Seismic Vulnerability of the Highway System

Task F1-1: Post-earthquake Assessment of Retrofitted Bridges
Technology Review
Subject Area: Special Studies

Principal Investigator(s) and Institution(s)
John Mander, University at Buffalo

Following a strong earthquake, perhaps the most important question facing a bridge owner is whether or not a particular bridge has been damaged and to what extent. If there are visible signs of damage, is the bridge safe enough to allow continued traffic on it? Answering this question is relatively easy for an experienced engineer when assessing a damaged bridge that has not been retrofitted, as the damage is generally self-evident. However, if a bridge has been retrofitted with either cable-restrainers, column jackets, foundation improvements, or a combination thereof, the damage may not be visible or easy to assess. In this case, the engineer may need to resort to nondestructive evaluation (NDE) techniques as a means of damage detection and to assess the severity of damage.

Although there are many different nondestructive evaluation (NDE) methods, those that are appropriate for rapid post-earthquake evaluation of bridges have not been specifically identified nor have they been, for the most part, validated against retrofitted bridge components. The objective of this task is to conduct a review and assessment of NDE methodologies that could be used for assessing damage in reinforced concrete columns that have been retrofitted using steel or composite jackets.

Concerns have been raised as to the ability of bridge columns that have been seismically-retrofitted using steel or composite jackets to continue to perform adequately after an earthquake. Jackets confine the column core concrete and, if properly designed and constructed, should relocate damage elsewhere in the structure - either in the connection, the gap between the jacket and the joint, or the longitudinal reinforcing steel. As a result of low cycle fatigue effects, the longitudinal rebar may be plastically strained to the extent that they may be close to fracture. The concern is further complicated if the jacket is a composite material that can deteriorate through aging. As a result, methodologies need to be investigated and developed which can determine if a retrofitted bridge column has been damaged during an earthquake and, if so, to what extent. In addition, the technology should enable remaining life or capacity assessments of jacketed columns. This task will review and assess the capabilities of various NDE techniques along with other possible inference methods for these purposes.

NDE Survey - A state-of-practice survey will be made of contemporary NDE methodologies to critically evaluate and ascertain the viability of various approaches for this application. It is envisaged that a number of technologies will be considered including acoustic emission, radar, ultrasonic methods, radiography, and microscopy. It is known that Caltrans has sponsored limited studies in this regard with the Aerospace Corporation and Lawrence Livermore National Laboratories. Information from these and other studies will be gathered and assessed. In addition, vendors of NDE technologies will be approached and asked to provide other research and application results. The merits of classical visual and manual NDE methods will also be evaluated, specifically for those situations where damage has been forced to occur in visually-inspectable areas (e.g., in the gaps at the top and bottom of the column jacket).

Computational Inference - Earthquake damage to bridge columns results, in some cases, from low cycle fatigue effects. Therefore, a companion assessment strategy based on a computational methodology to predict the remaining life of earthquake-damaged jacketed columns will be developed. This approach will use ground motions records to assess previous earthquake-induced inelastic displacement history and, hence, damage to pier columns. On this basis, the remaining life of the columns can be inferred. This work will draw heavily on previous energy-based theoretical developments made by Mander, Chang, and Dutta, under FHWA/MCEER Project DTFH61-92-C-00106.

If feasible, one or more of these technologies will be assessed in Year 1 by applying it to a fabricated component representative of a steel or composite jacketed column in limited laboratory testing. Technologies and methods which show promise in their potential application to jacketed columns will then be selected for further research in Year 2. It is anticipated that a series of laboratory and field evaluations will be conducted and, based on their results, improvements or modifications will be suggested for those that show promise.

Anticipated Start Date and Duration
January 1, 1999 - 12 months