CYCLIC BEHAVIOR OF SCREWED HEAD ANCHOR SYSTEM FOR APPLICATIONS IN NUCLEAR POWER PLANTS

Construction industry is undergoing considerable changes in recent years: continuous request for more flexible systems, as well as strengthening and repair of existing structures are leading to the development of new anchoring solutions for both structural and non-structural applications. Thousands of fasteners are installed everyday all around the world, but knowledge about their structural response is still limited. Construction codes in Europe and United States recently adopted a specific approach for the design of fastening to concrete, which accounts for two main steps: derivation of forces acting on the fastener and, successively, verifications at limit states for different failure modes and load directions. Specific product characteristics are required, particularly, for seismic resistance verification. Strictly speaking, it can be assumed that the basic seismic resistance of a post-installed anchors is lower than the basic static resistance. For cast-in anchors, nonetheless, the dimensions are usually such big to believe that the bearing capacity is not affected by the presence of the cracks. However, no codes for the seismic assessment of cast-in place anchors are currently available. Within such a framework, cyclic behavior of cast-in place anchors, named “screwed head anchors” is experimentally investigated. A screwed head anchor is basically composed of an embedded steel plate connected to a threaded rod by means of nuts and washers. Such an anchoring solution is specifically designed to fasten heavy equipment in nuclear power plants. They are installed in combination with other systems designed to transfer shear and torsion. Screwed head anchors are intended to transfer tensile load only, thus assuming no interaction with the above-mentioned systems. Unconfined pullout tests were carried out in cracked concrete conditions as expected during a seismic event. Test samples were prepared by embedding screwed head anchors in reinforced concrete members designed with “crack inducers” to force the passage of the crack plane through the anchor’s axis, once loaded in tension. The anchors were tested adopting protocols developed to simulate the effect of a seismic event. In particular, three different test series were carried out: i. Reference static pullout tests with fixed crack opening displacement; ii. Pulsating tensile tests with fixed crack opening displacement; iii. Tests with constant tensile load and with varying crack width. Results are presented and commented demonstrating how the available theoretical models for the evaluation of the load bearing capacity are rather conservative. Evolution of permanent displacements during the cyclic part of the tests are commented as well.