The Collapse of Willow Island Cooling Tower

The second willow island cooling tower in West Virginia collapsed while still under construction on April 27, 1978, killing all 51 workers. It is regarded as the deadliest construction accident in the whole of US history.

The Cooling Towers

In the late 1970s, as part of the Pleasants Power Station, two cooling towers known as the Willow Island cooling towers were built, the first being completed in 1977. The towers are formidable reinforced concrete structures with a base diameter of 109m, standing 131 m high. Their diameter and thickness varied with height, and their hyperbolic profile allows for a natural design. The tower design and construction were carried out by Research-Cottrell, Inc., Hamon Cooling Tower Company, which used its proprietary lift-form system consisting of a formwork and scaffolding system that ascended the tower in what was known as a sequence of ‘lifts’ during the construction. Prior to starting the Willow Island project, Research-Cottrell had successfully used this system in the construction of 36 other towers.

The lift-form system consisted primarily of four levels of scaffolding attached to jack frames, which were in turn supported by jump-form beams attached to the shell of the tower (Figure 2). The jump-shaped beams mounted both inside and outside the frame, were connected by anchor bolts that transferred the framework and loaded directly into the shell of the tower. Every jump-shaped beam consisted of an upper and lower portion, with the lower beam effectively leapfrogging over the upper beam as the tower ascended the structure.

In order to jump the system, the formwork of the previous day’s lift was removed; the lower jump-formed beams were detached from the tower shell, moved upward, and reassembled above the upper jump-shaped beam. Meanwhile the jacket frame was jacked upwards. When in place, the formwork will be mounted for that day’s pouring or ‘lifting,’ with the four scaffolding levels providing access to these activities. Below Lift 10, construction materials were lifted by stand-alone cranes, but above this level, six cathead gantry cranes, assisted by the lift-form system and positioned around the tower’s circumference at 60 ° angles, took over the lift function.

One leap or raise every day was done by the system: it was jumped up by 1.5 m in the morning and a 1.5 m high portion of the tower shell diameter was completed. The Process was repeated each day. Therefore, workers arrive on-site at 6:30 a.m. on a normal day and remove formwork and the lower beam jump. Then leapfrogged up the attached lower jump-form beam, and the machine jumped. The formwork was then reinstalled and the steel and concrete positioned for the lift. The jobs will stop at 3 pm, with the concrete gaining strength all night. Therefore, above Lift 10, the entire lift-form structure was supported by concrete poured in the lifts of the preceding days, along with the hoisting loads and freshly installed steel and concrete.

The Collapse

On the morning of 27 April 1978, preparations for the second tower’s Lift 29 began. The lift-shaped system was about 52 m above ground level, following Lift 28 the day before. The Lift 28 formwork and the lower jump-form beam were removed, the framework was jumped and the beam and formwork for the Lift 29 were mounted. Reinforcement was installed and concrete placement for Lift 29 commenced.

The Lift 28 concrete which was about 20 hours old now completely supported the system. There were 51 workers in the scaffolding. As cathead gantry cranes No. 4 and No. 5 were hoisting the third bucket of concrete into place, eyewitnesses reported hearing a loud crack coming from the vicinity of cathead gantry crane No. 4. The scaffolding at this location suddenly tilted inward, tearing down concrete from the tower. The progressive collapse had begun. The entire scaffolding system, along with the Lift 28 concrete, began to peel away from the top of the tower, both clockwise and counterclockwise. The collapse took less than a minute, with the 51 workers dropping 52 m down to the ground, as well as the lift-form structure and tons of concrete (Figures 2). There were no survivors. 

Cause of Failure

The case was investigated by the Occupational Safety and Health Administration (OSHA) on behalf of the National Bureau of Standards (NBS). The inquiry concentrated on three aspects of the collapse: the crane system, the lift-form system for the scaffolding and the tower shell itself.

The crane system was ruled out quickly: it didn’t show any harm and testing confirmed that its components met the necessary specifications. The lift-form system itself was also found to play no role in the failure. The inquiry then turned to the construction of the shell tower itself.

To assess the strength of the Lift 28 concrete at the time of the failure, the NBS investigators performed experiments using identical concrete that had been subjected to identical temperatures for 20 hours – the approximate time that had elapsed between the Lift 28 concrete installation and the collapse. The findings were disturbing. The concrete had approximately 1.5MPa cube strength-surprisingly weak. The cold temperatures at the site were to blame – the temperature actually resulted in the concrete not having enough time to develop the required strength to support the lift-form device and the loading applied.

However, Lev Zetlin Associates (LZA), who had been engaged by the contractor in the aftermath of the disaster, disagreed with the official findings upon the publication of the NBS report. They found that prior to the collapse, many vital anchor bolts had been removed from the structure. Such bolts were in the jump-form beams-and thus played a role in moving the load from the scaffolding to the Lift 27 tower shell concrete. LZA argued that if the bolts were not removed, the failure would’ve been avoided.

In response to this report, two of the original NBS investigators carried out further investigations, which were released in 1980. In the end, they concluded that the loss would have happened regardless of the bolts removed. They established that failure was inevitable if the concrete strength was below 6.9MPa. We know that the strength at the time of failure was estimated at 1.5MPa, a long way from 6.9MPa.

Also see: Hartford Civic Centre Roof Collapse

Lesson Learnt

This failure highlights the dangers of governing a concrete construction schedule based on time considerations alone, and it is not an isolated incident in the history of failure. Several other structural failures have been attributed to concrete being given insufficient time to develop necessary strength. In this case, the construction schedules were governed by time and did not consider the environmental conditions that the concrete had to contend with.

This failure also emphasizes the importance of having safety engineers on the ground. The OSHA was criticized for lax inspections. It didn’t have enough safety inspectors on the ground to enforce regulations in West Virginia. Nevertheless, it adopted new guidelines to protect future workers, including the requirement to produce detailed safety manuals for construction projects. It also made amendments to the US Construction Safety Act, such as transferring more responsibility for formwork decisions from the designer to the contractor and allowing compulsory testing of concrete samples before formwork removal when the concrete was being relied upon to carry the load.

References

1) National Bureau of Standards (1979) Investigation of Construction Failure of Reinforced Concrete Cooling Tower at Willow Island, West Virginia [Online] Available at: www.nist.gov/customcf/get_pdf. cfm?pub_id=908824 (Accessed: January 2015)

2) Delatte N. J. (2009) Beyond failure: Forensic case studies for civil engineers, Reston, USA: ASCE

3) S. Brady (2015) Willow Island cooling tower collapse: a lesson from nature–Professional Guidance, Institution of Structural Engineers

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