Strengthening of RC Beams and Frames by External Prestressing

Abstract

The use of external post-tensioning in strengthening continuous beams in reinforced concrete frames was investigated. First, a direct method to determine the required external tendons and to evaluate the load-carrying capacity of strengthened beams was proposed. The method was verified with test results of 124 simple-span beams and 23 continuous beams. Seven of the continuous beams were tested in this research. Elastic moment redistribution due to difference in section stiffness and cracking was observed up to the first yielding of internal steel reinforcement, beyond which plastic moment redistribution also occurred. Secondary moments led to lesser moment redistribution at ultimate. Four other frames were tested and analyzed. Secondary moments were beneficial if the frame was subjected to gravity loads only. Tertiary effects due to supporting columns reduced the effective prestress, leading to lower load-carrying capacity and larger deflections. In addition, moments and shear forces were introduced in the supporting columns which must be designed for.