Nonlinear spring modelling approach for the evaluation of anchor groups

Abstract

This thesis presents a nonlinear spring model based approach for the realistic assessment of anchorages loaded in tension, shear or combined tension-shear in the case of concrete breakout failure modes. First, the nonlinear spring model was conceptualised for anchor groups subjected to (1) tension loads, (2) shear loads, (3) combined tension-shear. The concept of the spring model for tension is based on the assumption that within an anchor group, anchors resist the tension forces, while the compression forces are transferred directly by the base plate to the concrete. Nonlinear tension-only springs are used for modelling the anchor behaviour in order to account for the distribution of forces among the anchors of the group. While defining the characteristics of the nonlinear anchor springs, due consideration is given to the vicinity of the edge and the neighbouring anchors through a tributary area approach. The contact between the base plate and concrete is modelled using compression-only springs, while the base plate is modelled using finite shell or solid elements to realistically consider the base plate stiffness. An analogous model was conceptualised for shear loaded anchorages. However, the tributary area approach was replaced by a tributary volume approach to enable the consideration of different edge distances within an anchor group. The model for interaction is a superposition of the nonlinear spring model for tension and shear. A comprehensive experimental program was designed and performed to verify the postulates made during the conceptualisation of the nonlinear spring model. The aim of this experimental program was to investigate the behaviour of tension and shear loaded anchor groups of various configurations undergoing concrete cone and concrete edge breakout failure, and to understand the load-displacement behaviour of single anchors and anchors as a part of an anchor group. The tension tests on anchor groups were aimed to investigate the following influences: (i) different geometric configurations, (ii) varying stiffness of the base plate, (iii) loading eccentricity, (iv) vicinity of con-crete edge and (v) crack pattern. The shear tests were carried out on single anchors and anchor groups of different configurations to obtain information on (i) the group behav-iour, (ii) the crack initiation and propagation, (iii) the influence of the displacement behaviour of single anchors on the behaviour of anchor groups (iv) the influence of hole clearance pattern and (v) the influence of crack pattern. The nonlinear spring models for tension, shear and interaction were verified against a vast number of experimental results on diverse anchor groups performed within the framework of this thesis and those available from the literature. The results of the experiments and the spring model display excellent agreement in terms of ultimate loads as well as load-displacement behaviour due to considering the change in stiffness and the corresponding redistribution of forces among the anchors of a group. Recommendations for the calculation of resistances of anchorages are given for using the nonlinear spring modelling approach including the field of application, the safety concept, the modelling rules and the verification of the results for Ultimate Limit State (ULS) and Serviceability Limit State (SLS).