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The basic requirement in the design of any structure is that it should safely support all the loads during construction and in service. During the construction of the cooling tower at the Pleasants Power Station located at Willow Island, West Virginia, U.S., the designers made a huge blunder by overlooking the evaluation of peak construction loads, including dynamic effects, that would be imposed upon the partially completed concrete tower.
On 27th April 1978, during the construction of the second cooling tower, the formwork supporting the construction workers failed, causing the death of 51 workers. An investigation conducted by the Occupational Safety and Health Administration (OSHA) concluded that the required strength of the concrete was not achieved to support the imposed loads. The concrete structure couldn’t resist the additional loads which were not incorporated in the design.
The construction methodology adopted for the construction of the Willow Island Cooling Tower had been used successfully 36 times, including once at the same site. Yet, the cooling tower failed making it one of the worst construction accidents in the history of the U.S.
In this article, the construction details of the cooling tower, the main causes behind the failure of the structure, and the lessons learned have been discussed.
- 1. Construction Details of Cooling Tower
- 2. Failure of the Cooling Tower
- 3. Legal Consequences on the Designers
- 4. Future After the Failure of the Cooling Tower
- 5. Lesson Learned
1. Construction Details of Cooling Tower
Two natural-draught hyperbolic cooling towers were to be built for the Pleasants Power Station, a coal-fired power station in Willow Island. These chimney-like towers were designed to be large, with a distorted hourglass shape, which would allow air to circulate without fans because the warmer air inside would rise naturally. While the base diameter was a constant 109 m, both the diameter and shell thickness were changing with the height of the tower.
These two towers were designed and built by the Hamon Cooling Tower Division of Research-Cottrell, Inc. (R-C), Bound Brook, New Jersey. R-C was an environmental control company that designed and built air and water pollution control systems for utility companies.
R-C held a $12 million subcontract for the pair of 131-m tall towers. The general contractor of the project was United Engineers and Constructors.
1.1 Construction Methodology
The towers were built using a patented lift-form technique that had been successfully used for construction of cooling towers. The lift-form scaffolding was made up of five basic components: jump-form beams, anchor assemblies, jacking frames, formwork, and scaffolding platforms.
A four-level high system of scaffolding was used, and working platforms were suspended from the inside and outside jacking frames. At the top level, the construction materials were received by the hoisting system, steel reinforcement was distributed, and concrete was poured. The second level was used only during the formwork adjustment process.
The level three and four of the scaffolding systems provided access to the jump-form beams, and final surface preparation such as patching and grouting was done from these levels. The scaffolding system was entirely supported by the previously completed portion of the tower. Each day, a 1.5 m lift was completed, and the entire scaffolding system moved with the jacking frame to a new elevation.
The daily routine for concrete placement consisted of four procedures. First, workers loosened the forms from the last concrete lift by removing the wedging from the formwork. Next, the forms were adjusted to accommodate the changing diameter of the shell, and the jacking of the entire formwork and scaffolding system took place at the next elevation. Then, the lowest jump-form beam was unbolted and moved to its new location at the top. Finally, the formwork was wedged into position.
After the tenth lift was completed, the concrete and construction materials were carried to the working platforms by an elaborate hoisting system. Six cathead gantry cranes powered by twin drum hoists delivered the materials. The legs of each cathead gantry were attached to the aluminum jump-form beams, which were then attached to the wall at rib locations approximately 3.7 m apart.
The catheads moved up the lift form scaffolding as the construction advanced. A static line guided all of the materials as they were hoisted to the working platforms. The static line was attached to the slide plate at the interior end of the cathead at one end and secured to an anchor point on the ground at the other end. During construction, because of the changing geometry of the tower, both the catheads and static line had to be adjusted periodically.
2. Failure of the Cooling Tower
The construction of the first cooling tower was completed in August 1977, but on April 27, 1978, during construction of the second tower, the formwork system failed. While concrete was being placed for lift number 29 (52 m above the ground), lift number 28 collapsed, leading to the death of workers.
On the day of the failure, cathead gantries number four and five were being used to deliver reinforcement and concrete. Failure at cathead gantry number four initiated when the third bucket of concrete was delivered to the platform, and the tower began to collapse inward. The platforms, formwork, fresh concrete, and most of the old lifts collapsed.
The interior work platform peeled away like someone opening a can. The exterior platform, 51 workers, and tons of concrete, were pulled inside the tower and collapsed into a heap of jumbled debris. The formwork ripped loose simultaneously in both circular directions and met directly across from the starting point.
2.1 Cause of the Failure
Immediately after the collapse, the National Bureau of Standards (NBS) began an investigation on behalf of the Occupational Safety and Health Administration (OSHA) to determine the cause of the accident. The entire system was divided into three sections to be examined. First was the hoisting system, second was the scaffolding system, and third was the tower itself.
The hoisting system was investigated because the failure initiated at the location of the cathead gantry number four. Once all the parts were collected from the debris, they were examined to see which of the components failed first. Lab tests were performed on the hoisting cable, static line, chain hoist, and anchor device of the system. These results, along with the field observations, indicated that the hoisting system did not initiate the failure.
The next section was the scaffolding system. According to the NBS investigation, the scaffolding system was not the most probable cause of the failure. The lab tests determined that the bolt failure was not the cause behind the accident because the hoisting system could not produce a large enough force to cause bolt failure.
Next, the NBS looked at the tower itself as the most probable cause of the collapse. To do so, the strength and other mechanical properties of the concrete in lift number 28 had to be determined. Concrete specimens were made using the materials supplied by the same company that supplied concrete to the construction site. These specimens were then cured in a chamber that simulated the temperature at the site for 24 hours before the collapse.
The weather that week had been cold and rainy, with temperatures just above 16°C during the day and just above 0°C at night. Estimating the collapse to have taken place approximately 20 hours after the completion of the tower section located near cathead gantry number four and curing the concrete at 4.4°C. The compressive strength of this section was approximately 1,500 kPa. It was concluded that this section on the tower did not have adequate strength to resist the applied construction loads.
2.2 Results of the Investigation by NBS
The following points describe the overall conclusion of the investigation conducted by NBS:
- During the failure of the tower, the concrete bucket was supported from the base of tower to gantry number four. Eyewitness accounts and measurements indicated that the bucket was approximately 18 m below the cathead beam. Therefore, it is not believed that the concrete bucket hit the cathead causing it to fail.
- The cables for catheads number four and five were broken after the onset of the collapse. Therefore, the breakage of the cables did not initiate the failure.
- Field and lab tests showed that the collapse did not occur because of a component failure of the hoisting, scaffolding, or formwork systems.
- The compressive strength of the concrete near cathead gantry number four was estimated to be 1,500 kPa at the time of the collapse.
- Analysis showed that the resultant stresses at several points along lift number 28 equaled or exceeded the shell strength in compression, bending, and shear. Failure would have propagated at any of these points and caused the overall failure of the structure.
- Imposition of the construction load on the shell was determined to be the most probable cause of the failure of structure because the load was applied prior to the adequate gain in the strength of concrete of lift number 28.
2.3 Investigation by Lev Zetlin Associates
While the NBS was investigating for OSHA, Lev Zetlin Associates (LZA) was performing another investigation, on behalf of the general contractor.
LZA’s findings disagreed with those of the NBS investigation. LZA claimed that the most probable cause of failure was the early removal of anchor bolts and cones from the lower portion of lift number 27. LZA believed that if the anchor bolts had been left in place and stayed attached to the jump-form beams, the collapse would have never occurred.
2.4 Conclusion of Investigation
Because of the contradictions found in these investigations, the authorities of NBS analyzed three remaining questions after the investigations were completed.
The first question was what should have been the strength of the concrete in lift number 28 so that the shell could have resisted the applied construction loads. The second question dealt with the bolts that should have been in lift number 27. The third question dealt with the location of the static load line at the time of the collapse.
After analyzing the hoisting loads, including dynamic effects that were inherent in the hoisting system, the stress resultants in the shell were determined. These values were then compared to the resistance of the shell throughout each shell profile. The results showed that crushing failure in lift number 28 would initiate if the strength of the concrete was 6,900 kPa or less.
To answer the second question, models were analyzed with and without the lower bolts in place. They found that because of the additional bolts in lift number 27, the magnitudes of the critical stress resultants were reduced, but were still well beyond the ultimate capacity of the concrete section in lift number 28.
For the answer to the third question, the experiment was performed with the ground anchor point of the static line at different distances from the shell of the tower. It was determined that if the base of the static load line was not moved nearer to the center of the tower (its location at the time of the collapse) the critical stress resultants would have been less than the ultimate strength of the concrete in lift number 28.
Therefore, the NBS investigators concluded that the concrete in lift number 28 would probably not have failed and consequently, collapse would not have occurred if the base of the static line was not moved.
3. Legal Consequences on the Designers
OSHA issued ten willful citations and six serious citations against R-C. Five of the willful citations appeared to be directly related to the collapse. The charges included:
- Failing to test field-cured concrete specimens before removing the formwork
- Not properly anchoring the scaffolding and formworks
- Improperly designed specifications
- Not having proper erection instructions on site
- Employees were not adequately trained
The other five citations were considered unrelated to the accident. Criminal charges were dropped, but R-C paid substantial amount in damages and fines imposed by OSHA.
Also, OSHA issued two serious citations each to Pittsburgh testing laboratory, the company that performed concrete testing for R-C, and the Criss Concrete Co., which supplied the concrete for the tower. These companies also faced lawsuits from the families of the victims, as well as from R-C.
4. Future After the Failure of the Cooling Tower
The three companies that faced legal repercussions were not the only ones under fire after the disaster. OSHA was criticized for not applying strict enforcement of regulations. At the time of the accident, the number of qualified federal safety inspectors for construction projects in the entire state of West Virginia was seven. This amount was considered absurdly low by the state commission.
As a result of this disaster, OSHA adopted new guidelines, one big step that OSHA took toward protecting future construction workers was making changes in the U.S. construction safety act. One of these changes shifted more responsibility from the engineer to the contractor for formwork decisions.
Another major change was that OSHA removed a table that provided a schedule for formwork removal. Now, the act requires that the concrete specimens be tested before removal of formwork or any system that relies on the strength of the concrete.
Other guidelines that OSHA adopted after the accident included having a specialist review construction plans for cooling towers and requiring that a detailed safety manual be developed as part of the construction plan. Also, OSHA improved their inspection procedures by increasing the number of items checked at cooling towers and adding an inspection for compliance with the construction plan.
5. Lesson Learned
This case study shows the importance of safety standards during the construction process. A process that had been used successfully 36 times, including once at the same site, drastically failed, causing the death of 51 workers. The men were paid for only eight hours a day, regardless of how long it took to do a day’s work, which provided an incentive to rush the work.
OSHA now has added many requirements, such as a construction plan, and has increased safety inspections to make sure that another disaster like this does not take place.
Another lesson that was learned by this accident was not to tie together all of the formwork. Not too long after the collapse of the Willow Island Cooling Tower, at a cooling tower site in Satsop, Washington, two workers were killed and a third seriously injured when a steel form pulled away from the concrete. If these forms had all been tied together, the loss of life would have been much greater.
The Willow Island Cooling Towers failed drastically and caused the death of 51 workers. Thus, it is considered as the worst accident in the history of US because the number of deaths of worker in this accident is considered as the highest death in the construction industry of US.
Hamon cooling tower division of research-cottrell, Inc. and Bound brook, New Jersey were the designers of the Willow Island Cooling Tower.
Two towers were constructed with a height of 131 m.