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Large reinforced concrete cooling towers have been constructed since the 1950s; they are one of the largest man made constructions at present. They are usually constructed in power generating plants or chemical plants to cool water used in the cooling of a vital process. Notice that a football stadium could fit inside any of those towers. Failure of a cooling tower may have disastrous consequences to the industry and to the environment.
On November 1, 1965, three of eight large reinforced concrete cooling towers collapsed at the Ferrybridge Power Station in England, under severe wind conditions. The three towers collapsed within one hour: the first one at 10:30 a.m., the second one ten minutes later, with more people observing the collapse, and the third tower collapsed less than one hour after the first one, with many workers and reporters watching the event.
Construction of this group of towers started in 1962 and the shell structures were completed in 1964, with pack completion in the first half of 1965 (only months before the collapse). The towers were very large in their own time, with 125 meters high, base diameter of 100 meters, and shell thickness of 0.125 meters. The diagonal distance between tower centers was 1.42 times the diameter, somehow less than the standard practice of the same company (1.5 times).
Most of the information that we have comes from the Report of the Committee of Inquiry into the Collapse, which investigated the failure. The Committee did not attempt “to apportion the responsibility for the collapse, but has confined itself to the technical appraisal of the several factors which it considers were contributory to the failure”.
In general terms, this was a group of well designed and constructed towers according to the state of the art in the 1960s. The shells were designed using membrane theory, and the Committee accepted that the method used to convert wind loading into membrane stresses was adequate. There was no unusual feature in the local topography that could have significantly influenced the wind velocities.
Some limitations were found in the design and construction processes: First, there were deficiencies in estimating wind action associated to misinterpretations of the code requirements. The shells were designed assuming 63 miles per hour winds at 13 meters above ground level, which is less than the maximum wind gusts estimated to have occurred on the day of the collapse, which were of 76 to 84 miles per hour at 10 meters aboveground and 81 to 90 miles per hour at 125 meters aboveground. Second, the design philosophy of this group of towers applied safety factors to the strength of the materials, instead of to the loads. Third, vertical and horizontal cracking had been detected prior to the collapses and in the months following them.
The Committee considered five possible causes by which the failure could have been initiated (they call them “modes”). The first cause was foundation failure. They had access to records before and after the collapse, and were able to show that there was no subsidence in the cooling tower area. They also found “appreciable reserve of capacity” in the foundation piles. The conclusion was that it was highly improbable that a foundation failure contributed to the collapse.
The second cause investigated was shear failure of the shell at about the top of the ring beam level. However, photographic evidence and subsequent inspection of the debris did not bear out this hypothesis. “The form of break was apparently a combination of vertical tension with appreciable bending”.
Third, instability failure of the shell was investigated. Evidence was collected from wind tunnel tests, debris observation and buckling calculations. The Committee concluded that the compressive stresses were too low to induce structural instability.
Fourth, vibration failure (resonance) was considered. A natural frequency of 0.6 cycles per second was found from tests and models. The conclusion was that no simple resonance vibration can be attributed to the vortex shedding at the high wind speeds on the day of the collapse.
The fifth cause investigated was tensile failure within the fabric of the shell. The shell had circumferential reinforcement of 0.15 percent by area over the whole shell height. Vertical steel reinforcement was originally calculated based on static wind pressures (not considering fluctuating forces) leading to a small quantity of steel to resist vertical uplift. “Fluctuating and steady forces within the group of towers could have induced substantial vertical cracking and horizontal lifting of the construction joints during the life of the tower”.
The Committee concluded that tensile failure of the reinforcement in the vertical direction due to wind uplift on the windward side was the dominant initial mode of failure. This conclusion was backed by numerical modeling and by inspection of the failure surfaces and reinforcements in the collapse debris.