DESIGN & CONSTRUCTION TEAM

Owner
Hudson River Park Trust

Architect
Dattner Architects/MKW + Associates Landscape Architects, New York

Marine engineers
DMJM+HARRIS, Inc., New York

Construction manager
HPA Engineers PC, Long Beach, Calif.

Contractor
Spearin, Preston & Burrows, Staten Island, N.Y.

The floating bridge at the Hudson River Park

By Boris Levintov, P.E.; Abbas Sarmad, P.E.; and Beth Greenberg, AIA

The development and growth of New York Citys west side was outlined in a 1997 Master Plan to create a 5-mile park along the Hudson River waterfront, with free access to the river extending the use of the park from the land onto the water. The design team of Dattner Architects / MKW+Associates Landscape Architects, with marine engineers DMJM+Harris, was selected to design the northern portion of the park. Among the elements of waterfront recreational activities along the Hudson River shoreline at pier 96 (56th Street) are a boathouse and floating bridge to a boat-launching pontoon for kayaking and canoeing.This unusual bridge was designed and constructed to connect a new boathouse pier and boat-launching pontoon (shown here) in the Hudson River Park.

The boathouse, on a new pier, is a long-term storage facility with a floating pontoon from which canoes and kayaks are launched. The facilities are connected by a bridge, supported at the waterside by the pontoon and at landside by the ledge of the pier deck.

Challenges

The waterfront bridge support is in permanent motion from tidal variations, waves, and boat wakes, while the landside support is stationary. Simple in concept, however, the engineers faced numerous challenges in the execution.

American Disability Act (ADA) guidelines - As a general rule, bridge and pontoon slopes may not exceed 8.3 percent (1:12). But, according to ADA guidelines for the marine facilities of the proposed size and water fluctuation, the maximum slope may be increased to 12.5 percent (1:8). The fixed-pier elevation established that bridge support on the pontoon had to be 3 feet above the water level to satisfy ADA requirements at mean low water. For the same reason, the pontoon structure at the bridge support had to be recessed to receive the bridge frame, providing smooth transition (no steps) from the bridge to the pontoon.

Water Club (tenant) requirements - The facility was designated for kayak and canoe service and launching. Oneperson outrigger canoes and kayaks are relatively small in size and easy to accommodate. However, six-person outrigger canoes that are 45 feet long and 8 feet wide presented a real challenge.

The design provides room for placing at least three such canoes on the pontoon and bridge during their preparation for launching. A 60-foot-long bridge configuration (which tapers from 45 feet wide to 17 feet wide), trapezoidal in plan, and a 45-foot by 50-foot pontoon were the most rational solution to provide ample staging room and a generous space for group instruction.

However, the slope necessary for ADA accessibility proved insufficient to achieve the low freeboard requirement at the launching area. The solution was to provide a step down launching platform, which effectively reduced freeboard to an acceptable level. By this arrangement, the pontoon area was divided into the staging and get-down docks.

In conventional design, a pontoon of the proposed size would be held in place by four spud piles placed close to the corners. This solution turned out to be unworkable because launching sixperson canoes from the pontoon's three, river-facing sides requires open area without any obstructions. The only place available for spud piles was on the land side next to the bridge. The two spud piles did not allow adequate redundancy, so the designers used the bridge structure to hold the pontoon in place in case of pile failure.

Tidal variations - Regular changes of tide levels in the range between mean higher high water (MHHW) and mean lower low water (MLLW) were in the limit of 5 feet. But the high observed water level (HOWL) exceeded the MHHW level by more than 5 feet. At this level, the pier will be under water and the floating pontoon may lift the bridge off its supports. A sloped recess in the pontoon's land side was required to accommodate the bridge at this level.

Design loads - The bridge structures and pontoon were designed for 75 pounds per square foot (psf ) of live load.

In addition, dead load, waves, current, passing vessel wakes, and ice were included into the structural analysis.

Wave heights that can reach the site are limited by the fetches (the open water distance available for wave generation in the area). Fetch calculations were performed and parameters of wave heights and associated periods and length were established. The results were graphically analyzed and a 5.7-foot water head was recommended for the pontoon side and bottom shell design.

Vessel wakes resulting from the passage of ships or barges were on the order of 2 to 4 feet, which is less than from fetch.

Bridge structure

Two, 62-foot-long tubular trusses on the outside bridge borders are the main load-carrying members. They are interconnected by the floor beams, which are braced by diagonals. Moment connections are provided between floor beams and truss verticals at the truss bottom chords. A combination of truss and frame structures was created to carry loads in both horizontal and vertical planes. Frame action between truss verticals and associated floor beams was necessary to account for lateral buckling forces at truss top chords and for horizontal wind load on the bridge trusses.

The bridge is positioned on four wheel supports located at the frame corners. On the land side, the supports are on the concrete pier ledge, and on the river side, the support is intentionally at the float's center line, where a recessed horizontal platform is provided. With this arrangement, any pontoon rolling associated with waves or river vessel wakes in the east-west direction does not affect the bridge, and only pontoon pitching has to be accounted for. Such arrangement also eliminates possible pontoon listing from bridge dead and live loads.

Another important element of the bridge support system is a vertical pin connection on the land side located on the same center line as the wheels. The pin assembly affixes the bridge structure to the pier ledge, and any horizontal translation from the bridge angle change related to tidal fluctuations takes place on the pontoon side. An ample space is provided at the recessed platform within the pontoon for these bridge movements.

However, bumper restrainers have been installed on the platform at all four sides to limit excessive bridge movements.

All shear forces from the pontoon in the east-west direction will be transmitted by the bridge structure to the corner bumpers bearing against a pier concrete wall or to the pin. In the north-south direction, shear forces will be transmitted by floor beam trusses to the pin. A combination of right or left land side corner bumpers and the pin will resist the moments.

First, a 3-D analysis of the bridge checked the dead and live loads acting in combination with wind, wave, and current loads acting on pontoon. The engineers assumed that the bridge was standing on four corner supports.

However, in case of pontoon pitching, displacements of bridge water-side supports may result in twisting of the bridge structure. The engineers estimated conservatively that one of the supports on the pontoon may be up to 18 inches higher or lower than the adjacent support. The structure was checked under full dead and live loads with 18-inch displacement of one of the water front supports.

For long-term protection, all steel elements of the bridge were hot dip galvanized.Tubular, welded trusses were prefabricated and galvanized as completed units. Bolted splices were provided at quarter points of floor beams for assembling bridge parts after galvanization.

IPE timber - an incredibly durable Brazilian hardwood - bridge decking (both planks and joists) was the selected finish and was placed on the floor beams.

Pontoon structure

The deck configuration, sloped on three waterfront sides and recessed on the land side, complicated the design of the pontoon steel structure. The pontoon hull was divided by watertight bulkheads into six compartments accessible by manholes. A desirable freeboard, including pontoon tilting, may be achieved by individual ballasting of compartments. Initial freeboard was intentionally established higher than required. All bottom and side shells are 0.375-inch-thick plates stiffened by angle ribs spaced at 2-foot intervals.

Diagonal trussing is provided along the sloped deck intersections.

A step-down launching platform with IPE timber deck on angle brackets is provided at the waterfront sides, and is faced with timber fenders and equipped with cleats. A stainless steel perforated shield at the front of the platform prevents capsized boaters from getting under the pontoon. Detachable pile guide assemblies are affixed to the land side of the pontoon.

Zinc anode weights are attached to the hull bottom shell enhancing corrosion protection. These anodes resist the formation of hard, dense corrosion products and continue producing protective current until complete consumption.The extended service life results in fewer replacements and reduced operating cost.

The pontoon deck, similar to the bridge deck, is wood planking on sleepers.

Conclusion

In order to satisfy the architectural vision, the functional program, and the specifics of the Hudson River site, considerable engineering ingenuity was required for these structures. Though not readily apparent, this engineering expertise has contributed to the success of the project and proven rewarding to the designers.

Boris Levintov, P.E., is a principal engineer and Abbas Sarmad, P.E., is a vice president of DMJM+HARRIS, Inc., in New York. Beth Greenberg, AIA, is a principal of Dattner Architects in New York. Levintov can be reached at boris.levintov@dmjmharris.com.