Rosemont horizon arena

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Rosemont Horizon Arena Timber Roof Collapse; Chicago

Carlos Nazario UAB REU Site 2000

The Rosemont Horizon Arena was 90% completed when suddenly its wood roof frame collapsed, on Monday, August 14, 1979 in Chicago, Illinois. In the accident, five construction workers were killed and 16 others injured. The construction cost of the 20,000 seats sport complex was $ 8 million and the estimated damages caused by the collapse was $ 3 million.

At the time of the collapse, one third of the roof had been placed. The roof was to have been supported by 16 glue-laminated timber arches 6.1 feet deep and spanning 288 feet. The timber arches were made up of three separate pieces and erected in three different stages. Concrete columns at the ends supported them, and concrete buttresses bearing on underground thrust blocks braced 14 of the 16 arches. Timber was selected to diminish the noise produced by air traffic on the nearby Chicago's O'Hare Airport.

The arches were tied by 3.1 feet deep girders interspersed by sets of three purlins. Angle irons were supposed to hold the arches and girders together. The angles were connected to the arches with several bolts and to the girders with three bolts. During the construction stage, the supervising engineer approved the plan of leaving two of the three bolts out of the girder-arch connection in order to allow the timber elements to deform downward as much as possible under the purlins and deck loads before installing the other bolts. To compensate for the missing bolts the roof erector subcontractor designed steel plates that were supposed to connect temporarily the arches and girders with lag bolts.

A post-collapse investigation carried by the Occupational Safety and Health Administration revealed that the cause of the collapse was the unstable condition of the wood roof frame. Over 53 percent of the required connection bolts were missing from the building's roof. Of the 944 girder bolts required for the connections already installed, only 444 were in place. Of these, 338 had no nuts, and some of the nuts in place were only finger-tight. OSHA also discovered that only 27 percent of the "compensating steel plates" were properly installed. Although the missing bolts were found the triggering cause, it was proved that inadequate bracing and the stockpiling of materials in the roof contributed to the collapse. Several other violations were attributed to the roof erector, who was severely fined by OSHA. The project's architect and other subcontractors were also fined for diverse irregularities. Even the independent engineering firm retained by the city to investigate the collapse was fined by OSHA for unnecessarily exposure of their employees to fall hazards during field inspection.

In an interesting note, one year after the roof collapse, concrete stands under construction also collapsed at the Rosemont Horizon, dumping 34 tons of concrete to the ground. No fatalities or major injuries resulted from this accident.

Lessons Learned:

The three main construction materials, concrete, steel, and wood were used in the construction of the Rosemont Horizon Arena taking into consideration their unique physical and mechanical properties. On recent years, the combined use of construction materials, including composite materials, has been very well accepted by the construction industry in general.

It is important to understand how the materials will interact with each other when tied or arranged together. On the case previously discussed the supervising engineer wanted the timber arches to deflect as much as possible before making the final connections, what seemed like a good idea. A steel plate was designed to take care of the loads affecting the arches, what was also the correct procedure. But he underestimated the wind load effect on the timber arches and didn’t take into consideration their weight and flexibility, what resulted on a very unstable structure. The steel plates were not placed correctly and there were too many bolts missing.

On any kind of construction project important decisions have to be made, sometimes in a short amount of time. Before making these decisions the engineer should be completely sure of the accuracy of his plan.

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