Last week, on July 27, 2023, the working arm or jib of a tower crane and its 16 ton bucket load of fresh concrete came crashing down onto a busy street in the Hells Kitchen neighborhood of Manhattan, from 45 stories above. The mishap appears to have resulted from a fire behind the operators cab atop a luffing crane, ignited by hydraulic oil leaking onto the hot casing of an electric motor. Heat from the fire appears to have caused the hydraulic pistons supporting the jib to fail, forcing both jib and load to collapse onto the street below, Thankfully there were no fatalities, but twelve people, including 3 firefighters suffered non-life threatening injuries from falling debris.
To understand this failure - along with the flurry of other tower crane mishaps over the last several years - requires some basic background information. For starters, every tower crane is erected in a fixed location during construction, differing from mobile cranes mounted on wheels or tracks, that can move about freely. Next, every tower crane used during construction has five major components: 1.) a large heavily reinforced concrete foundation, 2.) a vertical mast, embedded in and arising from the foundation, comprised of modular units, often including a self climbing module, 3.) a turntable, atop the mast, including an operators cab, hydraulic pumps for luffing cranes, and up to 3 or 4 electric motors operating the crane functions, 4,) a working arm or main jib, a tail jib, and/or counter weights extending off opposing sides of the turntable, and 5.) a hook block and steel cables hanging off the jib used to pick, lift, and locate loads.
Finally, there are two types of tower cranes in common usage at construction sites throughout North America. One type is the hammerhead crane, with a working arm or jib fixed in a horizontal position. [See tower crane pictured on left side of header]. The hook and block used to lift loads is hung from a mobile trolley traveling in and out along the length of the jib. The trolley moves along the jib guided by steel cables reeled in and out by electric motors. One motor and cable retrieves the trolley, another motor and cable sends the trolley out along the length of the jib. By moving the mobile trolley in and out along the jib, the tower crane operator locates the hook block as needed to pick, lift, and locate loads.
The other type is the luffing crane, (like the crane in the recent Manhattan mishap) with a working arm or jib hinged at the turntable. [See tower crane pictured on right side of header]. The hook and block used to lift and locate loads is hung from the end of the jib. The jib can be lowered to a near horizontal position or raised to a near vertical position. The jib moves up and down guided by hydraulic cylinders operated by an electric pump. By moving the jib up and down, the operator locates the hook and block as needed to lift and locate loads.
There are several major differences between hammerhead cranes and luffing cranes. The jib of a hammerhead crane is supported by the turntable and fixed in a horizontal position. In contrast, the jib of a luffing crane is hinged at the turntable, can move from a near horizontal to a near vertical position, and is raised and lowered by hydraulic pistons. However, the differences do not end there. The size of a construction site typically dictates, the type of tower crane used. Hammerhead cranes have a long swing radius and are typically used on building sites with a large footprint. [“ See, “Does Your Building Project Need a Real Estate Easement for struction?”, posted 3/1/23]. Once again, in contrast, the swing radius of a luffing crane is adjustable and is typically used on building sites with a small footprint. Both types of cranes must allow the jib to "weathervane" when not in use, meaning the jib can move with the wind and avoid stressing the mast and toppling the crane.
Many recent crane mishaps, including the recent Manhattan mishap, involved luffing cranes. On May 23, 2023, counterweights of a luffing crane in Atlanta fell onto the building under construction. The mishap appears to have resulted from a failure of the structure supporting the counterweights. On February 11. 2021, a luffing crane in Brooklyn, partially collapsed, dangling its jib alongside a 31 story building. The mishap occurred during routine maintenance and appeared to result from a mechanical failure. On August 6, 2020, a luffing crane suffered a partial collapse when its jib fell onto the street adjacent to the construction site. Just three weeks earlier, on July 16, 2020, a luffing crane in Toronto, partially collapsed, when its jib fell onto the roof of an adjacent building. Both mishaps appear to have resulted from a mechanical failure during crane operations. Thankfully, none of these mishaps resulted in fatalities.
However, hammerhead cranes have also suffered recent mishaps, two of which resulted in fatalities. On June 1, 1023, a hammerhead crane in Del Ray Beach Florida, toppled over, with the entire crane falling onto the newly excavated construction site. The mishap appears to have resulted from a an undersized insufficient foundation, that toppled over along with the entire tower crane. On February 19, 2021, a hammerhead crane in Atlanta, became unstable, listing to one side requiring the closure of surrounding streets, This mishap appears to have resulted from the failure of a hydraulic cylinder within the climbing module. On June 1, 2019 a hammerhead crane in Dallas Texas toppled onto an adjacent apartment building killing its occupant during a severe thunderstorm. The mishap appears to have resulted from the failure of the operator to allow the crane to "weathervane" when not in use. On April 27. 2019, a hammerhead crane in Seattle collapsed onto the street below when its mast failed during dismantling operations. The mishap resulted from the premature removal of pins connecting the modules within the vertical mast. Unfortunately, this latter mishap resulted in the death four people - two ironworkers working on the mast fell to their death and two individuals in cars on the street below were crushed by falling mast.
A world wide study shows the causes of tower cranes accidents vary wildly: 42% result from erection, dismantling or climbing errors, 27% occur during operation, 13% are caused by operator error, 10% from mother nature, and 8% from combinations thereof. This same study indicates a vast majority of these causes result from a failure to follow the crane manufacturer’s instructions, while a minority result from structural failures or electrical and mechanical malfunctions. Given most tower cranes are leased from manufacturers or equipment companies and erected and dismantled by specialty contractors, the failure to comply with manufacturers specifications seems alarming. Perhaps even more alarming is that only sixteen States currently require the operators of tower cranes to be certified and licensed.