Analysis On Compression Performance And Study On Bearing Capacity For RC Composite Circular Column Subjected To Secondary Load

Cylindrical concrete compressive component is the basic foundation form for pier and bored pile. The method of increasing the section area through reinforced concrete enclosure is commonly used, which is effective reinforcement method for reinforcing the defect pier and enhancing the technical level. The characteristics of stress and deformation show that the RC pier with initial stress belongs to composite structure subjected to secondary load after reinforced by concrete enclosure.Concrete compressive circular column subjected to secondary load can effectively improve the bearing capacity and section stiffness of the reinforced column. It is applied widely in reinforcement for bridge pier and the economic benefit is remarkable. When subjected to secondary load, the strain of enclosed concrete and reinforcement lags and core concrete and reinforcement advances in secondary loaded composite column. Therefore, the calculation on the ultimate capacity and deformation analysis should consider the mechanical characteristics in different stages.The related studies and tests on composite column subjected to secondary load carried out at home and abroad mainly concentrated in axial compression for rectangular section and circular section, reinforcement calculation in eccentric compression for rectangular section. And the studies on circular section in eccentric compression are less. However, the studies on secondary effect for mid-length circular column considering buckling deformation are much less. As one of the main force components, the issues of mechanical performance analysis on composite compressive circular column and ultimate bearing capacity calculation and secondary effect analysis on mid-length column considering buckling deformation are urgently needed to solve in practical engineering. Aiming at these problems, combined with the special research fund for the doctoral program"Theoretical analysis and experimental research on mechanical characteristics for cylinder pier of RC circular column reinforced by enclosed concrete"(200801830033) and the project of"Strengthening Calculation Theory and Practical Technique Study for Superstructure of Highway Bridge"( 2004-1-3)which belongs to traffic science and technology development fund in JiLin province.the following four aspects of research work are carried out based on the existing researches in the paper. 1. Axial compressive performance analysis and capacity studyThe bearing capacity under axial compression is the upper limit value of ultimate bearing capacity, which is often used in engineering design as the important means to preliminary estimate section and the check strength and has important theoretical significance.Through the analysis on mechanical behavior for composite column subjected to secondary load, the paper puts forward two possible ultimate states for composite axial compressive circular column. For ultimate state one, the damage sign for this ultimate state is that the stain of core concrete achieves compressive peak strain , now the compressive strain for enclosed concrete is lower than peak strain. This ultimate state is the most common ultimate case for composite column. For ultimate state two, the damage sign for this ultimate state is that the stain of core concrete achieves compressive peak strain , now the compressive strain for enclosed concrete has exceeded compressive peak strain , but has not reached the ultimate compressive strain .According to the assumption that constraint force of core concrete and annular tensile force of enclosed concrete are balance, theoretic minimum thickness and stirrup reinforcement ratio for axial compressive composite circular columns reinforced by enclosed concrete subjected to secondary load are deduced. In case of normal reinforcement, assuming that before concrete cracking, normal stirrup inside enclosure is not liable for annular tensile force and the annular tensile force is bore all by enclosed concrete. Core concrete constraint stress resulted by lateral restraint can be calculated according to the deformation condition. The minimum thickness of enclosure can be determined according to the assumption. In case that enclosure has certain thickness, strengthen reinforcement for enclosed concrete can also be used to make the encrypted stirrup play its role in hooping action at its ultimate state so as to improve the bearing capacity for composite structure. Stirrup minimum ratios can be calculated according to this assumption that annular tensile force bore by enclosure stirrup resulted by the transverse deformation of concrete and core concrete constraint stress are balance.Superposition method is adopted to calculate the bearing capacity for axial compressive composite column. The bearing capacity for axial compressive composite column also include the improvement value for core column after restrained by core concrete except the capacity of core concrete in circular section and enclosed concrete in annular section. In ultimate state one, the strength value for the enclosed concrete and steel should be multiplied by reduction factor less than 1. In ultimate state two, the bearing capacity of core concrete should be multiplied by the reduction factor less than 1. Also,the negative influence due to the action of tension-pressure composite stress and make the compressive strength lower should be taken into account.2. Study on the bearing capacity under eccentric compressive loadThis paper only studies on the ultimate bearing capacity calculations for the core section subjected to the first stage load and composite section subjected to the same directional eccentric secondary load. As the lower limit of bearing capacity of composite column, it also has important theoretical significance.According to the maximum compressive strain position, this paper divides the calculation process on composite column under eccentric loading into two kinds of ultimate states. Ultimate state one is corresponding to the case that the maximum strain in compressive zone of core column under the first stage load has reached a certain value or the thickness of enclosure is smaller, and at this moment the maximum strain in core concrete compressive zone will firstly reach the ultimate strain . Ultimate state two is corresponding to the case that the maximum strain in compressive zone of core column under the first stage load is smaller or the thickness of enclosure is bigger, and at this moment the maximum strain in enclosed concrete compressive zone will firstly reach the ultimate strain and the maximum strain in core column in compressive zone has not reached ultimate compressive strain.Base on the analysis on ultimate state under eccentric compression, the paper makes a simplification for the neutral axis of strain figure for core concrete respectively under the first stage load and the second stage load. The relation between height coefficients of core concrete and enclosure concrete in compressive zone is established and theoretical formula of relevant force and moment used to calculate the ultimate bearing capacity are deduced using integral.3. Simulation calculation on bearing capacityThe main influence factors on bearing capacity for composite axial compressive circular column subjected to secondary load are studied through the simulation analysis using software SBPES (Strengthening Bridge Piller by Enlarging Section) worked by ourselves. The paper analyzes enclosure thickness, reinforcement ratio of enclosure, strength of enclosure concrete, the initial stress level for core concrete and the influence of Poisson's ratio for core concrete on the ultimate bearing capacity through an example. Through ANSYS finite element simulation for two examples, the paper also verifies that the calculation accuracy on the bearing capacity of composite axial compressive circular column using of algebra algorithm and calculation accuracy for bearing capacity of eccentric compressive composite using theory formula can satisfy the engineering requirements.4. Secondary effect analysis on mid-length columnIn this paper, the actual bridge pier is simplified into a cantilever column with that one end free and one end fixed. The finite element iterative method considering influence of secondary moment is used to calculate the section deflection under the first and the second order bending moment to analyze the deformation behavior considering secondary effect for small eccentric compressive composite column under external load.Based on the assumption that incremental strain meets plane section under the second load, the paper puts forward the layered finite element theory and stiffness matrix is deduced in condition of taking material nonlinearity into consideration. Nonlinear program is worked out to solve the deflection for composite circular column with small eccentric cantilever column through many times iterations. Through an example, the simulation is carried out on many groups of small eccentric compressive circular columns with different initial eccentricity, different initial stress level indexes, different slenderness ratios and different concrete strength grades and different reinforcement ratios. The eccentricity incremental coefficient obtained by simulation is compared to the value calculated with the formula in"Concrete Structure Design Specification"(GB50010-2002).The systematic mechanical performance analysis and bearing capacity for RC composite compressive circular column under secondary load in the paper provides a simple and practical calculation formula for design calculation in reinforcing engineering and also provides theoretical basis for cracking and deformation calculation in ultimate state of normal use, which is of great significance for establishing and perfecting uniform design and calculation method for composite column under secondary load corresponding to the current design specification.