Theoretical Analysis On Secondary Effect Of Small Eccentric Compressive Circular Column With Mid-Length By Enclosed Reinforcement

At present, large-scale bridge construction has already past. The bridge in service may satisfy the demand of national economy development in a long period besides a few engineering constructed lately. The following task is to maintain and strengthen the civil engineering in service. A great many methods can be used to strengthen in totally. The enlarging section method is a traditional method among the methods mentioned above. The structure after strengthening belongs to secondary loading structure. Because of load has already exerted on the structure before strengthening, the stress and strain exist in the original structure. However, the material newly added just begins to bear load while the secondary load is exerted. Therefore, the total level of stress and strain is lower than that in original structure. The material newly added can not be utilized fully in ultimate resisting capacity state. Exactly because of the difference mentioned above makes the structure strengthened to have great difference with the common structure that have not yet to be strengthened. Thereafter, the research on various performances in strengthened structure can't follow to use the formula in exist completely.The paper relies on the project of"Strengthening Calculation Theory and Practical Technique Study for Superstructure of Highway Bridge"(serial number 2004-1-3), which belongs to traffic science and technology development fund in JiLin province. The paper carries out theory study on secondary effect of small eccentric compressive circular column with mid-length by enclosed reinforcement which is one of the enlarging section manners. As one of the series study, the paper mainly studies the deform regulation of column under external load with finite element theory, and workouts the procedure. And then many groups of columns are analyzed using the procedure. The factors influencing the secondary effect of mid-length circular column with small eccentricity are analyzed. The paper consists of five chapters. Among these chapters, chapter three and chapter four are the most important part from the whole paper.Chapter one is the introduction. It mainly introduces the principle of concrete structure strengthening and secondary effect. At present time, the calculation of secondary effect of strengthening column still uses the eccentricity incremental coefficient formula applied in the common structure in current without taking the differences between the strengthened and common structure into consideration. The eccentricity incremental coefficient obtained by the formula has limitation in some extent. The paper purpose is to analyze the factors influencing secondary effect of mid-length strengthening column with small eccentricity. Furthermore, the comparison is made between the simulation result of the eccentricity incremental coefficient and the value calculated by the formula according to the criterion. The paper puts forward rational suggestion for strengthen design.Chapter two mainly introduces the theory on secondary effect of the common component with eccentricity. The thought of calculating secondary effect of the common component is still adapted to strengthening structure. Thereby, taking the standard hinged column as the base, this chapter introduces three kinds of approximate methods to calculate secondary effect. The first is to solve the differentiation coefficient equilibrium equation by using the elastic stability theory. And then the eccentricity incremental coefficient is obtained. The second is the"graphic multiplication method"which is more convenient and feasible. The third is curvature method, which is also the most widely used at present. Finally, the chapter introduces concrete formula used to calculate the eccentricity incremental coefficient of common reinforced concrete structure in"Concrete Structure Design Specification"(GB50010-2002) currently used in our country.Chapter three is one of the important parts in the whole paper. This chapter refers to the stress-strain constituent relation model of new and old concrete and reinforcement. The assumption is made that the incremental strain meets plane section under the new load from the aspect of the new section. The marginal strain of the original column is firstly calculated in case that the stress level index is known. Then a series of assumption is made to deduce the stiffness matrix considering material nonlinearity. The layer method is used to solve the stiffness in the stiffness matrix. In addition, the chapter works out FORTRAN procedure to solve the nonlinear equation. The deflection in critical section is obtained by times after times calculation. Finally the total deflection is gained.Chapter four is the other significant part. This chapter carries on simulation test with many groups of columns which have different parameters. Then the analysis is made on slenderness, initial eccentricity, and steel ratio before and after strengthening, intensity grade of new and old concrete, the loading level while strengthening and etc. All above factors can influence the deflection and eccentricity incremental coefficient of mid-length circular column with small eccentricity by enclosed reinforcement. Still, the simple analyze is made in ultimate resisting capacity. In addition, the eccentricity incremental coefficient obtained by simulation is compared to the value calculated with the formula in the specification. The comparison result shows that the factors largely influence the secondary effect of mid-length circular column with small eccentricity by enclosed reinforcement are stress level index, steel ratio, initial eccentricity and slenderness. The intensity grade of new concrete has indistinct effect. At the same time, the analysis on ultimate capacity of the column after strengthening indicates that the influence degree of initial stress level index in mid-length circular column with small eccentricity is not obviously than that in axial pressed short column.Chapter five is the last part. And this chapter is summarization of this paper in which the content and direction are also pointed out for further study.