A design weakness caused three hammerhead tower cranes to collapse within hours of each other as winds in the wake of Hurricane Irma moved through Miami and Ft. Lauderdale, Fla., in September 2017.

A report by the Occupational Safety and Health Administration (OSHA) said that the failure involved the model SK-315 crane. There were no injuries at any of the three sites and property damage was reported to be minimal.

Television news image of one of the crane collapses. Credit: WPLGTerex Peiner GmbH manufactured one of the three cranes. The other two were manufactured by Noell Service and Maschinenetechnick GmbH. The report said that more than two dozen other cranes were deployed on construction sites in the Miami and Fort Lauderdale area, but none collapsed except the SK 315 models. One other SK 315 model crane in the area did not collapse, but OSHA says that crane was at a much lower height and was shielded by tall structures. In addition, there were Terex-manufactured cranes other than SK 315 in the area that did not fail.

According to the report, on Sunday, Sept. 10, 2017, the three tower cranes collapsed within hours of each other. Miami and Ft. Lauderdale were experiencing tropical storm conditions as Hurricane Irma made landfall on the west coast of Florida.

Structural engineers from the Office of Engineering Services in OSHA’s Directorate of Construction visited the incident sites, the Maxim Crane storage yard in Kentucky, and the grounds in Orlando and Miami where the remnants of the failed cranes were stored.

Met Existing Standards

Based on its investigation, the OSHA report concluded that the crane design generally met U.S. and European standards. The crane was designed for a maximum wind speed of 95 mph at the jib level for the out-of-service condition in effect during the period of high winds. Beyond 95 mph, failure could occur as stated in the manufacturer’s own investigation.

If the wind speed at the jib level exceeds 45 mph, the crane must be placed out of service and allowed to “weathervane,” or turn freely, as directed by the manufacturer’s manual. During the storm, all three cranes were put out of service and allowed to freely weathervane.

OSHA says that U.S. and European codes do not require that turbulent wind, suction or vortexes be considered in crane design. It said that all three cranes failed due to turbulent winds. The point of failure occurred at a weak spot where the jib connected to the turntables. As the cranes experienced turbulent wind loads, the crane jibs came apart from their turntables.

“Under the turbulent wind loads in either lateral or upward direction, the forces on the turntable cradles at the jib feet exceeded their capacities,” the OSHA report said. “The separation of the jibs at the turntable caused the jibs to fall in all three cases.”

In all three crane failures, only the jibs, counterjibs and the tower top collapsed. The crane masts, ties and foundations remained intact.

Recommendations

As part of its findings, OSHA recommends that on coastal areas with tall buildings and locations prone to frequent turbulent winds, local governing bodies should consider asking engineers to conduct simulations based on dynamic analysis to get a more realistic wind evaluation than quasi-static analysis. Instead of conducting costly wind tunnel tests, engineers could conduct a computational fluid dynamics analysis, which has become more affordable due to the advance of software and computing technology.

It recommended that Terex should evaluate the design of jib-to-turntable connections to prevent possible detachment of crane jibs in future high turbulent wind conditions. To prevent catastrophic collapses of the crane tower top, OSHA recommends that tower tops be evaluated and designed with a higher safety margin, and the design should also be based with loads applied from only the counterjib.

The jibs in all three cranes were suspended by single pendants. In the event that the jib begins to sway to the sides, the single pendants could not be expected to secure the stability of the jib. Therefore, OSHA recommended that two symmetric pendants or cables be considered in tower crane designs, and that Terex should consider adding pendants to the existing SK315 models.