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/297006
A C C E L E R AT E D B R I D G E C O N S T R U C T I O N Sliding and rolling bridges into place offers key benefits to owners, designers, and contractors. As a result, more bridges are being designed and built using these techniques. This series of articles looks at some of the key considerations when using these approaches to construct bridges. The first two parts of this series examined key design considerations that impact construction. This part addresses construction considerations. Pads Pads are a simple, low-cost solution. They offer significant directional flexibility, as the direction of movement is not tied to the orientation of the pads. Pads also allow the use of an unguided system that will not bind if ends of the bridge move at different rates. Normally, the superstructure is lifted prior to the slide, to allow the skid shoes and bearings to be cleaned and to apply a non-petroleum-based synthetic grease onto the sliding surface. Continuous lubrication of the pads is critical during the slide, especially to overcome the initial inertia and achieve the breakaway force. Fortunately, many types of inexpensive lubrication can be used. A variety of biodegradable lubricants are available that won't damage the pads. A popular option is dish soap, readily available from any mass-merchant store, but it dries quickly if left for any time. One contractor uses bananas as lubricant. Whatever is readily available, inexpensive, and has been found to be effective can be put into service for this need. Skid shoes should be braced during construction to maintain a level slide surface. Normally, the sliding surface of the shoe consists of polished stainless steel. There are many ways to construct skid shoes. One method uses concrete-filled steel shapes using steel as thin as ¼ in. Other methods use steel box or I-sections with the base plate at least ¾ in. thick. Thinner shoes can deflect or warp during construction, creating an uneven sliding surface. Beveled ends should be used to ease sliding over the pads and limit friction. Rollers Rollers are more costly than pads but have a longer service life. They are often used on bridge projects with larger load requirements. When properly sized, the slide resistance is more predictable and requires less force to start and stop the bridge. Undersized rollers can dimple the slide surface, which can significantly increase the starting force. Rollers are almost always guided with troughs or channels that align the rollers during the move. Roller-guide surfaces can be flush or approximately 2 to 3 in. wider than the rollers to allow room as the jacks push or pull the bridge. These channels must be kept clean and clear at all times to ensure no obstructions for the rollers. by Craig A. Shutt Sliding and Rolling Bridge Solutions–Part 3 Construction of falsework for the new bridge begins on the right side. All photos: Hilman Rollers. Construction of widened substructure begins on the left side. The deck is ready for casting concrete. The old bridge has been removed and the new bridge is ready to be moved into place. The new bridge is in its final location. ASPIRE , Summer 2013 | 31 Book_Sum13.indb 31 7/1/13 7:00 AM