May 2006
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Interstate Bridges — How They’ve Changed by , Editor-in-Chief |
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Like the chicken and the egg, it’s impossible to separate Interstates from their bridges. Interstates created the need and opportunity for all types of bridges from simple short-spans to major long span river crossings. But, says HDR’s Ed Power, Interstates and their growing use brought about many of the advances in bridge design and construction. Power, senior vice president and national technical director for bridges at HDR, has seen and participated in those changes. In the beginning Early Interstate project called for many bridges and their rapid construction. Many of these were overpasses of a simple, almost cookie-cutter design that let local roads pass over the new controlled access interstate. In some cases the new road passed over local roads. As a rule, these were short structures moving perhaps four lanes over a two-lane street or road. These early overpass designs were mostly functional with little regard for aesthetics, Power says. |
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With use As the Interstate system grew and traffic numbers began to increase steadily, bridge design, materials, and construction methods had to change to keep pace. Problems encountered brought about many of the solutions developed in the industry. Bridge deterioration, and particularly deck deterioration, from salt was one of the earliest problems to be faced. This led to research of ways to control salt penetration, Power says. First impermeable decks with epoxy-coated re-bars were developed to prevent early deck deterioration and then impermeable deck joints were developed to prevent salt deterioration to other parts of the structure below. Instead of the original simple spans with joints that allowed deck drainage and salt to pass through to deteriorate piers, designers began to use sealed joints or to eliminate them from the design altogether by designing continuous structures. Fatigue problems pushed for better bridge design, too, Power says. Originally, steel-welded designs were used as welding replaced riveting as the most economical way of fabricating steel girders. But welded girders with certain details developed cracks as heavy truck traffic grew.
This brought about the study of fatigue behavior in design at Lehigh and other institutions. Brittle fracture on long spans was another aspect studied and the concept of a fracture-critical structure or member was developed. To avoid fracture-critical structures where possible, designers began to incorporate redundant load paths to provide safety if fractures destroyed some paths. Seismic research originally helped California build flexible bridges that would better withstand earthquakes. As these designs improved, they spread to other parts of the country where fault lines signal their possible need. Methods of retrofitting older bridges to withstand seismic events without collapse were developed and implemented. As traffic continued to grow, bridge designs themselves changed. A key direction was more lanes and longer spans, says Power. Changes were implemented to accommodate not only more vehicles but to increase the load-carrying ability as America switched from rail to truck haulage. Added lanes also meant wider bridges. Overpass piers were moved away from road shoulders both for safety and to allow for future widening. Longer girders were developed, including longer precast prestressed concrete girders. Materials change High-performance steel and high-performance concrete developments made the longer spans and shallower girders more feasible, Power says. This provided both economy and lower bridge profiles. Better materials and longer life became an industry theme.
In recent years, most Interstate bridge work has been replacement, rehabilitation, or widening — all while traffic keeps rolling alongside. As a Federal Highways Administration slogan says, “Get in, get out; stay out.” Rapid work led to new methods such as precast units that can be set in place quickly. On some projects, a new bridge is build adjacent to the original structure and then quickly rolled into place once it’s completed. Along with Rapid Construction to minimize traffic disruption, Rapid project delivery is also the name of today’s bridge game, Power says. Design-build is a perfect example. Rather than moving laboriously from concept to design to bid to building, the design and construction phases work together. The designer needs to understand the construction method to give the contractor what’s needed. The bid comes up front. Beauty too Context-sensitive design, which includes aesthetics,public involvement and environmental awareness is current, too. Many want signature bridges to give their city a unique look. Cable-stayed bridges became popular as modern, practical, and aesthetic solutions, pleasing the public. Curved concrete girders and curved steel girders allow construction of aesthetically and practical flyovers connecting Interstate to Interstate. Steel box girders and concrete segmental designs provide aesthetic and practical options. Cost volatility New methods such as design-build may be pushed along by today’s extreme volatility in prices, Power says. On some major projects with traditional design-bid-build methods, bids have come in at 20 to 50% over the engineers’ cost estimates. Rapidly increasing costs of steel and concrete, as well as site restrictions, and unique design details lie behind the problem. In a volatile price market, bid price histories are no longer valid for general cost estimating, particularly on special designs. Design-build can be used to develop constructable concepts that stay within an owner’s specified budget. Funding for major projects is a real problem today, with tolling, and other innovative financing methods, including public private partnerships being frequently considered. State legislatures and city authorities need to pass laws needed to use rapid-delivery methods, and innovative financing where current restrictions are in place. The financing situation demands it. Alternative financing, including private-public partnerships and the increased use of toll bridges may become matter-of-fact solutions, rather than an occasional exception. |
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Design Tools Changed Too Tools to design Interstate bridges have changed as much as the bridge designs themselves, Power says.
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Reprinted from Better Roads Magazine |
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Copyright © 2006 James Informational Media, Inc.
All rights reserved.
