An Innovative Approach of Vibration Testing of Concrete Structures Using Performance Based Evaluation Techniques

Abstract

Many of the current state of the art structural health monitoring (SHM) techniques are reliant on ambient excitation to determine the modal behavior of a structure. Common sources of ambient excitation are wind, water flow through dams, traffic on bridges, or seismic activity. However, it is possible for none of these sources of ambient excitation to be available for a structure. For example, dams can have strict limits on how much water can be released over certain time periods and may not allow traffic on them. In these cases, it can be necessary to induce modal excitations from other sources such as mechanical shakers or a cold gas thruster (CGT). Mechanical shakers allow specific modes to be activated but are heavy and require space that may not be available on all structures. The U.S. Army Corp of Engineers (USACE) has many large concrete dams that require such excitation to obtain modal information. This information may then be used to directly obtain changes in fixity or to calibrate finite element models of the structure. To determine the limits of the CGT for characterizing a structure, a reinforced concrete test structure was subject to the CGT pulse force in several locations with 8 accelerometers recording the motion of the structure. Using results from this effort, a shock response spectrum can then be used to compare a model’s behavior to that of the structure itself and to validate or match a model. Additionally, knowledge of a structure’s dynamic behavior in both an intact state and a damaged state can directly identify a change in boundary conditions, indicating separation of monoliths or foundation sliding The reinforced concrete test structure was used to test the CGT, as well as to establish the methods used to match the finite element analysis (FEA) models to a physical test. A small CGT was used to accelerate the structure in several configurations, which included changing the location and direction of the load. The Shock Response Spectrum (SRS) is used to compare the response of the structure to both different loads and to the modeled behavior of an FEA model built in LS-Dyna. The material attributes of the concrete and soil beneath the slab are unknown, but the LS-Dyna model matches closely to the physical tests. Peaks in the SRS can indicate modal frequencies, while double peaks in a symmetric structure can indicate some asymmetry, either from additional mass, or more likely, from reductions in stiffness. The addition of mass or reduction in stiffness may be added to the model to determine exactly how or where damage has occurred. This will discuss how the models are matched to the structure using the shock response spectrum and how damage may be indicated by shifts in the shock response spectrum, as well as the appropriate structures for this application.