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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.
Keywords: anchor testing; concrete anchorage; concrete anchors; ductility; stretch length.
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INTRODUCTION
ACI 318-11 introduced a variety of changes to the design requirements for concrete anchorage subject to seismic forces. One of the most notable changes was that in lieu of designing anchorage for the maximum transmissible force or the amplified seismic force, designing for the standard seismic forces was allowed so long as a sufficient stretch length was provided. This design approach is particularly attractive for structural systems that have limited capacity for energy dissipation or ductility elsewhere in the structure, such as tall vertical vessels, large storage tanks, elevated storage tanks, large single-story buildings, and some types of ordinary braced frames used to support industrial equipment.
The default value of stretch length required by ACI 318 is eight times the anchor diameter (8D). This value has historical antecedents in previous building code requirements regarding ductile versus brittle anchors; however, it lacks a theoretical or analytical basis. Instead, the current inclusion of 8D is based primarily on good performance of industrial structures, as observed in past earthquakes. The 8D requirement may be waived in lieu of a rational analysis; however, methods and information needed to perform such an analysis are currently lacking in the literature.
RESEARCH SIGNIFICANCE
The current requirements of ACI 318 regarding stretch length are prescriptive by...