Seismic Strengthening of Reinforced Concrete Frames using Post-Tensioned Bracing Systems

Many existing buildings in the United States designed and constructed according to past standards are often found inadequate to withstand major earthquakes. Non-ductile reinforced concrete frames have been identified among the types of structures that pose the greatest hazard to society. Accordingly, significant research effort has been devoted to developing and evaluating different techniques for the seismic retrofit of such buildings.

The present study focuses on the use of a technique that involves the addition of high strength, post-tensioned braces (steel strands or rods) as an alternate retrofit scheme for non-ductile reinforced concrete frames. The main objectives of the study are to evaluate analytically the performance of the post-tensioned bracing system as a retrofit technique and to identify the benefits and inadequacies of the system.

To study the behavior of the post-tensioned bracing system, inelastic dynamic and static analyses are conducted on three non-ductile reinforced concrete structures. The buildings represent typical low and medium rise construction of the 1950's and 1960's in the United States. Dynamic analyses are conducted using five ground motions representative of major earthquakes on firm and soft soil conditions. For buildings located on soft soils, the effect of soil-structure interaction are included in the analyses using a simplified procedure.

Overall, the results indicate that the post-tensioned bracing system can control lateral drifts and prevent collapse of low and medium rise structures on firm and soft soils, the performance of the system depends on the characteristics of the earthquake record. A high level of initial brace prestressing maximizes energy dissipation of the system and reduces overall response of the building. For low and medium rise buildings, the technique provides a performance level comparable to that of X-bracing and to the addition of a structural wall.