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Case Study On The Ronan Point Tower Block History Essay


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During the Second World War, London was in a state of crisis after large portions of the city were demolished by German war planes. In response to this, many high rise apartments were constructed in order to take people off the streets and out of the slums and give them a basic means for living again.

Around the same time, there had been huge cutbacks in the amount of skilled construction workers as the large majority of them were opting for easier and safer jobs on factory floors around the country. Also at this time, a national policy change allowed the density of occupancy to double from seventy to one hundred forty persons to the acre. This all conveniently coincided with the development of prefabricated construction techniques and sparked the assembly of high rise apartment buildings all over the city of London. This new style of housing could accommodate large numbers of people, save land and labor, and was reasonably quick to construct.

Fig ? Ronan Point after Collapse (Courtesy of ???)The method utilized for this high rise apartment was the Larsen-Neilsen system. This system was first initiated by Larsen Nielson in Denmark in 1948 in order to reduce the amount of construction work required to be undertaken on-site. In this system, components such as walls, staircases and floors are prefabricated. All of these elements are load bearing, meaning they provide support for the elements which lie directly above. In the case of Ronan Point, which was the second of six similar sister towers constructed in the London area after the war, each floor section was supported solely by the load bearing wall beneath it. Gravity load transfer occurred only through these load-bearing walls. These external and internal walls and floors formed by large panels, approximately 150-175mm thick, of steel reinforced precast concrete. These floor and wall elements were slotted together, bolted and then filled with cement to secure the connection.

The construction of Ronan Point, which was located in the West Ham area of East-London, began on 15th July 1966. Construction of the project was undertaken and fully completed inside two years and at the time it cost approximately £500,000 to build. When fully assembled, Ronan Point measured in at 80ft by 60ft in area and 210ft in height. It was twenty-two stories tall with a total of 110 apartment units housed in the building (five per floor). It was made up of 44 2-bedroom flats and 66 1-bedroom flats. Tenants began flooding out of the slums in West Ham and into their new "homes in the sky". The building was occupied for less than three months before the explosion, leading to failure occurred.

2 Collapse

On the 16th of May 1968, at approximately 05:45, the southwest corner of Ronan Point Tower completely collapsed. The explosion occurs in Flat 90, which was located on the 18th floor, situated on the south-east corner if the tower. The explosion blew out complete sections of the outer wall, resulting in a type of domino style collapse of the wall and floor sections, right from the top of the buildings as far as the ground floor. The partial collapse of Ronan Tower claimed the lives of four people and injured a further seventeen but the death toll could have been so much higher.

Due to the timing of the disaster, luckily all other residents apart from one were sleeping in their bedrooms. Another stroke of luck was that, in the entire tower, only three apartments still remained unoccupied and these apartments were all contained in the south-east corner of the tower. Only one apartment which was situated directly above the explosion was occupied.

Figure 6. Close-up of collapse courtesy of (Bignell 1977).

Floor 18;

Apartment 90One lucky survivor, unable to sleep, had risen quietly leaving her husband in bed and was laying on a couch in her living room when the outer wall collapsed inwards and she was forced to the doorway as the floor disappeared leaving her on a narrow ledge unable to get through the door now jammed by rubble. Her husband reached through the opening and managed to grab and hold his wife with one arm while desperately clearing the rubble from the door with the other, he succeeded in bringing her to safety quite badly injured but alive.

The initial explosion in Flat 90 was caused by a gas-stove leak, and was sparked off by the occupier of the flat striking a match in order to brew her morning cup of tea. The force of the resulting explosion caused the woman to be flung to the other side of the room and knocked unconscious but miraculously she escaped without any further injuries.

The force of the explosion knocked out the opposite corner walls of the apartment. These walls were the sole support for the walls directly above. This created a chain reaction in which floor nineteen collapsed, then floor twenty and so on. Four floors fell onto level eighteen, which initiated a second phase of progressive collapse. This sudden loading on floor eighteen caused it to give way, smashing floor seventeen and progressing until it reached the ground

The collapse sheared off only the living room portion of the apartments, leaving the bedrooms intact with the exception of floors seventeen through twenty-two, where all the fatalities occurred. The explosion blew out the internal walls of the flat and also, unfortunately, the load-bearing external flank walls of the living room and bedroom, which left the floor above unsupported and this collapsed.

3 Failure Hypothesis

It was initially understood that Ronan Point had collapsed due to a gas explosion in the 18th floor due to the fact that a substandard brass nut which connected the hose to the stove failed.

4 Investigation & Cause of Failure

Shortly after the explosion, and partially due to pressure from the general public, the government formed a panel to investigate into the cause of the collapse. The panel quickly determined that the gas leak which caused the explosion was responsible for the initial collapse of the building. Further investigation showed that a substandard brass nut had been used to connect the hose to the stove. The nuts flange was thinner than the required standard and also had an unusual degree of chamfer. When this nut failed it lead to the gas flowing at an estimated rate of 120 ft cubed and hour through open doors and accumulate in the flat. Tests were carried out on a replica nut of same dimensions in order to determine how much force was required to break it in tension.

The findings of these tests showed that a force of 15.6 kN or 3,500 pounds, was all that was required to break the connections. It was also found that the hose connecting the stove to the gas would have actually failed before the nut at a force of 1.6 kN or 360 pounds. Prior to this investigation, it was commonly assumed that the nut had been fractured previous to the explosion due to over-tightening during instillation. It was believed that this over-tightening may have caused the nut too break which would have lead to gas leaking into the apartment.

It is thought that the gas may have rose over night and accumulated at the ceiling, which would explain why the resident did not smell it that morning. In terms of magnitude, it was not a very colossal explosion. The resident, who was flung across her kitchen, did not suffer any damage to her hearing which would suggest that the pressure of the explosion was less than 70 kPa. It was also proven, by testing items taken from the kitchen of this apartment, that these items were not subjected to pressure over 70 kPa.

A variety of different tests were carried out by The Building Research Station & Imperial College of London in order to determine how much internal force the building could withstand. These results indicated that the walls could have been displaced by a pressure of only 19.3 kPa (Levy 1992). It was also estimated that the kitchen and living room walls were moved at a pressure of only 1.7 kPa, while the exterior wall was moved at a gas pressure of 21 kPa (Griffiths et al., 1968).

5 Technical Aspects

Subsequent to these investigations carried out by the government's assigned panel of experts, it was revealed that failure on Ronan Point may have occurred in a number of different ways. It was found that both strong gale winds and/or the effects of a fire on the building could have resulted in a progressive collapse. The building was only designed to withstand wind velocities of a mere 100 kph. It is more than likely, that the building would have been subjected to wind speeds of up to 170 kph, 200 ft above ground level, within the buildings life expectancy of sixty years.

More findings on the Ronan Point tower collapse told that, the building codes which were used for the design of Ronan Point as well as its sister apartments were not up to scratch and had not been kept up to date. Relevant knowledge was available at that time from the National Physical Laboratory stating that "Higher than stated winds were known to occur in that area". It was noted that, "…the structure had been designed to comply with fifteen year old wind load codes that did not take into account current building heights (Britain 1970)." According to the inquiry, "the suction effect of the pressures applied by such winds in particular the opening of the joints as the tower block bent in the wind, would have similar effect to the explosion." 

It was also found that if a fire had ignited in the tower, that the structure would have suffered a similar effect. The inquiry stated, "it is estimated that fire could so expand and 'arch' the floor slab and bend the wall panel, as to displace or rotate an H-2 joint to a dangerous degree." (Wearne, 2000).

A fire test on the building had in fact been conducted by the request of the architect Sam Webber sixteen years prior to the collapse. The results of this test verified that the buildings elements would almost have certainly have failed under the impact of fire. Webb also predicted that after approximately sixteen years the tower would develop serious structural problems, especially with the joint connections.

Another major flaw in the building's design was that there were sizeable gaps in the connections between the walls and floors through which smoke could easily pass. As well as this, these gaps would have also enabled sounds such as television sets and conversations to pass between the floors.

Webb performed some simple tests in some of the Ronan Point apartments. According to Webb, "One of the simplest tests was to get a sheet of paper, tear a strip off, put it against the skirting board, and let it go at one end.  The loose end was coming out at ceiling level in the apartment below.  Another basic test was to put a coin up against the wall and let it go.  It fell through the gap as if going into a slot machine." (Wearne, 2000).

Webb was requested by the chair of Newham's Housing Committee to carry out a variety of test in order to fully gauge the condition of Ronan Point. He compiled an investigation team and together they surveyed and carried out tests on over half of the towers apartments. Their findings revealed cracks in the central stairwell and elevator shaft, which indicated movement throughout the building. After these cracks had been analyzed it was confirmed that Ronan Tower block was moving quite dramatically in conditions of high winds. As a result of this the building was slowly being broken apart. The building was on its lifting bolts and was held up by the 'blast angles' fitted after the public inquiry. It was found that the drypack mortar which was used had either been crushed, or had never even been used in the first place.

With all these negative test results, as well as growing concerns form Ronan Tower residents over the building's structural integrity, it was almost inevitable that the building would be required to be demolished. This came to pass in May 1986 when the controversial tower block was evacuated and taken apart. This building, which had a life expectancy of sixty years, served just eighteen of these years. The building was not tore to the ground as is often the method of demolition for these high rose apartments. In order to prove Webb's theories of poor workmanship in the towers construction, it was ordered that Ronan Point tower block be dismantled floor by floor in order for a variety of experiments and testing to be performed on its joints. These tests quickly verified Webb's suspicions of poor workman ship.

However, not even Webb himself could believe the extent of the shoddy workmanship. Latter to these experiments a shocked Webb commented, "I knew we were going to find bad workmanship - what surprised me was the sheer scale of it.  Not a single joint was correct.  Fixing straps were unattached: leveling nuts were not wound down, causing a significant loading to be transmitted via the bolts: panels were placed on bolts instead of mortar.  But the biggest shock of all was the crucial H-2 load-bearing joints between floor and wall panels.  Some of the joints had less than fifty percent of the mortar specified." (Wearne, 2000)

4 Professional and Procedural Concerns

Subsequent to the findings of Webb and his team of young architects, which displayed just how badly the workmanship at Ronan Point was flawed, it was almost immediately ordered that all remaining towers which were built under the Larsen-Nielson system be demolished. During the era of these towers construction, there were no building codes or strict regulations to govern over the types of issues that made this system of building so deeply flawed.

Large concrete panel construction was the height of innovation at this time, and little was known about how it would perform.  The building regulations in effect at the time contained a 'catch all' clause known as the 'functional requirement on structure'.  This clause contained no mention of redundancy or progressive collapse (Bignell 1977).

The collapse or the southeast corner of Ronan Point tower brought about some much needed changes to the existing regulation codes at the time. The possibility of progressive collapse is now largely taken into consideration in the building codes when construction a high rise building. The codes also require minimum amounts of ductility and redundancy. As a result of this, in 1970, the "Fifth Amendment" of the English building regulations was developed.

According to Arnold W. Hendry, it "applies to all buildings over four stories and requires that under specified loading conditions a structure must remain stable with a reduced safety factor in the event of a defined structural member or portion thereof being removed.  Limits of damage are laid down and if these would be exceeded by the removal of a particular member, that member must be designed to resist a pressure of 34 kN/m² (51 lb/in²) from any direction.  Of special importance in relation to load bearing wall structures is that these conditions should be met in the event of a wall or section of a wall being removed, subject to a maximum length of 2.25 times the story height." (Hendry, 1978).

Massive lessons were learned after the collapse of Ronan Point, and in many ways the industry probably needed a disaster of this magnitude to expose how radically the building codes needed to be amended. Britain proceeded to conduct research into the subject of progressive collapse and was followed shortly by the United States who implemented their own new design criteria.

The British government put a mandate in place which would aid against the failure known as progressive collapse. The main component of these guidelines was a necessity to include a failsafe mechanism is all such large panel buildings. Floor and wall connections would be required to be bound with steel bracing and contain a minimum tensile strength of 21 MPa (3000 psi) across the length and width of the roofs and floors.

Guidelines against progressive were also drawn up by The Portland Cement Association and the Pre-stressed Concrete Institute. These regulations also proposed that by tying building elements together, it would increase the buildings ductility therefore better sustaining the building elements deformations, following the failure of a portion of the buildings structure. Transverse ties create cantilever action from adjacent walls.  Vertical ties provide suspension from panels above, peripheral ties hold floors together, and longitudinal ties string floor planks-large prestressed panels-together" (Ross 1984).

The engineering profession was reminded of the need for redundancy in design to prevent a progressive collapse.  It is of utmost importance that building designs contain some measure of continuity (Shepherd and Frost, 1995). 

After extensive research into the subject, engineers and architects in the UK were provided with data for load bearing walls. The Larsen - Nielson system was not initially developed to be utilized in the construction of buildings more than six floors in height. But somehow in the UK this system was used to build these tall structures.

The inquiry concluded that the codes governing construction and design methods needed immediate reevaluation. It was included in the report that under no circumstance of imagination was Ronan Point considered to be any ways near an acceptable building to be occupied. And yet the building was designed and constructed in compliance with the Newham by-laws, which was the Building Regulations in operation at the time.

This is so manifestly an unsatisfactory state of affairs that it is necessary to enquire how it came about and to consider remedies for the future" (Griffiths et al., 1968).

After dismantling Ronan Point, the concept of quality control was really called into question. Although it was immediately recognized that it was the design of Ronan Point that primarily lead to the buildings collapse, it must also be noted that due to poor construction quality in the tower it is almost certain that there would have been future problems with the buildings structural integrity. At the time there was very little or no efforts to ensuring effective quality control methods were enforced in the construction process.

According to Feld and Carper (1997),  "As with all other construction materials, the best designs in precast and prestressed concrete can be ineffective unless the work done in the field is of high quality.  If the design is marginal, construction deficiencies can compound the errors increasing the potential for serious problems…"  "Skilled supervisors who understand the design intent and can communicate it clearly to the field workers are needed full-time at the construction site while all prestressed concrete work is erected."  (Feld and Carper, 1997).

5 Ethical Concerns

Initially, substandard workmanship had been detected prior to the collapse of Ronan Point. Even though it was later determined that this was completely negligible as to how the south-west corner of the tower collapsed, this information was not made public at the time which brought about a whole range of ethical and political concerns.

The government felt compelled to keep its promise on getting people out of the slums and providing affordable housing in the city. As a result of this, the findings on the substandard workmanship on Ronan Point were not published until 1968. By this time, many large panel concrete building had already been completely constructed under the Larsen-Nielson system. It was never a factor in the governments thinking that these building could ever be demolished. This is largely due to the fact that six of these types of buildings had already been completed and to demolish them would have been detrimental on the nation's economy. There was not even enough money to carry out works on these towers to make them more structurally safe.

Sam Webb done all in his powers to make people aware of the potential hazards associated with these new tower-blocks. He informed officials how these building could easily surcome to a progressive collapse in the event of fire or high wind speeds. He strongly advised for the demolition of these towers before they took the lives of unsuspecting occupants.

6 Conclusion

The collapse of Ronan Point revealed the vulnerability of the Larsen-Neilsen system of construction. Post-accident enquiries proved that the large panel construction system, in which panels of prefabricated concrete formed the walls, floors and roof slabs, was particularly vulnerable to progressive collapse due to gas explosion or other accidental damage. The incident revealed a fundamental error in the design concept.

Testing carried out on Ronan Points structural elements prior to the collapse proved that the tower would almost certainly have failed, even is the gas explosion in question had never taken place. Substandard gas nuts had been used, the hose connections were flawed, and the load bearing walls were unable to withstand relatively low pressures. As it was a relatively small explosion which led to the progressive collapse of the structure it had to be concluded that Ronan Point collapsed due to its lack of structural redundancy. There was no fail-safe mechanism designed into the building, and no alternative load paths for the upper floors should a lower level give way. Due to the absence of any type of structural frame, the upper floors had zero support and fell onto floor seventeen. The panels which formed floor seventeen could not support the sudden weight load of these five upper floors and consequently gave way. This collapse was repeated on every floor until it finally reached ground level.

The entire south east corner of the building was torn to the ground. It was subsequently rebuilt as a separate section and joined to the existing building by walkways. Gas was banned from Ronan Point and the building was reinforced with blast angles.

Even though it is quite clear that Ronan Point Tower collapsed due to a lack of structural redundancy it is also understood how difficult it would have been to build in more improved redundancy into this type of structure. Improving local resistance by installing higher strength precast concrete wall panels, which had been blown out to cause the initial collapse, would not have prevented the disaster. Testing proved that these walls would have blown out regardless of their strength.

Better interconnection of structural components would have been the key for the survival of Ronan Point Tower-block. Stronger and more positive connections between the wall panels and the floors, with less reliance on friction due to weight to hold everything together, is likely to have greatly reduced the scale of the collapse of the Ronan Point building.

7 Recommendations

From our investigations, it seems that Ronan Point Tower was destined for failure from day one. Since the progressive collapse of this structure, many codes and standards have attempted to address the issue of this type of collapse. It is now general practice that all the building elements be tied together and the ductility increased so building elements may withstand deformations from the failure of a portion of the building's structure.

However, before the initiation of these new building regulations there could have been a method utilized in Ronan Point's construction that may have saved the structure from progressive collapse.

J. Perf. Constr. Fac.. (May 2005). Ronan Point Apartment Tower Collapse and its Effect on Building Codes. Journal of Performance of Constructed Facilities. 19 (1), 172-177.

Rouse and Delatte, Lessons from the Progressive Collapse of the Ronan Point Apartment Tower, Proceedings of the 3rd ASCE Forensics Congress, October 19 - 21, 2003, San Diego, California


Griffiths, Hugh; Pugsley, A. G.; Saunders, Owen, (1968).  Report of the Inquiry into the Collapse of Flats at Ronan Point, CanningTown.   Her Majesty's Stationery Office, London.

Britain.  (1970).  "Britain tightens building standards, moves to stern 'progressive collapse.'"  Engineering News-Record.  April 16, 1970, 12.

Bignell, Victor; Peters, Jeoff; Pym,Christopher. (1977). Catastrophic Failures. Open University Press, Milton Keynes, New York.

Cook, Lal. (1997). Ronan Point. Available: http://www.lalamy.demon.co.uk/ronanpnt.htm. Last accessed 20 Jan 2010.

Feld, Jacob and Carper, Kenneth (1997). Construction Failure. John Wiley and Sons, Inc., USA.

Masayuki Nakao. (????), Chain Reaction Collapse of a High Rise Apartment due to a Gas Explosion, Available: http://shippai.jst.go.jp/en/Detail?fn=2&id=CA1000634, Last accessed 18/02/10.

Yataro Hatamura. 2003, Applying Lessons Learned from Failures, Kodansha Ltd.


1 Background

The collapse of L'Ambiance Plaza is said to be the most colossal disaster in modern Connecticut history. The collapse, which occurred on 23rd April, 1987, was responsible for claiming the lives of 28 construction workers. At this stage, the proposed structure was only a partially erect steel frame. It was proposed that L'Ambiance Plaza was to be a 16-story building, housing 13 apartment levels, with three parking levels underlying these apartments. Just prior to the collapse, the structure was comprised of two offset rectangular towers, each 63 ft by 112 ft in diameter, which were connected by an elevator shaft. The buildings structural frame was comprised of 178mm thick post tensioned, concrete slabs and steel columns. Post tensioning overcomes the tensile weakness of concrete slabs by placing high strength steel wires along their length or width before the concrete is poured. After the concrete hardens, hydraulic jacks pull and anchor the wires compressing the concrete (Levy and Salvadori, 1992).

Fig.?? Floor Plan of L'Ambiance Plaza (Moncarz Et Al. 1992)

Initially, at least six failure hypothesis theories were established. However, due to a prompt legal settlement these investigations were kept from being completed. It is important that was discuss and analyze all of these original hypothesis theories as well as the leading theory, where the finger of blame for the failure is pointed at the lift-slab construction method.

This method of slab placement was patented in 1948 by Youtz and Slick and was utilized in the construction of L'Ambiance Plaza. Each of the sixteen floor slabs which the building was to be composed of were all constructed at ground level and placed on top of each other using bonds breakers. Slabs packages of about two or three were then elevated into temporary positions using the hydraulic lifting apparatus and fixed using steel wedges. This process was carried out using a hydraulic jack on top of each column and a pair of lifting rods extending down to lifting collars cast in the slab. Slabs were then permanently attached to the steel columns. Lateral restraint for the top two floors were to be provided by two shear walls, one situated in each tower. These two floors depended on the rigid joints between the steel columns and the concrete slabs for their stability. Because the shear wall played such an indispensable role in the lateral stability of the building, the structural drawings specified that during construction the shear walls should be within three floors of the lifted slabs (Heger, 1991).

This accident prompted a major nationwide federal investigation into this construction technique as well as a temporary moratorium of its use in Connecticut.

At the time of collapse, the building was a little more than halfway completed. In the west tower, the ninth, tenth, and eleventh floor slab package was parked in stage IV directly under the twelfth floor and roof package. The shear walls were about five levels below the lifted slabs (Cuoco, 1992).

2 Collapse

The collapse of L'Ambiance Plaza occurred while the building was still under construction. It was little over halfway through its construction process meaning there was heavily occupied by construction workers on the day of the collapse. The ninth, tenth, and eleventh floor slab package were being stored in stage IV of the west tower, directly under the twelfth floor and roof package. This layout can be observed in fig. ??. The shear walls were located about five levels below the lifted slabs. Workmen were tack welding temporary wedges in order to hold the ninth, tenth, and eleventh floor package in position when all of a sudden they heard a loud metallic sound followed by rumbling. One of the ironworkers who was installing these wedges at the time looked upwards and observed the slab above him "cracking like ice breaking".

Level of Construction at time of Collapse, (Cuoco, 1992)

Suddenly, the slab fell on to the slab below it, which was unable to support this added weight and in turn fell. The entire structure collapsed, first the west tower and then the east tower, in 5 seconds, only 2.5 seconds longer than it would have taken an object to free fall from that height. Two days of frantic rescue operations revealed that 28 construction workers died in the collapse, making it the worst lift-slab construction accident. Kenneth Shepard was the only one on his crew to survive (Levy and Salvadori, 1992).


3 Failure Hypothesis

The failure hypothesis was base on two possible factors namely, overloading and instability of components:

1) An overloaded steel angle welded to a shearhead arm channel deformed, causing the jack rod and lifting nut to slip out.

2) The instability of the wedges holding the twelfth floor and roof package caused the collapse.

4 Site Investigation

Journal of Performance of Constructed Facilities, Vol. 6, No. 4, November 1992, pp. 211-231, (doi 10.1061/(ASCE)0887-3828(1992)6:4(211))

The City of Bridgeport's chief consultancy firm at the time of the incident was Thornton-Tomasetti Engineers (T-T). T-T were retained immediately after the collapse to investigate into the cause of this devastating structural failure. T-T were provided with unrestricted access to project documentation, to witness statements, to the collapse site, and to the collapse debris. Throughout the entire rescue operation, team members of T-T were present at the site of the collapse, recording and documenting every movement which took place, as well as preserving as much perishable evidence as possible. This document which I am about to investigate is a compilation of background information, field observations, the results of laboratory tests and computational analyses, and findings concerning the most probable cause of the collapse.

4.1 Eye Witness Reports

Any accounts given by eye witnesses all agreed that the collapse of the building was extremely rapid. Estimates of total collapse ranged from 2 sec to 10 sec. The majority of eye witnesses claimed that the event was first drawn to their attention by a loud bang which appeared to have came from the west building. Most of these witnesses stated that they saw or heard the slabs in the west building collapsed first. The majority of witnesses who had a good vantage point of the collapse believed that the center of the west building was the first element to collapse.

An important account of how the collapse transpired was provided by an ironworker working on the stage IV of the west tower. These men had the unfortunate job of tacking weld wedges underneath the doomed floor package, intended for the ninth, tenth, and eleventh story of the building. Suddenly the claim to have heard a "loud metallic sound followed by rumbling". One of the ironworkers who was working on this slab-package described how he looked upwards to observe the slab over his head "cracking like ice breaking". All of a sudden the slab fell upon the slab directly below it. As a result of this extra loading which the slab was unable to support, it also fell. This caused a domino effect to the ground floor. The entire structure collapsed, first the west tower and then the east tower, in 5 seconds.

This surviving member of the wedging crew mentioned that at the time of the collapse he and his crew were somewhere near the center of the west building. He and his partner, both of whom were on top of a scaffold, had just inserted both wedges. He turned his head to shield his eyes while his partner began to tack weld the wedges, at which point he heard the initial bang. The sound appeared to come from immediately above him or from within 25 ft west of him. He then heard a crumbling sound, observed a lot of dust, and noticed that the ceiling directly over his head was cracking like "ice breaking". The slabs then came down around him, driving him inside the cage of the rolling scaffold, which somewhat protected him during the fall.

4.2 Curing Concrete Test Cylinders

When L'Ambiance Plaza collapsed to the ground, it was only halfway through its construction, so the curing concrete test cylinders still remained on site, some of these concrete cylinders had already been transported to the laboratory for testing. Since just after the collapse, the investigators team were try to collect all curing concrete cylinders immediately. These cylinders (Figure 3.14) would provide the investigators with invaluable information results regarding the stability of the concrete which had been poured in L'Ambiance Plaza. However, the compressive test had shown that the quality of concrete was not the factor contributing to the collapse of building.

Figure 3.14 These curing concrete cylinders were discovered at the site of the L'Ambiance Plaza collapse and were instrumental in ruling out substandard concrete strength as a possible cause of the collapse. (Robert T Ratay, 2000)

4.3 Laboratory Test on the Capacity of Shearhead and Lift Angle

The collapse of L'Ambiance Plaza was suspected to be due to the overload of the shearhead and lift angle. This hypothesis was made based on finding of the investigation. According to the evidence obtained, it was shown how just prior to the collapse the 9th, 10th, and 11th floor package was hoisted into place and ironworkers began tack-welding the steel wedges into place. The lifting jack was mounted on top of column E4.8 or E3.8 (Figure ???) in order to make a slight adjustment to the position of the slab which was causing overloading on the lifting angles. Once the packet containing three 320-ton slabs had been lifted into place by the shearheads and lifting angles, they were limiting their maximum capacity. The addition of even the tiniest of loads may have strained them to failure.

The reason for this was that the two jacks used for this hoist did not have the required lifting capacity to support the 960-ton package being lifted. The regular jacks have a maximum load of 89-tons, while the super jacks have a maximum load of 150-tons.

Subsequent investigations carried out by the investigation team showed how the shearhead and lift angle tended to twist as the loads approached 80 tons. Although they were strong enough, they were in no means rigid enough for the lift. This excess force caused the deformation of the lifting angle which in turn transferred the excess load to the column. This in what effectively caused the collapse of L'Ambiance Plaza.

4.4 Computational Analysis of Shear Gaps

Shearhead gap is to allow clearance between the shearhead and the weld blocks during the lifting operation. It is necessary that the distance between the shearhead headers be larger than the out to out distance between the weld blocks.

Gap is defined as the distance from the outside face of the weld block to the inside face of the header, or header bar, as illustrated in Figure 3.15. The larger the gap, the more difficult it is for the wedge to remain stable and to transfer loads from the shearhead to the column. Also, the larger the gap, the smaller the amount of bearing area provided to the shearhead.

Based on the lifting shop drawings, investigators calculated the actual shearhead gaps at all erected columns in the east and west buildings for the case in which the shearhead is perfectly centered on the column. They found that the shearhead gaps on column 3E-3.8E, 16mm (0.628 in) were much larger than the rest of the building, 5.92- 8.31mm (0.233 in-0.327 in) and other building built with the lift slab technique, 6.35- 9.53mm (0.250in-0.375in). The results are summarized in Table 3.4.

Figure 3.15 Slab to column connection (Cuoco et al, 1992

Table 3.4: Shearhead gaps of column


Shear Gap (mm)

Column 3E -3.8E of L'ambiance Plaza


Other column of L'ambiance Plaza


Other building column


5 Cause of Failure

The collapse of the L'Ambiance Plaza were due to the failure occurred at the building's most heavily loaded column E4.8 or the adjacent column E3.8 as a result of a lifting assembly failure. Since the shearhead and lifting angle were overloaded, the excess force deformed the lifting angle, allowing the jack rod and lifting nut to slip out of the lifting angle and hit the column with 333kN (75,000 lb) of force (Figure 3.16). After this initial slip, the jack rods and lifting nuts in the entire E line progressively slipped, causing the ninth floor slab to collapse, initiating the collapse of the entire building. (Levy and Salvadori, 1992).

However, as is often the cases in the majority of structural failure, there were other parameters which lead to the progressive collapse of L'Ambiance Plaza. The wedges at column 3E were highly instable which caused the 12th floor/roof package to fall, which initiated the collapse. Computer analysis of the shear gaps proved that the distance of the shear gap of column 3E and 3.8E were well over the normal acceptable tolerance.

As well as these abnormally large shear gaps, the shearheads on these two columns in question did not contain any addition of "cut outs" in their lifting angles in order to resist relative shifting, plus were also installed eccentrically. The system is completely reliant on friction to hold it together until a wedge is welded into place. In most cases, this is usually sufficient. However, due to the large shearhead gaps on column 3E and 3.8E, and the presence of hydraulic fuel on these wedges, an extremely high coefficient of friction would have been required to hold the wedges in place.

On the day of collapse, the lateral load from the hydraulic jack exerted on the heavily loaded wedges had caused the west wedge to roll. Then the local adjustments to slab elevations caused the remaining wedge to roll out initiating the collapse of the 11th floor/roof package and the west tower. Forces transmitted through the pour strips or the horizontal jack, or the impact of the debris from the west tower triggered the east tower collapse (Cuoco et al, 1992)

Fig ?, Lifting Assembly (Levy and Salvadori, 1992).

6 Legal Repercussions

The legal repercussions from this case were overwhelming. In the end, a settlement between almost 100 parties was resolved by a panel of two judges. Over twenty of these parties were found guilty of "widespread negligence, carelessness, sloppy practices, and complacency." A total figure of $41 million was paid out, in varying amount, by these parties found guilty. $30 million of this was allocated to those injured and also to families which had suffered a loss of loved ones as a result of the collapse. $7.6 million was allocated for all claims and counter claims which were expected to arise between the designers and contractors on the project.

While this settlement kept hundreds of cases out of court and provided rapid closure to a colossal collapse, it also ended all investigations prematurely, leaving the cause of collapse undetermined (Korman, Nov 24, 1988).

7 Technical Concerns

It is widely understood that once a building has been fully constructed using the lift-slab method, it is generally completely safe. However, if relevant care and attention is not paid during the its construction it can lead to disastrous consequences. The following measures should be taken into consideration in order to insures lateral stability and safety are utilized during construction;

During all stages of construction, temporary lateral bracing should be provided.

Concrete punching shear and connections redundancy should be provided in the structure (Kaminetzkv, 1991)

Cribbing (temporary posts which support the concrete slab until it is completely attached to the column) should be used

Sway bracing (cables which keep the stack of floors from shifting sideways) should be used. This was required but not used in L'Ambiance Plaza (Levy & Salvadori, 1992).

Due to the terms of the settlement, many of the technical lessons that could have been learned from this incident have been lost forever.

8 Procedural Concerns

Many procedural methods were highlighted as being flawed and not up to the relevant standards after the collapse of L'Ambiance Plaza was highlighted. One area where health and safety measures suffered was that the responsibility for design was spread across such a large range of subcontractors that many problems which were arising from design went undetected. If responsibility had been placed solely on the engineer and whoever he chose to delegate as his representative, for the overall design of the project, then it is almost certain that these design flaws would have been detected at an early stage.

Procedures for lift-slab construction should also have been standardized into a simple step-by-step method which would have ensured a more smooth method of work as well as the safety of those who were carrying out the work.

As well as this, it is proposed that a licensed professional engineer should be present at all times during the construction process to ensure that these guidelines are followed.

9 Ethical Concerns

While there is no doubt that L'Ambiance Plaza would have been a perfectly safe structure once totally constructed, while it was under construction the safety regulations which were in operation on site were highly flawed.

Canon 1, of the American Society of Civil Engineers (ASCE) Code of Ethics states, "Engineers shall hold paramount the safety, health and welfare of the public and shall strive to comply with the principles of sustainable development in the performance of their professional duties" (ASCE Code of Ethics, 1998).

This Code of Ethics ensures the safety of those working on site. However, these regulations do not provide sufficient structural safety procedures for when a project is under construction.

There is a necessity for these regulations to be altered to ensure a higher standard of safety is practiced during the construction stage. These standards should also be maintained after the buildings construction, into its serviceability stage and beyond.

10 Modes of the Failure

The collapse of L'Ambiance Plaza which was under construction was caused by the lack of stability of the wedges. The structural failure occurs suddenly.

11 Conclusion

L'Ambiance Plaza collapsed while half-way through its construction. This was primarily due to the breaking in the load path of the structure. Load path can be explained in simple way that any structure touches the ground in certain areas, which are the only places from which it can derive support. It will also experience forces from loads, such as traffic and wind pressure. The load path is the route by which forces applied by the ground can exert the forces necessary to balance the loads.

In structural analysis, the continuous load path is very important in design. It is because the continuous load path is able to transfer loads from structural component to another structural component until they are transferred into the earth.

If the continuous load path is disturbed like what had happened at L'Ambiance Plaza, the structure member will not able to carry load, hence damage or collapse of the structure will happen.

The continuous load path of the structure can be achieved by adequate design of the element and connection of structure. To prevent the structural collapse entirely due to the disturbing of the load path, the alternate load path can be designed for the structure.

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