Analytical Studies in Support of an Improved Approach to the Design of Acceleration-Sensitive Nonstructural Elements

This paper summarizes a series of analytical studies that were conducted in connection with an improved approach for the design of acceleration-sensitive nonstructural elements. In the new approach, bracing to secure nonstructural elements to the structure is designed and detailed to experience nonlinearities to limit forces acting not only in the nonstructural elements but also in the attachments to the structure and in the attachment(s) to the nonstructural element. The project was sponsored by the Seismology and Earthquake Engineering Research Infrastructure Alliance for Europe (SERA) and involved shake table testing at the University of Bristol to validate the proposed novel approach as well as analytical studies. Prior to the testing, a series of analytical studies were conducted to examine the feasibility of the proposed approach and for selecting motions to be used in the shake table tests. By using exclusively motions recorded in instrumented buildings in California it is shown that acceleration demands in nonstructural elements can easily exceed 2 or 3g, but that by allowing nonlinearity to occur in the bracing element, acceleration and forces can be greatly reduced even with small levels of nonlinearity. In particular, it is demonstrated that given the frequency content of floor motions, which correspond to ground motions amplified and filtered by the structure, the reductions in accelerations and forces are much larger than those that are produced under ground motions for similar levels of nonlinearity. Furthermore, it is shown that the proposed approach not only results in large reductions in forces and accelerations, especially for elements tuned to any of the modal frequencies of the supporting structure but, simultaneously, it can also achieve substantial reductions in lateral deformations with respect to those that would occur on nonstructural elements remaining elastic. Yet, another important advantage of the proposed approach is that force and deformation demands become far less sensitive to the period of vibration of the nonstructural element.