Prior to the invention of the computer, or even the hand held calculator, bridge designers used what was termed “Engineering Judgment” to ensure the public that their design was competent. Engineering judgment can be defined as the ability to make sound design choices based on experience and intuition; it is not summarized in a list of rules. Bridge failures were in abundance in the mid 1800’s, a time when in a ten year span one out of every four bridges built collapsed. Ensuing investigations would provide greater understanding to what went wrong, and through engineering journals, the entire bridge design community could gain “Engineering Judgment”.
In 1854 The Wheeling Suspension Bridge located in Wheeling, WV was subject to a strong windstorm which destroyed the deck of the bridge through torsional movement and vertical undulations that rose almost as high as the towers. The news of this failure quickly reached an engineer building a similar type bridge in Niagara, NY. The engineer, John Roebling, who later went on to design the Brooklyn Bridge, defended his design by pointing out that the Wheeling bridge deck lacked the inherent stiffness to prevent deck undulations unlike his Niagara design. The Niagara Bridge was successful though became functionally obsolete and was replaced in 1886.
Eighty six years after the Wheeling Suspension Bridge collapse, the Tacoma Narrows Bridge collapsed for the simple reason that it lacked necessary stiffness. Othmar Ammann, a leading bridge designer and member of the Federal Works Agency Commission investigating the collapse of the Tacoma Narrows Bridge, wrote: “The Tacoma Narrows bridge failure has given us invaluable information...It has shown [that] every new structure [that] projects into new fields of magnitude involves new problems for the solution of which neither theory nor practical experience furnish an adequate guide. It is then that we must rely largely on judgment and if, as a result, errors, or failures occur, we must accept them as a price for human progress.” Rather than ban the use of suspension bridges, the collapse of the bridge led bridge engineers to take a look at aerodynamics (really for the first time) and resulted in dramatic changes in the engineering principles used to design decks for future suspension bridges.
Eleven years after the Tacoma Narrows Bridge collapse, in 1951, the Walnut Lane Memorial Bridge was opened to traffic as the first prestressed concrete girder bridge built in the United States. While prestressed concrete bridges were not able to span the lengths of suspension bridges, they quickly became the preferred choice of shorter span bridges. The newly developed non-composite adjacent box beam bridge was built in abundance after 1956 and the inception of the Interstate Highway System. These bridges were typically constructed with beams butted next to each other with an asphalt overlay and a minimum under clearance of 14ft. The design life of the structure was about 40 years and typical maximum permissible axle weights were 18,000lbs.
The early beams were constructed using stress relieved 240ksi prestressing cables with about 1.25” of concrete cover to the bottom of the beam. The shear reinforcement was 40ksi uncoated and positioned above the first row of strands. Concrete strengths varied; however, mix designs were approved based on strength and slump which was typically 5000psi and 4” respectively. The interior form of the box beam was fabricated out of wood and cardboard and had to be held to a specific location under wet concrete. Because there was no standard quality control program, beams were fabricated to differing degrees of quality depending on the fabricator.
Today adjacent box beam bridges have been getting a bad rap. Driving under most of the early prestressed concrete bridges, it is not hard to understand why. A lot of the ends of the beams are cracked with rusty steel showing. Some of the beams have damage from over height trucks impacting the bottom. In a few isolated cases, bridges collapsed or were taken out of service. As was done with suspension bridges over 100 years ago, a collapse would lead to an investigation which would be used as a teaching tool to increase our knowledge and aid in preventing future collapses. PCI is instrumental in ensuring the investigations are performed in a nonbiased manor.
From the beginning, the industry was focusing on improvement to products and quality. To attempt to limit “fly-by-night companies” and create a standard of quality, in 1958, the Precast Concrete Institute proposed a volunteer plant certification program and in 1963 issued the first PCI Quality Control Manual. Around 2000, PCI mandated that producer members be part of the certification program. As construction and performance issues are raised, the industry has adopted solutions for future projects. This is evident in the details of adjacent box beam construction today. Bridges are now designed for the larger trucks that are on the road today. Details were changed to prevent road salts from penetrating the beams. Design life is now considered 75 to 100 years. Strands are now low relaxation 270ksi and located to provide cover based on exposure to corrosive environments. Shear reinforcement is 60ksi and can be coated to prevent corrosion. Stirrups are also placed around the strand to provide confinement to the strand and designed to prevent bursting of the ends of the beams during the fabrication detensioning process. Concrete mix design strengths are up to 10,000psi and slumps are now measured as a spread rather than “slump”. More importantly, materials used in the concrete mix are tested for various properties that are now known to drastically reduce the service life of the concrete. The mix design is also run through a range of performance testing to verify that it will have long term durability. Void material is now made of solid light weight polystyrene that absorbs negligible water and can withstand the heavy buoyancy force of the curing concrete. The voids still have to be held in place at a specific location; however the system to hold them in place has improved greatly over the past 60 years.
The idea of a Suspension Bridge has been around since the 7th century and has evolved with time to meet the demands of the traveling public. In the short history of the prestressed bridge beam, it too has evolved to meet these needs. The traveling public does not want to see any bridges closed for any reason let alone safety reasons, therefore one failure is considered one too many. Until these aging bridges, which are mostly functionally obsolete, are replaced we will continue to have to defend the box beam.
