Port of San Diego chose to replace dock with precast concrete modules.
By David Holzel
The Ferry Landing Marketplace in Coronado, Calif., is a waterfront shopping and dining area with a beautiful view of downtown San Diego, a mile across San Diego Bay. Until recently, a wooden floating dock received pleasure craft that tied up for the day in front of Peohe’s Restaurant, a local landmark. But when the dock had deteriorated to the point it needed to be torn down, the Port of San Diego chose to replace it with a floating dock assembled from precast concrete modules.
The owners are clearly thinking of the long term. A precast concrete dock is more durable than timber or timber-concrete systems, and requires a fraction of the care a steel dock needs. “They tend to be very stable and very durable,” says Jim Trefren, a project engineer with the Port of San Diego. “They don’t deteriorate. They don’t rust.” And the modular design allows a few standard-sized sections to create a variety of configurations at many load-carrying capacities.
The Port of San Diego is relying on all of the precast system’s advantages to counteract the rough conditions that led to the timber dock’s demise. “The wave climate in the bay is quite severe,” Trefren says. “It’s an open bay, not like in a marina. There are a lot of swells from passing ships and a considerable amount of wind waves.” The new dock was designed to withstand wave heights of 3 feet and wave periods of 3.1 seconds. It’s able to do so because it uses a system of 50 wave-attenuation panels that calms the water to such an extent that it allowed the Ferry Landing to become the first wheelchair-accessible dock on the bay.
On the waterfront The dock is shaped like a five-fingered E, with a 170-foot-long walkway that looks over the bay to San Diego. The finger piers themselves extend from the walkway toward the Ferry Landing. The outside fingers measure 48 feet long by 10-feet-6-inches wide. Two small inside fingers are 30 feet long and 6 feet wide. A 60-foot aluminum gangway with fixed ramps on both sides connects the decking on the Ferry Landing to the dock’s fifth finger, which is 45 feet long and 13 feet wide. Lights line the walkway and 24 20-inch-square precast/prestressed concrete piles provide spots for mooring.
The system consists of nine monolithic precast and prestressed concrete dock sections measuring 5 to 12 feet wide, 36 inches tall and up to 60 feet long. The sections weigh up to 55,000 pounds each. The 50 precast wave-attenuator panels, weighing 4,400 pounds each, are attached by rods to the outboard side of the dock system and hang 6 feet below the system. The precast sections were constructed of concrete around a foam core.
“The structural strength of the section is significant in both vertical and lateral loading directions for either negative or positive moments and shears,” Concrete International magazine wrote about a similar precast design. The saltwater of the bay adds to the floating dock’s buoyancy. But it also encourages corrosion. To compensate, the concrete products used in the dock were mixed with corrosion inhibitors, according to Gary Taylor of Oldcastle Precast, Inc., which supplied the piling and floats for the project. Oldcastle manufactured the modules in the spring of 2000. The entire project was then staged and shipped to the waterfront. The system was assembled in two weeks, and the mechanical portion, including plumbing and electrical work, took another month. Installation was complete for the ribbon-cutting ceremony June 28, 2000. The total cost of the installed project was about $700,000. The cost to produce the precast modules was $340,000.
Steady as she goes
“The most important key to success for an effective concrete dock system is to provide strong and fatigue-resistant connections,” according to Moffat and Nichol, the Long Beach, Calif., firm which performed the engineering evaluation of the system used at the Ferry Landing. The manufacturer estimates the dock’s lifespan will be 25 years. In that time, the greatest wear and tear will be on the bolt connections. Simply replacing them will extend the system’s life at a fraction of the cost of replacing the entire system. For those used to designing and building projects on terra firma, construction on or in water requires its own set of skills.
A 10-person crew from Marathon Construction Corp. of San Diego assembled the dock using a 40-ton-capacity crane on the shore and a 235-ton-capacity crane aboard a floating barge. “We did the bulk of the work with that (barge) crane,” says Dave Cunningham, Marathon’s project engineer.
“The large crane suspended the dock in a partially submerged state. All of the wave attenuation panels – six to eight per section – were installed this way,” he continues. “Before the large crane released the dock section after all of the panels were mounted, we tested the dock section by lowering it to see if it floated level or listed to one side.” Most of the sections initially had some list, says Cunningham, but the crew corrected that by repositioning the added flotation blocks. This could only be done by lifting the dock section back out of the water to the point where the foam blocks could be pushed around. This was repeated until each section floated level. “It was a fairly intricate little dance.”
Because of the rough conditions at the Ferry Landing, the crew first created a calm-water site nearby to begin assembly. “We utilized one of the quay walls at another Port terminal across the bay. All of the dock components were delivered to the water’s edge there,” Cunningham says. “We created the calm water by positioning our floating crane and barge such that a 15-to-20-foot work area existed between the barge and the quay wall.” A second barge was placed against the quay wall to the bay side of the crane barge to block the wave action from the bay. The fourth side of the newly created rectangular work area faced shore and was left open so that workers could move the assembled components in or out as needed.
Once the dock was assembled, the crew towed it to its permanent site to be installed. “The long outer edge was installed first,” Cunningham says. “Our barge then moved around between the shore and this section. Each finger was then installed one at a time.” The sequencing was such that the finger at the far end, which had wave-attenuation panels, was installed first. This was essentially a rough-water installation.
“Next we moved the barge back to install the next section,” Cunningham continues. A calm water area was created in front of the barge because the installed long outer section and the first finger both had wave-attenuation panels, which calmed the water. The barge itself calmed the water from the other direction and again the only open side was toward the shore. The second finger installed was designed to accommodate the gangway. The gangway was installed once the second finger was in place. “All work prior to this only had access via water. The gangway provided shore access,” he says. When the Ferry Landing dock opened, it was touted as the first in San Diego Bay with barrier-free access for people using wheelchairs.
“When we built this, there were no adopted guidelines,” Jim Trefren says. So the designers followed guidelines proposed under the Americans with Disabilities Act. The proposed guidelines called for 60-foot gangways with fixed ramps on both sides, and slopes of 1 inch per 12 inches or less, at least 92 percent of the time, Trefren says. The specifications ultimately adopted were looser than what the designers used at the Ferry Landing, which would have made the gangway “less accessible a greater amount of the time,” he adds. “What you’ve got is a very stable floating dock. It’s not bobbing up and down like a cork. That’s important if it’s a wheelchair-accessible dock.”
The Ferry Landing dock will not be the last precast concrete system built in San Diego Bay. Five additional docks already have been built and another three are under construction, Trefren says. Most are public boarding or dinghy docks. The Harbor Police and the Customs Service use one; another offers access to a deck used by the San Diego Maritime Museum. When mariners decades from now step out of their boats and onto one of these docks, they’re likely to notice the stability of the landing and few signs of age. At the Ferry Landing, the scene on shore may be completely different, but the dock itself, with its five fingers and 50 wave-attenuation panels, will still be doing its job.
Project: Floating Dock at Ferry Landing Marketplace, Coronado, Calif.
Owner: Port of San Diego, Port Coronado Associates
Contractor: Marathon Construction Corp., San Diego
Precast Company: Oldcastle Precast Inc., Fontana, Calif.
Leave a Reply