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Highway Bridge Deterioration from Climate Change Will Affect U.S. Military Mobility and Deployments

Bridge #3 with Deteriorated Deck _WI

By Marc Kodack

The overall state of infrastructure in the U.S. is very poor. Whether it’s energy, transit, drinking water, or inland waterways, these and other types of infrastructure are all aging and deteriorating at different rates. Climate change exacerbates the condition of many of these types of infrastructure. For the Department of Defense (DoD), infrastructure, such as bridges, roads, rail, ports, and aviation structures, is important to move personnel and equipment in response to disasters within the U.S. or for deploying overseas for humanitarian and/or combat operations. The declining state of bridges across the U.S. may impede the DoD’s ability to meet the timely execution of its assigned mission. Bridges will experience climate change effects, including those from rising temperatures that can lower physical performance leading to degradation in their life span.

The U.S. has over 615,000 bridges with an average age of 43 years. As of 2018, almost 8 percent of bridges were rated as structurally deficient, i.e., in poor condition, with an average age of 60 years. Regardless of their present condition, bridges are an integral part of the U.S.’ national highway system and its’ component sub-systems. Without functioning bridges alternative routes must be identified leading to lengthened trips and opportunity costs.

The National Highway System (NHS) “consists of roadways important to the nation’s economy, defense, and mobility.” The NHS has several subsystems including (1) the Dwight D. Eisenhower Interstate System; (2) other principal arterials that connect an arterial with a port, an airport, or other intermodal transportation facility; (3) the Strategic Highway Network are highways “important to the United States strategic defense policy and which provide defense access, continuity and emergency capabilities for defense purposes”; (4) major strategic highway network connectors “provide access between major military installations and highways which are part of the Strategic Highway Network,” and (5) intermodal connectors.

The Strategic Highway Network (STRAHNET) are “roads deemed necessary for emergency mobilization and peacetime movement to support U.S. military operations…Together STRAHNET and the Connectors define the total minimum defense public highway network needed to support a defense emergency.” STRAHNET includes all 50 states and Puerto Rico. The Surface Deployment and Distribution Command Transportation Engineering Agency is the designated DoD agent for addressing DoD’s public highway needs.

The Surface Deployment and Distribution Command Transportation Engineering Agency identifies “the minimum public highway infrastructure that DoD needs to fulfill its mission; then integrates these public highway needs into civil policies, plans, and programs [and] ensure the defense readiness capability of public highway infrastructure and establish policy on how DoD uses the public highway system.”

Bridges across the U.S. are deteriorating because of weather, traffic, and climate change. Deterioration in bridge deck expansion joints is a common structural element that is affected. “In spite of being small components, if expansion joints do not perform properly, it can affect major structural elements of a bridge. This problem becomes even more significant given the abundance of deck joints bridges in the country.”

Within the interstate highway system, the simply supported steel girder (SSSG) bridge type represents approximately 60% of all the bridges in the system. It has expansion joints that allow the superstructure to expand and contract longitudinally when the temperature changes. These joints clog with debris (e.g., gravel, sand) preventing the joints from expanding as the temperature rises. As the joints deteriorate, debris, water, and deicing salts migrate under the bridge deck affecting how loads are transferred to the sub-structure. The sub-structure will begin to deteriorate, leading to thermal stresses not accounted for in the design. With increases in temperature forecasted because of climate change, these sub-structure thermal stresses will begin to be reflected in the superstructure. Structural deterioration throughout the bridge will eventually lead to its’ failure. For several forecasted climate change scenarios, SSSG “bridges located in the northern portion of the United States, such as [the] Northern Rockies & Plains, Northwest and Upper Midwest are the most vulnerable due to more pronounced temperature range…South Dakota and North Dakota are the [most] critical states…regardless of the climate scenarios.

Bridges are one of many types of infrastructure that DoD relies on to fulfill its national security mission. Most of this infrastructure is not owned or controlled by DoD. Given that bridges, such as the SSSG type in the national highway system, are included in STRAHNET, bridges are likely included in the Defense Critical Infrastructure Program (DCIP). Critical infrastructure are systems and assets that when unavailable or have been destroyed “would have debilitating impact on security.” Bridge unavailability would one of these debilitating events.

Transportation related infrastructure, e.g., roads and bridges, sub-sets of which are in DCIP, that DOD relies on is included in the Transportation Systems Site-Specific Plan (TSS-SP) part of the National Infrastructure Protection Plan. The goal of the TSS-SP is to “guide and integrate efforts to secure and strengthen the resilience of transportation infrastructure.“ The TSS-SP mentions climate change risks, including sea level rise, extreme heat, wildfires, flooding, and droughts, and the effects these risks have on transportation systems. However, the plan was written in 2015 and has not been updated since then. It is unknown if any activities associated with addressing the plan’s priorities are on-going.

Climate change effects will not discriminate among different types of infrastructure, whether it is bridges, roads, ports, or rail, that DoD uses. Not including these effects in their initial construction, major rehabilitation, and/or operations and maintenance, will lessen their performance over the decades that they will be in service. With many other competing financial demands, the non-federal owners of this infrastructure will be challenged to fund the critical infrastructure that DoD relies for mobility and deployments in the U.S. DoD can be a proponent for including climate change effects into local decisions on infrastructure because of its multi-decade dependence on this infrastructure’s availability.

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