Abstract
This paper reviews four different civil engineering failures, highlighting common issues between them and will discuss why each failure occurred. It will conclude that failures are the result of a critical network of human factors, decisions and actions and a trade-off between safety and cost. The most common error was the lack of consideration of every failure mode, as in every case, the failure was not foreseen during design.
INTRO
The purpose of this report is to examine past engineering failures, understanding how they occurred and analyse contributing factors. The chosen failures are: The Millennium Bridge, Charles-de-Gaulle Airport Terminal 2E, the Alfred Murrah Building and the Sampoong Department Store. A short description of each will be given then the failures will be compared and contrasted including any lessons learnt, with particular emphasis on issues common to each.
TERMINAL 2E
On 23rd May 2004, terminal 2E at Charles-de-Gaulle airport collapsed, killing 4 people and injuring 3. The collapse left a 50x30m hole in the curved roof, which fell onto the departures lounge and storage areas below.
The terminal was designed by French architect Paul Andreu as an “eggshell” structure made up of three layers; plate glass, a metal support structure and reinforced concrete blocks.
An official investigation concluded that there were four different issues. Firstly, the metal reinforcing in the concrete was insufficient, caused either by poor design or incorrect placement during fabrication, making it weak. The footbridges entering the terminal on one side made the structure asymmetrical meaning that the stresses couldn’t transfer from one part of the structure to another easily. This caused large stresses in certain parts of the structure which may have been greater than the failure stress of the concrete. The passage of ventilation ducts meant that the main roof beam was weak. The struts separating the outer glass and metal layer from the concrete were embedded too deeply into the concrete. This coupled with the rapid thermal expansion of the metal structure forcing the concrete to expand and contract, caused the concrete to crack and therefore weakened the roof.
The French government's choice to use the same company, state-owned ADP, for both the design and construction of the terminal possibly contributed to the failure, as it led to a lack of distinction between the client and architect. This decision allowed the government to ensure a French architect would design the building, therefore the terminal would be a showcase of French design and engineering prowess. Using the same company, however, meant that there was a lack of detailed analysis during both the design and the construction of the terminal.[2]
The design process was not rigorous enough for the structure, as the complexity of the design was underestimated. “The models employed to predict the behaviour of the complex three-dimensional structure were much too simple to capture fully and predict accurately how the real one would work”[1]. Independent structural analysis was not conducted on the design due to its classification as a building and not a civil engineering structure. This meant that none of the calculations were checked and allowed ADP to overlook critical design faults.
Another terminal at Charles-de-Gaulle airport, terminal 2F, is very similar in design to 2E but the structural elements differ. Terminal 2F is supported by steel arches which make the structure much stronger, but construction was much more complicated and expensive. To keep costs down, 2E was constructed with reinforced concrete. Terminal 2F did not collapse, which stands testament to why settling for ease of construction and cost of the project over the integrity of the design is an unacceptable practice. As an isolated incident, the collapse of terminal 2E wasn’t enough to create or reinforce too much new regulation.[3]
SAMPOONG
The collapse of Sampoong Department Store occurred on June 29th, 1995, killing 502 people and leaving 937 injured, resulting in the largest loss of life in a single construction disaster. The failure of the store, located in Seoul, South Korea, was easily preventable [1] and arose due to personal shortfalls of the store’s owner, Lee Joon, and errors made by designers and contractors [2].
The department store was built on a former landfill site [3] by two companies. Originally, Woosung Construction were contracted to build a four-story office building on the site, however after the foundations and first floor had been completed they were fired for refusing to drastically change the construction plans, including adding a fifth floor. Joon thus hired his own company to complete the superstructure [1].
The building had been constructed as a flat-slab structure, with no beams or girders present. Consequently, large spans could not be achieved due to the low tensile strength of concrete. However, the store was constructed with a span of nearly thirty-six feet between each support, dangerously lowering the strength of the structure to maximise floor space and profits [1]. When the building's design was altered from an office block to a department store, numerous support columns were removed to accommodate the addition of escalators, leading to a further increase in the spans [5]. To meet fire regulations, floor columns had been reduced from around 33 inches thick to less than 24 inches thick [1].
Initially, the building's fifth floor had been designed as a roller skating rink, to withstand a dead load of 860kgm-2 and a live load of 240kgm-2 [4]. However, executives altered the design to a restaurant with large refrigerators and heated floors adding four feet of thickness to the floor slab [1]. This extra weight equated to a 50% increase in the working load. With columns on the fifth floor being out of alignment with the columns beneath, weight was not transferred from column to column but through the slab.
Additionally, the air conditioning units on the east side of the roof were moved to the west; the 45-tonne combined weight of these units was four times the design load of the structure. The main support columns were forced downwards [1], and cracks formed in the roof which were exacerbated by the vibrations from the air conditioning units [3].
on 29th June, the fourth-floor ceiling began producing cracking sounds and starting sinking. That afternoon the roof gave way to an air conditioning unit which fell onto the overloaded fifth floor. The columns collapsed, and the south wing of the store pancaked into the basement, trapping over 1,500 people [4].
ALFRED MURRAH
The Alfred P. Murrah Federal Building in Downtown Oklahoma City, USA was subject to the biggest ever act of domestic terrorism on American soil by a home-grown terrorist on 19th April 1995. The attack took 168 lives including those of nineteen children.1 A lorry parked 10 feet away from the entrance to the building containing a bomb made primarily of ammonium nitrate fertilizer and other high street ingredients detonated resulting in the progressive collapse of almost 50% of the nine storey structure.2
After the collapse an investigation by FEMA, a Building Performance Assessment Team (BPAT), found that the architect Wendell Locke of Locke, Wright and Associates and the constructors of the building had complied with the legislation and codes available for the construction of buildings of reinforced concrete of the time in the 1970s3. The building had an ordinary moment frame design which had been unable to withstand the loading from the blast. Three main columns had failed on the north side of the building including a principle exterior column (G20) through brisance. The failure of these columns disrupted the alternative load-transfer path through three-dimensional Vierendeel frame action and thus led to its progressive collapse4. In addition to this, the blast wave travelled through the building creating uneven vertical forces on the floor slabs. In accordance with codes of practice of the time of construction the floor slabs had only been reinforced at their base and so had no resistance to the upward loading from the blast wave. The slabs underwent catenary action due to gravity loading after the blast wave which led to them failing in punching shear at some column lines from Level 5 and below. The failing of these floor slabs is thought to have led to the buckling and failure of columns G16 and G243. The abrupt pressure from the blast exceeded the allowable shear in the concrete composite and resulted in its brittle failure5. This led to the loss of the transfer girder which instigated the progressive collapse of the Murrah building.
The catastrophic failure of the Murrah Building led to calls for changes in legislation. The reports investigating the collapse found that an increase in lateral load resistance was not needed, rather different detailing would be required to provide structural integrity following damage caused by a sudden blast load such as a bomb. Suggestions for structural improvements from the official report included special moment frame constructions and dual systems with special moment frames as these provide a higher toughness through ductile detailing with which to resist blast forces3.
MILLENNIUM
In June 2000, a new footbridge spanning the river Thames in London was opened to the public. Designed by a team including renowned sculptor Sir Anthony Caro and Britain’s leading architect of the time, Lord Norman Foster, the Millennium Bridge (Figure 1) was intended to resemble ‘a blade of light’ and thus had an extremely shallow profile. [1] The shallow suspension bridge, with two groups of four cables not rising more than 2.3 metres above the deck, was designed to allow for unobstructed views of the city.
As upwards of 100,000 people streamed across the 333m long structure for its opening, almost immediately, the bridge began to develop excessive lateral movement. [2] The worst case of this was encountered at the central of the three spans where the displacement of the deck was up to 70mm in the horizontal plane. [3] As the structure began to oscillate, to the point that people found it hard to maintain their balance, the instinctive behaviour for pedestrians was to match the lateral rhythm of the bridge. With more pedestrians adjusting their gait to the motion of the bridge, increased footfall forces were applied at the bridges resonant frequency which further increased the motion. As the amplitude increased, lateral forces exerted by individuals increased and this positive force feedback caused magnified motion. [4]
Ultimately, fearing public safety, city authorities were forced to close the bridge only 2 days after opening due to this unanticipated motion. Subsequent research was undertaken by Arup, the engineering firm responsible for design and construction of the bridge, to investigate this phenomenon and mitigate the problem.
Reviewing video evidence, (footnote) it is obvious that the vibration of the bridge was caused by lateral pedestrian loading. The oscillations were caused by the lateral frequency from crowd movement coinciding with several lateral and torsional mode frequencies of the bridge. This phenomenon, where the forcing frequency matches the natural frequency, is known as resonance. [5]
While the bridge was not in danger of immediate structural collapse, the deflection was excessive and therefore considered a failure. At times the movement was sufficient enough that pedestrians had to stop walking or hold onto the balustrades in order to retain balance (Figure 3). Clearly, the design engineers are at fault since they overlooked a whole mode of failure in design. Whilst they followed protocol for structural and dynamic design, they did not foresee the effects of synchronised footfall. The system was sensitive to the way it was used and had the bridges maximum capacity not been exceeded on its opening day, the affectionately named ‘Wobbly Bridge’ may have not have wobbled. However, in today's design, bridges and other structures are becoming increasingly slender. Research undertaken has now resulted in changes to the codes for bridge building worldwide and this is a direct example of how the engineering process and professional practice has directly evolved from new knowledge gained as a result of the failure. [6]
EVALUATION
The causes of each of the aforementioned failure cases differ significantly, however, it is possible to draw similarities between the cases. A common theme that runs through all four cases was that in the initial design phase, a major factor was not considered or ignored – essentially a human error. For the case of Sampoong Department Store, an extra floor was added with no consideration for this increased loading. Ultimately this came down to negligence rather than a lack of knowledge. Whilst for the Millennium Bridge, ignorance of a failure mode was the causation of failure and similarly, with Terminal 2E, the design was oversimplified. There was also no prior consideration to the impact of a bomb explosion with the Alfred P. Murrah Federal Building.
A feature that can distinguish the four cases is the severity of the failure. The extremity ranges vastly from the fairly minor and non-harmful failure of the Millennium Bridge to the grievous Sampoong Department Store failure which led to a loss of approximately 500 lives and left 937 people injured. It is noteworthy that the level of severity tends to closely relate to the new legislation that is put in place as a result of a failure. Restrictions are often imposed in the wake of highly visible or particularly tragic failures with the intent of implementing legislative technological fixes. The Millennium Bridge was particularly in the public eye and thus Arup needed to demonstrate they were solving the problem at hand. Therefore, after exploration into the cause of the failure, legislation was implemented which altered how bridges must be designed. For the particularly catastrophic case of the Alfred P. Murrah Federal Building, new legislation designed to increase the protection around federal buildings to deter future terrorist attacks.
Particularly in the cases of Terminal 2E and Sampoong Department Store, politics and corruption played large roles leading to their failures. While Sampoong Department store was governed by greed and Terminal 2E was driven by a political agenda, both caused a certain degree of negligence. As the French government were adamant to showcase French design, they failed to diligently identify errors in the design process. Similarly, the greed of the safety officers reviewing the design of Sampoong Department store allowed the build to go ahead, ignoring critical issues.
Ultimately, it comes down to a trade-off. Whilst engineers are required to design safely, they are forced to realise that increasing reliability is costly. In order to progress as an industry, suitable risk management must consider the balance between cost and robustness of a structure. In the rare case that something goes wrong, where a structure or system fails catastrophically, it is essential that on reflection the industry develops its understanding and improves so that failures do not repeat themselves. This is particularly important because it can not only be costly in a financial sense but also to lives, the environment and to reputations.