Imagine taking one of the tallest buildings in the city of Charleston, putting it on a floating barge and maneuvering it across Charleston Harbor, under the Arthur Ravenel Bridge and up the Wando River to Mount Pleasant.

Then imagine making that trip twice, and you'd have a fair idea of the task the State Ports Authority is preparing for -- to move two "super-post-Panamax" container cranes from the Columbus Street Terminal in Charleston to the Wando Welch Terminal in Mount Pleasant.

Each of the towering cranes is 236 feet tall, or 32 feet taller than the nearby 18-story Dockside condominiums. Each crane weighs about 1,500 tons and they are nearly the tallest structures on the peninsula; only St. Matthew's Church reaches higher.

"This is the first time we've moved any this tall," said David Smith, the

SPA's chief port engineer.

When they are running all-out, those cranes can move loaded containers the size of tractor-trailer bodies from a ship to a waiting truck (or truck to ship) in about 90 seconds, guided by a single operator perched in a small cab 16 stories up.

Now that the SPA has shifted its container operations to the Wando Welch terminal, that's where the biggest, fastest cranes are needed. Moving the pair of them will cost about $5 million, which is about half the cost of buying one new one.

For several weeks, crews have been working to prepare the cranes. They are far too tall to fit under the Ravenel Bridge, so the apex and supporting structures on the top of the cranes must be taken down, with supports added to brace the structure. The apex is the top section, which is painted white.

Even after being partially dismantled, the cranes will have about 15 feet of wiggle room -- at low tide -- to squeak under the bridge, which has about 203 feet of clearance.

"I think it's extraordinary," said William Laurence, a project engineer with Shaw GBB, the company contracted for the move. "You're picking up 1,500 tons and moving it."

The first of the two cranes is expected to make the 3 1/2-mile crossing during the next several days. The second will follow about a week later.

Once the cranes are in place, there will be 11 cranes at the Wando terminal, which can handle as many as four container ships at one time. Of those, eight will be able to load and off-load the largest, super-post-Panamax class of vessels.

"Big-ship traffic in Charleston continues to increase, so positioning these assets at Wando helps us over the long term," said Allison Skipper, the SPA's public relations manager. "In a port that prides itself on productivity, this is a big plus."

Post-Panamax and "big ship" means container ships that currently can't squeeze through the Panama Canal, a vital shortcut for the shipping industry.

These monstrous vessels can carry the equivalent of more than 8,000 20-foot containers, a common industry measurement known as a TEU, or "twenty-foot equivalent units."

Several ships capable of carrying more than 9,000 TEUs call on Charleston now, and even larger ships are expected after the Panama Canal expansion is completed in about two years.

Currently, all the cranes at Wando can handle the largest ships that call here, which are 18 shipping containers wide. The super post-Panamax cranes can handle ships 22 containers wide, the kind of ships Charleston expects to see in the future, and the reason why the SPA is pressing hard to have the shipping channel deepened.

When the crane move is completed, the SPA will have 11 cranes at Wando Welch, six at the North Charleston Terminal and three at Columbus Street Terminal.

Columbus Street is now primarily a noncontainer terminal best known for its role in exporting BMW vehicles that are made near Spartanburg, delivered by rail to Charleston and exported around the world.

Reach David Slade at 937-5552.