestressing steel wire and non-prestressed steel bar after fire is obtained respectively. The experiment results show that: the strength of prestressing steel wire with initial stress after fire is lower than that without initial stress. For the prestressing steel wire with the ratio of initial stress to ultimate strength is 0.6 at normal temperature, the 0.2% yield strength of prestressing steel wire after fire is 7.0% lower than that without initial stress. But the strength of non-prestressed steel bar with and without initial stress after fire does not vary obviously.(2) Specific fire design method for PC structure has not been contained in Code for design of concrete structure (GB50010-2002) in China. 38 PC simple and continuous beams and slabs are test on the fire resistance. The influences of key factors such as concrete cover, load level and global reinforcement index to the fire resistance performance of PC beams and slabs are explored. The calculation formulas of the stress in the unbonded tendons and the deflection at mid-span of PC beam and slabs at elevated temperature are proposed, and the variation of support reaction at elevated temperature is obtained.(3) The constitutive model of concrete, prestressing steel wire and non-prestressed steel bar and the mechanical performance of PC beam and slab differ with different temperature-load paths. Based on the constitutive model of concrete, prestressing steel wire and non-prestressed steel bar considering the influence of thermal-mechanical coupling, and transient strain, compressive creep in concrete at time t+1 can be calculated through stress in concrete at time t, and then the coupled constitutive model of concrete can be decoupled, and prestressing steel wire and non-prestressed steel bar are disposed by the same way. Therefore, the whole progress of moment-curve analysis of PC beam and slab subjected to fire is completed. Based on the deflection compatibility equation at the support, the support reaction can be calculated by secant stiffness matrix iteration method, and the deflection of continuous beam and slab can be obtained by curve integral of cross section. Then the nonlinear finite element analysis of loading process for prestressed concrete continuous beam and slab at elevated temperature can be completed.(4) Structural fire design should ensure that the structure is safe during fire and after fire. 19 PC simple and 16 PC continuous beams and slabs are tested on mechanical performance after fire. Crack distribution and growth, deflection development, support reactions, stress in unboned prestressing steel wires, flexural carrying capacity as well as failure characteristics are obtained. The experiment results show that: the carrying capacity of bonded PC slab decreases grteater than that of the unbonded PC slab after fire under the same condition. Based on the experiment data, calculation formula of residual stress, ultimate stress in prestressing steel wire and crack width is put forward, and deflection analysis method of PC beams and slabs after fire is proposed.(5) In order to meet the requirement of fire design and repair method after fire for PC beam and slab, proposition on fire design and repair method after fire for PC beam and slab is proposed, which comprises general, effects of actions, characteristic values of resistance and fire design principles, thermal properties of material, mechanical properties of matireial during/after fire, calculation for temperature field, verification methods for crack, defletion and carrying capacity, concrete cover thickness for fire resistance of PC beams and slabs, detailing rules, check method on spalling of concrete of prestressed slabs and beams at elevated temperature and detailing rules to avoid concrete spalling etc.