Summer 2019

ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.

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44 | ASPIRE Summer 2019 F H WA I n my position as director of the Federal Highway Administration's Office of Bridges and Structures, I have repeatedly been asked by both internal and external stakeholders what potential impacts I foresee from truck platoons on highway bridges and tunnels. My immediate response is to ask in return: What is a truck platoon? This may seem to be a flippant and unsatisfying reply, but until a definition exists that describes what constitutes a truck platoon and how platoons will be allowed to operate, we cannot forecast their impact with any certainty. As you may know, advances in technology have made it possible to electronically connect vehicles so that they can safely operate in unison with very little headway (distance between the front of the trailing vehicle and the back of the leading vehicle). In a manner similar to drafting vehicles on a car racetrack, platooned trucks could col- lectively achieve signicant e•ciencies, and those e•ciencies could lead to a lower demand for fuel and corresponding drops in operating costs and adverse environmental e•ects. •ese are not the only potential benets of truck platoons, but they are likely the ones most evident to the public. Highway bridges are currently designed for a notional live-load model that envelops the force e•ects that current legal truck tra•c creates on bridges. Once in operation, highway bridges are evaluated (load rated) for all individual legal and unrestricted truck congurations. Without a clear understanding of the possible loading models that platoons might create, engineers cannot determine whether design loads are adequate, nor can they conduct valid load rat- ings to determine the impact of truck platoons on the 615,000 existing bridges in the National Bridge Inventory. To meet those obligations, sev- eral questions need to be answered. How Many Trucks Can Be in a Platoon? In demonstrations in the United States and elsewhere, platoons have included two, three, or four trucks, but present and future technology will certainly accommodate more. •e number of trucks in a platoon needs to be determined for two reasons. First, from a load-rating perspec- tive, a truck or a platoon of trucks is modeled as a series of axle loads and spacings. •us, the platoon will be treated as one long truck with in- dividual and tandem axle weights and spacings. Second, from a safety perspective, we need to determine how long a platoon can be before it begins to pose unacceptable risks to the traveling public. For example, if a platoon is passing a highway exit, how does that a•ect other vehicles' access to the exit? Also, what happens when a platoon is exiting? Can the exit ramp accommodate the length of the platoon, or does the platoon back up onto the bridge or cause other tra•c to back up onto the bridge? What Truck Congurations Can Be Platooned? Although most demonstrations to date have focused on three- to ve-axle semi-tractor- trailer combinations, will all legal congurations (including twins and triples in states where those combinations are legal) be permitted to platoon? Will platoons only be composed of similar congurations, or will mixed congurations be permitted? Once platooned, what is the minimum headway between trucks? What is the minimum spacing between platoons? What is the minimum spacing between a platoon and a nonplatooned truck? We need answers to these questions (and likely others) to understand the possibilities of axle spacings in a platoon, and for a platoon operating in tra•c. From a capacity (strength) perspective and considering only gravity loads, the operation of platoons may not signicantly a•ect bridges Truck Platoons and Highway Bridges by Dr. Joey Hartmann, Federal Highway Administration A three-truck platoon. All Photos and Figures: U.S. Department of Transportation.

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