Bridges in America are aging and deteriorating, causing substantial financial strain on federal resources and tax payers’ money. Of the various deterioration issues in bridges, one of the most common and costly is malfunctioning of expansion joints, connecting two bridge spans, due to accumulation of debris and dirt in the joint. Although expansion joints are small components of bridges’ superstructure, their malfunction can result in major structural problems and when coupled with thermal stresses, the demand on the structural elements could be further amplified. Intuitively, these additional demands are expected to even worsen if one considers potential future temperature rise due to climate change. Indeed, it has been speculated that climate change is likely to have negative effect on bridges worldwide. However, to date there has been no serious attempts to quantify this effect on a larger spatial scale with no studies pertaining to the integrity of the main load carrying girders. In this study, we attempt to quantify the effect of clogged joints and climate change on failure of the superstructure of a class of steel bridges around the U.S. We surprisingly find that potentially most of the main load carrying girders, in the analyzed bridges, could reach their ultimate capacity when subjected to service load and future climate changes. We further discover that out of nine U.S. regions, the most vulnerable bridges, in a descending order, are those located in the Northern Rockies & Plains, Northwest and Upper Midwest. Ultimately, this study proposes an approach to establish a priority order of bridge maintenance and repair to manage limited funding among a vast inventory in an era of climate change.

Data Availability: The bridge data was obtained from the 2017 National Bridge Inventory (NBI) from the U.S. Department of Transportation – Federal Highway Administration (FHWA) repository, [ https://www.fhwa.dot.gov/bridge/nbi/ascii2017.cfm ]. The datasets on climate and projected temperatures generated during and/or analyzed during the current study are publicly available in the NOAA repository, [ https://www.ncdc.noaa.gov/cag/national/time-series ]. The authors did not have special access privileges. All temperature data has been stored in Dryad and can be downloaded from [ https://doi.org/10.5061/dryad.vdncjsxpz ].

Introduction

It is not a surprise that infrastructure in America and other countries around the world is aging and deteriorating, as a result in increase in demand due to population growth and limitation in resources required for proper inspection and maintenance. According to the National Academy of Engineering, the urban infrastructure restoration and improvement is ranked among the greatest challenges for the Engineers of the 21st century [1]. In order to keep track of the infrastructure condition in the U.S., every four years the American Society of Civil Engineers (ASCE) evaluates sixteen fundamental categories of the U.S. infrastructure such as energy, drinking water, schools, roads, dams, bridges, among others, and issues a report card, assigning a grade to each category based on the physical conditions and investments needed for improvement [2]. In 2017, bridges in the U.S. received a grade C+ [2], as a reflection of their current condition. Undeniably, since the first report card was issued in 1998 the grade for U.S. bridges has been incrementally increasing but hovering around the C range for the last twenty years [3].

Historically, since the collapse of the Silver Bridge over the Ohio River in December of 1967, which resulted in 46 casualties, more attention has been given to establish sound procedures for inspection and management of U.S. bridges [4,5]. The collapse that occurred during the rush hour was attributed to failure of a structural element and in part due to poor inspection [4,5]. As a consequence of that catastrophe, the Federal-Aid Highway Act of 1968 created the National Bridge Inspection Program and established a unified bridge proper safety inspection standard [6,7]. Despite such effort, in 1983 the Mianus River Bridge in Greenwich, Connecticut, collapsed, due to insufficient maintenance, causing three fatalities and resulting in more stringent regulations regarding inspections and safety of bridges [6].

Since 1968, the Federal Highway Administration (FHWA) has developed and maintained the National Bridge Inventory (NBI)–a substantial database that currently contains comprehensive information of every bridge longer than 6 m (20 feet) on all public roads. The inventory is annually updated with the aim of guaranteeing public safety through identification and evaluation of bridge deficiencies [7]. According to NBI 2017 [8], the United States possess 615,002 highway bridges. These bridges are part of the National Highway System comprising of 76,564 km (47,575 miles) of Interstate Highways plus 289,119 km (179,650 miles) of major roads, which carries most of the highway passenger traffic and freight in the U.S. [9]. Fig 1 shows the distribution of every highway bridge in the U.S. The cartographic boundary of U.S. was obtained from the United States Census Bureau [10].

The 2017 NBI reveals that four in ten bridges are 50 years or older, reaching or even exceeding their design life with the average age of bridges in America being 45 years old [8]. In addition, it is noted that 54,560 bridges in U.S. were characterized as being structurally deficient [8] where ‘deficient’ implies that elements of the bridge structure were found in poor conditions due to deterioration or damage [2]. Despite the poor conditions of these bridges, there were approximately 188 million trips across them each day in 2016 [2]. Undoubtedly large spending is required to address a host of deterioration issues in these bridges. These issues include clogging of expansion joints with road debris, scour of foundations caused by water flow, corrosion of structural elements and components due to improper drainage or leakage through damaged expansion joints, deck deterioration due to standing water and deicers, decay or misalignment of bearings, cracks in bases due to uneven settling of foundation, among others [4]. It is important to note that the effect of these listed problems on bridge performance will vary in terms of their level of impact. For example, deck deterioration is expected to cause traffic delay while large scour could threaten the integrity of the structure. Of course, it is important to acknowledge that the criticality of each of these problems might change due to their coupling nature. In this study, we chose to focus on expansion joints in bridges. This is because even though innumerable components of the bridges throughout the country require maintenance or replacement, the deterioration of bridge deck expansion joints, is one of the most common issues [11]. In spite of being small components, if expansion joints do not perform properly, it can affect major structural elements of a bridge [12]. This problem becomes even more significant given the abundance of deck joints bridges in the country. Such frequency is the result of the widespread adoption of simply supported spans design type, which facilitated the construction of a large quantity of roadways in U.S. after the 1956 Federal-Aid Highway Act. However, at that time, potential issues and costs associated with maintenance of deck joints were overlooked [13,14]. As such, the maintenance cost to keep expansion joints clean and functional has been a burden to the American transportation agencies [15]. Fig 2 shows what is typically known as major potential damages to structural elements of simply supported bridges due to the combination of clogged joints condition and unpredicted thermal stresses. These include local buckling of the main girder flanges, spalling of concrete of the abutments, and cracking and crushing of the roadway deck. Some of these issues could arguably fall under the category of serviceability limits. Meaning, local buckling for example would not compromise the safety of the structure. In this study we focus on what is typically an overlooked but critical issue. That is to evaluate the effect of joint clogging on increasing the demand on bridge girders and the deck slab which are the main load carrying elements of the superstructure. The criticality of evaluating this criterion stems from the fact that failure of the girders and crushing in the concrete deck could simply compromise the functionality and structural integrity of the bridge. The integrity, or lack of residual capacity, will depend on the level of redundancy in the bridge and the presence of alternative load path. That is to say failure of the girders and crushing of the concrete may or may not present major safety concerns; however, immediate actions should be taken.

Expansion joints are transversal spaced gaps along the bridge length, with the purpose of allowing longitudinal movements of expansion and contraction of the superstructure when it is subjected to a temperature variation. Nevertheless, they easily become clogged with debris, preventing the bridge from expanding when it is exposed to a temperature rise. In addition, deteriorated joints allow debris, water and deicing salts to infiltrate underneath the bridge deck and accumulate where the bearings are located. This could cause the sliding bearings, which are components that transfer the loads from the superstructure to the substructure of the bridge and allow thermal movement of the girders as well, to corrode and lock up, further preventing the bridge from accommodating thermal movements [12]. As a result, unpredicted thermal stresses for which elements of the bridge were not designed for are ultimately imposed on the superstructure [13,14,16]. This effect can even be amplified considering the projected future temperature changes [17]. According to the recently released Climate Change Adaption Guide for Transportation Systems Management Operations and Maintenance (U.S Department of Transportation–Federal Highway Administration), bridges with joints are more susceptible to damage due to their sensitivity to temperature [18]. Therefore, more attention should be given for these joints since change in climate may require different maintenance and rehabilitation approaches. This was further highlighted by the ASCE Committee on Adaptation to a Changing Climate who not only noted the importance of adapting transportation infrastructure to a changing climate but also emphasized that a changing climate may affect bridge expansion joints [19].

One should note that the vulnerability of infrastructure in general to climate change has recently become a topic of debate and research among engineers, researchers, and policy makers [20–28]. Here, we evaluate the vulnerability of 89,089 simply supported steel girder bridges (hereinafter SSSG bridges) in the U.S. due to clogged joints and climate change effects by focusing on the capacity of the main load carrying girders. The superstructure of the 89,089 bridges analyzed comprise of steel-concrete composite sections (see Materials and methods). The NBI database classifies bridges by the type of material they were built with–steel, concrete, wood, masonry as well as the type of their structural design such as girder, truss, arch, suspension, stayed, box girder, among others [29]. The girder type, which consists of two or more longitudinal beams that span over the piers to support the superstructure weight and the traffic load, is by far the most common design type of bridges built in U.S. as shown in Fig 3. For instance, among the Interstate highway bridges, which carry the largest volume of the nation’s traffic, the girder bridge type corresponds to approximately 60% (see S1 Appendix). We acknowledge that other types of steel bridges are configured with expansion joints and in such case clogging of the joints in these bridges could give rise to global as well as local problems. This could include for example rib-to-deck joints in orthotropic bridge systems [30–32]; however, these types of bridges were not the focus of this study.

Girder-type bridges are highlighted not only by their vast number compared to the other types of bridge design, but also by the fact that more than 50% of all deficient bridges in the U.S. are girder type, with the majority of them being SSSG [8] (see S1 Appendix). The total number of SSSG bridges in the U.S. is 97,393 [8] and the average age of this kind of bridge is 50 years old, surpassing the national average bridge age of 45 years (see S1 Appendix).

In this study, we evaluate the vulnerability of SSSG bridges in the U.S. under the combined effect of clogged joints and projected temperature rise for 2040, 2060, 2080 and 2100, considering the climate model from the NOAA’s (National Oceanic Atmospheric Administration) Geophysical Fluid Dynamics Laboratory GFDL-CM3 under three Representative Concentration Pathway (RCP) scenarios, which are named for the approximate radiative forcing in year 2100: the lower forcing scenario RCP 2.6, a moderate scenario RCP 6.0 and the higher forcing scenario RCP 8.5 [33] (see Materials and methods and S1 Appendix). One should be aware that this study does not intent to examine all the existing climate models in order to exhaustively minimize uncertainties for future temperatures prediction. Instead, the present study correlates potential temperature rising with the vulnerability of infrastructure. Ultimately, it aims to offer insights on vulnerability of a massive bridge inventory so that management policies can be devised. Fig 4 shows the variation of the projected daily maximum temperature from 2020 to 2100 as well as the location of SSSG bridges that belongs to interstates and U.S. highways based on the mentioned climate model and the higher forcing scenario RCP 8.5.

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larger image TIFF original image Download: Fig 4. SSSG bridges on Interstate and US highways and projected daily maximum temperature change from 2020 to 2100 under the higher forcing scenario RCP 8.5. https://doi.org/10.1371/journal.pone.0223307.g004

As one can note, for this particular climate scenario RCP 8.5, the augment of daily maximum temperature along the future years is projected to occur with more intensity in the climate regions: Northern Rockies & Plains, Upper Midwest, Ohio Valley and South. Nevertheless, the amount of thermal stresses developed into the bridge superstructure when expansion joints are clogged do not depend on those future temperatures only, but also on the temperature at the time of construction of the bridge. In other words, the induced thermal stress is a function of the temperature variation and not of the absolute temperature value for which the bridge is subjected. This temperature variation is calculated as the difference between the future daily maximum temperatures projected by the climate model (2040, 2060, 2080 and 2100) and the temperature of the bridge at the time of construction (see Materials and methods). Since the only available information in 2017 NBI [8] is the year in which construction was concluded, the temperature during bridge construction at the stage of expansion joints installation is considered according to four possible scenarios: average of minimum temperatures in the winter (Scenario 1), spring (Scenario 2), summer (Scenario 3) and fall (Scenario 4) for the respective year of each bridge construction conclusion. Each scenario can be interpreted as a hypothetical temperature condition, where Scenario 1 (winter) is the worst case, Scenario 3 (summer) is the most optimistic and Scenarios 2 and 4 present intermediate ranges of temperature (see Materials and methods and S1 Appendix). In addition, bridges are classified to belong to one of the nine U.S. climate regions: Northwest, Northern Rockies & Plains, Upper Midwest, Ohio Valley, Northeast, West, Southwest, South and Southeast (see S1 Appendix). Using this classification, the average of minimum temperatures of each season scenario is taken from the historical NOAA Regional Time Series [34] and attributed to each bridge in the inventory according to its respective year of construction and its regional location (see S1 Appendix). Hence, the temperature range that each bridge is subjected to depends on of the year in which it was built, its geographical location and the future projected temperature.