The chapter on seismic behavior and retrofit of existing reinforced concrete shear walls is still wide open. Researchers and engineers around the world are trying hard to develop new materials and innovative techniques for seismic upgrade of concrete structures. But why is this so important?
Reinforced concrete shear walls have long been recognized as suitable structural systems, providing both lateral resistance and drift control in RC buildings. However these older shear walls were typically designed for combined actions of gravity loads and wind loading. Seismic loading and design were not considered in day to day mid-height building structures. In fact seismic provisions were only introduced in the 1970’s; therefore, most of the structures designed an built earlier, have deficiencies in design and detailing. The deficiencies in these shear wall structures make them vulnerable to seismic hazard.
General Deficiencies of Thin Reinforced Concrete Shear Walls
The following general deficiencies can be seen in many existing buildings:
- inadequate wall thickness with only one curtain of distributed horizontal and vertical reinforcement;
- inadequate lap splice lengths of the longitudinal reinforcement;
- lap splices located in regions of potential plastic hinging;
- inadequate confinement of the end regions of the walls;
- lack of control of the buckling of the flexural reinforcement;
- Insufficient amounts and poorly detailed transverse (shear) reinforcement.
To learn more about these deficiencies, refer to the Applied Technology Council (ATC 40) Seismic Evaluation and Retrofit of Concrete Buildings report.
Experience from recent earthquakes (Wallace, 2012; Saatcioglu, 2013) demonstrates that these thin reinforced concrete shear walls were severely damaged. Laboratory studies (Hamed et al. 2012, Paterson 2003) on full scale thin shear walls show that the ductility of these wall elements is near to nothing. In fact, some of these walls show brittle response when subjected to cyclic loading. Layssi et al. showed that inadequate length of lap splices at the critical sections (plastic hinges at the base of the wall) results in a brittle failure of the walls along the lap splices. Inadequate length of lap splices results in reduced ductility, and prevents the section from reaching nominal flexural capacity.
The positive effect of confinement around the boundary element reinforcement is studied by Paterson et al. Their research show that slightly thicker walls with a basic stirrup around the flexural bars in the boundary element helps the concrete section reach the nominal flexural capacity of the section, and show limited ductility.
How to improve
In order to improve the response of these deficient walls, several repair and retrofit schemes have been proposed and tested. Some of these methods are:
Carbon-Fiber Reinforced Polymer (CFRP) wraps:
In this method, the shear wall is wrapped with a layer of CFRP sheet. This layer slightly improves the confinement around the boundary elements. While this confinement effect is minimal, it improves ductility of the section, and avoids brittle failure of lap splices.
Fibre-Reinforced Self Consolidating Concrete Jacketing
A combination of steel fibers and reinforcement can be used to increase the flexural capacity of concrete section, and the same time, improves the ductility of the wall the new plastic hinge location.