Research On Fire Resistance Performance Of Full-scale Prestressed Concrete Beams And Slabs

Bonded prestressed concrete beams and slabs are commonly structural member forms and have been applied widely in civil engineering field. But the fire resistance performance mechanism of them is not clear, the beams and slabs have the risk of collapse in fire. So it is necessary to research the fire resistance of bonded presressed concrete beams and slabs. At present, most research of bonded prestressed concrete beams and slabs subjected to fire is focused on experimental aspect. But owing to the high cost of fire testing and the limitation of test conditions, the test specimen size is smaller than actual size. And due to the thermal inertia of concrete, the problem of similarity ratio is difficult to solve. So the fire resistance performance of the full-scale bonded prestressed concrete beams and slabs can not been reflected by the fire resistance performance of small-scale specimens. In view of the status in quo, the following research work is carried out in this paper.(1) The process of heat transfer is simulated using ANSIS software in fire environment. And the temperature field of bonded prestressed concrete beams and slabs in given temperature-time curve is achieved. Based on the analysis results of the temperature field, the structural response of the beams and slabs under high temperature is analyzed using written FORTRAN programs. In the process of analysis on structural response, fiber element model is adopted and the coupling effects of load-temperature are taken into accounted. The moment-curvature relationship of all sections along the span of beams and slabs is got through the moment-curvature analysis of cross section. Then the deflections of the beams and slabs are obtained by using the method of conjugate beam.Using the above method, the analysis of spreading parameters to structural response of bonded prestressed concrete beams and slabs under high temperature is adopted. The influence on ratio M_u~T /M_u between ultimate moment under high temperature and ultimate moment at normal temperature of the beams and slabs are explored, including the section size of beams and slabs, composite reinforcement index, prestress degree, effective prestress, concrete cover of prestressed reinforcement and so on factors. The calculation results indicate that composite reinforcement index, prestress degree and effective prestress have little effect on M_u~T /M_u of the beams and slabs. The calculation formulas about M_u~T / M_u of bonded prestressed concrete beams and slabs are established. The established formulas reflect the decreasing extent about section bearing capacity of the beams and slabs under high temperature. When the loads and reinforcement of full-scale bonded prestressed concrete beams and slabs are fixed on, the fire resistance of the beams and slabs under high temperature is achieved.(2) With the same analysis method under high temperature, the structural response of bonded prestressed concrete beams and slabs after high temperature is analyzed using written FORTRAN programs. The analysis of spreading parameters to the beams and slabs is adopted. The calculation formulas about ratio M_u~A hbetween ultimate moment after high temperature and ultimate moment at normal temperature of beams and slabs are established. The formulas reflect the decreasing extent about section bearing capacity of the beams and slabs after high temperature. The establishment of the formulas has the significance to evaluate the damage in fire and restoration after fire.(3) Aiming at the issue in existence that the fire resistance of bonded prestressed concrete simply slabs is controlled by deflection, the deflection of full-scale bonded prestressed concrete simply slabs in fire is got through nonlinear calculation analysis. And the influence on fire resistance controlled by deflection of the slabs are explored and discussed, including effective span, concrete cover thickness, load level, prestress degree, effective prestress, composite reinforcement index and so on factors. The calculation results indicate that prestress degree and effective prestress have little effect on the fire resistance controlled by deflection of the slabs. The calculation formulas of fire resistance controlled by deflection are established, in which, effective span, concrete cover, load level and composite reinforcement index are rationally considered.(4) In order to meet the requirement of fire design for bonded prestressed concrete beams and slabs, proposition on fire design for bonded prestressed concrete beams and slabs are proposed, which comprises general, temperature-time curve, temperature field calculation, fire resistance of bonded prestressed concrete beams and slabs effects of actions, the calculation of capacity, the calculation of fire resistance controlled by capacity, the calculation of deflection, detailed rules of reinforcement arrangement, checking of concrete spalling and detailed measures to prevent concrete from spalling etc.