Redundancy: As stated by Wearne (2000, p. 142), a buildings "ability to carry loads by more than one mechanism if something critical fails (in other words, its fail-safe margin).
Continuity: A structural system that is able to transfer loads and stresses from member-to-member as if there were no connections.
Progressive collapse: The failure of a primary structural element that results in the failure of adjoining structural elements.
Figure 1: Ronant Point after the collapse
Introduction
The partial progressive collapse of Ronan Point Apartment Tower in London occurred on May 16, 1968. The collapse was the result of a gas explosion caused by a gas-stove leak in flat 90, located on the 18th floor of the 22 story apartment building. The force of the explosion blew out the opposite corner load bearing walls of the apartment that were the only source of support for the levels above. The first phase of the progressive collapse occurred when the walls of the southeast corner on floors 19 through 22 collapsed. The second phase of the progressive collapse occurred when floor 18 was suddenly over-loaded by the impact of the collapsed floors above. This loading caused the southeast corner walls of floor 18 to collapse onto floor 17, and so on, until the ground floor was reached (Figure 1). Four people were killed and another 17 were injured in the partial progressive collapse of the Ronan Point Apart ment Tower. Upon investigation of the collapse, it was found that Ronan Point was flawed in both design and construction. Ronan Point was constructed using the Larsen-Neilsen system, a system of factory-built, precast concrete components, originally intended for structures of no more than six stories. Precast panels used in the design were joined together without a structural frame. Building codes used in designing Ronan Point were approximately 15 years old and were considered out-dated. No alternate load-paths existed that allowed for redistribution of loads in case of a partial collapse. After questioning the building’s structural integrity, Ronan Point was demolished in May 1986, and poor workmanship was found at critical connections. Ronan Point led to the reevaluation of the codes governing construction and design methods, in terms of safety and unusual loads. The importance of continuity in joints of buildings and of redundancy in structures was brought to attention by the collapse of Ronan Point (Levy and Salvadori 1992).
Design and Construction
During World War II, much of London’s housing stock was destroyed by enemy attacks. After the war, until the mid-1950’s, low density flats were constructed to replace the dwellings demolished (Griffiths et al. 1968). However, a lack of skilled laborers, a change in policy allowing for higher occupancy densities, and the development of prefabricated construction techniques (or system building) led to the popularity of high-rise apartment buildings (Pearson and Delatte 2005, p. 172).
The Ronan Point Apartment Tower was constructed using the Larsen-Neilsen system. Developed in Denmark in 1948, the Larsen-Neilsen system was “composed of factory-built, precast concrete components designed to minimize on-site construction work. Walls, floors and stairways are all precast. All units, installed one-story high, are load bearing” (ENR 1968), p. 54). The ‘know how’ of the Larsen-Neilsen system is a combination of production techniques, erections methods, and jointing details (Griffiths et al. 1968). The system was not intended for buildings over six stories (Wearne 2000, p. 140).
Ronan Point was the second of nine identical high-rise apartment buildings to be built as part of an expansive housing program of 1,000 Larsen-Neilsen built dwellings (Griffiths et al. 1968). Designed as a precast concrete flat-plate structure, each floor of Ronan Point was supported by the load-bearing walls directly beneath. The only path of gravity load transfer was through these loadbearing walls. Tooth-edged floor panels were fitted into slotted wall panels, and the joints were then bolted together to lock the panels together, providing continuity and mutual interaction. Connections were filled with dry-pack mortar for further security (Levy and Salvadori 1992, p.79). Ronan Point Apartment Tower was 22 floors of flats that rested on a concrete podium. Overall, Ronan Point was 80 feet by 60 feet, andhad a height of 210 feet (Griffiths et al. 1968).
Collapse
Figure 2: Layout of Flat 90
The southeast corner of Ronan Point collapsed at 5:45 a.m. on May 16, 1968. The collapse was initiated by a gas-stove leak in apartment 90 on the 18th floor. The tenant of apartment 90, Ivy Hodge, struck a match and was knocked unconscious by the resulting explosion. The extent of the damage and progressive collapse was due to both the effects of the explosion and the structural characteristics of the building (Griffiths et al. 1968).
The force of the explosion knocked out the non-loadbearing walls of the kitchen and the living room, as well as the external loadbearing walls of the southeast corner of apartment 90 (Griffiths et al. 1968). These external loadbearing walls on the 18th floor acted as the si ngle support of the 19th floor. As a result, the first phase of the progressive collapse of the southeast corner occurred; floor 19 collapsed, and then floor 20, and so on. Consequentially, floor 18 was suddenly overloaded by the impact of the collapsed floors above, which initiated the second phase of the progressive collapse of the southeast corner; floor 18 collapsed onto floor 17 and progressed downward until ground level was reached. The collapse sheared off a portion of the living room of the southeast apartments, leaving bedrooms intact, aside from floors 17 through 22 where the fatalities occurred (Figure 2). Due to the time that the collapse happened, most of the tenants were still sleeping in their bedrooms, considerably reducing the fatality rate (Delatte 2009, p. 101).
Causes of Failure
All accounts report that the explosion from the gas leak initiated the collapse of Ronan Point. The investigation into the collapse, performed by a panel formed by the government, was issued just months after the failure (Griffiths et al. 1968). A substandard brass nut was used to connect the hose to the gas stove in apartment 90. In comparison to a standard brass nut, the nut that was used in the connection had a thinner flange and an unusual degree of chamfer. A replicate of the substandard nut was made and tested to determine the amount of force needed to break it. The results of the test indicated that a force of 3.5 kips would break the nut, but a considerably smaller force of 0.360 kips would fail the hose before the nut. Therefore, it was assumed that the nut had been previously fractured by over-tightening during installation, causing the nut to break and allowing gas to leak into apartment 90 (Delatte 2009, p. 102).
Evidence suggests that the explosion was not large. Ivy Hodge’s hearing was not damaged in the explosion, indicating that the pressure was less than 10 lb/in2 (Delatte 2009, p. 102). Nevertheless, extensive testing suggested that a mere pressure of 3 lb/in2 could have displaced the exterior walls of Ronan Point, a pressure less than one-third that of the explosion (Levy and Salvadori 1992, p. 80).
Evidently, the progressive collapse of Ronan Point was due to its lack of fail-safe mechanisms and alternate load paths. Without a structural frame the upper floors had no redundant support. When the southeast walls of the 18th floor blew out, and the upper floors failed, the process continued downward until the ground level was reached (Delatte 2009, p. 102).
Technical Aspects
Figure 3: Exterior horizontal joint between floor slab and flank wall as built
Public inquiry into the collapse revealed strong winds or the effects of a fire could have also caused a progressive collapse. Ronan Point was designed using building codes that were fifteen years old. These codes did not take into account wind loads occurring at current building heights. Designed to withstand 63 mph winds, Ronan Point, 210 feet in height, could see up to 105 mph winds at 200 feet above ground level. In addition, fire could “arch” the floor slabs in a way that would displace or rotate the joints (Delatte 2009, p. 103). Some researchers now believe that a fire in one of the apartments could cause such extreme thermal expansion that it could have caused a failure similar to the explosion.
Though the southeast apartments of Ronan Point were rebuilt as a separate section of apartments, and Ronan Point at large was reinforced with blast angles, its demolition occurred in May 1986. Because poor workmanship was suspected, Ronan Point Apartment Tower was dismantled floor by floor to inspect the joints. This process uncovered joints, between walls and slab, full of voids and rubbish (Levy and Salvadori 1992, p. 83). Further, the steel tie-plates connecting the walls to the floor slabs were further evidence of negligence. All the tie plates inspected showed that laboreres failed to tighten the nuts of the connecting studs (Wearne 2000, p. 145). Figure 3 illustrates the connection as built, and the poor workmanship that was uncovered. Due to the magnitude of poor workmanship discovered upon dismantling Ronan Point, the remaining Larsen-Nielsen system built towers were demolished (Delatte 2009, p. 103).
Professional and Procedural Aspects
The importance of continuity in joints of buildings and of redundancy in structures was brought to attention by the collapse of Ronan Point (Levy and Salvadori 1992). Building codes, regulations, and guidelines, as well as design methods were revised accordingly throughout the world:
The United Kingdom amended their building regulations in 1970. Buildings more than four stories in height were to be designed to resist progressive collapse (Pearson and Delatte 2005, pp. 175).
The British government mandated guidelines that required fail-safe mechanisms in large-panel system structures, steel bracing with floor-to-wall connectors, and a minimum tensile strength of 3000 lb/in2 across the length and width of roofs and floors (Delatte 2009, p. 104).
The Portland Cement Association and the Prestressed Concrete Institute developed guidelines that required building elements be tied together (Delatte 2009).
Changes were also made to U.S. model codes, that required structural systems remain stable, even after sustaining local damage (Pearson and Delatte 2005, p. 175).
The collapse of Ronan Point Apartment Tower reminded professionals that redundancy and continuity is most important in building designs. Furthermore, dismantling Ronan Point proved the need for quality control in the construction process (Delatte 2009, p. 105).
Conclusion
Ronan Point Apartment Tower is an excellent example of the need for redundancy and continuity in structural designs as well as quality control during the construction process. As Delatte (2009, p. 106) states, “The relatively low overpressure from the gas explosion should have led to localized damage at most, not a partial progressive collapse and the loss of four lives.” Overall structural integrity is of utmost importance in the prevention of progressive collapses. Though building codes, regulations, and guidelines were reevaluated, Ronan Point was not the last progressive collapse to occur. For another example of a failure that occurred due to a lack of redundancy, refer to Kemper Arena. For examples of failures due to quality control during construction, refer to Skyline Plaza in Bailey's Crossroads, 2000 Commonwealth Avenue, or Harbour Cay Condominium.
References
Delatte, N. J. (2009). Beyond failure: Forensic case studies for civil engineers. American Society of Civil Engineers (ASCE), Reston, Virginia, 97-106.
Engineering News Record (ENR). (1968). “Systems built apartment collapse.” ENR, May 23, 1968, 54.
Griffiths, H., Pugsley, A. G., and Saunders, O. (1968). Report of the inquiry into the collapse of flats at Ronan Point, Canning Town. Her Majesty’s Stationery Office, London.
Levy, M., and Salvadori, M. (1992). Why buildings fall down: How structures fail. W.W. Norton, New York, 76-83.
Pearson, C.,and Delatte, N. J. (2005) Ronan Point Apartment Tower Collapse and Its Effect on Building Codes. J. Perf. of Constr. Fac., 19(2), 172-177.
Wearne, P. (2000). Collapse: When Buildings Fall Down, TV Books, L.L.C., New York, 137-156.
Additional References
Bignell, V., Peters, J., and Pym, C. (1977). Catastrophic failures. Open University Press, Milton Keynes, New York.
Cagley, J. R. (2003, April). The design professional’s concerns regarding progressive collapse design. Building Sciences, 27, 4-6.
Pearson, C., and Delatte, N. (2003). Lessons from the Progressive Collapse of the Ronan Point Apartment Tower. In Forensic Engineering, Proceedings of the Third Congress, edited by Paul A. Bosela, Norbert J. Dellate, and Kevin L. Rens, ASCE, Reston, VA., pp. 190-200.
Shepherd, R., and Frost, J. D. (1995). Failures in Civil Engineering: Structural, Foundation, and Geoenvironmental Case Studies, ASCE, New York.
Ronan Point Apartment Tower (May 16, 1968)
Table of Contents
Hali F. Voycik, BAE/MAE, Penn State, 2009
Key Words
Introduction
The partial progressive collapse of Ronan Point Apartment Tower in London occurred on May 16, 1968. The collapse was the result of a gas explosion caused by a gas-stove leak in flat 90, located on the 18th floor of the 22 story apartment building. The force of the explosion blew out the opposite corner load bearing walls of the apartment that were the only source of support for the levels above. The first phase of the progressive collapse occurred when the walls of the southeast corner on floors 19 through 22 collapsed. The second phase of the progressive collapse occurred when floor 18 was suddenly over-loaded by the impact of the collapsed floors above. This loading caused the southeast corner walls of floor 18 to collapse onto floor 17, and so on, until the ground floor was reached (Figure 1). Four people were killed and another 17 were injured in the partial progressive collapse of the Ronan Point Apart ment Tower. Upon investigation of the collapse, it was found that Ronan Point was flawed in both design and construction. Ronan Point was constructed using the Larsen-Neilsen system, a system of factory-built, precast concrete components, originally intended for structures of no more than six stories. Precast panels used in the design were joined together without a structural frame. Building codes used in designing Ronan Point were approximately 15 years old and were considered out-dated. No alternate load-paths existed that allowed for redistribution of loads in case of a partial collapse. After questioning the building’s structural integrity, Ronan Point was demolished in May 1986, and poor workmanship was found at critical connections. Ronan Point led to the reevaluation of the codes governing construction and design methods, in terms of safety and unusual loads. The importance of continuity in joints of buildings and of redundancy in structures was brought to attention by the collapse of Ronan Point (Levy and Salvadori 1992).Design and Construction
During World War II, much of London’s housing stock was destroyed by enemy attacks. After the war, until the mid-1950’s, low density flats were constructed to replace the dwellings demolished (Griffiths et al. 1968). However, a lack of skilled laborers, a change in policy allowing for higher occupancy densities, and the development of prefabricated construction techniques (or system building) led to the popularity of high-rise apartment buildings (Pearson and Delatte 2005, p. 172).The Ronan Point Apartment Tower was constructed using the Larsen-Neilsen system. Developed in Denmark in 1948, the Larsen-Neilsen system was “composed of factory-built, precast concrete components designed to minimize on-site construction work. Walls, floors and stairways are all precast. All units, installed one-story high, are load bearing” (ENR 1968), p. 54). The ‘know how’ of the Larsen-Neilsen system is a combination of production techniques, erections methods, and jointing details (Griffiths et al. 1968). The system was not intended for buildings over six stories (Wearne 2000, p. 140).
Ronan Point was the second of nine identical high-rise apartment buildings to be built as part of an expansive housing program of 1,000 Larsen-Neilsen built dwellings (Griffiths et al. 1968). Designed as a precast concrete flat-plate structure, each floor of Ronan Point was supported by the load-bearing walls directly beneath. The only path of gravity load transfer was through these loadbearing walls. Tooth-edged floor panels were fitted into slotted wall panels, and the joints were then bolted together to lock the panels together, providing continuity and mutual interaction. Connections were filled with dry-pack mortar for further security (Levy and Salvadori 1992, p.79). Ronan Point Apartment Tower was 22 floors of flats that rested on a concrete podium. Overall, Ronan Point was 80 feet by 60 feet, andhad a height of 210 feet (Griffiths et al. 1968).
Collapse
The force of the explosion knocked out the non-loadbearing walls of the kitchen and the living room, as well as the external loadbearing walls of the southeast corner of apartment 90 (Griffiths et al. 1968). These external loadbearing walls on the 18th floor acted as the si ngle support of the 19th floor. As a result, the first phase of the progressive collapse of the southeast corner occurred; floor 19 collapsed, and then floor 20, and so on. Consequentially, floor 18 was suddenly overloaded by the impact of the collapsed floors above, which initiated the second phase of the progressive collapse of the southeast corner; floor 18 collapsed onto floor 17 and progressed downward until ground level was reached. The collapse sheared off a portion of the living room of the southeast apartments, leaving bedrooms intact, aside from floors 17 through 22 where the fatalities occurred (Figure 2). Due to the time that the collapse happened, most of the tenants were still sleeping in their bedrooms, considerably reducing the fatality rate (Delatte 2009, p. 101).
Causes of Failure
All accounts report that the explosion from the gas leak initiated the collapse of Ronan Point. The investigation into the collapse, performed by a panel formed by the government, was issued just months after the failure (Griffiths et al. 1968). A substandard brass nut was used to connect the hose to the gas stove in apartment 90. In comparison to a standard brass nut, the nut that was used in the connection had a thinner flange and an unusual degree of chamfer. A replicate of the substandard nut was made and tested to determine the amount of force needed to break it. The results of the test indicated that a force of 3.5 kips would break the nut, but a considerably smaller force of 0.360 kips would fail the hose before the nut. Therefore, it was assumed that the nut had been previously fractured by over-tightening during installation, causing the nut to break and allowing gas to leak into apartment 90 (Delatte 2009, p. 102).Evidence suggests that the explosion was not large. Ivy Hodge’s hearing was not damaged in the explosion, indicating that the pressure was less than 10 lb/in2 (Delatte 2009, p. 102). Nevertheless, extensive testing suggested that a mere pressure of 3 lb/in2 could have displaced the exterior walls of Ronan Point, a pressure less than one-third that of the explosion (Levy and Salvadori 1992, p. 80).
Evidently, the progressive collapse of Ronan Point was due to its lack of fail-safe mechanisms and alternate load paths. Without a structural frame the upper floors had no redundant support. When the southeast walls of the 18th floor blew out, and the upper floors failed, the process continued downward until the ground level was reached (Delatte 2009, p. 102).
Technical Aspects
Though the southeast apartments of Ronan Point were rebuilt as a separate section of apartments, and Ronan Point at large was reinforced with blast angles, its demolition occurred in May 1986. Because poor workmanship was suspected, Ronan Point Apartment Tower was dismantled floor by floor to inspect the joints. This process uncovered joints, between walls and slab, full of voids and rubbish (Levy and Salvadori 1992, p. 83). Further, the steel tie-plates connecting the walls to the floor slabs were further evidence of negligence. All the tie plates inspected showed that laboreres failed to tighten the nuts of the connecting studs (Wearne 2000, p. 145). Figure 3 illustrates the connection as built, and the poor workmanship that was uncovered. Due to the magnitude of poor workmanship discovered upon dismantling Ronan Point, the remaining Larsen-Nielsen system built towers were demolished (Delatte 2009, p. 103).
Professional and Procedural Aspects
The importance of continuity in joints of buildings and of redundancy in structures was brought to attention by the collapse of Ronan Point (Levy and Salvadori 1992). Building codes, regulations, and guidelines, as well as design methods were revised accordingly throughout the world:The collapse of Ronan Point Apartment Tower reminded professionals that redundancy and continuity is most important in building designs. Furthermore, dismantling Ronan Point proved the need for quality control in the construction process (Delatte 2009, p. 105).
Conclusion
Ronan Point Apartment Tower is an excellent example of the need for redundancy and continuity in structural designs as well as quality control during the construction process. As Delatte (2009, p. 106) states, “The relatively low overpressure from the gas explosion should have led to localized damage at most, not a partial progressive collapse and the loss of four lives.” Overall structural integrity is of utmost importance in the prevention of progressive collapses. Though building codes, regulations, and guidelines were reevaluated, Ronan Point was not the last progressive collapse to occur. For another example of a failure that occurred due to a lack of redundancy, refer to Kemper Arena. For examples of failures due to quality control during construction, refer to Skyline Plaza in Bailey's Crossroads, 2000 Commonwealth Avenue, or Harbour Cay Condominium.References
Additional References