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.
Issue link: http://www.aspiremagazinebyengineers.com/i/306894
Most of the county's bridges, like the MB Road Bridge, use adjacent box beams with composite concrete deck. The new concrete Race Track Road bridge, so named because it leads to a racing track, features the decorative touch of the straining head of a racehorse on the railings. ASPIRE , Winter 2007 | 45 C O U N T Y Thinking Long TERM By Edward J. Binseel F or many years, designers in Prince Georges County, Maryland, built bridges using steel components. But about 12 years ago, we re-evaluated the long-term costs, taking into account maintenance and other factors that were not always being considered during the initial design stages. As a result, we went entirely in the opposite direction and now specify precast, prestressed concrete beams for the superstructure of almost all of our bridges. Over the past 12 years, we have standardized the design of our bridges, allowing us to pursue a cookie-cutter approach when replacing old, obsolete structures (although we do sometimes dress them up for special occasions). The bridges typically span streams rather than roadways and average 60 to 80 ft in length. Today, the majority of those bridges are built with precast, prestressed concrete box beams with a composite concrete deck. They typically include massive foundations, with the goal of making the bridges as permanent and as maintenance-free as possible. The desire to obtain a minimum life span of 75 to 100 years for our bridges while minimizing their maintenance was the driving force behind our switch to concrete bridges. This change began in 1994, when we designed a standard steel bridge and advertised the bridge for construction. We were surprised when the bids received far exceeded the engineer's estimate due to a recent spike in the cost of steel beams. We immediately redesigned the bridge using precast, prestressed concrete box beams. We never looked back. At the same time, I had been reading about the European approach to bridge design, which focused on making the structures long-term, permanent parts of the environment. That philosophy not only helped the bridges become area landmarks, but it minimized maintenance costs and extended the life of the bridges. Those attributes allowed the County to begin to manage the life-cycle costs of its bridges and to more effectively use the limited funds available. It also eliminated the problems arising with detours and high user costs during the performance of future bridge repairs or maintenance. At the same time as these philosophies and steel prices were directing us toward concrete, other factors began to become apparent, too. We realized that trucks were likely to continue to become heavier, resulting in the need to load post bridges that had once carried full legal loads. Designing for HS20 loads wasn't feasible any more—and the use of the HS25 or HS27 design vehicles was now necessary. We realized that we were also retrofitting bridges that were functionally obsolete and were not capable of bearing full legal loads, even after updating. Maintenance Budget Cut Meanwhile, the budget for bridge maintenance continued to be cut, making us realize that we needed to create more durable designs within the original construction budget. This was brought home when the repainting of a large steel beam bridge superstructure required us to clean the lead painted steel beams down to bare metal before they could be re-coated. That wake-up call to our limited maintenance budget then resulted in still further changes in the way we design our bridges. To make the bridges more durable, the d e c i s i o n w a s m a d e t o u s e e p o x y - c o a t e d r e i n f o r c e m e n t t h r o u g h o u t t h e b r i d g e 's substructure and superstructure. We increased the minimum concrete cover usually specified over reinforcement by 1 in. for both cast-in-place and prestressed concrete components. And we incorporated the use of silane-penetrant sealers (rather than linseed oil), and we restricted the water-cementitious materials ratio of the concrete to 0.40 for all superstructure concrete and substructure beam seats. These changes are all intended to make the bridges less permeable to salt intrusion. While we initially received criticism for "wasting" concrete as a consequence of the thicker cover over the reinforcing steel, we calculated that our approach added only $3,000 to $5,000 to the typical bridge budget of $1.5 to $2 million. We saw we could get a lot of protection for a tiny incremental cost. We can't mobilize a contractor to fix even minor concrete spalls on a bridge for that much! The upshot of our switch to concrete bridges is that we have not been back to repair any of the 20 concrete bridges that we've built in the past dozen years, a testament to this approach. The designs also have allowed us to incorporate interesting aesthetic touches, adding the icing to this concrete cake. _____________ Edward J. Binseel is Associate Director, Office of Project Management, Department of Public Works & Transportation, Prince Georges County, Maryland. Offcials in Prince Georges County, Maryland, shifted to concrete bridges after a variety of factors made it apparent the benefts will pay off. ASPIRE_win07.indb 45 12/29/06 2:35:24 PM