Design of a Steel Stop Log Gate and Basin Operations

Trevor Lighty, P.E.1 Andrew Bennett, P.E.2 Stuart Moore, P.E.3 1KPFF Consulting Engineers, 101 Stewart Street, Suite 400, Seattle, WA 98101; PH (206) 382-0600; email: tlighty@kpffspd.com 2KPFF Consulting Engineers, 1601 5th Avenue, Suite 1600; Seattle, WA 98101; PH (206) 622-5822; email: andy.bennett@kpff.com

3Kiewit Infrastructure West, 33455 6th Avenue South, Federal Way, WA 98003; PH (253) 943-4200; email: stuart.moore@kiewit.com ABSTRACT

In February 2011, construction began on the first dry dock to be built in Washington State since 1962. Eight months later the final piece, a three-part steel stop log gate, was lowered into place, sealing off the 4-acre basin from the waters of Grays Harbor in Aberdeen, Washington. This massive, stop log gate spans 33.5 meters (110 feet), is 9.1 meters (30 feet) tall and weighs 130 tonnes (300 kips).

Most dry docks utilize floating gates to seal the basins, but these large structures take hours to remove and require the use of tug boats and a local mooring site when not in use. They also need a ballast management and transfer system to raise and lower them as needed. In contrast, the three piece steel stop log gate can be removed or installed in under two hours. The steel gate was designed in separate sections to allow a 300-ton crane on shore to install and remove the segments.

This paper will focus on the specific design and construction challenges encountered with the steel gate.

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INTRODUCTION The existing State Road 520 (SR 520) Floating Bridge opened in 1963 to carry traffic across Lake Washington. The bridge utilizes over 2,100 meters (7,000 lineal feet) of floating concrete pontoons anchored by high-strength cables to carry four lanes of traffic to and from Seattle. The aging pontoons and anchors are vulnerable to windstorms and the bridge is required to be closed in high wind events. After years of alternatives analysis, the Washington State Department of Transportation (WSDOT) decided to replace the floating bridge. A 222,500 square meter (55-acre) project site was chosen in Aberdeen, Washington, for constructing the pontoons. Before work on the pontoons could begin, a new dry dock large enough for building multiple pontoons at one time had to be designed and constructed. The dry dock required a gate wide enough to allow removal of the pontoons after being constructed in a dry condition. Thirty-three concrete pontoons will be built in the dry dock over the a period of three years, with largest of the pontoons weighing over 9,990 tonnes (22,000 kips) and measuring 109.7 meters (360 feet) long, 22.9 meters (75 feet) wide, and 8.5 meters (28 feet) tall. Several unique concepts in the dry dock design were developed to promote efficiencies during pontoon construction and float-out. One of these was the design and construction of the three-piece steel stop log gate to reduce the float-out cycle time and reduce construction costs (see Figure 1).

Figure 1. Stop log gate concept.

CONCEPT DEVELOPMENT During design of the casting facility, several gate options were examined, including steel and concrete floating gates and several gateless options. Floating gates are the most common types used for dry docks constructed in Washington State.

STEEL SPACE TRUSS

CRANE ACCES TRESTLE FOR PICKING GATE

(3) PART STOP LOG GATE WITH RUBBER SEALS AT JOINTS 8’ x 12’ CONCRETE

JAMBS EACH SIDE OF GATE

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Traditional floating gates take on ballast and “sink” into the closed position to seal the graving dock. To open the graving dock, the ballast is pumped out and the gate is “floated” to an off-site storage location. The sinking and floating processes require a ballast management and transfer system and must be coordinated with the ebb and flood of the tides. To decrease the time required for pontoon float out, the stop log gate concept was developed. This gate consists of three individual steel space trusses with a maximum weight of 49.9 tonnes (110 kips) each to allow for removal by crane. This design approach allows the gate to be installed or removed in under two hours, significantly reducing the time required for pontoon float-out. The reduction in cycle time is important because the pontoons are not permitted to set back down on the dry dock floor after they float off. The reduction in gate removal time allowed for the total basin depth to be reduced by over 0.6 meters (2 feet), saving over 9,940 cubic meters (13,000 cubic yards) of soil excavation. To allow the gate to be handled using available cranes, a lightweight steel space truss was designed, using hollow rectangular steel sections, to support a stiffened plate barrier wall. These truss members were completely welded at the web connections to the main truss chords to prevent water intrusion and create full moment connections. The trusses then span over 33.5 meters (110 feet) to bearing points on concrete jambs. The barrier wall is a three-piece stiffened steel plate which holds back the waters of Grays Harbor. The barrier wall transfers over 7.6 meters (25 feet) of hydrostatic pressure, 44.7 m/s (100 mile-per-hour) winds, and 100-year wave event loads into the main truss sections. Assembled, the gate (trusses and barrier walls) can resist a 1,000 year return period earthquake including hydro-dynamic reactions with the retained water. GATE SEALING Two separate seals were used to prevent water intrusion into the dry dock. Perimeter seals (see Figure 2) are mounted vertically to the sides of the barrier wall and horizontally along the bottom of the wall. The perimeter seals react against stainless steel bearing plates set into the concrete sill and jambs, and prevent water intrusion around the perimeter of the gate. The second seal type, intermediate seals (see Figure 3), are mounted to the top of each barrier wall section to prevent water from flowing between each of the gate sections. Both seal types are constructed with molded natural rubber and a stainless steel plate is embedded in the base of the seal. The perimeter seals had to be flexible enough to seal under the differential hydrostatic pressure created between the outside and inside of the basin. The intermediate seals must seal under the gravity load of the gate alone. The seals utilize their compression to resist the applied shear due to the hydrostatic pressure at each seal location.

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Both seal types use seal stops to prevent over-compression, which causes excessive cracking or permanent deformation of the rubber and reduces a seal’s lifespan.

Figure 2. Gate perimeter seal. Figure 3. Gate intermediate seal. DISPLACEMENT COMPATIBILTY Due to the flexibility of the lightweight space truss, two differential displacements had to be considered. The first differential displacement issue occurs between truss sections. Since there is a seal between each barrier wall section, each truss must have a similar deflection wherever it is located. In addition, the plane between each truss section must be nearly vertical. Each truss section is designed to deflect the same amount despite the fact that the load is significantly higher on the bottom truss. Shear transfer fittings are used to transfer forces between the truss sections in both the longitudinal and transverse directions. These fittings are constructed of steel and Ultra High Molecular Weight (UHMW) faces, and also served to align each truss section during installation. There are 24 fittings between each truss section. The second differential displacement issue occurs between the barrier wall of the lower truss section and the fixed concrete sill. The flexible trusses will deflect under the hydrostatic loads; however, the barrier wall is not permitted to move at the seal-to- concrete connection. This issue was solved by de-coupling the lower barrier wall and the lower truss.

PERIMITER SEAL

SEAL STOP

GATE BARRIER WALL

UPPER BARRIER WALL

INTERMEDIATE SEAL

LOWER BARRIER WALL

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Figure 4. Belleville Springs Stainless steel Belleville springs were used to maintain tension in the pinned connections between the bottom of the barrier wall and bottom chord of the lower truss. Belleville springs are constructed via several opposing dish-shaped plates. These springs have a very low force-deflection coefficient, allowing the required deflection of the barrier wall without imparting a significant load into the trusses. As the trusses deflect and the lower barrier wall rotates, the threaded rod tensions, compressing the Belleville Springs. This allows the lower perimeter seal to remain in contact with the concrete sill. The springs were also used to compress the lower barrier wall seal during the first placement to account for the sill/gate tolerances. REDUNDANT DESIGN Each of the three gate sections is an individual three-dimensional space truce with four main chord members. Each truss, when viewed individually, is a fracture-critical, non-redundant system (see Figure 5). However, due to the addition of the shear keys, and the vertical tie rods joining the front and back of each truss, the system as a whole behaves as a single truss with twelve individual chord members. The shear keys are able to transfer any residual force to the next truss section and the tie rods prevent the trusses from separating or rotating vertically. Analysis was done to ensure that the truss would remain stable and not collapse if a main tension member failed. The fracture-critical analysis was completed by removing a single main tension chord at the center of the truss span where the tension in the chord is the highest. The trusses were then loaded with a Mean Higher High Water (MHHW) hydrostatic load and the member forces were examined. All

BELLEVILLE SPRINGS

BELLEVILLE SPRING TENSION ROD

FRONT TRUSS CHORD

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members and connections remained elastic when the failure of each truss was simulated.

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Figure 5. Three Piece Gate System. FABRICATION AND ASSEMBLY The individual truss sections were fabricated under cover at Jesse Engineering in Tacoma, WA, using standard welding and assembly processes.

To ensure quality control of the welding of the closed sections, pressure tests were conducted on selected members to demonstrate that were no leaks that could allow seawater to enter the section and cause corrosion. The test ports were left in place to allow future testing if necessary. Although the barrier walls were designed to withstand significant hydrostatic pressure, they were relatively flexible and great care was required to move them from the fabrication floor into the paint shop and then to lift them into position on their respective trusses. The stainless steel Belleville springs had a long lead time and were not available at the time of initial assembly. To finish the gate, temporary Belleville springs, made of more common carbon steel, were delivered. The temporary springs had the same performance characteristics as the designed springs, but the carbon steel did not have the same expected service life, particularly in the salt spray environment of Grays Harbor.

UPPER GATE ASSEMBLY

GATE VERTICAL TIE RODS

SHEAR TRANSFER FITTING

BELLEVILLE SPRINGS

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A test fit-up of all three sections was conducted at the fabricator’s yard. The three pieces fit together well and the shear keys performed as expected to simplify the alignment during assembly (see Figure 6).

Figure 6. Gate Initial Assembly & Fit-Up in fabricator’s yard.

After the test fit-up, the gate was disassembled and each of the three sections (truss plus barrier wall) was loaded onto a special trailer. During transit, special supports were fabricated to stabilize the cantilevered Belleville springs. Due to the length and width of each section, rolling road closures were required to transport the sections the 137 kilometers (85 miles) between the fabrication yard in Tacoma and the job site in Aberdeen. Given the length and weight of the load, the fully loaded trailers could not travel at normal highway speeds, so the delivery trip was made in the middle of the night to minimize its impact on other highway users. INSTALLATION Because the excavation and dredging of the waterway access channel was not complete at the time of the initial installation, the basin was dry and both sides of the gate were visible. As a result, the operators were able to carefully guide the gate into position, almost perfectly centered between the jambs. The hydraulic rams were able to move the gate into final position and push the gate back out as needed to make the final adjustment to the shim pads that controlled the compression of the vertical seal (see Figures 7 and 8). With the gate in the closed position, the uncompressed length of each Belleville spring was measured and recorded on the compression plate at the inboard end of the spring. After recording the uncompressed lengths, the Belleville Springs were adjusted to achieve the desired compression of the bottom perimeter seal, using the seal stops on the outside of the seal as guides. The compressed length of each spring was then recorded and the force in the springs was calculated and compared to the design values, with good agreement between the two.

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Figure 7. Truss #2 being installed. Figure 8. Gate hydraulic cylinders on Truss #1.

Once the excavation of the basin access channel was complete and the waters of Grays Harbor were being held back by the barrier wall, a small amount of weepage appeared at several locations on the gate. Although there was visible weepage, it was generally less than that observed occurring on more traditional caisson gates at other dry docks in Puget Sound. OPERATION Given the restrictions in the pontoon construction specifications, which include a prohibition on the pontoons touching down on the basin floor once they floated off, the gate removal, pontoon float-out, and gate reinstallation had to be completed within a 14 hour window. The first pontoon float-out and gate removal occurred on 30 July and 31 July, 2012. The basin flooding was started on a rising tide at about 09:00 on 30 July 2012, and the gate was opened by pushing it away from the sill and jamb at approximately 16:30, shortly before the following low tide. If the pontoon with the deepest draft, Pontoon W, was not out of the basin by 03:30 on 31 July, it would touch down as the tide receded. To ensure this did not happen, and allowing one hour for each pontoon to be towed out of the basin, the gate had to be removed by 20:00, which gave the gate removal crew 3.5 hours to install the gate guides then rig, lift, land, and unrig all three sections. Figures 9 through 12 illustrate the removal and re-installation process.

To ensure the gate removal was done safely and on schedule, a removal procedure was developed using 15 minute increments to schedule all events, which used a Liebherr LR1300 mobile crane for all gate section movements. The gate removal and re-installation planning also included preparation of an environmental analysis, job hazard analysis, and fall protection work plan.

The top section of the gate was removed with very little sediment on the barrier wall stiffeners. The middle and lower barrier wall sections had significantly more sediment trapped on the barrier.

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Structurally, none of the three sections had any visible damage. The shear keys showed very little wear and even the UHMW bearing pads used to maintain alignment between the jambs showed very little wear, due in large part to the care taken by the rigging crew. Inspections of the gate by the engineer and contractor showed that no major repairs were necessary and that the gate could be reinstalled when the tide started rising again.

During the inspection, small particles of sand or rock were observed between the leaves of the Belleville springs. Because this grit could cause local deterioration of the spring leaves, it was agreed that it would be washed out prior to reinstallation. Overall, no damage or deformation of the springs was observed other than minor discoloration due to surface rust. It was recommended that the springs be carefully inspected again prior to the second float-out to determine whether the permanent stainless steel springs would need to be installed prior to the third float-out cycle.

Figure 9. Bottom gate section being removed.

After the last pontoon was towed from the basin, and before the gate was reinstalled, a fish screen was placed across the entrance to the basin. Three small boats towed a net down the length of the basin to move any fish that had entered the basin into the fish trenches near the dewatering pumps. From the trenches, the fish could be caught and returned to Grays Harbor, outside the basin.

Once the basin was cleared of fish, the gate sections were lowered back into place, starting around 03:30 on 31 July 2012. The shear keys and UHMW guide blocks worked as designed and the gate assembly was completed slightly ahead of schedule, at approximately 07:30.

Once all three pieces of the gate were reinstalled, the closing hydraulic cylinders were extended, bringing the perimeter seal in contact with the sill and jambs, then compressing it. Immediately, the water that had been swirling around the gate from the rising tide stopped. Shortly thereafter, the dewatering pumps were started to

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empty the basin in preparation for building the second set of pontoons. By 08:15, there was approximately a 0.5 meter (18 inch) difference between the water level on the outside of the basin and that inside the basin. The seals appeared to be working effectively and the dewatering process continued until the basin was empty. After the basin was emptied, the screw jacks were engaged and tightened against the back of the jamb and the vertical tie rods were put in place and tightened down, thereby locking the gate in place and securing it until the next pontoon float-out cycle.

Figure 10. Gate sections on shore for inspection.

Figure 11. Bottom section of gate Figure 12. Gate re-installed. being re-installed. CONCLUSION

The steel stop log gate has performed as expected to date. The gate has been in place for approximately 18 months, and has been cycled for float out one time. The gate lightweight design allows for easy removal and assembly, significantly reducing the time that the dry dock will be flooded. The next pontoon float out is scheduled for May 2013.

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