The Department of Defense (DoD) controls an estimated 90,000 structures that are over 60 years old. Many are considered historically significant, and federal statutes dictate that  all agencies must make every effort to preserve and rehabilitate these resources. According to the Code of Federal Regulations 36 CFR Part 60, federal buildings are evaluated for eligibility for the National Register of Historic Places (NRHP) normally when they reach approximately 50 years of age. Preservation requirements under several federal and state statutes require these assessments to be incorporated into Integrated Cultural Resources Management Plans (ICRMP) for DoD installations. If the evaluation deems a structure historically significant, then the condition is assessed to determine if the structure can be renovated economically. Continually constrained infrastructure budgets are forcing owners to use cost-efficient means of evaluating building condition that meet cultural resource legal requirements.

The objective of this project was to determine the feasibility of identifying the modal parameters (i.e., natural frequencies, mode shapes, and damping ratios) of a reinforced concrete structure using a portable modal analysis test system. The performance-based non-destructive (ND) modal test system determines the actual structural capacity without damaging historically significant building components.

A prototype modal test system was assembled and applied on a historically-eligible DoD building to demonstrate method feasibility. The prototype included a portable ND impact device, accelerometers, and data acquisition and signal processing software assembled from off-the-shelf (OTS) products. The prototype was tested on a reinforced concrete floor slab to obtain its modal parameters. Test results were analyzed to obtain modal parameters for numerical model validation. The validated numerical model was used to assess load capacity.

Results showed that a ND method using OTS impulse hammer, data acquisition, and signal processing software, combined with a Finite Element (FE) analysis program, can determine the modal response of a floor slab. In addition, the results showed that a test-validated FE model can determine the allowable load capacity of a building’s floor slab; however, some issues remain that need to be resolved or pursued further to ensure a reliable assessment method.

The ND method was able to determine:

• Natural frequencies of first five modes within two percent. Other modal parameters, such as damping ratios and modal shapes, were not determined because of time and resource constraints. The determination of these parameters requires additional software and processing time.
• Verification of Live Load Rating: The test-validated FE model showed that the denoted live load rating for the test floor slab was within current flexural strength requirements for reinforced concrete according to American Concrete Institute (ACI) 318−08.
• Defects in floor slab. The tests showed that some observable surface delaminations can be detected, but not reliably. Many surface delaminations were detected, but not all. The extent of defects may affect the response. The research emphasized the identification of the structural response, not defect detection; however, distinguishing defects from the structural response helps to explain anomalies in the data.

Performance-based ND load assessment determines actual capacity, which optimizes rehabilitation strategies and avoids environmental impacts that are inherent in facility demolition. This preserves historical aspects of a building while maintaining DoD installation readiness. By knowing the actual load capacity, DoD stakeholders can accurately formulate strengthening or repair estimates for military construction (MILCON) funding. This study showed that the ND load assessment method is a viable non-destructive assessment tool for historical DoD buildings. The combined application of modal and impact load testing can determine the load carrying capacity of reinforced concrete floor structures. The system’s portability facilitates usability in confined spaces that are not accessible with conventional performance-based methods. With further time and resources, a complete modal test and analysis of the building’s floor structure can be achieved. In addition, the modal method can be further developed to assess other structural components such as walls and columns.