John+Hancock+Center+(Chicago)+Suspended+Scaffold+Collapse

=Introduction= toc At 1:45 p.m. on Saturday March 9, 2002 a scaffolding accident occurred at the John Hancock Center in Chicago, Illinois. The scaffold platform was suspended by two outriggers on the roof of the building, and on the day of the collapse, the north outrigger overturned and fell to the ground below. The south outrigger remained intact, causing the scaffold platform to swing back and forth along the west facade of the building. When this happened, the scaffold broke multiple windows of the John Hancock Center and the debris from the scaffolding structure fell onto East Chestnut Street below. Three motorists were killed by the falling scaffold pieces and several other people were injured. At the time of the collapse, there were no workers on the scaffold platform due to the inclement weather conditions. High winds caused the contractor to suspend work for the day, and the scaffold platform was being stored approximately half way up the building when the collapse occurred. It was determined that many factors contributed to the collapse of the scaffolding, including design and procedural errors by multiple parties.
 * John Hancock Center - Suspended Scaffold Collapse (March 9, 2002)**
 * Ryan Dalrymple AE537 Fall 2011 **

=Keywords= Suspended Scaffold, Outrigger, John Hancock Center, Scaffold Standards, Cable Truss, Scaffold Failure

=Background= The John Hancock Center is one of the tallest skyscrapers in Chicago, Illiniois (see Figure 1). The 1,127-foot tall, 100-story building consists of residential condominiums and a commercial portion. In 1999 it was decided by the owners of the building to restore the facade of the building, which included replacing all window sealants and cleaning the aluminum cladding on the exterior of the building. AMS Architectural Technologies Inc. was the contractor that was hired to do the restoration (Roe 2002). Beginning in 2000, AMS began work on the John Hancock Center. In order to perform the work, a suspended scaffold system was used by AMS that was provided by a scaffold supplier. The scaffold supplier designed a system of platforms constructed primarily out of aluminum. Figure 2 shows a 3-dimensional view of the scaffold platform suspended from the face of the John Hancock Center. Simpson Gumpertz & Heger Inc. holds the rights to this image. The platforms used were 100 feet long and 4 feet wide. Each platform was supported by two outriggers located on the roof of the John Hancock Center. The outriggers that supported the suspended scaffold platforms were designed to roll on the existing tracks for the rig used for window-washing.

=Collapse= On the day of the collapse, there were wind speeds of 35 mph with gusts of up to 56 mph. Due to the windy conditions, the contractor decided not to work that day. The suspended scaffold platform was stored on the outside of the building at the 42nd floor of the Hancock Tower. This was done because there are access doors at this level of the building. At 1:45 p.m. the north outrigger which supported the suspended scaffolding fell from the roof and landed on the sidewalk below. When this outrigger failed, the scaffold platform was no longer supported on the one side which caused it to swing along the facade of the building. When the scaffold platform impacted the building, a portion of the aluminum frame disintegrated and the debris fell onto the street 42 stories down. The falling wreckage killed 3 women that were in their cars at the time of the accident, and eight other people were injured from the falling debris (USA Today 2002). One woman died three years later in 2005, due to the brain and spinal injuries that she suffered on the day of the accident (Claiborn 2006).

=Investigation= The investigation of the collapse lasted over two years. Shortly after the accident, the debris from scaffolding was taken to a warehouse where it was sorted and weighed (Zimmer, Bell 2006). Many lab tests were done on the scaffold remains to determine the actual cause of the accident. Wind tunnel testing was done to simulate the conditions on the day of the collapse. In addition, the design of the outrigger that supported the scaffolding was checked in order to determine if it met code requirements. Pieces of the scaffold were examined in order to determine the condition of the scaffolding. The fabrication quality of the components and conformance with the design drawings were also looked at during the investigation. The under-flange wheels on the track roller assembly were tested because it was determined that they were crucial components in the collapse of the scaffold system.

=Findings= It was found that the north outrigger failed due to the combination of dead and wind load that it was subjected to. It was found that the design of the outrigger was inadequate for the design loads. In addition, the original undercarriage rollers had been replaced with smaller ones and the outrigger A-frames were not substantially secured to the roof track. Errors by multiple parties were found and ultimately led to the collapse of the scaffolding.

The contractor had the scaffold platform stored at the 42nd floor of the Hancock Center at the time of the collapse. The operations manual provided by the scaffold supplier clearly stated that the platform should have been lowered to the ground or raised to the roof when it was not in use. The contractor did not follow this operation procedure due to amount of time that it took at the beginning and end of the work day (Zimmer, Bell 2006). By mooring the scaffold platform at the 42nd floor, the contractor saved thousands of dollars each week. The scaffold supplier also provided friction clamps for the scaffold platform that were supposed to be used in windy conditions. These clamps were never utilized by the contractor.

The Occupational Safety and Health Administration requires that a scaffold structure must be able to resist four times the sum of the dead and live loads (OSHA 3150 2002). When the design of the outrigger that supported the suspended platform was analyzed, it was determined that the outrigger was inadequate for the design loads (Zimmer, Bell 2006). It was ultimately found that multiple deficiencies existed in the design of the outrigger and its components.

The mechanism of the collapse was determined to be the slipping of the fist-grip splice in the north outrigger's cable truss (Zimmer, Bell 2006). In turn, this made the cable truss ineffective and caused the entire outrigger to become unstable. This led to a large amount of uplift on the inboard track roller carriages which became overstressed and ultimately failed causing the outrigger to overturn. Figure 3 shows a 3-dimensional view of the scaffold outrigger and all of its components. Simpson Gumpertz & Heger Inc. holds the rights to this image. The cable truss and fist-grip clips that failed during the collapse can be clearly seen at the top of the outrigger.

The wheels used to secure the outriggers to the roof track were not the ones that were originally intended for the original design of the scaffold system. The design called for 3-1/2 in. diameter wheels, although wheels as small as 1-1/2 in. were used (Van Hampton 2006). The insufficient wheels failed during the collapse of the north outrigger. Figure 4 shows a diagram of the track roller carriage. Simpson Gumpertz & Heger holds the rights to this image. The actual wheels that were installed were less than half the diameter of the wheels that were designed for the connection to the track.

=Code Changes= The scaffold collapse at the John Hancock Center caused the code to change in the city of Chicago with respect to scaffolding design and safety. Prior to the collapse, Chicago did not require permits for scaffolding, and only required that scaffolding "be so constructed as to ensure the safety of persons working on or passing under or passing by the scaffold" (Khudeira 2009). After the collapse, code officials looked at the code and determined that changes needed to be made. In July of 2002, a scaffolding ordinance was passed which included more stringent requirements for the design and construction of scaffolding. In addition, this ordinance required the users and erectors of scaffolding to take training courses.

It is estimated that 65 percent of workers in the construction industry use scaffolding on a regular basis (Maxwell 2005). For this reason, OSHA published a document titled A Guide to Scaffold Use in the Construction Industry. This document was updated in 2002 after the scaffold collapse at the John Hancock Center. The guide outlines OSHA's Safety Standards for Scaffolds Use in the Construction industry in a question and answer format. The OSHA standard is extensive and it can often be difficult to address every requirement in the standard. This guide addresses common questions about the scaffold standard in order to make it easier for contractors, engineers and scaffold suppliers to be able to follow the standard. The ultimate goal of this document is to prevent scaffold failures like the one that occurred at the John Hancock Center in 2002.

=Similar Cases= The scaffold system of 4 Times Square in New York City collapsed on July 21, 1998. When the scaffold structure collapsed, a woman was killed by falling debris that ended up in her apartment, and many others were injured by debris that fell onto the street. When scaffolding is used above a busy city, extra precautions should be taken in order to make sure that the scaffolding is safe and does not injure the large number of people that are passing by the scaffolding throughout the day. The scaffold structure that collapsed at 4 Times Square was unlike the scaffold system that collapsed at the John Hancock Center in that it was not suspended from the roof of the building. The scaffolding consisted of many platforms that were stacked on top of one another. It was found that this scaffold tower failed due to the lack of lateral bracing to the structure. This lateral bracing was designed by the scaffold provider although was never installed by the contractor. A detailed description of the scaffold collapse at 4 Times Square and the causes of the failure can be found here: Four Times Square - Scaffold Collapse.

=Conclusions= While scaffolding is a temporary structure, it should be taken as seriously as any permanent structure. The scaffold collapse at the John Hancock center is an example where many technical and procedural errors ended up in a failure. The scaffolding was improperly designed and erected. In addition, the scaffolding was not being used by the contractor as it was intended. This accident could have been prevented if the proper precautions were taken and the obligations of all parties were met.

=Bibliography= This article gives information about the lawsuits that were filed due to the accident and the settlements that were made.
 * Claiborn, Lindsay (2006). "$75 Million Settlement in Hancock Scaffolding Collapse." //Chi Town Daily News// (Chicago), February 2, 2006.** [].

This USA Today article summarizes the accident and gives information on the investigation of the parties at fault for the failure.
 * (2002). "Investigators Probe Scaffolding Accident in Chicago," //USA Today//, March 10, 2002.** [].

This report published by ASCE discusses the changes that were made to Chicago’s Scaffolding Code after the John Hancock Center scaffolding collapse.
 * Khudeira, Soliman (2009). "Scaffolding on High-Rise Buildings", Pract. Period. Struct. Des. Constr. 14, 11.** [].

This magazine article discusses the safety precautions that should be taken when using scaffolding in the construction industry.
 * Maxwell, Mike (2005). "Scaffold Safety." //Constructioneer//, December 2005, 14.**

This informational booklet published by OSHA provides standards and requirements for scaffolding used in the construction industry.
 * Occupational Safety and Health Administration (2002). "A Guide to Scaffold Use in the Construction Industry" OSHA 3150.** [].

This ENR article looks at the current codes that relate to the type of scaffolding used on the John Hancock Center and the possible causes of the collapse.
 * Roe, Andrew G. (2002). "Fatal Scaffold Collapse Prompts Scrutiny of Code." //Engineering News-Record//, March 25, 2002.** [].

This ENR article discusses the causes of the scaffold accident at the Hancock Center.
 * Van Hampton, Tudor (2006). "Experts Say Improper Rigging Caused Hancock Accident." //Engineering News-Record//, October 23, 2006.** [].

This report that was authored by the orginal investigators of the collapse and published by ASCE provides a summary of the collapse of the scaffolding at the John Hancock Center and investigates the causes of the failure.
 * Zimmer, Alec S. and Bell, Glenn R. (2006). "John Hancock Center Scaffold Collapse." ASCE Conf. Proc. 217, 10.** [].