Pier Groups
A floating precast concrete dock
system shows its strength in the rough waters
of San Diego Bay.
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 Profile
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.
