Research On Solution Of The Temperature And Shrinkage Crack Control In Overlong Concrete Structure

The characteristic of overlong concrete structure is that its length exceeds the code limits and the stress caused by temperature and shrinkage is rather large, which usually brings crack in structure. Sometimes the crack is so wide that the structure can't be normally used. So the temperature and shrinkage crack has been a pivotal problem to be solved urgently. Based on the theory of the temperature and shrinkage stress, several methods of controlling the crack are studied. The contents are as follows:1. The concrete structure deforms because of the temperature's change and shrinkage. The beam and slab's deformation will be constrained by the column and shear wall, and then stress takes place in the structure which is called the temperature and shrinkage stress. Two main influencing factors are the temperature's change (including the equivalent temperature's change by shrinkage) and constraints. There are two kinds of ideas in the crack control. One is to enhance the concrete's tensile capability, such as exerting prestress and adding tensile fiber into concretes. The other is to minish the stress by weakening the constraints, such as setting deformation slots and installing the rubber bearing.2. Several important affecting factors on the temperature and shrinkage stress are analyzed, and the distribution characteristic of the stress in the structure is studied. The whole temperature decline is a slow, seasonal course. When it happens, the stress will be rather large in the bottom two floors while it is quite small in the other floors. The top floor's temperature decline is a rather quick course. When it happens, the stress will be rather large in the top two floors while it is quite small in the other floors. Moreover, the peak stress of beams and columns at the ends is quite large (especially of the column, sometimes it will cause the column to break), which greatly exceeds that of the slab and happens no matter how the temperature changes. However ,this is usually ignored in the past.3. As for the prestressed concrete structure's crack control, firstly, special attention must be paid to structure disposal, secondly, the prestress needs to be exactly calculated, moreover, importance must be attached to the detailing measures. In the construction of overlong concrete structure, the after-pouring zone is set to release the forepart shrinkage stress, and prestress is exerted subsection by subsection. The study indicates: proper simulation of after-pouring zone is of great importance to evaluate the effect of the prestress. By means of the finite element analysis software ANSYS, the computing method is proposed which considers the construction course and is further expatiated combined with numerical cases.4. By referring to the bridge engineering experience, the method of installing the rubber bearing in the concrete frame is put forward, which is a meaningful try on crack control. After installing the rubber bearing, the temperature and shrinkage stress drops greatly. The stress in the beam and column minishes apparently so that the problem of column's large stress is solved. Setting the rubber bearing on the top of top columns can minish the stress caused by temperature decline of the top floor and meetthe aseismic requirement. Setting the rubber bearing on the top of bottom columns can minish stress caused by the whole temperature decline, also the aseismic requirement can be met by means of setting shear wall at certain location. The study indicates: the larger of the pressure of the rubber bearing, the weaker of the gliding capability of the rubber bearing. However, because the rubber bearing can rotate freely, the effect of minishing the stress of temperature and shrinkage is also very apparent.5. Following the second idea of crack control, a new method–"disconnection"method, is put forward to solve the the temperature and shrinkage crack problem in the frame-shear wall structure. Shear wall is the primary vertical component in the frame-shear wall structure. It is usually strong to resist the lateral force and limit the horizontal displacement. If slots are set between the shear wall and both the second and the third floor, the two will disconnect, the shear wall will no longer constrain the slab, which will greatly minish the stress of temperature and shrinkage. However, after disconnection, the integrity of the structure will weakened, which will cause doubt about the aseismic capability. The study indicates: after disconnection, the horizontal force of the top floors can be transferred to the base through shear wall, however, the horizontal force of the bottom floors is transferred thoroughly through the frame to the base. The horizontal force of bottom floors is quite small, so the bottom shear force of the shear wall will decline slightly, however, the shear wall also carries most of the base shear force, the structure can meet the aseismic requirement.