The Research On Vertical Differential Shortening Of Steel Frame Reinforced Concrete Corewall Structure When Considering Concrete Shrinkage And Creep

Under vertical gravity loads, the differential shortening between columns and corewalls in the steel frame-reinforced concrete corewall structure can not be ignored as a result of the difference of stress levels and concrete shrinkage and creep. The concrete shrinkage and creep have been studied for many years, and professional softwares for analyzing bridge stage construction have been developed, but investigation on differential shortening between columns and corewalls in the steel frame-reinforced concrete corewall structure is not so satisfied to guide construction practice. Sponsored by the Special Technical Development Foundation to National Scientific Research Institutes, this paper is aimed at analyzing the differential shortening problem.Controlled by time parameter, the displacement method equation considering construction procedure and concrete creep effect is deduced from the strain-stress basic relation when creep is included. The recurrence calculating mode is adopted to save memory space by transforming creep formula to specific format.A finite element program CSASRC is designed which can analyze the differential shortening between columns and corewalls and the internal forces of structural members when the concrete shrinkage and creep is considered in the stage construction of the steel frame-reinforced concrete corewall structure. To consider time effect, the calculating time can be until the structure construction completed or until 3 years after the structure construction completed. In the program CSASRC, several technical parameters such as the environment relative humidity (RH), the age of concrete starting shrinking (ts), the number of construction days of each storey (sday) and the storeys the concrete portion erected before the steel portion (spre) can be adjusted to take field humidity condition and different construction schemes into account. The program CSASRC can also provide two creep formula choices: the creep formula in JTG D62-2004 and the creep formula in JTJ023-85.Six calculating models of different plane layout and different storeys are designed. The influence of plane layout, the number of structure storeys, the environment relative humidity and construction scheme are studied. The analysis shows that the calculating results according to the creep formula in JTG D62-2004 and the creep formula in JTJ023-85 are basically the same. The differential shortening between columns and corewalls will decrease when the their stiffness difference decrease or when the beam stiffness increase. The cumulative shortening of steel columns is larger than that of concrete corewalls in the middle storeys of the structure, and the maximum cumulative differential shortening between them can approach to 20mm. The differential shortening will sharply decrease when environment relative humidity decreases, but will slightly increase when the number of construction days of each storey is reduced or when the storeys the concrete portion erected before the steel portion is increased. If considering the concrete shrinkage and creep until 3 years after the structure construction completed, the cumulative shortening of concrete corewalls may exceed that of steel columns in the top storey, and the maximum cumulative differential shortening between them can approach to 13mm. The differential shortening between two columns is far smaller than that between columns and corewalls, and the shortening of internal columns is larger than that of side columns. The axial compressive forces of columns will increase 10% when considering the concrete shrinkage and creep until the structure construction completed or increase 30% when considering the concrete shrinkage and creep until 3 years after the structure construction completed. If environment relative humidity decreases, the axial compressive force increase proportion of columns will be more significant. Normally, the axial compressive forces of corewalls will decrease when considering the concrete shrinkage and creep. When considering the concrete shrinkage and creep until 3 years after the structure construction completed, the axial compressive forces of corewalls will decrease in most storeys except top storeys in which the corewall axial forces may be tensional. The internal forces of steel beams are greatly changed when the beams lie in the storeys where the differential shortening between columns and corewalls is rather enormous, so that the bending direction of these beams may shift to affect the beams design.Several simplified methods based on concrete effective elastic modulus are introduced and compared with accurate method. Their feasibility and applicability are studied detailed. The research shows that the results of most simplified methods are not reliable for the problem of the differential shortening except the method which adopts integer calculating model and consider all the gravity loads, the concrete shrinkage and creep at the same time(not step by step).Several compensation schemes are introduced and the compensation effects are verified. The study points out that the optimal compensation scheme is fairly reasonable, not only ensuring the compensation precision, but also making construction more convenient after optimal calculation. When optimal compensation scheme is adopted, the maximum cumulative differential shortening between columns and corewalls is reduced sharply, which ensures beams and plates to maintain horizontal. In the compensation structure, the change of internal forces of all structural members is rather small so that the original structure design can still be applied. According to the optimal compensation scheme, tables of recommended compensation values to the models in this paper are listed to guide construction practice.