The 2007 Narrows Bridge, being constructed near Tacoma, Wash., will be supported by hollow reinforced concrete towers rather than steel towers as in the adjacent bridge. However, rebar lap splices are prohibited in numerous portions of the new towers due to seismic considerations. That restriction, combined with congestion and weight concerns, led engineers to use rebar couplers that save space, speed construction, and maintain the full strength of the reinforcing system.
A major difference between the previous two Tacoma Narrows bridges and the one under construction now is the towers. The new concrete towers are tapered, hollow-box structures that will rise 510 feet above water level.
Tacoma Narrows Constructors' type of concrete required special construction considerations. According to Reg Carson, the tower manager with TNC, the silica fume concrete being used has a very high viscosity, which causes excessive form pressure. Carson says, “We were shooting for 20-foot lifts when we started this project, but the [form] manufacturer had never exceeded 13 feet for concrete” with this particular system. “We worked with [the manufacturer] to design a form system to allow us to go to a height of 17.5 feet,” Carson continues.
The height of the forms and the tapering of the tower necessitated mechanical rebar splices. With a 35-foot rebar cage, lap splicing would have increased the cage size by almost a third and also would have caused a weight concern with the tower crane, according to Carson. Using couplers was seen as a way to reduce both congestion and weight.
Another challenge was the plastic hinge zone. “This area of the towers,” Carson says, “will see the maximum loading during a seismic event.” These no-lap-splice zones occur in six locations on each tower leg. To meet the “bar-break remote” requirement in these zones, the bars either had to be welded or joined with ultimate couplers. Carson says couplers were picked over welding to save cost and time.
The LENTON Plus taper-threaded couplers from Erico Inc., Solon, Ohio, being used on this project install in approximately four turns and are some of the slimmest couplers available.
A standard LENTON coupler is a mechanical taper threaded splice that develops the minimum specified ultimate strength of the rebar being spliced. The LENTON Plus taper threaded coupler ensures bar break remote every time by slightly cold forming the end of the rebar prior to threading and using a slightly longer sleeve to engage more threads on the reinforcing steel. The result is equivalent to an unspliced section of reinforcing bar.
The $849 million 2007 Narrows Bridge will have three 12-foot lanes, two 10-foot shoulders, and a 10-foot pedestrian/bike path. Plus, the bridge is being built to accommodate a second deck or light rail in the future if additional capacity is needed. Once the new bridge opens, the Current Narrows Bridge will be reconfigured and receive seismic upgrades expected to take 9 to 11 months.
Environmental concerns pose additional challenges on the project, according to Erin Babbo, public affairs manager for TNC. “We're building a huge structure underwater in 15-foot tidal swings, 7-knot currents 60 feet from an existing structure,” she says. In addition, TNC is continually battling gusting winds that can, at times, shut down construction. Nonetheless, TNC is keeping the project on schedule and on budget. When the new bridge opens in 2007, it will significantly improve traffic flow and reduce the three- to four-hour backups that commuters are currently facing.
—Material for this article provided by Julian Abrigo and Laurel Modic, Erico, Inc., Solon, Ohio.