Straightening a slouch

April 18, 2005

In February 2004, scaffolding supporting a 105-ft concrete span of the under-construction Clearwater (Fla.) Memorial Causeway Bridge buckled, and the span dropped 7 in. at one end. The concrete was sufficiently cured that it sustained no significant damage, but the challenge was to move the span, weighing hundreds of tons, back to where it belonged.

In February 2004, scaffolding supporting a 105-ft concrete span of the under-construction Clearwater (Fla.) Memorial Causeway Bridge buckled, and the span dropped 7 in. at one end. The concrete was sufficiently cured that it sustained no significant damage, but the challenge was to move the span, weighing hundreds of tons, back to where it belonged.

Getting jacked up

The 55 x 105-ft south span between piers 7 and 8 dropped 7 in. at the pier 7 end when falsework buckled. The bridge contractor installed six shoring beams transverse to the span, resting on a combination of 100 kip and 25 kip shoring towers. With the situation stabilized, Enerpac was presented with the challenge of pivoting the low end of the span back to its correct position.

Bridge contractor staff provided drawings and data indicating appropriate lift points and required forces. Within 10 days of go-ahead, Enerpac delivered a system employing 24, 100-ton jacks. The system design used eight primary jacks under computer control to perform the synchronized, proportional lift. The remaining 16 secondary jacks provided necessary additional support at specified points.

Because the contractor required a 10-day delivery on a complete system, there was not sufficient time to develop a complete 24-point PLC controlled proportional lift system. Therefore, Enerpac needed to devise a system from in-stock components that would provide the accuracy and control required to raise the span. To accomplish this task a hybrid system comprising the Enerpac Synchronous Lift System and dual-flow pumps was developed. The strategy was to balance the span on 16, 100-ton cylinders while using eight PLC-controlled, 100-ton cylinders to provide the additional force and control to lift the span.

Jacks were distributed over the six shoring lines, four to a line. The 24, 100-ton jacks ultimately supported the weight of the entire span, delivering a total force of approximately 800 tons at 3,350 psi hydraulic pressure. (A safety margin of 2:1 or better between jack rating and maximum expected load is the norm for this sort of work.) The jack cylinders were equipped with lock nuts for added safety.

The primary lifting was provided by the eight jacks on shoring lines C and F under control of a synchronous lift controller. However, a predetermined portion of the span weight was supported by the other 16 jacks, which were of identical size but supplied with slightly (about 2%) lower hydraulic pressure. The desired vertical movement of the span was zero at the pier 8 (“hinge”) end and approximately 7.6 in. at the pier 7 end.

Although supplied by a single hydraulic source, the eight primary cylinders were individually controlled by the PLC-based lift controller. Feedback to the controller came from a stroke length (displacement) sensor associated with each jack cylinder. The controller regulated hydraulic flow to each cylinder via poppet valves.

Shoring line C is approximately half way from the stationary, hinge end of the span, and shoring line F is close to the maximum-lift end. Thus, the amount of lift required at shoring line F was about double that required at shoring line C. In order to achieve the appropriate 2:1 proportional lift between shoring lines C and F, Enerpac used two different 4-20 mA stroke sensors. Shoring line C used sensors having a 500-mm range at each jack, and shoring line F used 1,000-mm sensors. This stratagem caused the PLC to think it was lifting uniformly, even though it was actually lifting at a 2:1 ratio.

The 16 primary cylinders were supplied in eight groups of two, using four dual-flow pumps. Each dual-flow pump provided two independent outputs with manually operated control and adjustable relief valves. Each of these eight outputs fed a pair of cylinders. The independently controlled cylinder pairs afforded the ability to alter the side-to-side cant of the span, should an unplanned course of events make that necessary.

Proper resting place

Concrete debris had to be removed from the topside crack at the hinge end of the span to allow for the crack to re-close as the span returned to its original position. Also, as a precautionary measure, the jacks were pressurized sufficiently to apply a preload to the span. Lock nuts were set, and the entire setup was left as-is for two weeks. No problems were noted, so the lift proceeded.

Five people controlled the system components: one at the synchronous lift controller and four at the dual-flow pumps feeding the support cylinders. Hydraulic pressure was raised in increments of 2.5% of the calculated value needed to lift the span, alternating between lift and support cylinders. A surveyor continuously monitored the position and orientation of the span. At each inch of lift progress, the cylinder locknuts were set and the process paused while roadway and waterway traffic were allowed to pass beneath. The lift cylinder stroke sensors assured that the support cylinders did not carry a disproportionate share of the load and begin lifting the span themselves. Such an event would be evidenced by the sensors indicating a span displacement without the lift cylinders having been advanced.

The time devoted to actual lifting was about one-half hour, although pauses for traffic stretched the overall operation to about five hours. Now the span once more rests in its proper location.

Another one on the way

The barrier island of Clearwater Beach seasonally attracts a large amount of retirement and recreational traffic. The traffic volume is presently carried across Clearwater Bay from the city of Clearwater on the mainland by a low-rise causeway supported by closely spaced piles and equipped with a drawbridge. This bridge is old, blocks views up and down the bay from the waterfront, and its drawbridge periodically gets stuck in the raised position, resulting in traffic jams.

The Florida Department of Transportation (FDOT) had planned to replace the old bridge at the end of this decade with a “plain-Jane” girder bridge. With Clearwater Beach serving as the city’s main attraction, officials decided that the schedule should be expedited and the bridge should be a signature structure that tourists would find memorable. Partnering with FDOT, the city funded right-of-way acquisition and a bridge design.

The design presents a graceful, cast-in-place segmental concrete bridge that soars 74 ft above the high-tide water level. Wide pedestrian walkways with periodic scenic overlooks will add to the aesthetics. The 2,340-ft-long bridge consists of nine twin box girder spans resting on eight sets of piers. Four of the spans are being constructed as balanced cantilevers using form travelers, and five are constructed on falsework.

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