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/319569
p recast segmental concrete design offers thinner cross section A precast concrete "channel" bridge offers significant opportunities for expanding the capabilities of post- tensioned segmental construction. The potential can be seen in the DCR Access Road Bridge over Route 24 in Randolph, Mass. The project will improve clearance for the heavily traveled road below, complement the scenic areas surrounding it, and reuse existing materials to reduce waste and lower costs. The project is located in the Blue Hills Reservation area, surrounded by land owned by the state Department of Conservation and Recreation (DCR). The existing bridge, built in 1958, consisted of a 247-ft-long, four-span steel I-girder structure supporting a 7.5-in-thick concrete deck and an asphalt wearing surface. The substructure featured two concrete stub-type abutments supported on steel piles and three reinforced concrete piers supported on spread footings. Partly due to its low 14 ft 3 in. vertical clearance, the existing bridge had become structurally deficient. In fact, the steel I-girders had repeatedly been hit by trucks driving below on Route 24. Officials at the Massachusetts Department of Transportation (MassDOT) wanted to increase the vertical clearance without having to perform extensive roadway work either underneath the bridge or at the abutments. Raising the profile of the approach roads at the abutments was not an option because of the scenic location, landscaping surrounding the site, and transitions to extensive woods and horse paths used by horse-rental farms on both sides. MassDOT officials also wanted to create a design that would blend with the scenic surroundings and minimize long-term maintenance needs. To reach those goals, MassDOT engineers selected the precast concrete "channel" bridge concept with post-tensioned, segmental construction. In addition to meeting the immediate goals, it will provide long-term durability through a minimum service life of 75 years. e dge Beams serve two roles The channel cross section features a precast concrete superstructure with an unusual U-shaped design. It consists of two edge beams that function as the main load-carrying elements, with the roadway slab supported between them. The two edge beams serve a second purpose by acting as traffic barriers. During erection, top flanges on the outside of the edge beams temporarily support the segments on erection beams that span between the abutments and piers. This system eliminates the need for a below-deck support system, minimizing vertical-clearance needs and construction time while reducing life-cycle costs. The DCR Bridge is 248-ft-long, comprising t w o 1 2 4 f t s p a n s . T h e t w o - s p a n continuous precast segmental concrete structure will increase vertical clearance over Route 24 by more than 2 ft to 16 ft 5 in. The substructure consists of two new, reinforced concrete stub-type abutments 'Channel' BrIdge Improves ClearanCe by matt card, purcell Associates and thomas cyran, International Bridge technologies DCR ACCESS ROAD BRIDGE Ov ER ROUTE 24 / RAND olPH, MASSAc HUSETTS engI neer oF recorD: Purcell Associates, Boston, Mass. superstructure engIneer: International Bridge Technologies Inc., San Diego, calif. constructIon engIneer: FINlEY Engineering Group Inc., Tallahassee, Fla. prIme contrActor: R. Zoppo corp., Sloughton, Mass. precAster: Unistress corp., Pittsfield, Mass., a PcI-certified producer profile This rendering of the DCR design shows the cross section of the precast concrete segmental "channel" bridge. Rendering: IBT. 32 | ASPIRE , Fall 2010 ASP10-1704.indb 32 9/17/10 2:06 PM