In May 1968, a small gas explosion at the top of a 22-storey tower block called Ronan Point in east London caused one entire corner of the building to collapse, killing four people and injuring 17. The spectacular nature of the disintegration – caused by poor design and construction – led to a serious loss of public confidence in high-rise residential buildings.
The past few decades have seen other building collapses, such as the World Trade Center and 130 Liberty Street in New York, caused by various factors but with one common point – the unexpected nature of the events and the accidental loading that triggered the sequence of the collapse.
There have also been several related cases involving other kinds of built infrastructure, one being the collapse of the I-35W Mississippi River bridge in Minneapolis, Minnesota in 2007. The bridge, with eight traffic lanes, was designed and built using steel arched trusses. Not even 40 years old, it unexpectedly collapsed one rush hour, killing 13 people and injuring 145. Along with the loss of life, the effects on business, traffic and transportation funding were enormous.
Since 1968, a raft of research has been carried out to mitigate what is known as the ‘progressive collapse’ of structures – in other words, structures that collapse, or part-collapse, due a chain reaction effect.
Almost half a century on, Ronan Point remains at the forefront of structural engineering because of multiple natural and man-made hazards, the ageing of the infrastructure and the catastrophic consequences associated with that.
Birmingham’s Charalampos (Lambis) Baniotopoulos, Professor of Sustainable Energy Systems at the School of Civil Engineering, is one of Europe’s top researchers working on how to make structures and buildings more robust. Indeed, his research is aimed at being incorporated into the next generation of Eurocodes (a set of harmonised technical rules for the structural design of construction works in the EU).
His studies include both steel and reinforced concrete structures, but a recent research paper on the subject focused on steel structures. Entitled ‘Progressive collapse mitigation of 2D steel moment frames: Assessing the effect of different strengthening schemes’, it was published in the prominent German construction journal Stahlbau (Steel Construction).
The paper focuses on two-dimensional moment-resisting frames, which are rectilinear assemblages of beams and columns with the beams rigidly connected to the columns (where resistance to lateral forces is provided primarily by rigid frame action) and examines collapse modes and design solutions to mitigate the collapse of these frames.
‘The collapse of Ronan Point was triggered by a gas explosion – caused when a resident on the 18th floor held a lit match to her stove to make a cup of tea – that blew out the load-bearing flank walls, removing the structural supports to the four flats above,’ explains Lambis. ‘It is thought the weakness was in the joints connecting the vertical walls to the floor slabs. The flank walls fell away, leaving the floors above unsupported and causing the progressive collapse of the southeast corner of the building.
‘After that, the USA, the UK and other countries started working on disproportionate collapse events. Guidelines and codes started to be developed and numerous research groups around the world began working on the robustness of structures.’
The terrorist attacks on the Alfred P Murrah Building in Oklahoma in 1995 and on the World Trade Center in 2001 further increased research interest in this field.
Progressive collapse is characterised by a triggering event that is usually disanalogous with the resulting consequences (which was the case at Ronan Point, but not in the Alfred P Murrah and World Trade Center cases). In certain cases, structures have shown high levels of vulnerability to the triggering mechanism – for example, Ronan Point – whereas in other cases they have proved to be resilient.
The paper, written with one of Lambis’s former PhD students, Simos Gerasimidis, investigates the effects of different strengthening techniques for mitigating the domino-effect phenomenon. These techniques are applied to a series of moment-resisting frames of varying heights and designs, using a finite element analysis involving both material and geometric non-linearities.
What the research shows is that no one strengthening method can be applied to all collapse mechanisms.
‘The results of this work demonstrate that, when analysed as 2D assemblies, realistic, similar steel frame structures undergo different collapse mechanisms and that a general strengthening scheme cannot be applied to all collapse mechanisms,’ Lambis and Simos explain in the paper.
‘This work has focused on the mitigation of the progressive collapse phenomenon in certain common 2D moment-resisting steel frames. A parametric study of different strengthening techniques has shown that the only possible way of strengthening a structure against a loss-of-stability/buckling-induced collapse mode is to strengthen the critical column elements that undergo buckling failure. It has also been shown that the strengthening/tying of the beams above column removal level has a negative impact on the capacity of the frame, also directing the collapse mechanism to the buckling of columns.
The findings of this work constitute an important step towards the comprehensive investigation of the relationship between loss of stability and progressive collapse mitigation.
Lambis, who has been at Birmingham for three years, adds: ‘There will be a new generation of Structural Eurocodes in the next few years, one of which concerns structural robustness. So, the impact of this research will be significant because it supports the new generation of Eurocodes under development.
‘Ronan Point happened 45 years ago, but until now there has not been a Eurocode to help mitigate the risk posed by that kind of scenario. Today, this is also important because of the threat of terrorist attacks. To have a method for designing buildings in a way that mitigates a disproportionate collapse is something new and very important for the safety of the built environment, and subsequently for the wellbeing of society.’
- This paper titled ‘Progressive collapse mitigation of 2D steel moment frames: Assessing the effect of different strengthening schemes’ was published in Stahlbau
- The paper was also awarded College of Engineering and Physical Sciences Best Publication for March 2015