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/740919
C O N C R E T E B R I D G E T E C H N O L O G Y 32 | ASPIRE Fall 2016 beams and 650 ft long with concrete beams. However, there are a number of examples where bridges over 1000 ft long have performed well without expansion joints. The discussion below presents several options available to eliminate expansion joints and provide jointless bridge superstructures. More details are available in the PCI publication The State-of-the-Art of Precast/Prestressed Integral Bridges, authored by the PCI Subcommittee on Integral Bridges of the Committee on Bridges. 1 Details at Abutments A bridge abutment has the dual purpose of resisting the loading transmitted from the supported superstructure and the pressure from the soil retained in transitioning from soil-supported roadway to "point"-supported bridge. Creating a totally integral abutment detail requires that the abutment carry the vertical loads from the end span as well as the lateral soil pressure from the adjacent soil. A simple integral abutment detail employed by Midwest states, including Nebraska, is to directly support the concrete beams on steel cross channels that are directly welded to steel HP piles at the required seat elevations (Fig. 2). The beams are secured in position on the channels until the abutment wall concrete is placed and cured. No bearing pads are used. If the expansion of an integral bridge due to thermal effects, for example, creates excessive stresses on the abu tment or excessive deformation of the supporting piles, another option may be used (Fig. 3). The detail is called a semi- integral or turn-down abutment. In this situation, the pile cap (or a b u t m e n t w a l l ) i s s e p a ra t e d from the abutment diaphragm by compressible filler, such as extruded Traditional bridges use expansion joints in conjunction with expansion (sliding) b e a rin g d e vi c e s t o a c c o m m o d a t e superstructure movement due to volume change effects. These effects are primarily due to creep and shrinkage of concrete and both daily and seasonal temperature variations. However, use of expansion joints, especially above the abutment and pier supports, may require significant maintenance expenses and may shorten bridge life. Leakage of contaminated water and freeze-thaw cycles can cause staining and cracking of the concrete surface and locking of the expansion bearings, which would further exacerbate concrete deterioration. Bridges with structurally continuous beams over the piers offer a number of advantages. Continuity for superimposed dead loads and live loads allows for relatively long spans. Such bridges also have better resistance to wind and seismic forces. They have significantly less d eflec tion and vibration than simple-span bridges, and thus improved durability. Ride quality is also improved if the "bump" at the piers caused by the expansion joint is eliminated. A number of owners have adopted measures to eliminate expansion joints on bridges, and limit their use to locations in the approach slabs only, as illustrated in Fig. 1. In addition, some owners have developed details that allow for use of simple elastomeric pads for erection purposes, or just wood blocking until the diaphragm concrete is placed. Bridges that utilize these features are sometimes called jointless or integral bridges. There are no requirements in the American Association of State Highway and Transportation Officials' AASHTO LRFD Bridge Design Specifications (AASHTO LRFD specifications) for maximum bridge length allowed without expansion joints. Many state highway agencies allow eliminating expansions joints for bridges that are less than 350 ft long with steel Eliminating Expansion Joints in Bridges by Dr. Maher K. Tadros, PE, e.construct.USA LLC Figure 1. Elevation of a typical jointless bridge. Figure: e.construct.USA. Deck Girder Pier Integral Abutment Flexible Piling Approach Slab Expansion Joint Sleeper Slab Wingwall Figure 2. Example of integral abutment detail. Figure: Reference 2. Deck Slab Steel Pile C12x30 Beam Seat C Bearing and Steel Pile End of Approach Slab C onstruction Joint Girder L 3'-0" 6'-6"