On July 21, 1998, construction workers noticed slight bowing near the 20th floor of the 49-story scaffold system located at the east end of the north face of the building on 43rd Street. Minutes later the scaffold above the 20th floor crashed and dropped approximately 8 feet weakening the ties between the personnel hoist system and the attached scaffold, which provided lateral restraint to the hoist structure. One woman was killed when debris fell into her apartment, while dozens of others were injured by the falling debris. The accident caused hundreds to be homeless for an extended period of time and the area around Times Square was paralyzed for days. The accident caused the shutdown of 150 area businesses, affecting thousands of people.
Key Words
Personnel Hoist System, Scaffold, Buckling, Collapse, Conde Nast Building, 4 Times Square, Temporary Structure, Failure During Construction
Introduction
On Tuesday, July 21, 1998, the morning rush hour in the Times Square area of New York City was interrupted by the partial collapse of an exterior scaffold/personnel hoist system being used to construct the 49 story tall Conde Nast Building at Four Times Square. Approximately five minutes before the collapse workers observed the scaffold buckling at the 20th floor. At 8:25 a.m. the scaffold buckled and collapsed 8 feet causing the top 24 floors of the hoist to fall to the ground and surrounding buildings below. It was later determined through a reconstruction of the scaffolding and a finite element analysis that the cause of the collapse was a lack of lateral bracing located between the 19th and 23rd floors. The bracing was designed for, but was not installed. This caused the unbraced length to be increased and the strength was reduced, causing the structure to collapse. The accident caused the death of one elderly woman who was in her neighboring top floor apartment building, while hundreds were left homeless for weeks while the area was made safe. It also forced the area to be evacuated and shutdown while clean up was being done. This ended up costing the surrounding businesses millions of dollars. (Pristin, 1998)
Description of the Scaffold Structure
Figure 2 - Based on Velivasakis, 2000
The 4 Times Square hoist structure was designed by a well known hoist supplier, who were also the designers of scaffolds and hoist towers for several other major projects in he New York City area. Each platform of the hoist tower was designed to carry a 100 psf load. Two platforms were assumed to be fully loaded at any one time. The hoist support tower structure is built up from stacks of 6.5 ft. tall, 6 ft wide knee braced frames made from 3.5 in diameter standard aluminum pipes
Figure 3 - Based on Velivasakis, 2000
with walls varying in thickness. These stacks can be seen in Figure 2. The bottom of one frame stacks into the top of another via an insert at the top of each leg. The frame stacks are connected to each other with cross bracing seen in Figure 3. Each of the hoist bays are 15 ft. wide by 6 ft. deep. The 15 ft. side is formed alternately by a channel at the floors that tie back to the building, or a truss like "landing frame". The hoist support structure was tied back to the building at two points at every even-numbered floor through strut anchorage's welded into spandrel beams. The hoist extended approximately 10 ft. from the building at ground level, increasing to 50 ft. at the top of the structure because building
Figure 4 - Based on Velivasakis, 2000/Rentschler, 2000
setbacks. At each floor level, the workers were able to exit the cars and walk across scaffold wood planking into the building. Figure 4 shows a plan view of the typical hoist back structure.(Velivasakis, 2000)
Events Leading to the Collapse
The hoist structure was topped out at 49 stories two days prior to the collapse. The day of the collapse, work started on the site at 7:00 a.m. that morning, and the personnel and materials hoists were busy transporting workers and materials up and down the 50 story tower.(Velivasakis, 2000)
At 7:30 a.m., a large delivery truck showed up at the construction site. Because of its length, in order to unload its cargo, construction workers had to temporarily close down 43rd street to vehicular traffic, for the truck to back up to a temporary loading ramp near the building.(Velivasakis, 2000)
At 8:15 a.m., the operator of the western most cab of the personnel lift, on its way up with 10 passengers, felt severe vibrations. He stopped the cab at the 23rd floor, looked up and down, and saw nothing out of the ordinary, other than some water dripping above. He then noticed workers on the adjacent material hoist platform shouting at him to get off. He opened the cab side door, but the platform side door had jammed against the scaffold. He managed to pry it open, discharge his passengers, and leave towards the building. In the middle of the runway, however, he remembered that he had left his bag in the cab. On the way back to pick up his bag, the runway dropped from under him. He grabbed a piece of rope and, with the help of others, pulled himself into the building.(Velivasakis, 2000)
The Collapse
On July 21, 1998, at approximately 8:20 a.m., workers noticed a slight buckling near the 20th floor and alerted other workers and pedestrians on 43rd Street to clear the area. Approximately five minutes later, severe buckling near the 20th floor caused the scaffold above to drop approximately 8 ft. The alternate level links
Figure 5 - Debris From the Collapse - Permission Pending
from the hoist mast to the hoist support tower rotated downward and pulled the mast and its then upper cab in toward the scaffold. While the cab wedged itself into the scaffold and the lower mast of the hoist leaned toward the scaffold, the upper 24 stories of the 3 ft. by 2 ft. lattice hoist shaft toppled and fell. The hoist debris landed as far away as 44th Street as seen in Figure 5. One portion of the shaft went through the roof of a neighboring hotel, killing a woman in her 12th floor apartment. On the east corner of the building along 43rd Street, the 25 ft. wide upper half of the scaffold ended up hanging from the struts originally planned to stabilize it back to the building frame. These struts rotated down and acted as hangers which in combination with the leaning mast and the wedged cab, kept the upper portion of the scaffold from completely collapsing. Figure 6 shows how the upper half of the scaffolding hung off of the building by the struts. (Velivasakis, 2000)
Cause of Failure
Figure 6 - The Scaffolding Above the 22nd Story Can Be Seen Hanging - Permission Pending
On July 28, after the failure, the New York City Department of Buildings hired Wiss, Janney, Elstner and Associates to determine the cause of the collapse. WJE, along with other investigators representing other involved parties, labeled each piece of the scaffold between the 19th and 29th floors as it was removed from the site so that it could be reassembled at an offsite location. The scaffold was then transported to a pier in Brooklyn where investigators laid out and closely examined the members between floors 19-23. It was observed after comparing the scaffold to the construction drawings provided by the scaffold supplier that a number of braces were missing. The missing braces can be seen in Figure 7. (Rentschler, 2000)
The missing braces caused the unbraced length of the scaffold frame legs on these floors to be increased thus reducing the strength of the brace. A finite element analysis confirmed that the missing bracing severely reduced the strength of the scaffold to resist vertical loading. As a result of the missing braces the
Figure 7 - Based on Rentschler, 2000
frame legs located at the 21st floor buckled. The buckling can be seen in Figure 8. It was determined that the legs would buckle at a load 34% under the design load. This theory was also confirmed by the construction workers who reported that the collapse originated around the 20th and 21st floors.(Rentschler, 2000)
It was determined, based on the sequence of construction, that the braces were never installed to begin with. The type of lateral bracing found to be missing was essential to providing lateral stability of the
Figure 8 - Based on Rentschler, 2000
scaffold system to prevent buckling form occurring. Since the bracing was not installed, the frame legs buckled under normal loading conditions, thus compromising the structure and causing a partial collapse. (Rentschler, 2000)
Stabilization of the Structure
In order to stabilize the structure, a plan was developed in order to prevent any more damage to the surrounding buildings. The plan called for outriggers at floors 48, 43, 41, 39, 37, 31, 27, 25, 23, and 21. The outriggers were slowly pushed out to cantilever 35 feet into the wreckage to support the scaffold both vertically and laterally. A sling cable was also called for under the 5,000 lb. wedged cab. During the construction of the outriggers, workers began to sling a 5/8 in dia. steel cable under the cab and attach it to steel building columns at the 26th floor. The plan also called for two horizontal cables around the leaning mast and 14 sling cables at odd floors beginning at the roof. To stabilize the inward leaning mast, the workers wrapped horizontal cables at floors 10 and 22. Other diagonal cables, also tied back to the building columns, looped around the scaffold from the roof down at odd numbered floors.(Velivasakis, 2000)
Workers also immediately began work on hanging a reinforced nylon mesh net curtain. This curtain was intended to keep any small falling objects inside the scaffold's footprint and prevent further damage to surrounding buildings or injury to workers.
Clean Up Procedure
With the scaffold strapped back to the building and wrapped with a net, the demolition process began in early August. The process involved using a tower crane at the top of the building to hang a 40 cu. yd. debris container outside the net, and two rooftop stiff leg derricks. One derrick was used to hang a 15 ft. square aluminum work platform inside the net and a second derrick for two hooks to use as necessary to bundle large sections of the scaffold components and other debris. During the process it was necessary for workers to wear full body harnesses, rope grips, and safety lines while they unbolted or cut components from top to bottom. As each piece of the scaffold was deconstructed, it was labeled by investigators and transported to an offsite location so it would be examined later to assist in the determining the cause of the collapse could be determined. (Velivasakis, 2000)
Effects on the City
The 4 Times Square Scaffold Collapse had a large impact on the city. During the collapse, a 50 ft mast section plunged through the roof of a 12 story hotel across the street, causing the death of an elderly woman in her top floor unit. The debris also damaged the building's roof and 12th floor, requiring some beams to be replaced. Debris also damaged an area parking garage that
required repairs before it could reopen. (Pristin, 1998)
The collapse could have been much more damaging however. Construction workers, after receiving warnings by radio, rushed to clear pedestrians from 43rd street seconds before the debris hit the street. Fearing a secondary collapse and more falling debris, city officials evacuated surrounding buildings and close streets for several blocks all around. These surrounding buildings and businesses were kept closed for the following four weeks until clean up of the collapsed hoist was complete. This had a detrimental effect on the local businesses costing some businesses millions to keep their doors shut. A local theater missed 35 performances during that time causing them to lose more than $1.5 million in ticket sales. (Terry, 1998)
Preventative Measures
This tragedy could have been avoided had all the engineers regarded their duty to the public welfare as paramount. They would have not taken the chances they did by allowing the workers to randomly remove X-braces that were essential for the balance and support of the structure. The on-site engineer should have paid more attention to the procedures and happenings on the site and he/she should have informed his/her workers of the consequence of their actions. (Skourou, 2001)
Conclusion
On July 21, 1998, approximately five minutes before the collapse workers observed the scaffold buckling at the 20th floor. At 8:25 a.m. the scaffold buckled and collapsed 8 feet causing the top 24 floors of the hoist to fall to the ground and surrounding buildings below. The debris caused the death of one elderly woman, as well as damage to surrounding buildings. It was determined through a reconstruction of the scaffolding and a finite element analysis that the cause of the collapse was a lack of lateral bracing intended for location between the 19th and 23rd floors. The bracing was correctly designed, but was never installed. This caused the unbraced length to be increased while the strength was reduced, causing the structure to collapse. An extensive plan was created to stabilize the structure and remove the remaining scaffold. The process took weeks to complete and caused area businesses to be closed while work was completed.
This resource contains a newspaper article written a month after the collapse. It details some of the damages caused, both direct costs (damage) and indirect costs (loss of customers for surrounding businesses.)
This resource contains background information regarding the collapse.
The Times Square Scaffold Collapse. Glen P. Rentschler and Stephanie Wallup, ASCE Conf. Proc. 280, 9 (2000), DOI: 10.1061/40482(280)9.
This resource is a report created by WJE detailing the investigation and results of the investigation.
4 Times Square Hoist Collapse - The Handling of a Potential Disaster. Emmanuel E. Veliuasakis, ASCE Conf. Proc.280, 8 (2000), DOI: 10.1061/40482(280)9.
This resource is a report detailing the events leading up to the collapse and the events immediately following the collapse, and details the clean up procedure.
Paul Stewart, BAE/MAE, Penn State, 2010
Note: Pictures are pending permission.
Abstract
Figure 1: Elevation of the Scaffold System at 4 Times Square - Permission Pending
Table of Contents
Key Words
Personnel Hoist System, Scaffold, Buckling, Collapse, Conde Nast Building, 4 Times Square, Temporary Structure, Failure During Construction
Introduction
On Tuesday, July 21, 1998, the morning rush hour in the Times Square area of New York City was interrupted by the partial collapse of an exterior scaffold/personnel hoist system being used to construct the 49 story tall Conde Nast Building at Four Times Square. Approximately five minutes before the collapse workers observed the scaffold buckling at the 20th floor. At 8:25 a.m. the scaffold buckled and collapsed 8 feet causing the top 24 floors of the hoist to fall to the ground and surrounding buildings below. It was later determined through a reconstruction of the scaffolding and a finite element analysis that the cause of the collapse was a lack of lateral bracing located between the 19th and 23rd floors. The bracing was designed for, but was not installed. This caused the unbraced length to be increased and the strength was reduced, causing the structure to collapse. The accident caused the death of one elderly woman who was in her neighboring top floor apartment building, while hundreds were left homeless for weeks while the area was made safe. It also forced the area to be evacuated and shutdown while clean up was being done. This ended up costing the surrounding businesses millions of dollars. (Pristin, 1998)
Description of the Scaffold Structure
The 4 Times Square hoist structure was designed by a well known hoist supplier, who were also the designers of scaffolds and hoist towers for several other major projects in he New York City area. Each platform of the hoist tower was designed to carry a 100 psf load. Two platforms were assumed to be fully loaded at any one time. The hoist support tower structure is built up from stacks of 6.5 ft. tall, 6 ft wide knee braced frames made from 3.5 in diameter standard aluminum pipes
Events Leading to the Collapse
The hoist structure was topped out at 49 stories two days prior to the collapse. The day of the collapse, work started on the site at 7:00 a.m. that morning, and the personnel and materials hoists were busy transporting workers and materials up and down the 50 story tower.(Velivasakis, 2000)
At 7:30 a.m., a large delivery truck showed up at the construction site. Because of its length, in order to unload its cargo, construction workers had to temporarily close down 43rd street to vehicular traffic, for the truck to back up to a temporary loading ramp near the building.(Velivasakis, 2000)
At 8:15 a.m., the operator of the western most cab of the personnel lift, on its way up with 10 passengers, felt severe vibrations. He stopped the cab at the 23rd floor, looked up and down, and saw nothing out of the ordinary, other than some water dripping above. He then noticed workers on the adjacent material hoist platform shouting at him to get off. He opened the cab side door, but the platform side door had jammed against the scaffold. He managed to pry it open, discharge his passengers, and leave towards the building. In the middle of the runway, however, he remembered that he had left his bag in the cab. On the way back to pick up his bag, the runway dropped from under him. He grabbed a piece of rope and, with the help of others, pulled himself into the building.(Velivasakis, 2000)
The Collapse
On July 21, 1998, at approximately 8:20 a.m., workers noticed a slight buckling near the 20th floor and alerted other workers and pedestrians on 43rd Street to clear the area. Approximately five minutes later, severe buckling near the 20th floor caused the scaffold above to drop approximately 8 ft. The alternate level links
Cause of Failure
On July 28, after the failure, the New York City Department of Buildings hired Wiss, Janney, Elstner and Associates to determine the cause of the collapse. WJE, along with other investigators representing other involved parties, labeled each piece of the scaffold between the 19th and 29th floors as it was removed from the site so that it could be reassembled at an offsite location. The scaffold was then transported to a pier in Brooklyn where investigators laid out and closely examined the members between floors 19-23. It was observed after comparing the scaffold to the construction drawings provided by the scaffold supplier that a number of braces were missing. The missing braces can be seen in Figure 7. (Rentschler, 2000)
The missing braces caused the unbraced length of the scaffold frame legs on these floors to be increased thus reducing the strength of the brace. A finite element analysis confirmed that the missing bracing severely reduced the strength of the scaffold to resist vertical loading. As a result of the missing braces the
frame legs located at the 21st floor buckled. The buckling can be seen in Figure 8. It was determined that the legs would buckle at a load 34% under the design load. This theory was also confirmed by the construction workers who reported that the collapse originated around the 20th and 21st floors.(Rentschler, 2000)
It was determined, based on the sequence of construction, that the braces were never installed to begin with. The type of lateral bracing found to be missing was essential to providing lateral stability of the
scaffold system to prevent buckling form occurring. Since the bracing was not installed, the frame legs buckled under normal loading conditions, thus compromising the structure and causing a partial collapse. (Rentschler, 2000)
Stabilization of the Structure
In order to stabilize the structure, a plan was developed in order to prevent any more damage to the surrounding buildings. The plan called for outriggers at floors 48, 43, 41, 39, 37, 31, 27, 25, 23, and 21. The outriggers were slowly pushed out to cantilever 35 feet into the wreckage to support the scaffold both vertically and laterally. A sling cable was also called for under the 5,000 lb. wedged cab. During the construction of the outriggers, workers began to sling a 5/8 in dia. steel cable under the cab and attach it to steel building columns at the 26th floor. The plan also called for two horizontal cables around the leaning mast and 14 sling cables at odd floors beginning at the roof. To stabilize the inward leaning mast, the workers wrapped horizontal cables at floors 10 and 22. Other diagonal cables, also tied back to the building columns, looped around the scaffold from the roof down at odd numbered floors.(Velivasakis, 2000)
Workers also immediately began work on hanging a reinforced nylon mesh net curtain. This curtain was intended to keep any small falling objects inside the scaffold's footprint and prevent further damage to surrounding buildings or injury to workers.
Clean Up Procedure
With the scaffold strapped back to the building and wrapped with a net, the demolition process began in early August. The process involved using a tower crane at the top of the building to hang a 40 cu. yd. debris container outside the net, and two rooftop stiff leg derricks. One derrick was used to hang a 15 ft. square aluminum work platform inside the net and a second derrick for two hooks to use as necessary to bundle large sections of the scaffold components and other debris. During the process it was necessary for workers to wear full body harnesses, rope grips, and safety lines while they unbolted or cut components from top to bottom. As each piece of the scaffold was deconstructed, it was labeled by investigators and transported to an offsite location so it would be examined later to assist in the determining the cause of the collapse could be determined. (Velivasakis, 2000)
Effects on the City
The 4 Times Square Scaffold Collapse had a large impact on the city. During the collapse, a 50 ft mast section plunged through the roof of a 12 story hotel across the street, causing the death of an elderly woman in her top floor unit. The debris also damaged the building's roof and 12th floor, requiring some beams to be replaced. Debris also damaged an area parking garage that
required repairs before it could reopen. (Pristin, 1998)
The collapse could have been much more damaging however. Construction workers, after receiving warnings by radio, rushed to clear pedestrians from 43rd street seconds before the debris hit the street. Fearing a secondary collapse and more falling debris, city officials evacuated surrounding buildings and close streets for several blocks all around. These surrounding buildings and businesses were kept closed for the following four weeks until clean up of the collapsed hoist was complete. This had a detrimental effect on the local businesses costing some businesses millions to keep their doors shut. A local theater missed 35 performances during that time causing them to lose more than $1.5 million in ticket sales. (Terry, 1998)
Preventative Measures
This tragedy could have been avoided had all the engineers regarded their duty to the public welfare as paramount. They would have not taken the chances they did by allowing the workers to randomly remove X-braces that were essential for the balance and support of the structure. The on-site engineer should have paid more attention to the procedures and happenings on the site and he/she should have informed his/her workers of the consequence of their actions. (Skourou, 2001)
Conclusion
On July 21, 1998, approximately five minutes before the collapse workers observed the scaffold buckling at the 20th floor. At 8:25 a.m. the scaffold buckled and collapsed 8 feet causing the top 24 floors of the hoist to fall to the ground and surrounding buildings below. The debris caused the death of one elderly woman, as well as damage to surrounding buildings. It was determined through a reconstruction of the scaffolding and a finite element analysis that the cause of the collapse was a lack of lateral bracing intended for location between the 19th and 23rd floors. The bracing was correctly designed, but was never installed. This caused the unbraced length to be increased while the strength was reduced, causing the structure to collapse. An extensive plan was created to stabilize the structure and remove the remaining scaffold. The process took weeks to complete and caused area businesses to be closed while work was completed.
References