Assessment and Evaluation of Existing Structures by Means of In-Situ Load Testing and Structural Monitoring

J. Gustavo Tumialan (Fig. 1) is a senior staff engineer in the Boston office of Simpson Gumpertz and Heger Inc. (SGH). He is involved in the investigation, evaluation, and repair of concrete, masonry, and steel structures of all types. His expertise includes structural condition appraisal, in situ strength evaluation, restoration engineering, and preparation of contract documents for remedial work. He received his B.S. degree in civil engineering from the Pontificia Universidad Catolica del Peru and his M.S and Ph.D. in civil engineering from the University of Missouri–Rolla. Dr. Tumialan is a member of ASCE, the American Concrete Institute (ACI), and the American Institute of Steel Construction. He is an active member of ACI Committee 440–FRP Composites, ACI Committee 437–Strength Evaluation of Concrete Structures, and RILEM Committee–Masonry Strengthening with Composite Materials. He has published and presented various technical papers on the evaluation and rehabilitation of structures.

The idea of this special issue on Assessment and Evaluation of Existing Structures by Means of In-Situ Load Testing and Structural Monitoring was first suggested by Prof. Antonio Nanni, Chairman of the Civil, Architectural and Environmental Engineering Department of the University of Miami, at the ASCE Editors’ Workshop held in Chicago at the end of October 2006. Since then, and with the assistance of Prof. Ken Carper (editor of this journal), we embarked on the task of gathering technical articles for this special publication. The papers included in this special issue address different technical challenges and present a variety of case studies on the in situ evaluation of structures by load testing and structural monitoring methodologies. These methodologies are not exclusive of each other but can be brought together and in a synergetic manner achieve a more robust structural evaluation.

The primary goals of any structural investigation are to establish the existing condition of the structure, identify issues affecting the structural performance; and develop and implement any remedial actions required. When conducting a structural investigation, engineers proceed much as medical doctors do in treating patients. Physicians review clinical histories, make observations, conduct examinations, and analyze test results to arrive at a diagnosis and establish treatment. We engineers perform similar tasks to determine the health of our “patients” of concrete, steel, wood, or masonry. We also “review clinical histories” by examining construction documents and interviewing owners and facilities personnel; “observe the patient and conduct physical examinations” by performing walk-through inspections, probing and sampling; “analyze test results” by evaluating results of in situ tests and materials testing, performing structural analyses; and finally, “recommend a treatment” by formulating a set of remedial actions and their implementation.

In certain instances, it is not possible to compute the true load-carrying capacity of a structure with reasonable certainty due to the degree of deterioration or uncertainties in analyses. In such instances, in-situ load testing and structural monitoring may be necessary to arrive at a meaningful diagnosis.

In-situ load testing and structural monitoring can provide valuable information about the structural performance of existing structures and their components. Both techniques can be used to gain knowledge on the static and dynamic behavior of a structure; determine the safety and actual load-carrying capacity of a structure; assess the performance of a structure with alleged deficiencies such as design or construction errors; determine the adequacy of structural components damaged because of overloading or deterioration; monitor a deteriorating structure; validate design approaches and retrofitting schemes; account for the beneficial effects of “hidden” load paths; supplement, validate, or “tuneup” analytical work aimed at understanding the behavior of a structure, and so on.

In-situ load testing as a methodology to evaluate structures is not new; reports of load tests performed on structures in the United States date back to the early twentieth century. Examples of early in-situ load testing include the Deere and Webber Buildings and the Powers Building in Minneapolis, the Franks Building in Chicago, and the Barr Building in St. Louis. In-situ load testing typically involves loading the structure with dead weights such as water, sand, or steel plates, or loading the structure by mechanical means, such as hydraulic jacks or actuators, in a way that simulates the critical structural effects of the structure in use at and beyond service-load levels.

Structural evaluation by in-situ load testing is based on the analysis of displacements and deformations. In a load test, the response of the structure to the applied loads is monitored during a time period, and the recorded measurements are used to evaluate the performance of the structure. Depending on the sophistication of the instrumentation, measurements can be obtained periodically, typically done with manual dial gauges, or continuously, by using electronic instruments connected to data-acquisition equipment. The latter advantageously displays in real time the behavior of the test structure and the possibility of making determinations about the structural performance on the spot.

Many times the assessment of a structure by in-situ load testing is not practical or feasible, for instance, in situations where critical members are not easily accessible or are concealed under architectural finishes, making it almost impossible to apply loads by mechanical means. Furthermore, many times the structural assessment requires the continued evaluation of several members and connections of a structure over a long period of time to obtain information at different operational loads. Structural “health” monitoring methodologies are a powerful tool in such situations.

Structural monitoring methodologies provide information to identify and characterize the performance and/or possible deterioration of a structural system. The fundamental principle of the method is that the presence of damage results in changes in the dynamic properties of the structure, such as damping and stiffness. Basically, a change in the dynamic response can pinpoint distress or damage at a local level that can compromise the global performance of the structure. The evaluation of the dynamic response is made at service-load level; the excitation of the structure is achieved either by actual loads, such as human or vehicular activity, or artificially by vibration-inducing devices. The most widely used sensors for structural monitoring are accelerometers, but crack monitoring and strain measurements also can be achieved with localized sensors.

The first paper presented in this special publication describes the techniques for measurement and monitoring movement and thermal response of the Milwaukee City Hall Tower. This masonry structure exhibited distress for many decades. The paper discusses the monitoring results and supplemental high-end structural analyses that formed a rational basis for selection of the most appropriate repair strategy.

The next two papers are companion papers dealing with the in situ strength evaluation of a posttensioned concrete slab system at a parking garage and a reinforced concrete slab in a building. In-situ load testing and acoustic emission techniques were used to evaluate the structures. The first companion paper provides background on the load test motivations and rationale, describes the load test protocols recommended by the American Concrete Institute (ACI) Committee 318 ( $24\mathrm{h}$ load test) and Committee 437 (cyclic load test), and describes the load test equipment, techniques, and instrumentation. The second companion paper presents the application and interpretation of the evaluation criteria adopted for assessing the in-situ load tests performed on the two slab systems. The paper presents the load test results in the context of the evaluation criteria recommended by ACI318 and ACI437, discusses their limitations, presents evaluation criteria based on acoustic emission testing, and finally introduces the new concept of global index as an acceptance criterion.

Next are two papers that focus on the evaluation of structural performance using measured dynamic response. The first describes a year-long monitoring program of the Giussepe Meazza stadium in Milan, one of the largest and most important stadia in Italy. The vibration data were recorded during soccer games and music concerts. The objective of the study was to define intervals in which an anomalous situation can be identified as possible damage. The analyses were performed using the operational modal analysis approach. The other paper presents three case studies and formulates a methodology to detect structural damage. The proposed methodology was validated in case studies, one of which was a highway bridge tested by using a vibration-shaking machine. As the paper explains, the methodology consistently predicted the location and the amount of damage.

The last group of three papers deals with transportation structures. The first investigates the field performance of pipe-arch culverts under static and dynamic loads. The steel culverts showed different degrees of corrosion-related deterioration. Different parameters were considered in the evaluation of the culverts, including backfill height, loading conditions, and geometry. The loads were applied using fully loaded trucks, and the load tests allowed identification of the areas with the largest deformations.

The remaining two papers present the structural evaluation of two bridge structures. One paper addresses the field evaluation of extreme overloads (referred to as superloads) that crossed the Bonnet Carré Spillway Bridge, a reinforced concrete bridge structure in Louisiana. The bridge was instrumented before the passage of three superloads to obtain actual deformations of critical members. The results were used to refine analytical models and thereby determine more realistic bridge ratings than those achieved by conventional rating approaches alone. The final paper focuses on the evaluation of a repaired steel girder of a bridge in Missouri damaged by a vehicle’s impact. The structure was load tested with fully loaded trucks. Strains were collected using fiber optics installed on the structure, and displacements also were measured using a total station system. The project demonstrated the potential of fiber-optic sensor technologies to assess the performance of large-scale structures.

We hope this is the first in a series of journal issues devoted to the in situ structural evaluation of civil infrastructure. We believe that this publication will interest both the academic and engineering communities, since the papers presented provide background and theoretical understanding of the subject illustrated with case studies. We also believe that the case studies and structural evaluation methodologies published in this collection of papers can be a valuable resource in university courses dealing with rehabilitation and evaluation of existing structures.