Special Issue on Field Testing of Bridges and Buildings

Understanding the behavior of structures subjected to complex environmental and mechanical loading is essential for their design, safety, and reliability. In many instances, behavioral understanding is developed using laboratory tests of structural components and systems that are at or close to full scale. As a result, a relatively recent trend in experimental structural engineering research has centered on building larger and more complex testing laboratories. Example facilities include those affiliated with the Network for Earthquake Engineering Simulation (NEES) in the United States (NEES 2014) and E-Defense Shake Table in Japan (NIED 2014). Several successful research projects have been executed at these facilities, with results being published in special issues of the ASCE’s Journal of Structural Engineering (Vol. 139, Issues 7 and 8) and elsewhere (e.g., Lignos et al. 2011; Kawashima et al. 2012; Lowes et al. 2012). Despite offering several advantages, which can include better control of loads during testing and dense instrumentation, there are also limitations to experiments conducted in a laboratory setting. These limitations can include costs associated with completing tests, correct simulation of realistic loading, boundary and environmental conditions, adequate reproduction of size effects, and ensuring that material properties and construction techniques used in the lab are representative of actual properties, techniques, and age.

As our infrastructure ages, the cost to maintain and replace it continues to rise. The ASCE’s 2013 Report Card for America’s Infrastructure states that 24.9% of the nation’s bridges are either functionally obsolete or structurally deficient, with the total price tag for repair or replacement estimated at $76 billion (ASCE 2013). Therefore, the need to better understand the performance and current health of actual structures in situ is becoming of paramount importance. As a result, structural health monitoring and structural field testing have emerged as important components of structural engineering and prominent research areas. Recent research efforts related to real-world applications of structural identification and health monitoring were documented in the October 2013 Special Issue of the ASCE’s Journal of Structural Engineering (Vol. 139, Issue 10). A similar effort related to documenting the state of the art on field testing and evaluation of large-scale, in situ structural components and systems has not been undertaken by the ASCE for some time.

This special issue is intended to fill this knowledge gap by documenting research results on large-scale field testing of structures and contains manuscripts that focus on in situ destructive testing of decommissioned and large-amplitude forced testing of in-service bridges and buildings as well as monitoring under ambient vibration conditions or during construction. A total of 13 papers are included, 7 of which are related to bridges and 6 to buildings. Researchers and engineering practitioners from seven countries located on four continents contributed to this special issue. Brief summaries of the published manuscripts, in the order in which they are published, are provided in what follows.

Seven articles related to bridge field testing are presented in this edition of the Journal of Structural Engineering. The first two papers are categorized under testing under ambient traffic conditions. The following three papers are categorized under forced testing, while the last two papers are categorized under destructive testing.

Fasl et al. instrumented and monitored stresses experienced during girder erection and concrete deck placement by a horizontally curved steel I-girder bridge in Austin, Texas. This work was motivated by the complex torsional behavior of curved bridges that pose unique challenges to designers and construction contractors. It was found that the stresses in the girders were relatively low during steel erection due to their high strength-to-weight ratio. However, high flange warping stresses were observed during ratcheting of the cross frames into position. Higher stresses were recorded during concrete deck placement compared to the steel erection process.

Zhou et al. report on a behavioral investigation and ensuing retrofit related to the discovery of fatigue cracks in floor beam end connections on a double-deck, cantilever-suspended, steel truss bridge in Philadelphia. The cracks were found to be a result of distortion-induced fatigue in the floor beam web due to interacting deformations under live load and temperature variations. As a retrofitting strategy, fatigue-susceptible weld terminations were removed and local areas of the floor beam web were reinforced against out-of-plane distortion. The response of the structure was monitored and the performance was found to be satisfactory since the repairs were made.

Maguire et al. conducted a field study to verify that simplified analysis procedures, namely one-dimensional beam models and equivalent frame analysis, produce satisfactory results for segmental concrete bridges. Two single-cell segmental box girder bridges, one in Daytona, Florida, the other in Blacksburg, Virginia, were investigated. Findings indicate that the use of a so-called beamline frame model, which simplifies a single-cell, segmental box girder into line elements placed at the centroid of the girder, could conservatively predict longitudinal deflections, strains, and rotations under live load for bridges that did not exhibit significant shear lag. The applicability of the beamline method to bridges having excessive shear lag was not studied and should be investigated in the future.

Murià-Vila et al. performed field tests on four recently built, elevated, post-tensioned, multispan concrete viaducts in Mexico City. The objective of their research was to study the response of the elements at representative locations along the viaducts to ascertain the assumptions made in the design and make necessary corrections if needed. The superstructure behavior for all of the viaducts, particularly in terms of lateral stiffness, indicated that the post-tensioned members may be considered monolithic and continuous under service loads. Additionally, field test data collected during the study facilitated the design process by recommending that prefabricated column footings be used at one viaduct site.

Manos et al. tested an in situ, reduced-scale model bridge pier to study dynamic soil–structure interaction and how this interaction changes as a result of structural damage in the pier. The pier was supported on soft soil deposits, and radiating waves generated by the vibration of the pier through the surrounding soil medium were investigated. It was found that the equivalent maximum damping ratios measured from the nonlinear, dynamic response of the model pier were notably higher when compared to those in the linear elastic range. Data from the experiments were used to verify the analytical expression of Mylonakis et al. (2006) to predict the elastic compliance of near-surface footings.

Puurula et al. assessed the effectiveness with which near-surface mounted, square, carbon fiber–reinforced polymer reinforcing (CFRP) bars enhanced the capacity of a 50-year-old reinforced concrete (RC) railway bridge located in Örnsköldsvik, Sweden. One of the spans was tested to failure, and the failure mechanism was observed to be a combination of bending, shear, torsion, and localized bond failures between the concrete and CFRP bars. The authors used data obtained from the test to calibrate finite-element models that helped predict the capacity of the unstrengthened bridge. Analyses and testing indicated that the unstrengthened and strengthened versions of the bridge could sustain a total load 4.7 and 6.5 times greater, respectively, than the current load requirements.

McConnell et al. discuss results from field tests of a decommissioned, skewed, steel, I-girder bridge that were conducted to determine its capacity. A load 17 times the design load was applied to the bridge with no sign of yielding of the girders, and a finite-element model was created to predict behavior at higher load levels. A significant reserve capacity was observed, relative to both current bridge design and rating specification values and to the maximum load that could physically be applied to the structure. The high capacity resulted mainly from a transverse redistribution of forces. The authors proposed two potential conceptual approaches to better quantify bridge ultimate strength.

Six papers related to the field testing of buildings are summarized in this section. The first two papers are categorized under testing under ambient conditions. The third paper is categorized under forced testing, while the last three papers are categorized under destructive testing.

Kosnik and Dowding presented a study that examined the effectiveness of a field monitoring management system that can autonomously acquire, communicate, aggregate, and graphically report data for decision support via a Web site. System effectiveness was demonstrated using three case studies of in-service structures to address concerns associated with construction and blast vibration–related damage. The studies indicated that the autonomous monitoring system was suitable for the measurement of structural responses to random events.

Valla et al. tested the effectiveness of a multipath Lidar vibrometer as an approach to system identification. Five buildings in Grenoble, France, were used as case studies, and velocimeter measurements were used to benchmark frequencies and mode shapes obtained from the vibrometer. Although the noise level was higher for the vibrometer than the velocimeter, good correlation was reported between measured frequencies and mode shapes. It was concluded that, despite drawbacks, such as the single direction of sight and operating difficulties in dense urban areas, a multipath Lidar vibrometer could be a good structural health-monitoring alternative involving shorter testing times and producing accurate measurements.

Giongo et al. studied the effect of the in-plane behavior of flexible timber diaphragms on the global response of unreinforced masonry buildings (URMs). A two-story URM building in Whanganui, New Zealand, was used as a case study. A series of cyclic and snap-back tests were performed in a direction orthogonal to the floor joists. It was determined that the development of the full strength of timber floor diaphragms required displacements that were much larger than those that caused out-of-plane instability of the URM walls. It was also found that existing guidelines used to predict diaphragm stiffness in timber buildings yielded overly conservative results.

Della Corte et al. conducted field tests on a damaged RC building in Naples, Italy, strengthened with all-steel buckling restrained braces (BRBs), to examine the effectiveness of this strengthening approach. Masonry infill panels were used to hide the BRBs for aesthetic purposes, and built-up bolted connections allowed for an easily connecting solution. The available theoretical models were found to be adequate for the proper design of BRBs to control the failure mode to bulging of the casing. Test results indicated that connections between the BRBs and the existing RC frame may have a significant influence on system stiffness during loading and unloading.

Shih et al. performed in situ tests of a typical two-story, confined, masonry school building having RC columns and beams in Tainan City, Taiwan, to investigate the effectiveness of external steel framing as a retrofitting system. The structure was divided into a control section that did not have external framing and a section that had an external framing retrofit. Each test structure was loaded with increasing monotonic static lateral force along its weak axis to simulate typical structural failure due to an earthquake. The retrofitted specimen satisfied updated seismic regulations, and the failure mode for the retrofitted columns shifted from a shear failure, as seen in the unretrofitted structure, to a flexural-shear failure. It was found that external steel framing resulted in substantial improvement in building stiffness, strength, and ductility.

Kernicky et al. examined the behavior and damage levels of a full-scale, masonry, infill wall in an existing building subjected to an internal air blast. Combined stochastic-deterministic system identification was used to interpret vibration data to help estimate modal parameters for pre- and post-blast conditions. Measurable changes in natural frequencies and mode shapes were observed at low levels of visible distress to the wall, and these changes were more noticeable for higher-order modes.

The need for a special issue on field testing of structures was first identified by the ASCE Subcommittee on Experimental Methods in Earthquake Engineering. The special-issue editors wish to thank Dr. Bassem Andrawes, Chair of the Subcommittee, and Dr. Sherif El-Tawil, Editor of the ASCE Journal of Structural Engineering, for their support in association with the development, review, and publication of this special issue. We would also like to express our gratitude to the authors for the quality of their manuscripts, making editorial work a pleasant experience. Thanks also go to ASCE Experimental Methods in Earthquake Engineering Subcommittee members and all other reviewers who, under tight time constraints, carefully evaluated the submitted manuscripts and provided valuable feedback.