Designing Bridges for Durability
Last summer's collapse of the I-35W Mississippi River bridge in Minnesota has moved the care and maintenance of our nation's bridges to center stage. State transportation departments and federal agencies are re-examining the mechanisms for funding comprehensive maintenance programs, given years of under budgeting for basic upkeep and the realization that the situation is reaching a critical juncture. Clearly, a lack of adequate funding for maintenance has affected the safety and quality of life of all Americans.
Realizing that maintenance funding is always shortchanged, engineers and public officials are developing design strategies for new and rehabilitated bridges to significantly reduce future maintenance costs. Design innovations and new materials have proven highly successful in minimizing overall maintenance needs.
Inspection and Maintenance
The first step in incorporating low maintenance into the planning and design of bridges is to design the structure so that inspections are easier. This is accomplished by ensuring that the design addresses bridge details critical to the inspection process, such as stay-cable connections at tower and deck surfaces; inspection platforms at deck level and below deck; stairs and elevators to hard-to-reach places; proper and safe circulation within a steel or concrete box girder; and adequate heightinside a box girder. Such measures allow regular maintenance to be performed quickly and with less cost.
Look Another design approach is to avoid or reduce the number of bearings on abridge, which can in many cases improve structural integrity and overall bridge performance. Eliminating bearings reduces the maintenance cost since fewer bearings have to be inspected, maintained, and replaced over time.
Inspections can also be made more efficient if the bridge is designed as a continuous structure by eliminating as many expansion joints as possible, which improves structural redundancy and prevents the deterioration that regularly occurs in expansion joints. Fewer expansion joints means less water seepage; without water seepage there is little or no corrosion or deterioration.
Another key to minimizing future maintenance needs is to protect the deck slab. Overlays of asphalt, microsilica (silica fume) concrete, high-performance concrete, or latex-modified concrete add to upfront costs, but nonetheless they dramatically extend the life of the deck slab. Overlays are the first line of defense against chloride penetration in to the concrete deck slab, so they should be regularly inspected and repaired if needed. Lower maintenance costs can also be achieved by introducing compression into the deckslab with post-tensioning cables. This compression eliminates the hairline cracks that develop in bridge decks, thus preventing water penetration.
Designing For Strength
The materials used in a bridge's designand construction can result in lower maintenance. For example, on steel bridges, the use of weathering steel (which extends the steel's life with a dense oxide formation on the surface) can significantly reduce corrosion and enhance a structure's durability.
There are also new materials for cable bridges that lower maintenance. These include post-tensioned cable strands and galvanized cable strands. Post-tensioned cable strands use polyethylene ducts or sheathing to inhibit corrosion by preventing water and weather damage. Galvanized cablestrands also use polyethylene ducts, but the galvanizing adds another layer of protection,which lowers cable maintenance.
On cable stay bridges, maintenance can be reduced by using ungrouted cablestays. With grouted cable stays, bleed water from the grout enters the steel strand and starts the corrosion process. By eliminating the grout from the cable stays, corrosionis delayed. Ungrouted cable stays offer another advantage in that they allow the replacement of any particular strand in a cable at any time. Replacing a single strand on an as-needed basis is more cost effective than replacing the entire cable. The ability to do single-strand replacement further improves the long-term structural integrity and life cycle cost of a bridge.
Even with the increased public scrutiny on infrastructure improvement, the maintenance needs of our nation's bridges will continue to outstrip the available funding. A careful strategy to design structures for lower maintenance can be a highly effective tool for dealing with this issue.
Vijay Chandra is senior vice president of PB, New York