Threaded bar has a long history of use in civil engineering projects,particularly for long ground anchors,where the ability to join short straight lengths is achieved using thread couplers.Threadbar is also widely used in mining applications for the control of static and dynamic loads.
The effects of changes in embedment lengths and surface fixtures on tha performance of 20mm threadbar to dynamic loading are investigated.
The WASM model uses the double embedment test configuration to examine thw performance of complete reinforcement systems in response to a single axial dynamic load.
WASM static and dynamic tests are carried out under axial load conditions,and are not necessarily representative of the shear loading that can occur underground.Shear loading of a reinforcement system underground is particularly uncertain.The performance will be significantly affected by the quantity of available dilation on the shearing structure,how intact the rock remains that is applying the shear loading,the rate of debonding/fracturing between the threadbar and encapsulation medium.An important influence for performance is whether the shear displacement is concentrated at one location on the bolt, or multiple locations.Concentrated shear will result in failure at a smaller displacement.
Three embedment configurations for the 20mm diameter threadbar were examined;fully encapsulated,centrally debonded and jumbo installed with a toe anchor.The testing also involved various collar plate,washer and nut combinations.
WASM testing has shown that for high strain rates the work by Malvar and Crawford on the dynamic increase factors the strength of steel reinforcing bars is applicable.The program identified that,for all geometries tested,the type of encapsulation medium,toe and collar embedment lengths,surface hardware and loading conditions all can be critical to the performance of tha complete reinforcement system.
The testing program identified a significant change in capacity and performance between loading threadbar with sufficient energy to fail the reinforcement system on the frist load,i.e. critical loading,compared with,the repetitive loading of the reinforcemeng system at a lower energy which does not fail the bolt,but rather progressively plastically deforms the bar.The loading at a subcritical strain rate that does not snap the bar results in yielding of the bar and the generation of a debonded length about the simulated discontinuity;for fully encapsulated bolts this changes the configuration and performance of the reinforcement system in reponse to the next loading.