Chicago post office
Chicago U.S. Post Office Collapse
Carlos Nazario UAB REU Site 2000
On October 1995, a new $317 million U.S. Post Office facility was inaugurated in the downtown area of Chicago, Illinois. The building is five stories tall and occupies a site of 10,600 square feet, making it one of the largest in downtown Chicago. On November 5, 1993 the construction process of the imposing building was marked by the fatal collapse of a large portion of its structural steel frame. More than 50 tons of structural steel members fell, killing 2 ironworkers and injuring 5 others. The collapse was attributed to the failure of a temporary beam to column connection.
The Architect/Engineer of the project designed the steel structure of the building as well as its connections. When the steel fabricator/erector reviewed the contract drawings and specifications, he decided to change the construction sequence of some of the beam to column connections to simplify the construction work. This seemed reasonable since the structural design of the connections would not be altered.
The beam-to-column connection construction sequence as redesigned by the steel erector consisted of connecting two plates to the web of the beam using 30, 1-in.-diameter bolts in the shop, then connect a temporary erection angle to the web of the column with two, 1-in.-diameter bolts with nuts in the shop. Then, in the field, the bottom flange of the beam was to be placed on the temporary erection angle at a distance of 1-in.-diameter from the face of the column web and connected the beam flange to the temporary erection angle with two, 1-in.bolts with nuts. At the end, to finish the connection, the two connection plates (previously bolted to the web of the beam) were to be welded to the web of the column and the temporary erection angle disconnected. This connection construction sequence is typical for low-rise structural steel frames.
After an in depth investigation it was revealed that important changes were made to the temporary beam to column connection construction. First, the beams were placed 1 1/4 in. from the face of the columns instead of the specified 1 in. on the erector design. The 1/4 in. difference increased the eccentricity of the load from the beam on the erection angle and prevented the use of the specified two 1-in.-diameter bolts with nuts to connect the beam to the temporary erection angle. Only one 3/4-in.-diameter bolt was used to connect the inferior flange of the beam to the temporary erection angle. The 3/4-in.-diameter bolt was installed without the corresponding nut; this considerably reduced the safety of the connection. On-site investigation revealed that on 44 of 47 similar connections only one 3/4-in.-diameter bolt without nut was placed.
Finite element analyses showed that if the 3/4-in.-diameter bolt would have been used with a nut the ultimate capacity of the erection angle would have been increased and the failure might have been prevented.
The steel fabricator/erector of the project was fined by the Occupational Safety and Health Administration, charged with two misdemeanors by the U.S. Department of Justice and faced criminal charges leveled by the U.S. Department of Labor. Claims by the families of the killed and injured workers in the collapse were settled.
The Architect/Engineer of the project was cleared of any liability for the accident.
Lessons Learned:
It is a normal procedure for the steel fabricator/erector to change the construction method of steel structures designed by the engineer, as long as the original design is maintained and engineering principles are followed. It is essential to communicate the details of the changes and the consequences of not following them.
