RC large displacements: Optimization applied to experimental results

A large number of towers for telecommunication were installed during the implantation of cellular telephony services in Brazil. Some of those towers presented problems as excessive displacements, residual displacements, cracking and some accidents happened. On the other hand, the computation of large displacements in slender reinforced concrete structures is a very difficult task as the flexural stiffness of the sections changes continuously as the bending moment increases, due to the very non-linear material behavior of concrete, involving such phenomena as formation of cracks and plastification. The goal of this paper is to present some initial results of the application of optimization techniques to experimental data relative to the determination of the effective bending stiffness of transverse sections of reinforced concrete structures. The objective is to determine parameters of reduction of the stiffness of unstressed sections for the correct calculation of the displacements of those structures. The results of a test with a reinforced concrete tower for telecommunications of 30 m of length, circular ring cross-section with 50 cm diameter, were used. For several cross-sections along the axis of this structure the effective stiffness was computed. For analysis purposes, the structure was discretized and the differential equation of the elastic line integrated to obtain the rotations and displacements. The values of the effective stiffness of the cross-sections were obtained using optimization techniques. The effective stiffness is presented in graphs as function of the solicitation level (the ratio between the characteristic bending moment and the ultimate moment of the cross-section). The section where the largest stiffness loss happened is the section that indeed collapsed in a real similar structure. Directions for future researches are presented.