Ocean Tower One

Engineering News Record: June 12, 2000 – ”IMPLOSION SPARES FOUNDATIONS”

By Nadine M. Post, with Richard Korman

Several missions were accomplished during a June 2 implosion of a condemned 12-story concrete skeleton at a partially complete condominium complex in Key Biscayne, Fla., say sources. The 9:30 a.m. “shoot” of Ocean Tower One, two months after a job site fire that caused the failure of much of the structure, did not damage any nearby buildings. And preliminary data indicates the blast successfully brought down the 42,000-ton frame without compromising the existing auger-cast pile foundation. The intention was to salvage the foundation for a replacement-in-kind building, says the general contractor.

“Our feeling is that the foundations should be fine,” says geotechnical engineer David K. Miller, an associate in the Bethesda, Md., office of Schnabel Engineering Associates Inc. The firm is a consultant to implosion contractor, Controlled Demolition Inc., Phoenix, Md., which installed vibration and other monitors on the piles. A final decision about the condition of the foundation, however, will be made by a group of engineering consultants retained by the owner, he says.

Blast data also will be used to help CDI predict blast and debris-impact vibrations beneath future implosions, says William J. Murphy, a Schnabel principal. Murphy says this could help in planning implosions of structures in urban areas, which might have underground utility lines and vehicular or transit tunnels running beneath them.
Ocean Tower One’s structural system consisted of a ground-level supported slab, reinforced concrete shear walls in both directions for wind resistance, reinforced concrete columns and post-tensioned concrete flat slabs, says the project’s North Miami, Fla.-based structural engineer, Herbert L. Gopman Consulting Engineers Inc.

Troubles at the jobsite started on the evening of April 6. John Hinson, president of Coral Gables, Fla.-based developer Ocean Club Development Co., says he believes the local fire department will determine that the fire started in a pile of construction debris, left in the adjacent parking garage. The likely source of ignition was a cigarette.
Eugene Santiago, chief building official of the Village of Key Biscayne, says high winds fanned the flames. They jumped to a tool and equipment shed on the second level of the main structure, and from there to wooden reshores and oiled formwork on the third level, near a “pour strip” about 120 ft from the east end of the building. The strip was one of three at each level of the 433 x 90-ft plan, left out during casting of the 8-in.-thick slab. Voids eliminate expansion joints and allow the slab to be post-tensioned in sections, says Gopman.

The fire visibly affected a total width of structure of 40 ft around the pour strip, to the first full-width shear wall west of the pour strip, says Gopman. However, heat caused the tendons at the pour strips to release tension. That triggered progressive failure of the post-tensioned slab well beyond the zone of visible damage. Most half the slabs on levels three to six, and possibly seven, lost integrity, says Gopman.

Santiago ordered the owner to shore the building within seven days of April 7. A week later, he says, Ocean Club told him that no shoring contractor would venture into the damaged structure.

Key Biscayne declared the structure unsafe on April 14th and issued a demolition order April 20. According to Santi-ago, the South Florida Building Code requires the owner to demolish any damaged structure whose cost of repairs exceeds 33% of the structure or 50% of the value of the completed structure. In this case, the general construction contract with Morse Diesel International Inc. was about $46 million, says Ocean Club.

According to Ocean Club and CDI property insurer Reliance Group Holdings Inc., New York City, failed several times to have the demolition decision reversed. The owner, which wanted the structure razed quickly so that rebuilding could begin, called in CDI. It then developed a U.S. Occupational Safety and Health Administration-compliant safety plan to permit its forces to enter the condemned structure, says CDI President Mark Loizeaux. Crews took six days to install temporary emergency shoring that would allow implosion preparations, he adds. Loizeaux says his firm was challenged in preparing the structure to a 90% level for implosion without doing irreversible damage to the building while Reliance investigated the fire and went through its appeals. Further complicating CDI’s work was the compromised slab system, which prevented the modeling of load transfers during the shoot and forced a shorter delay sequence of 4.5 instead of 9.5 seconds. A longer shoot would have allowed smaller pieces of debris, says Loizeaux. That, in turn, would have reduced ground vibrations and air overpressure.

Crews placed explosives in more than 2,000 holes drilled in shear walls and columns in the lobby and the second floor, and 1,000 holes on the mezzanine, fifth and eighth floors. In ground-floor columns, shear-load mitigation charges were detonated to lightly fracture columns to prevent shear transfer to the piles. CDI scored the concrete in the two main 20-in.-thick shear walls that ran the width of the building, above dowels rising out of the pile caps. Above that, rebar was torch-cut to reduce the transfer of the bending moment, says Loizeaux. Work also included placing a 4-ft-thick bed of sand on the ground level, to cushion the impact of the debris on the foundations.

Despite the complexities, CDI “pulled” the structure away from newly occupied high-rise condominiums as close as 56 ft away, and from nearby utilities that serve the Ocean Club development. “The building came down with results that were even better than I had hoped,” says Loizeaux. At nearby properties, vibrations were less than 0.3 in. per second particle velocity, which is less than half of what was expected, says Miller. Vibrations from falling debris at the foundations measured less than 2 in. per second, rather than the 5 in. per Second expected.

The plan is to use the same construction methods for the rebuilding, including wooden forms blamed for the fire’s spread. “We could do it with steel forms, but it would be extremely expensive,” says John Monts, senior vice president in the Ft. Lauderdale, Fla., office of Morse Diesel. The contractor, which has continued work on the tower’s twin, now performs daily, end-of-the-work-day surveillance for fires, says Monts. The inspection now begins an hour after workers leave, he adds, because some fires smolder. If all goes well, reconstruction of the razed frame should start in August. Monts expects the delay to set back the move-in date by 11 months, to late 2001.