By its very nature construction is a challenging and dangerous industry. In contrast to most industries, construction requires travel to multiple sites, each with their own constantly changing hazards. Cranes play an essential role on construction sites throughout the United States on a daily basis. Cranes also add many complex risks to the construction process. Recently, numerous fatal crane collapses have grabbed national news headlines, increasing attention and scrutiny of construction crane operations. The most highly publicized being massive tower crane failures in New York, Miami, and Seattle. Estimates indicate that up to 33% of construction casualties and between 8 to 16% of construction fatalities involve cranes [1];[2]. Construction deaths as a whole declined sharply from 2007 to 2008, however, crane related construction deaths skyrocketed nearly 42% according to the U.S. Bureau of Labor Statistics [3]. This article reviews available information on crane system failures. Specifically, types and causes of crane failures, the major types of cranes, the effects of crane type on failures, safety measures, and case studies will be discussed.
Types and Causes of Crane Failures
The number and description of categories used to classify crane failures varies significantly in available literature. The list of mutually exclusive proximal causes of crane-related fatal events proposed by Beavers et al. is used below. The numerical data reported in the following discussion of the proximal causes of crane-related fatal events was compiled from OSHA fatality investigation case files from 1997 to 2003 [2].
Struck by Load (Other than Failure of Boom/Cable)
Being struck by a load accounted for 32% of fatal crane events. The most common contributing factors to this proximal cause in order were rigging failures, loads being dropped, equipment damage, unbalanced loads, and accelerated movement.
Electrocution
Electrocution accounted for 27% of fatal crane events. All recorded electrocutions involved mobile cranes and were the result of a cranes boom or wire rope getting to close to high voltage power lines.
Crushed During Assembly/Disassembly
21% of crane related fatalities were contributed to being crushed during crane assembly or disassembly. Lattice boom cranes were involved in all of these deaths. The most common contributing factors to this proximal cause in order were improper disassembly and pin removal, improper boom support, and improper assembly.
Failure of Boom/Cable
Failure of a boom or cable accounted for 12% of fatal crane events. The most common contributing factors to this proximal cause in order were overload, equipment damage, boom collapse, incorrect assembly, cable snapping, boom buckling, and two blocking, lifting a load to high and having the hook block strike the tip of the boom.
Crane Tip Over
Crane Overturning was the cause of 11% of fatal crane events. The most common contributing factors to this proximal cause in order were overload, loss of center of gravity control, outrigger failure, high winds, side pull, and improper maintenance.
Struck by Cab/Counterweight
Being struck by the crane cab or counterweight was only responsible for 3% of fatal crane incidents. Tragically all reported incidents were cause by intentional turntable rotation or by intentional motion of a bridge crane.
Falls
Falls were the cause of 2% of all crane related fatalities. Missing hand rails, improper crane operation, and improper maintenance were three contributing factors.
Types of Cranes
The two basic construction crane designs are mobile and tower, both having hundreds of different configurations. Mobile cranes can be mounted on crawlers, trucks, or barges, have telescoping or lattice booms, have lift capacities from 15 to 1000 tons, and have total boom lengths up to 600 ft. Tower cranes can feature climbing sections that let them grow in height with the building, be located inside or outside a building during construction, use a hammerhead or a luffing boom, have boom lengths of up to 250 ft., and have load capacities in excess of 22 tons [1]. Fig 1 and Fig 2 below show a typical tower crane, and mobile crane's respectively.
Fig 2 Mobile Cranes courtesy of U.S. Department of Transportation Federal Highway Administration
Fig 1 Tower Crane (stock photo)
Effects of Crane Type on Failures
Mobile cranes have been shown to represent 88.4% of all crane related fatalities and 100% of crane related electrocutions. [2]. As new crane designs continue to use lighter and lighter weight material in their lattice booms, crane tip over will account for a smaller proportion of crane failure [4].
Safety Measures
Safety Equipment
A plethora of safety equipment that if used properly could improve crane safety already exists. Equipment such as spirit levels, boom angle indicators, load charts and load moment indicators can give information to crane operators reducing the chances of crane failure due to operator error. Another such device is a load drum rotation indicator which sends a tactile, audible, or visible signal to a crane's operator informing them whenever the load supporting cable moves, helping protect riggers from loads moving unexpectedly. Addition of items as simple as safety latches to prevent load straps or chains from slipping off hooks unexpectedly can also greatly improve rigging safety. Outriggers, boom stops, anti-tow-block mechanisms, wind sensors, and breaks can increase crane stability reducing the risk of tip over failures. Limit switches can be installed which will prevent a crane from lifting loads greater than 110% of its maximum rated load further reducing the risk of overturning. Numerous devices, such as insulated boom cages, insulated lines, insulated barriers, and current proximity warning alarms, have been developed to reduce the risk of electrocution by preventing transmission of electrical current from energized power lines to crane components. Stringing nonconductive lines fitted with high visibility flags adjacent to power lines reduces the risks of electrocution and collision. Crane-crane collisions can be prevented by radar based devices that automatically halt cranes when potential interference from other cranes is detected. Flashing strobe lights can be attached to tall cranes to decrease the chances of aircraft-crane collisions [1].
CRANIUM
In addition to the currently available safety equipment mentioned above many new devices such as the CRANIUM are currently under development. The CRANIUM is a video system that allows a crane operator to see a live feed of what is happening at the lifting point on a screen in his cab. The prototype CRANIUM consisted of a high resolution color video camera with a motorized lens encased in a robust housing mounted on a damped gimbal at the tip of a crane's boom and a 9 in. color television monitor located in the crane operator's cab. Crane operators rely on information supplied to them through a set of hand signals by a signalperson near the point of the load to know how and when to move a load. Frequently the signalman is obscured from the crane operator's view and a tagperson in direct line of sight of both the signalman and crane operator must relay the hand signals. The use of a tagperson inherently creates a signal delay and increases the chances of a signal error. Crane operators account for these problems by using a adopting a tedious move-and-wait strategy in which they move loads a small increments then stop and wait for feedback out of necessity because signals to stop moving the load are also delayed. By allowing crane operators to see the signalperson directly the CRANIUM permits loads to be moved more quickly, increasing productivity. Direct communication between the crane operator and signalperson also reduce the chance of signal errors improving safety. New technological developments such as the CRANIUM hold promise for increasing the safety of crane operations in the near future [5].
Regulation
There is currently no up-to-date national crane regulation in the United States. The Occupational Safety Health Administration's crane rules have not been updated substantially since the late 1960s. As a result, they are no longer appropriate for current crane designs and industry practices. Some states and cities have passed their own more comprehensive regulations however, in some cases these regulations conflict with the old OSHA regulations and those of other locations. These conflicts make enforcement of and adherence to these codes confusing tasks [6]. OSHA spokesman Richard De Angelis recently said "We are confident that we will issue a final rule next year," raising expectations that OSHA's new crane standards which have been in the works for years are will finally be released . One of the most controversial elements of a draft version of OSHA's new crane safety standard is requiring crane operator certification. Certification requirements for signalpersons, riggers, and inspectors were not included in the draft safety standard [3]. Summarized eloquently by Professor Kevin Parfitt "Regardless of perspective, most in the industry agree that new regulations and cooperative agreements with OSHA on the topic of crane safety are long overdue." [6]
Regardless of what laws are in the books -- if enforcement is lacking -- conditions are not apt to improve. The recent arrest of Edward J. Marquette for allegedly falsify a business record and filing it with the buildings department has reduced the number of crane inspectors in New York to four. With only four inspectors being left responsible for 250 cranes in use, of which 30 are tower cranes, the abilty to fully enforce regulations is clearly questonable [7].
Certification
The United States currently has no national requirement for crane operators to be certified. Nevertheless, currently 17 states and 6 cities have licensing requirements for crane operators. Additionally, voluntary certification programs are available for crane operators, signalpersons, and riggers. The National Commission for the Certification of Crane Operators (NCCCO) is a national not-for-profit organization that administers both written and practical examinations that are required for or recognized by a majority of current licensure requirements. The NCCCO does not however provide training courses [8]. Training courses are offered by numerous companies nation wide such as the Crane Institute of America with a 5-acre training facility near Orlando, Florida [9].
Site Layout
Throughout the use of a robust site layout planning model constructions sites should be organized to maximize the safety of construction operations and minimize travel cost of resources. Safety measures as simple as locating high occupancy facilities like site offices outside the range of crane operations whenever possible. Construction planners can realize significant improvements in the safety and cost performance of construction facilities through use of multi-objective genetic algorithms. Such algorithms can help construction planners to identify site layouts that provide optimal trade-offs between safety and cost performance while still satisfying practical construction constraints. Serious consideration of crane operation risks when laying out construction sites can go a long way to decrease the number of construction casualties, especially of workers on site but not participating in crane operations [10].
Tower Crane Database
In the wake of several recent fatal tower crane collapses, the building departments of New York City, Chicago, and Philadelphia are setting up a database that will attempt to keep track tower cranes and their parts. Specifically the quarterly updated database will record the number of cranes active in a jurisdiction, the address where the crane is working, the crane's owner, the make, model, year and serial number, erection date, maximum height, dismantling date, and any accidents or other incidents involving the cranes. Several places have expressed interest in participating in database program including New York State, New Jersey, Connecticut, California, Ontario, and Dallas. The implementation of such databases holds the prospect of significantly reducing the number of crane accidents caused by the continued use of faulty equipment [11].
Case Studies
There have been countless catastrophic crane accidents responsible for a great loss of life of construction sites throughout the years. Many recent high profile crane collapses have brought much needed attention to the need for diligent crane safety precautions. Below case studies of two of these high profile crane failures are discussed at length. The first case study discussed is the collapse of a massive mobile crane in high winds during the construction of Miller Park in 1999. The second case history discussed at length is the 2008 collapse of the tower crane on East 51st Street in Manhattan. Other high profile crane failures will be discussed briefly.
The Miller Park Crane Mobile Crane Collapse (1999)
Situation Leading Up to the Failure
Late in the afternoon of July 14, 1999, the construction site of Miller Park, the Milwaukee Brewers new baseball stadium, was a windy place. Wind speeds were 22-24 miles per hour on average with gust of 32-34 miles per hour. A group of ironworkers had left the site at noon, refusing to work in elevated locations in the windy conditions. A Lamspon LTL 1500, Series IIIA, Transi-Lift crawler crane nicknamed Big Blue remained in operation under these adverse weather conditions. Big Blue was an immense crane with 340 ft main boom, 200 ft jib, and 190 mast lattice truss structures. It was powered by 11 diesel engines, had six miles of wire rope, 1150 tons of counter weights, and weighed in at 2100 tons total [12].
Mitsubishi Heavy Industries, the operator of Big Blue, was using the crane to lift a 300 member, 190x130x12 ft, 510 ton, curved open truss panel roof section to a 330 ft elevation on a carry-travel sequence to its final location [12].
Failure
The failure began with a loud bang as Big Blue's king pin bottom end cap assembly developed radial fractures in its top hat bushing flange. Big Blue was designed to have side load stability criterion, 2% of rated main hoist load capacity, dealt with entirely by the dead weight of the crane upperworks. Thereby allowing the king pin to serve only as a centering pivot for swing (slewing) rotations, and to experience no transverse or bending loads during all normal operations. In the adverse weather conditions that afternoon, in which the massive curved roof panel behaved similar to a sail, the lateral wind loads on the crane were nearly 250% greater than the 2% resistance to overturning provided by crane upperworks dead load. The resulting lateral displacements of the load and crane upperworks, in combination with out-of-level runway foundations and soft ground, exacerbated the overturning moment. Consequently, a bending moment was imposed on the king pin which it was not designed to resist. A second bang followed four seconds later as additional circumferential fractures developed in the top hat bushing. At the same time, the crane boom and jib and the suspended roof panel moved downwind noticeably, the front crawler driver fled for safety, and the upwind crawler track lifted off the ground. Moments later a third bang was heard as a 1/2 in. soft bronze spacer ruptured, a cloud of dust appears under Big Blue's crawler base as parts of the king pin bottom end cap assembly were forcibly ejected downward, and 500 tons of crane and roof panel fall into previously erected sections of the stadium roof [12].
An alternate theory of the failure placed the blame on the 1/2 in. soft bronze spacer added to the king pin end cap assembly when the crane was modified from its previous configuration as a barge mounted crane. This totally disparate theory was sided against in ensuing litigation [12].
By chance, the entire collapse of the crane was captured on video. An official source of this footage could not be located. Numerous websites including YouTube have the footage of the crane collapse posted. http://www.youtube.com/watch?v=HI-ENqgjQXM
Aftermath
The crane collapse resulted in the death of three ironworkers and financial damages in the hundreds of millions. The accident could have easily been prevented had the crane not been operated in obviously adverse weather conditions. The decision to go forward with the lift was determined to be solely the responsibility of the general contractor in the resulting litigation [12].
303 East 51st Street, Manhattan, Tower Crane Collapse (2008)
Situation Leading Up to the Failure
A collar connection to tie the ill fated crane to the building's structure was being installed just prior to the crane's catastrophic collapse. The building being tied into is 303 East 51st Street, a proposed 43-story concrete frame building designed by Garrett Gourlay Architect PLLC on behalf of Kennelly Development Company, L.C., it was 19 stories tall at the time of the collapse. The crane was an external self-climbing luffing tower crane, Model M440E, manufactured by Favelle Favco Cranes Pty. Ltd. and supplied by New York Crane. The tower crane was supported laterally by tie-ins to the building at the 3rd and 9th floors. The tie-in connections were comprised of a two part steel framed collar, which surrounded the four tower legs, and three W12x79 tie-beams. The tie-beams were anchored to the building by being welded to embedded steel base plates, and were connected to the collar with 3 in. pins. The day before the collapse the tower crane consisted of fifteen 13'-1" tall sections. Starting at 7:00 am, on March 15, 2008, the crane was extended (jumped) three additional sections. After all three sections were in place, installation began on a tie-in connection on the 18th floor. The two halves of the collar were lifted into place, connected together around the tower, and temporarily suspended from the tower [13].
Failure
When the failure occurred, the 18th floor collar was being temporarily suspended from the tower by a total of four polyester web slings. This temporary support was inadequate in numerous ways. The manufacturer's requirements called for eight chain blocks to be used, twice the number of supports provided and a more durable material. The slings that were installed were all located at positions inconsistent with the manufacturer's instructions. The method used to rig the slings to the tower was not in accordance with accepted industry practice and standards, it caused them to be chocked around the crane legs and seated in V-shaped grooves with unprotected edges. One of the slings used had been deteriorated by UV exposure and physical damage prior to use and should not have been installed. At the time of the collapse the erection crew was about to install the first W12x79 tie-in beam to the 18th floor collar connection. The tie-in beam was suspended from the crane, ready to be put in place shortly. At approximately 12:30 pm on March 15, 2008, the four polyester slings supporting the 18th floor collar broke. Then, the collar fell downward along the tower striking the 9th floor collar destroying its connection to the building. Proceeding to fall down, the 18th floor and 9th floor collars proceeded struck the 3rd floor collar damaging it. At this point only base friction and the damaged level 3 collar were left to resist overturning of the tower. While it was adequately designed to transfer vertical loads to the foundation through dunnage beams, the base was not designed to provide base friction in resistance to lateral loads. With inadequate base shear being provided the tower overturned the base sliding north towards the building and the top falling south away from the structure. The tie in beam that was suspended from the crane was flung through the air impaling and damaging an adjacent building. No alternate theories of the failure are apparent [13]. Figures 3 and 4 show the top and base of the tower crane after the collapse respectively.
Aftermath
The collapse resulted in the death of six construction workers and one civilian, the demolition of a four-story brownstone on East 50th street, and damage to more than a dozen buildings in the East Side neighborhood. The investigation into the collapse concluded that the crane failure initiated by four inappropriately used synthetic slings were the cause of the collapse as described above. Multiple charges of manslaughter, criminally negligent homicide, assault, and reckless endangerment have been brought against the tower crane rigger and his company. 18 buildings had to be vacated following the collapse, three of which remained vacated due to outstanding repairs required to make them safe for occupancy as of March 11, 2009. Inspections of tower cranes undergoing jumping operations have been increased. New York City enacted 12 new laws designed to increase construction safety and regulatory oversight [13];[14].
Fig 4 Base of tower after collapse. Photo courtesy of New York County District Attorney
Fig 3 Top of tower after collapse. Photo courtesy of the New York Police Department
Other High Profile Crane Failures
Bellevue, Washington, Tower Crane Collapse (2006)
On November 16, 2006 a 210-foot tower crane being used to construct the 20 story Tower 333 office building in downtown Bellevue, Washington collapsed crashing into three neighboring buildings, killing a man as it demolished his condo. The failure was determined to be caused by metal fatigue in the steel base frame which was drastically under-designed. The base frame needed to be at least four times stronger [15]. The structural engineering firm responsible for the crane foundation design and the contractor paid fines and settled out of court with the victims family [16].
Miami, Florida, Tower Crane Collapse (2008)
A 20 ft. section of crane tower being hoisted into place to jump a tower crane fell 30 stories killing two people on March 25, 2008. The tower crane was being heightened to continue construction of a high-rise condominium building on Biscayne Bay in Miami. As it fell, the crane tower segment crashed through a neighboring two story house on its way down. The previously erected crane segments remained intact [17].
No conclusive determination of the exact cause of the failure is currently publicly available. This deadly collapse occurred only days before new local crane regulations, a reaction to a deadly 2006 crane collapse in Miami, came into effect [17].
335 East 91st Street, Manhattan, Tower Crane Collapse (2008)
On Friday, May 30, 2008, a crane cab came loose from its mast and toppled down to the street below damaging neighboring buildings as it fell. Two men were killed by the collapse. This event happened less than three months after the Tower crane collapse on East 51st Street. Initial reports blame failure of a shoddy turntable weld for the cab coming loose from the tower. The crane was in operation but not being jacked, assembled, or disassembled at the time of failure [18].
According to a New York Daily News news report warnings about the safety of the weld were brought to the owner a year before the collapse. A Chinese company, RTR Bearings, was not confident in its welding technique and recommended the crane be welded by its owner in New York. The owner New York Crane then upped the price it would pay RTR bearings to perform the weld and suddenly RTR Bearings stopped trying to get out of the job. Even after the price had been raised RTR Bearings claimed it could do the weld for in only 46% of the time and for only 18% the cost stated by an Ohio-based firm in a competing bid. These huge time and price discrepancies should have sent a warning that things were too good to be true. Another unheeded warning that the weld in the crane was faulty came a month before the collapse. A radiographic analysis on another crane, owned by New York Crane, revealed a similar part made by RTR Bearings was unacceptable. Despite these warnings signs the crane was kept in operation and no test were performed to verify the safety of the weld [18].
Chicago, Illinois, Tower Crane Near Collapse (2009)
A potential disaster was avoided on January 21, 2009, in downtown Chicago by swift decision-making to suspend operations of a Potain MR605B luffing tower crane and investigate the cause of a loud popping sound. The crane was one of two located on top of an occupied 33 story Blue-Cross/Blue-Shield office building being used to erect a 24 floor vertical extension of structural steel to the existing building. Figure 5 shows the cranes located on top of the existing building during construction. The source of the popping sound was one of the four legs of the cranes mast cracking. Fortunately, the crane remained standing on three of its four legs for several days until the crane manufacturer provided a welding procedure to repair the crack. It was determined that the building did not need to be evacuated and the crane was already scheduled to be disassembled soon so the incident did not wreak major havoc on the building occupant's business, or on the construction schedule [19].
Testing of similar mast components on other cranes was conducted to ensure that material defects were not systemic at the crane manufacturer's expense. The results of the testing indicated that material defects were not widespread. While material defects have not been ruled out as the source of the crack, there are alternative theories claiming misuse of the crane was responsible. One such theory is that the mast was torqued lost strength because crane tie-ins were not readjusted as the steel building was plumbed. This potentially catastrophic event clearly showed that diligence and caution when operating cranes can prevent failures from becoming deadly [19].
Fig 5 Cranes on Chicago Blue Cross Building Photo courtesy of Matthew Haapala
Philadelphia, Pennsylvania, Mobile Crane Collapse (2009)
Making national headlines most recently, at 1:30 pm on October 12, 2009, a mobile cherry picker type crane toppled over and struck an apartment building in downtown Philadelphia, PA. The cranes operator fell 125 ft to his death. Three people were injured when falling debris struck their vehicles. Initial reports indicate the crane tipped over when a fiberglass street hole cover owned by Comcast Cable collapsed below one of the cranes wheels. The street hole cover was never intended to support large machinery, the crane's wheel should not have been placed over it. Preliminary assumptions are that operator error will be blamed for this failure [20];[21].
Conclusions
Cranes play an indispensable role in the construction industry, especially in high rise structures. It is a fact that cranes are involved with an alarmingly high number of casualties and fatalities, increasingly so in 2008. The sad truth is that the equipment and know-how exist to greatly reduce crane failure incidents but it's not used diligently throughout the construction industry. Furthermore, in some cases safe operation of cranes is hampered by outdated or contradicting regulations. While the proper use of current safety equipment, the incorporation of new devices such as the CRANIUM, and appropriate regulations can make it easier to use cranes safely, they can not put an end to crane failures themselves. As with most things, it comes down to the skills, motivation, and attention to detail of the people operating and caring for these sometimes gargantuan machines that will determine their safety.
OSHA should update its crane regulations so that they are an effective minimum code of safe conduct for current crane operations. Ample crane inspectors must be employed to assure compliance with any new regulations. Under either OSHA regulation or its own volition the construction industry must insure that crane operators are qualified because countless lives including their own depend on it. Structural engineers must find a way despite the continually faster track of design and construction to verify that crane foundations and tie-ins are adequate.
The father of a victim of 91st street crane collapse told reporters his question since his son's death had been "Why?". After it was revealed that saving time and money by knowingly choosing to use a poor quality products he said "Now I have some answers. My son was killed for one reason only - greed." When corners get cut things fall between the cracks. With the stakes as catastrophically high as they are, corners should not be cut with cranes.
References
[1] Neitzel, R.L., Seixas, N.S., Ren K.K. (2001). "A Review of Crane Safety in the Construction Industry." Applied Occupational and Environmental Hygiene, 16(12), 1106-1117
[2] Beavers, J.E., Moore, J.R., Rinehart, R., Schriver, W.R. (September2006). "Crane Related Fatalities in the
Construction Industry." Journal of Construction Engineering and Management, 132(9), 901-910.
[5] Everett, J.G., Slocum, A.H.(March/April 1993). "CRANIUM: Device for Improving Crane Productivity and Safety." Journal of Construction Engineering and Management, 119(1), 23-39.
[6] Parfitt, M.K. (March 2009). "Cranes, Structures under Constuction, and Temporary Facilities: Are We Doing Enough to Ensure They are Safe?" Journal of Architectural Engineering
[7] Sawyer, T.,Carlsen, R., Barner, C., Fulmer, B., Bodilly, L., Schwartz, E., Wood, D. (March 31, 2008). "Tower-Crane Fears Drive Regulations." Engineering News Record March 31,2008 p 11.
[8] (2009). "State Licensing Requirements." National Commission for the Certification of Crane Operators, <http://www.nccco.org/licensing/index.html > (Sep 24, 2009).
[10] El-Rayes, K., Khalafallah, A. (November 2005). "Trade-off between Safety and Cost in Planning Construction Site Layouts." Journal of Construction Engineering and Management, 131(11), 1186-1195.
| Introduction | Types and Causes of Crane Failures | Types of Cranes | Effects of Crane Type on Failures | Safety Measures | Case Studies | The Miller Park Crane Mobile Crane Collapse (1999) | 303 East 51st Street, Manhattan, Tower Crane Collapse (2008) | Other High Profile Crane Failures | Conclusions | References
Introduction
By its very nature construction is a challenging and dangerous industry. In contrast to most industries, construction requires travel to multiple sites, each with their own constantly changing hazards. Cranes play an essential role on construction sites throughout the United States on a daily basis. Cranes also add many complex risks to the construction process. Recently, numerous fatal crane collapses have grabbed national news headlines, increasing attention and scrutiny of construction crane operations. The most highly publicized being massive tower crane failures in New York, Miami, and Seattle. Estimates indicate that up to 33% of construction casualties and between 8 to 16% of construction fatalities involve cranes [1];[2]. Construction deaths as a whole declined sharply from 2007 to 2008, however, crane related construction deaths skyrocketed nearly 42% according to the U.S. Bureau of Labor Statistics [3]. This article reviews available information on crane system failures. Specifically, types and causes of crane failures, the major types of cranes, the effects of crane type on failures, safety measures, and case studies will be discussed.Types and Causes of Crane Failures
The number and description of categories used to classify crane failures varies significantly in available literature. The list of mutually exclusive proximal causes of crane-related fatal events proposed by Beavers et al. is used below. The numerical data reported in the following discussion of the proximal causes of crane-related fatal events was compiled from OSHA fatality investigation case files from 1997 to 2003 [2].Struck by Load (Other than Failure of Boom/Cable)
Being struck by a load accounted for 32% of fatal crane events. The most common contributing factors to this proximal cause in order were rigging failures, loads being dropped, equipment damage, unbalanced loads, and accelerated movement.Electrocution
Electrocution accounted for 27% of fatal crane events. All recorded electrocutions involved mobile cranes and were the result of a cranes boom or wire rope getting to close to high voltage power lines.Crushed During Assembly/Disassembly
21% of crane related fatalities were contributed to being crushed during crane assembly or disassembly. Lattice boom cranes were involved in all of these deaths. The most common contributing factors to this proximal cause in order were improper disassembly and pin removal, improper boom support, and improper assembly.Failure of Boom/Cable
Failure of a boom or cable accounted for 12% of fatal crane events. The most common contributing factors to this proximal cause in order were overload, equipment damage, boom collapse, incorrect assembly, cable snapping, boom buckling, and two blocking, lifting a load to high and having the hook block strike the tip of the boom.Crane Tip Over
Crane Overturning was the cause of 11% of fatal crane events. The most common contributing factors to this proximal cause in order were overload, loss of center of gravity control, outrigger failure, high winds, side pull, and improper maintenance.Struck by Cab/Counterweight
Being struck by the crane cab or counterweight was only responsible for 3% of fatal crane incidents. Tragically all reported incidents were cause by intentional turntable rotation or by intentional motion of a bridge crane.Falls
Falls were the cause of 2% of all crane related fatalities. Missing hand rails, improper crane operation, and improper maintenance were three contributing factors.Types of Cranes
The two basic construction crane designs are mobile and tower, both having hundreds of different configurations. Mobile cranes can be mounted on crawlers, trucks, or barges, have telescoping or lattice booms, have lift capacities from 15 to 1000 tons, and have total boom lengths up to 600 ft. Tower cranes can feature climbing sections that let them grow in height with the building, be located inside or outside a building during construction, use a hammerhead or a luffing boom, have boom lengths of up to 250 ft., and have load capacities in excess of 22 tons [1]. Fig 1 and Fig 2 below show a typical tower crane, and mobile crane's respectively.Effects of Crane Type on Failures
Mobile cranes have been shown to represent 88.4% of all crane related fatalities and 100% of crane related electrocutions. [2]. As new crane designs continue to use lighter and lighter weight material in their lattice booms, crane tip over will account for a smaller proportion of crane failure [4].Safety Measures
Safety Equipment
A plethora of safety equipment that if used properly could improve crane safety already exists. Equipment such as spirit levels, boom angle indicators, load charts and load moment indicators can give information to crane operators reducing the chances of crane failure due to operator error. Another such device is a load drum rotation indicator which sends a tactile, audible, or visible signal to a crane's operator informing them whenever the load supporting cable moves, helping protect riggers from loads moving unexpectedly. Addition of items as simple as safety latches to prevent load straps or chains from slipping off hooks unexpectedly can also greatly improve rigging safety. Outriggers, boom stops, anti-tow-block mechanisms, wind sensors, and breaks can increase crane stability reducing the risk of tip over failures. Limit switches can be installed which will prevent a crane from lifting loads greater than 110% of its maximum rated load further reducing the risk of overturning. Numerous devices, such as insulated boom cages, insulated lines, insulated barriers, and current proximity warning alarms, have been developed to reduce the risk of electrocution by preventing transmission of electrical current from energized power lines to crane components. Stringing nonconductive lines fitted with high visibility flags adjacent to power lines reduces the risks of electrocution and collision. Crane-crane collisions can be prevented by radar based devices that automatically halt cranes when potential interference from other cranes is detected. Flashing strobe lights can be attached to tall cranes to decrease the chances of aircraft-crane collisions [1].CRANIUM
In addition to the currently available safety equipment mentioned above many new devices such as the CRANIUM are currently under development. The CRANIUM is a video system that allows a crane operator to see a live feed of what is happening at the lifting point on a screen in his cab. The prototype CRANIUM consisted of a high resolution color video camera with a motorized lens encased in a robust housing mounted on a damped gimbal at the tip of a crane's boom and a 9 in. color television monitor located in the crane operator's cab. Crane operators rely on information supplied to them through a set of hand signals by a signalperson near the point of the load to know how and when to move a load. Frequently the signalman is obscured from the crane operator's view and a tagperson in direct line of sight of both the signalman and crane operator must relay the hand signals. The use of a tagperson inherently creates a signal delay and increases the chances of a signal error. Crane operators account for these problems by using a adopting a tedious move-and-wait strategy in which they move loads a small increments then stop and wait for feedback out of necessity because signals to stop moving the load are also delayed. By allowing crane operators to see the signalperson directly the CRANIUM permits loads to be moved more quickly, increasing productivity. Direct communication between the crane operator and signalperson also reduce the chance of signal errors improving safety. New technological developments such as the CRANIUM hold promise for increasing the safety of crane operations in the near future [5].Regulation
There is currently no up-to-date national crane regulation in the United States. The Occupational Safety Health Administration's crane rules have not been updated substantially since the late 1960s. As a result, they are no longer appropriate for current crane designs and industry practices. Some states and cities have passed their own more comprehensive regulations however, in some cases these regulations conflict with the old OSHA regulations and those of other locations. These conflicts make enforcement of and adherence to these codes confusing tasks [6]. OSHA spokesman Richard De Angelis recently said "We are confident that we will issue a final rule next year," raising expectations that OSHA's new crane standards which have been in the works for years are will finally be released . One of the most controversial elements of a draft version of OSHA's new crane safety standard is requiring crane operator certification. Certification requirements for signalpersons, riggers, and inspectors were not included in the draft safety standard [3]. Summarized eloquently by Professor Kevin Parfitt "Regardless of perspective, most in the industry agree that new regulations and cooperative agreements with OSHA on the topic of crane safety are long overdue." [6]Regardless of what laws are in the books -- if enforcement is lacking -- conditions are not apt to improve. The recent arrest of Edward J. Marquette for allegedly falsify a business record and filing it with the buildings department has reduced the number of crane inspectors in New York to four. With only four inspectors being left responsible for 250 cranes in use, of which 30 are tower cranes, the abilty to fully enforce regulations is clearly questonable [7].
Certification
The United States currently has no national requirement for crane operators to be certified. Nevertheless, currently 17 states and 6 cities have licensing requirements for crane operators. Additionally, voluntary certification programs are available for crane operators, signalpersons, and riggers. The National Commission for the Certification of Crane Operators (NCCCO) is a national not-for-profit organization that administers both written and practical examinations that are required for or recognized by a majority of current licensure requirements. The NCCCO does not however provide training courses [8]. Training courses are offered by numerous companies nation wide such as the Crane Institute of America with a 5-acre training facility near Orlando, Florida [9].Site Layout
Throughout the use of a robust site layout planning model constructions sites should be organized to maximize the safety of construction operations and minimize travel cost of resources. Safety measures as simple as locating high occupancy facilities like site offices outside the range of crane operations whenever possible. Construction planners can realize significant improvements in the safety and cost performance of construction facilities through use of multi-objective genetic algorithms. Such algorithms can help construction planners to identify site layouts that provide optimal trade-offs between safety and cost performance while still satisfying practical construction constraints. Serious consideration of crane operation risks when laying out construction sites can go a long way to decrease the number of construction casualties, especially of workers on site but not participating in crane operations [10].Tower Crane Database
In the wake of several recent fatal tower crane collapses, the building departments of New York City, Chicago, and Philadelphia are setting up a database that will attempt to keep track tower cranes and their parts. Specifically the quarterly updated database will record the number of cranes active in a jurisdiction, the address where the crane is working, the crane's owner, the make, model, year and serial number, erection date, maximum height, dismantling date, and any accidents or other incidents involving the cranes. Several places have expressed interest in participating in database program including New York State, New Jersey, Connecticut, California, Ontario, and Dallas. The implementation of such databases holds the prospect of significantly reducing the number of crane accidents caused by the continued use of faulty equipment [11].Case Studies
There have been countless catastrophic crane accidents responsible for a great loss of life of construction sites throughout the years. Many recent high profile crane collapses have brought much needed attention to the need for diligent crane safety precautions. Below case studies of two of these high profile crane failures are discussed at length. The first case study discussed is the collapse of a massive mobile crane in high winds during the construction of Miller Park in 1999. The second case history discussed at length is the 2008 collapse of the tower crane on East 51st Street in Manhattan. Other high profile crane failures will be discussed briefly.The Miller Park Crane Mobile Crane Collapse (1999)
Situation Leading Up to the Failure
Late in the afternoon of July 14, 1999, the construction site of Miller Park, the Milwaukee Brewers new baseball stadium, was a windy place. Wind speeds were 22-24 miles per hour on average with gust of 32-34 miles per hour. A group of ironworkers had left the site at noon, refusing to work in elevated locations in the windy conditions. A Lamspon LTL 1500, Series IIIA, Transi-Lift crawler crane nicknamed Big Blue remained in operation under these adverse weather conditions. Big Blue was an immense crane with 340 ft main boom, 200 ft jib, and 190 mast lattice truss structures. It was powered by 11 diesel engines, had six miles of wire rope, 1150 tons of counter weights, and weighed in at 2100 tons total [12].The current official Spec Shet for Lamspon LTL-1500 cranes can be found here http://www.lampsoncrane.com/PDFs/LTL1500Spec.pdf.
Mitsubishi Heavy Industries, the operator of Big Blue, was using the crane to lift a 300 member, 190x130x12 ft, 510 ton, curved open truss panel roof section to a 330 ft elevation on a carry-travel sequence to its final location [12].
Failure
The failure began with a loud bang as Big Blue's king pin bottom end cap assembly developed radial fractures in its top hat bushing flange. Big Blue was designed to have side load stability criterion, 2% of rated main hoist load capacity, dealt with entirely by the dead weight of the crane upperworks. Thereby allowing the king pin to serve only as a centering pivot for swing (slewing) rotations, and to experience no transverse or bending loads during all normal operations. In the adverse weather conditions that afternoon, in which the massive curved roof panel behaved similar to a sail, the lateral wind loads on the crane were nearly 250% greater than the 2% resistance to overturning provided by crane upperworks dead load. The resulting lateral displacements of the load and crane upperworks, in combination with out-of-level runway foundations and soft ground, exacerbated the overturning moment. Consequently, a bending moment was imposed on the king pin which it was not designed to resist. A second bang followed four seconds later as additional circumferential fractures developed in the top hat bushing. At the same time, the crane boom and jib and the suspended roof panel moved downwind noticeably, the front crawler driver fled for safety, and the upwind crawler track lifted off the ground. Moments later a third bang was heard as a 1/2 in. soft bronze spacer ruptured, a cloud of dust appears under Big Blue's crawler base as parts of the king pin bottom end cap assembly were forcibly ejected downward, and 500 tons of crane and roof panel fall into previously erected sections of the stadium roof [12].An alternate theory of the failure placed the blame on the 1/2 in. soft bronze spacer added to the king pin end cap assembly when the crane was modified from its previous configuration as a barge mounted crane. This totally disparate theory was sided against in ensuing litigation [12].
By chance, the entire collapse of the crane was captured on video. An official source of this footage could not be located. Numerous websites including YouTube have the footage of the crane collapse posted. http://www.youtube.com/watch?v=HI-ENqgjQXM
Aftermath
The crane collapse resulted in the death of three ironworkers and financial damages in the hundreds of millions. The accident could have easily been prevented had the crane not been operated in obviously adverse weather conditions. The decision to go forward with the lift was determined to be solely the responsibility of the general contractor in the resulting litigation [12].303 East 51st Street, Manhattan, Tower Crane Collapse (2008)
Situation Leading Up to the Failure
A collar connection to tie the ill fated crane to the building's structure was being installed just prior to the crane's catastrophic collapse. The building being tied into is 303 East 51st Street, a proposed 43-story concrete frame building designed by Garrett Gourlay Architect PLLC on behalf of Kennelly Development Company, L.C., it was 19 stories tall at the time of the collapse. The crane was an external self-climbing luffing tower crane, Model M440E, manufactured by Favelle Favco Cranes Pty. Ltd. and supplied by New York Crane. The tower crane was supported laterally by tie-ins to the building at the 3rd and 9th floors. The tie-in connections were comprised of a two part steel framed collar, which surrounded the four tower legs, and three W12x79 tie-beams. The tie-beams were anchored to the building by being welded to embedded steel base plates, and were connected to the collar with 3 in. pins. The day before the collapse the tower crane consisted of fifteen 13'-1" tall sections. Starting at 7:00 am, on March 15, 2008, the crane was extended (jumped) three additional sections. After all three sections were in place, installation began on a tie-in connection on the 18th floor. The two halves of the collar were lifted into place, connected together around the tower, and temporarily suspended from the tower [13].Failure
When the failure occurred, the 18th floor collar was being temporarily suspended from the tower by a total of four polyester web slings. This temporary support was inadequate in numerous ways. The manufacturer's requirements called for eight chain blocks to be used, twice the number of supports provided and a more durable material. The slings that were installed were all located at positions inconsistent with the manufacturer's instructions. The method used to rig the slings to the tower was not in accordance with accepted industry practice and standards, it caused them to be chocked around the crane legs and seated in V-shaped grooves with unprotected edges. One of the slings used had been deteriorated by UV exposure and physical damage prior to use and should not have been installed. At the time of the collapse the erection crew was about to install the first W12x79 tie-in beam to the 18th floor collar connection. The tie-in beam was suspended from the crane, ready to be put in place shortly. At approximately 12:30 pm on March 15, 2008, the four polyester slings supporting the 18th floor collar broke. Then, the collar fell downward along the tower striking the 9th floor collar destroying its connection to the building. Proceeding to fall down, the 18th floor and 9th floor collars proceeded struck the 3rd floor collar damaging it. At this point only base friction and the damaged level 3 collar were left to resist overturning of the tower. While it was adequately designed to transfer vertical loads to the foundation through dunnage beams, the base was not designed to provide base friction in resistance to lateral loads. With inadequate base shear being provided the tower overturned the base sliding north towards the building and the top falling south away from the structure. The tie in beam that was suspended from the crane was flung through the air impaling and damaging an adjacent building. No alternate theories of the failure are apparent [13]. Figures 3 and 4 show the top and base of the tower crane after the collapse respectively.Aftermath
The collapse resulted in the death of six construction workers and one civilian, the demolition of a four-story brownstone on East 50th street, and damage to more than a dozen buildings in the East Side neighborhood. The investigation into the collapse concluded that the crane failure initiated by four inappropriately used synthetic slings were the cause of the collapse as described above. Multiple charges of manslaughter, criminally negligent homicide, assault, and reckless endangerment have been brought against the tower crane rigger and his company. 18 buildings had to be vacated following the collapse, three of which remained vacated due to outstanding repairs required to make them safe for occupancy as of March 11, 2009. Inspections of tower cranes undergoing jumping operations have been increased. New York City enacted 12 new laws designed to increase construction safety and regulatory oversight [13];[14].Other High Profile Crane Failures
Bellevue, Washington, Tower Crane Collapse (2006)
On November 16, 2006 a 210-foot tower crane being used to construct the 20 story Tower 333 office building in downtown Bellevue, Washington collapsed crashing into three neighboring buildings, killing a man as it demolished his condo. The failure was determined to be caused by metal fatigue in the steel base frame which was drastically under-designed. The base frame needed to be at least four times stronger [15]. The structural engineering firm responsible for the crane foundation design and the contractor paid fines and settled out of court with the victims family [16].Miami, Florida, Tower Crane Collapse (2008)
A 20 ft. section of crane tower being hoisted into place to jump a tower crane fell 30 stories killing two people on March 25, 2008. The tower crane was being heightened to continue construction of a high-rise condominium building on Biscayne Bay in Miami. As it fell, the crane tower segment crashed through a neighboring two story house on its way down. The previously erected crane segments remained intact [17].No conclusive determination of the exact cause of the failure is currently publicly available. This deadly collapse occurred only days before new local crane regulations, a reaction to a deadly 2006 crane collapse in Miami, came into effect [17].
335 East 91st Street, Manhattan, Tower Crane Collapse (2008)
On Friday, May 30, 2008, a crane cab came loose from its mast and toppled down to the street below damaging neighboring buildings as it fell. Two men were killed by the collapse. This event happened less than three months after the Tower crane collapse on East 51st Street. Initial reports blame failure of a shoddy turntable weld for the cab coming loose from the tower. The crane was in operation but not being jacked, assembled, or disassembled at the time of failure [18].According to a New York Daily News news report warnings about the safety of the weld were brought to the owner a year before the collapse. A Chinese company, RTR Bearings, was not confident in its welding technique and recommended the crane be welded by its owner in New York. The owner New York Crane then upped the price it would pay RTR bearings to perform the weld and suddenly RTR Bearings stopped trying to get out of the job. Even after the price had been raised RTR Bearings claimed it could do the weld for in only 46% of the time and for only 18% the cost stated by an Ohio-based firm in a competing bid. These huge time and price discrepancies should have sent a warning that things were too good to be true. Another unheeded warning that the weld in the crane was faulty came a month before the collapse. A radiographic analysis on another crane, owned by New York Crane, revealed a similar part made by RTR Bearings was unacceptable. Despite these warnings signs the crane was kept in operation and no test were performed to verify the safety of the weld [18].
Chicago, Illinois, Tower Crane Near Collapse (2009)
A potential disaster was avoided on January 21, 2009, in downtown Chicago by swift decision-making to suspend operations of a Potain MR605B luffing tower crane and investigate the cause of a loud popping sound. The crane was one of two located on top of an occupied 33 story Blue-Cross/Blue-Shield office building being used to erect a 24 floor vertical extension of structural steel to the existing building. Figure 5 shows the cranes located on top of the existing building during construction. The source of the popping sound was one of the four legs of the cranes mast cracking. Fortunately, the crane remained standing on three of its four legs for several days until the crane manufacturer provided a welding procedure to repair the crack. It was determined that the building did not need to be evacuated and the crane was already scheduled to be disassembled soon so the incident did not wreak major havoc on the building occupant's business, or on the construction schedule [19].Testing of similar mast components on other cranes was conducted to ensure that material defects were not systemic at the crane manufacturer's expense. The results of the testing indicated that material defects were not widespread. While material defects have not been ruled out as the source of the crack, there are alternative theories claiming misuse of the crane was responsible. One such theory is that the mast was torqued lost strength because crane tie-ins were not readjusted as the steel building was plumbed. This potentially catastrophic event clearly showed that diligence and caution when operating cranes can prevent failures from becoming deadly [19].
Philadelphia, Pennsylvania, Mobile Crane Collapse (2009)
Making national headlines most recently, at 1:30 pm on October 12, 2009, a mobile cherry picker type crane toppled over and struck an apartment building in downtown Philadelphia, PA. The cranes operator fell 125 ft to his death. Three people were injured when falling debris struck their vehicles. Initial reports indicate the crane tipped over when a fiberglass street hole cover owned by Comcast Cable collapsed below one of the cranes wheels. The street hole cover was never intended to support large machinery, the crane's wheel should not have been placed over it. Preliminary assumptions are that operator error will be blamed for this failure [20];[21].Conclusions
Cranes play an indispensable role in the construction industry, especially in high rise structures. It is a fact that cranes are involved with an alarmingly high number of casualties and fatalities, increasingly so in 2008. The sad truth is that the equipment and know-how exist to greatly reduce crane failure incidents but it's not used diligently throughout the construction industry. Furthermore, in some cases safe operation of cranes is hampered by outdated or contradicting regulations. While the proper use of current safety equipment, the incorporation of new devices such as the CRANIUM, and appropriate regulations can make it easier to use cranes safely, they can not put an end to crane failures themselves. As with most things, it comes down to the skills, motivation, and attention to detail of the people operating and caring for these sometimes gargantuan machines that will determine their safety.OSHA should update its crane regulations so that they are an effective minimum code of safe conduct for current crane operations. Ample crane inspectors must be employed to assure compliance with any new regulations. Under either OSHA regulation or its own volition the construction industry must insure that crane operators are qualified because countless lives including their own depend on it. Structural engineers must find a way despite the continually faster track of design and construction to verify that crane foundations and tie-ins are adequate.
The father of a victim of 91st street crane collapse told reporters his question since his son's death had been "Why?". After it was revealed that saving time and money by knowingly choosing to use a poor quality products he said "Now I have some answers. My son was killed for one reason only - greed." When corners get cut things fall between the cracks. With the stakes as catastrophically high as they are, corners should not be cut with cranes.
References
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