Traditionally, damage identification techniques in bridges have focused on monitoring changes to modal-based Damage Sensitive Features (DSFs) due to their direct relationship with structural stiffness and their spatial information content. However, their progression to real-world applications has not been without its challenges and shortcomings, mainly stemming from: (1) environmental and operational variations; (2) inefficient utilization of machine learning algorithms for damage detection; and (3) a general over-reliance on modal-based DSFs alone. The present paper provides an in-depth review of the development of modal-based DSFs and a synopsis of the challenges they face. The paper then sets out to addresses the highlighted challenges in terms of published advancements and alternatives from recent literature.

https://www.mdpi.com/2076-3417/7/5/510/pdf

A State of the Art Review of Modal-Based Damage Detection in Bridges: Development, Challenges, and Solutions

The vibration responses of structures under controlled or ambient excitation can be used to detect structural damage by correlating changes in structural dynamic properties extracted from responses with damage. Typical dynamic properties refer to modal parameters: natural frequencies, mode shapes, and damping. Among these parameters, natural frequencies and mode shapes have been investigated extensively for their use in damage characterization by associating damage with reduction in local stiffness of structures. In contrast, the use of damping as a dynamic property to represent structural damage has not been comprehensively elucidated, primarily due to the complexities of damping measurement and analysis. With advances in measurement technologies and analysis tools, the use of damping to identify damage is becoming a focus of increasing attention in the damage detection community. Recently, a number of studies have demonstrated that damping has greater sensitivity for characterizing damage than natural frequencies and mode shapes in various applications, but damping-based damage identification is still a research direction ‘in progress’ and is not yet well resolved. This situation calls for an overall survey of the state-of-the-art and the state-of-the-practice of using damping to detect structural damage. To this end, this study aims to provide a comprehensive survey of uses and features of applying damping in structural damage detection. First, we present various methods for damping estimation in different domains including the time domain, the frequency domain, and the time-frequency domain. Second, we investigate the features and applications of damping-based damage detection methods on the basis of two predominant infrastructure elements, reinforced concrete structures and fiber-reinforced composites. Third, we clarify the influential factors that can impair the capability of damping to characterize damage. Smart Materials and Structures Smart Mater. Struct. 26 (2017) 043001 (14pp) doi:10.1088/1361-665X/aa550a 3 Author to whom any correspondence should be addressed. 4 Present address: Nanoscale Materials Science, Empa-Swiss Federal Laboratories for Materials Science and Technology, Dubendorf, Switzerland. Original content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. 0964-1726/17/043001+14$33.00 © 2017 IOP Publishing Ltd Printed in the UK 1 Finally, we recommend future research directions for advancing damping-based damage detection. This work holds the promise of (a) helping researchers identify crucial components in damping-based damage detection theories, methods, and technologies, and (b) leading practitioners to better implement damping-based structural damage identification.

https://iopscience.iop.org/article/10.1088/1361-665X/aa550a/pdf

Structural damage identification using damping: a compendium of uses and features

The near fault ground motion (NFGM) is characterized by a single long period velocity pulse of large magnitude.(Somerville et. al 1997) Extensive research for the far fault ground motion (FFGM) have been carried out in strong seismic region, but limited research have been done for NFGM in low or moderate seismic regions because of very few records. For reinforced concrete (RC) bridge piers constructed before implementing the 1992 seismic design code in Korea, longitudinal reinforcing steels were practically lapspliced in the plastic hinge region between column and footing. This research aims at investigating the seismic behavior of RC bridge piers with lap-spliced longitudinal steels subjected to NFGM in low or moderate seismic region. Five RC pier specimens with a diameter of 400 mm and height of 1400 mm were tested on a shake table, and two identical specimens were tested under a quasi-static load and a pseudo-dynamic load. The effects of lap-spliced longitudinal reinforcing steels are investigated. Shaking test results are compared to those by Quasi-static test and Pseudo-dynamic test. It was concluded that: a) displacement ductility was significantly decreased in RC column specimens with lap-spliced longitudinal reinforcing steels; b) bond failure in the lapped starter bars and buckling of the longitudinal steel bars initiated failure in RC specimens with and without lap-spliced longitudinal steels, respectively; c) more transverse reinforcing steels induced better displacement ductility; and d) a decrease of displacement ductility and energy dissipation was observed in the specimens on a shake table excitation in comparison with the specimens by the quasi-static and the pseudo-dynamic test.

/pdf/the-14th-world-conference-on-earthquake-engineering-2tucotmndz.pdf

The 14th World Conference on Earthquake Engineering

We present a methodology for transforming the structural and non-structural fragility functions in HAZUS into a format that is compatible with conventional seismic hazard analysis information. The methodology makes use of the building capacity (or pushover) curves and related building parameters provided in HAZUS. Instead of the capacity spectrum method applied in HAZUS, building response is estimated by inelastic response history analysis of corresponding single-degree-of-freedom systems under a large number of earthquake records. Statistics of the building response are used with the damage state definitions from HAZUS to derive fragility models conditioned on spectral acceleration values. Using the developed fragility models for structural and nonstructural building components, with corresponding damage state loss ratios from HAZUS, we also derive building vulnerability models relating spectral acceleration to repair costs. Whereas in HAZUS the structural and nonstructural damage states are treated as if they are independent, our vulnerability models are derived assuming "complete" nonstructural damage whenever the structural damage state is complete. We show the effects of considering this dependence on the final vulnerability models. The use of spectral acceleration (at selected vibration periods) as the ground motion intensity parameter, coupled with the careful treatment of uncertainty, makes the new fragility and vulnerability models compatible with conventional seismic hazard curves and hence useful for extensions to probabilistic damage and loss assessment.

/pdf/14th-world-conference-on-earthquake-engineering-3zh3xugox0.pdf

14th World Conference on Earthquake Engineering

Advances in construction materials and computational methods have made it possible to design and construct taller masts, buildings with increasingly slender frames, and bridges (and roof structures) with ever larger spans. In addition, masts, towers and new forms of construction such as offshore structures are being built in more hostile environments than previously contemplated. These evolving structures which keep extending the boundary of 'normal' designs require that the designers take into account vibration of structures at the design stage to a much greater extent than they have done in the past.
 
The slenderness of modern structures and the large magnitude of the loads that many of them must carry also make it imperative that such structures be designed for stresses induced by dynamic disturbances. The response of a structure to a dynamically applied load may be many times greater than its response to the same load applied statically. The relationship between a structure's static and dynamic responses depends primarily on its damping characteristics and on its natural periods of vibration. In fact, damping is one of the most significant contributors to the dynamic response of high-rise buildings, bridges, tall chimneys and other slender structures considered to be significantly affected by dynamic forces.
 
Under a severe lateral dynamic loading condition, the structure that is likely to survive is one whose members are sufficiently ductile to absorb and dissipate energy by elastic and/or inelastic deformation. This requires the designer to realistically assess the possible levels of strength in flexural and shear elements. Thus, in designing such a concrete structure, it is important to understand and determine the ability of the structure to absorb energy under an external impulsive force. At this stage, information in this regard is lacking in published literature and the ability of the constituent elements of the structure to absorb energy is not well understood. This, for example, is true for reinforced and partially prestressed concrete beam, especially the cracked ones. In particular, no simple and accurate formulae are available to evaluate the damping ratios of reinforced and partially prestressed concrete beams cracked or otherwise, for use in the dynamic design of civil engineering structures. It is this area which forms the primary focus of this research.
 
In this research, an extensive test programme has been carried out to study the cracking and damping behaviour of reinforced and partially prestressed concrete beams. The tests were carried out in two stages and involved a total of 30 reinforced and partially prestressed beams. Nine reinforced and 12 partially prestressed simply supported full-size box beams were tested at the first stage. Tested at the second stage were 2 simply supported and 3 two-equal span continuous reinforced full-size box beams and 4 solid rectangular full-size simply supported reinforced beams. For all the beams, at each level of loading, measurements were made of instantaneous and residual crack widths, instantaneous and residual concrete strains, and mid-span deflections. Each beam was also subjected to free vibration tests to measure its logarithmic decrement of damping corresponding to each load level.
 
Based on the experimental results, two empirical formulae have been developed for predicting logarithmic decrement of damping separately in reinforced and partially prestressed concrete beams. These formulae predict damping from the residual crack widths of the beams. For these formulae to be of practical use, a formula relating the residual crack widths of concrete beams to the instantaneous average crack widths was developed. In addition, a unified formula was derived for the prediction of the instantaneous average crack widths based on the general beam parameters. As an alternative, separate formulae are also presented for predicting residual crack widths using mid-span deflections of reinforced and partially prestressed beams. These further enhance the practicability of the proposed damping formulae.
 
In an effort to verify the accuracy and reliability of the proposed formulae, comparative studies are carried out based on the author's own laboratory test results as well as those available in published literature. In total, 104 full-size reinforced and prestressed concrete solid and box beams are involved in the comparative study. In general, good correlations are obtained for instantaneous and residual average crack widths and for logarithmic decrement of damping values. These are true for both reinforced and partially prestressed concrete beams.

Damping Characteristics of Reinforced and Partially Prestressed Concrete Beams

This paper reports the results of a series of experimental modal analyses that were performed to examine the similitude of the dynamic parameters of reinforced concrete replica models. Also reported are the similitude requirements for damping forces that were developed as part of this study. The similitude analysis shows that both material properties and system geometry must be considered when scaling the damping forces. Results of the experiments show that the modal frequencies and the mode shapes of a prototype structure can be accurately predicted from tests on scale-model structures. Variations in equivalent viscous damping ratios identified on models and prototype were greater than variations for the other measured dynamic parameters. However, all damping ratios were less than 2 percent of critical, and the observed variations would not significantly alter the dynamic response of the structures.

Dynamic parameter similitude for concrete models

This paper describes a series of experimental research efforts focused on characterizing the ultimate flexural behavior of lightly reinforced concrete intake towers. The main objective of this research was to provide empirical data to be used in the development of a simplified analysis procedure for seismic evaluation of this type of structures. The studies included an experimental investigation of the flexural behavior of 1/8-scale model rectangular concrete intake towers under monotonic and cyclic lateral loads. Further experimentation was conducted to quantify the influence of the parameters governing the deflection capacity of these structures. An empirical expression is presented for the ultimate deflection capacity of lightly reinforced towers with rectangular sections.

Richard C. Dove

Papers

Dynamic parameter similitude for concrete models

Ultimate Deflection Capacity of Lightly Reinforced Concrete Intake Towers