Reinforced concrete shear wall structures constructed in the 1970’s or earlier suffer from a number of deficiencies in design and detailing. This is around the time when seismic provisions emerged in the design codes, and changed the traditional strength-based design philosophy. In this article, I will briefly review common problems with deficient shear walls, and propose some basic techniques for improving the existing condition. The focus will be on concrete frame structures, with and without shear walls.
Deficient Shear Walls | General
The most common deficiencies observed in existing shear wall structure include short lap splice lengths of the longitudinal reinforcement in potential plastic hinge regions, insufficient and poorly detailed transverse reinforcement and consequently inadequate shear strength required to develop hinging. In addition, some of the older construction used to have a very thin cross section which make them vulnerable under lateral loading.
Lap Splice Issue
Inadequate length of lap splices for main reinforcing bars is a common deficiency in many RC frame structures. The lap splice lengths for columns were often only 30 bar diameter of less. This is very much shorter than what is currently asked by major design codes, ACI 318, and CSA A23.3.
Most lap splices occurred just above the foundation level, or right after the first floor level, when high flexural moment is expected in case of lateral load. This is the location of potential plastic hinging, and a failure in this region is unacceptable by design codes.
A major deficiency in the detailing of lap splices were the lack of confinement, as lap splice region were typically not confined with closely spaced ties. Often we see open ties, with large spacing that provide nearly zero confinement over the spliced bars. This can reduce the ductility of the wall where it matters most.
Walls without confined boundary elements is a general observation in the construction of 1970’s or earlier. In these walls, a minimum number of reinforcement were placed at the edges of the wall to provide the desired flexural capacity for the wall. The vertical reinforcement lap splices were usually designed for compression loads, and may be inadequate for the flexural tension that may develop under earthquake loads.
The thickness of shear walls, and the amount and spacing of transverse bars were often determined to resist code-specified lateral forces rather than the shear corresponding to development of the wall’s flexural capacity. This makes all walls in these older construction shear critical. Another issue with the transverse reinforcement is the lack of proper anchorage in the boundaries. The transverse reinforcement were single leg bars, with 90 degree hood that provides nearly zero confinement over main flexural bars, or is enough to generate the yield stress along the bar.
Sliding Shear / Construction Joint
A weak construction joint is a common observation when evaluating existing walls. Poor quality of construction joints during construction make these locations a weak plane in the vertical wall system.
Retrofit – Upgrade
Read this article to find some practical solutions for upgrading deficient shear walls
Article originally posted on fprimec.com