The infamous collapse of the World Trade Center in New York tragically showed the consequences of the total devastation of a skyscraper of these dimensions. "Despite the atypical causes and tragic consequences of this event, it can perhaps teach us something and we can use this widened knowledge to improve the design of similar structures in the future," conclude Professors Michael H. Faber and Mario Fontana. Together with their assistants, Oliver Kübler and Markus Knobloch, they are developing new models with which to judge the consequences of such a collapse, working on methods to reach the maximum level of safety.

Working out rational measurement bases for structures is one of the main duties of the Joint Committee on Structural Safety (JCSS) – an international group of experts in which Faber heads the working group "Risk Assessment of Civil Engineering Facilities". They provide a basis and support standards' commissions (e.g. Eurocodes, ISO standards) and civil authorities in their work and decisions.

A realistic and uniform assessment of the consequences of the collapse of special structures, such as the World Trade Center, a drilling platform or the Golden Gate Bridge, has not been carried out so far. "Until the end of 2003, in a project financed by The Swiss National Science Foundation (SNSF), IBK is looking into this question and developing a unique and rational basis on which decisions on necessary safety standards for similar structures can be met in the future," says Faber.

Graphic illustration of the effect of heat on a piece of steel with damaged isolation. large

Realistic assessment

The internationally presented model takes the most important consequences of an extraordinary event, which could also be caused by an earthquake or an explosion. The realistic assessment of such an event is a prerequisite in determining an optimal degree of safety for the structure, for example the cornerstone in the design basis of such structures. The aim of the proposed project, according to Michael Faber, is to analyse the collapse of the twin towers of the World Trade Center by analysing the consequences. Consequences in this context are, among others, the resulting injuries and fatalities to human beings, but also the rescue services, the clean-up costs , loss of property and inventories, as well as the reduction of functionality of the building in question or buildings in the vicinity.

Following events on 11th September 2001, Oliver Kübler, Markus Knobloch, Mario Fontana (holding a piece of distorted steel from the World Trade Center) and Michael Faber (from left to right) developed new methods to assess the safety and consequences of extraordinary events. large

Faber summarises: "The evaluation includes costs on a social basis, and takes into account the loss of human lives, the loss of a company and material losses. Based on detailed knowledge of these components an optimal degree of safety can be ascertained in this study." A basic framework has been developed to judge acceptance criteria, the aim of which is to maximise the yield of a structure over its lifetime, taking all possible consequences into account.

Fire destroys resistance

A study, currently being carried out at IBK, also motivated by 11 September 2001, under the charge of Mario Fontana, examines the structural behaviour and the development of methods to calculate the fire resistance of steel girders once fire protection isolation has been partially destroyed. Fontana explains: "We have determined that even a partial destruction of the fire protection isolation – even over a very small area – considerably lowers the resistance of protected iron girders." Standard methods of computation do not take the partial destruction of protective materials into account, which occurs for any number of reasons, for example, after a collision, inadvertent damage, incorrect usage, removal in the proximity of cables and installations, etc. Preliminary results of the study on structural behaviour and the fire resistance of steel supports – considering the local loss of fire protection insulation in a small area would show that fire, even if it only causes a local rise in the temperature of the steel where the protective material is damaged, can still considerably reduce the columns’ ability to resist fire. This is why a better understanding of the reaction of the column to a local loss of fire protection is important to arrive at an assessment of the reliability of fire resistant steel structures, and especially taking locations and infrastructure into consideration.

Design and analysis

Tragic events, such as earthquakes, explosions, fires or impacts, can cause the collapse of one or more floors in a multi-storey building. When this happens, the upper part of the building develops enormous kinetic energy. If the lower floors cannot withstand this energy the result is the progressive and total destruction of the building. Fontana, who is collaborating with Professor Dazio of IBK for this part of the study, sees a possible measure to prevent such a collapse by strengthening the structure or by installing a cushioning system. "The results of the analysis clearly show that as soon as one floor collapses, the installation of a shock absorbent system is the least expensive strategy to prevent the total collapse of the building." On the other hand, strengthening strategies, that are more expensive, lead to a positive change in the structure of the building. This lowers the probability of the collapse of any floor and thus the probability of the entire building collapsing.

In order to make practical and usable tools for engineers and architects, standards will be brought into line with normal structures. And to such an extent that buildings for normal use of normal dimensions and normal design, built with well-known materials and maintained so that an adequate and homogeneous degree of reliability can be achieved with an acceptable level of investment. For exceptional buildings or infrastructure that would have serious consequences if they fail, additional analyses and considerations must flow into the design. This study of the ETH Institute of Structural Engineering provides an essential basis for experts.