Developing Ductility Using Concrete Anchorage

The stretch length of an anchor is defined as the length over which plastic deformations are expected to occur during seismic loading. Providing system ductility via the stretch length is an attractive design philosophy, particularly for structural system types where energy dissipation and ductility are not easily integrated elsewhere. This paper presents a basis for stretch length design including data from a large testing program of commonly used anchor materials. More than 90 tension tests of all-thread and headed anchors were conducted to determine strength characteristics, the relationship between anchor deformation capacity and stretch length, and serviceability limit states. Subsequently, simple analytical methods to determine the required stretch length for common connection configurations, including building column baseplate connections and nonbuilding structures, are developed. The paper concludes by providing a rational stretch length design framework as an alternative to the current ACI 318 prescriptive requirement of eight times the anchor diameter.