by Daniel Whittemore, PE, Senior Structural Engineer
As a professional bridge engineer, I typically practice my profession from behind the scenes. I find most people take comfort in knowing that engineers are behind the bridge they are crossing, but think little about the efforts that go into designing or maintaining that bridge.
Public safety is always the number one priority. Recently, the term “Accelerated Bridge Construction” has bounced around news outlets in both positive and negative lights. As someone working in the industry, I thought it would help to give my take on what that term means and how it is used every day by bridge engineers and contractors.
Bridges are expensive in multiple ways. When compared on a square foot basis to any other section of a highway, it is a fact that bridges are several times more expensive to build and maintain than any other portion of a road. But even more significantly, it is important to remember that bridges are a critical piece of our interconnected economies. As an example, think of the travel delays that happen if the local bridge you cross daily closes or even necks down a lane or two for an extended period of time. Time spent sitting in traffic results in delays and cost overruns, not to mention the environmental impact of the wasted fuel spent idling in traffic.
There is inherent risk in working on a bridge next to or over live traffic. Bridge work is hazardous for the construction professionals that keep our infrastructure in good working order, and getting them off the structure and out of work zones near active traffic as fast as possible is in everyone’s best interest.
As a bridge design professional, it is my responsibility to make decisions in the best interest of the public – including the public working on our job sites. It is also my charge to keep bridges in good condition, with as little impact to the travelling public and local economies as possible. These two statements are often at odds with each other, such as when there is a bridge with significant maintenance needs that is so heavily used by the public that delays caused by construction would be a significant blow to the community. In these sorts of situations, one of the most powerful tools in the bridge engineer’s toolkit is “Accelerated Bridge Construction”, or “ABC”.
What ABC boils down to is the simple concept that we can make better, potentially longer lasting bridges by doing as little construction on a bridge site as we can, thus reducing the impact to the traveling public. But make no mistake about it – this is not a shortcut. Safety and proper engineering are not compromised with ABC. Let me give you an example: instead of casting concrete bridge components such as concrete decks or central supporting piers on site, we can make them away from the site and bring them in fully constructed. This reduces the time we would have used getting the site ready for placement, setting up concrete forms, or waiting for concrete to cure in the field. Days saved using ready-made components are days we can get out of the travelling public’s way.
Another advantage of ABC is the matter of component quality. Concrete or steel components cast or fabricated in controlled yards are usually of higher quality than components fabricated at the bridge site. This happens because casting yards are more controlled, have better equipment, are less subjected to vibrations such as might be found on a moving bridge deck, and aren’t subject to the whims of mother nature’s worst. By placing higher quality components into our new structures, bridge professionals can provide the public more durable, less maintenance-prone bridges.
ABC is a concept that has been in regular practice for 10 years or more – you are probably already using a bridge designed and constructed using ABC techniques. Though ABC can, at times, cost more up front in terms of initial construction budgets, the total cost of bridge work should include the economic, durability, and public safety aspects as well as the bottom line contractor’s fee. When the whole picture is evaluated against phased work with temporary bridges or long construction windows, the value of ABC makes a compelling case for its use in most projects we undertake.
ABC isn’t a magic wand that can instantly make every bridge project complete overnight, nor is it industry shorthand for quickly completing projects with a lack of attention to standards or quality. It is a tool, that when used appropriately, allows the public to move forward in the most efficient way.
About the Author:
Daniel Whittemore, PE has many years of diversified bridge design experience. He has worked with both private and public bridge owners spanning the gamut from bridge design to inspection, replacement, protection and rehabilitation. Dan’s experience touches the design, documentation, analysis, and management of bridge projects from the local to the signature scale. Dan is an internationally published author and scholarly cited leader in sustainable bridge design whose work has been incorporated into design standards by many agencies and municipalities, most notably the 2015 Iowa Department of Transportation’s Bridge Design Manual.
Contributions from John Byatt, PE
John’s technical experience includes preliminary bridge investigations and rating analysis as well as the design of steel and concrete bridge structures, a variety of substructure designs, bridge inspection, and seismic analysis. John is familiar with VTrans, New Hamphire DOT, and MassDOT design standards and procedures.