4A6 CASE STUDY PRESENTAION World Trade Centre Collapse

NEW YORK SKYLINE, 2000

The Twin Towers Two 110-storey towers: North Tower 1,368ft tall South Tower 1,362ft tall North tower struck between floors 94 & 98 with an impact speed 470mph South tower struck between floors 78 & 84 with an impact speed 590mph North tower collapsed 56 minutes after it was struck South tower collapsed 1 hour 43 minutes after it was struck

What Happened... The planes struck each tower causing extensive structural damage, including localised collapse at the several floors directly impacted by the aircraft. As the planes impacted each building jet fuel on board ignited, causing intense fires throughout the upper portions of the building. The steel columns, softened in the heat, separated from each other and buckled. Weighted down by debris, furniture, concrete and steel, floors progressively collapsed. The structural damage sustained by each tower from the impact, combined with the ensuing fires, resulted in the total collapse of each building.

The Technical Factors Contributing to the Disaster WTC was in effect a vertical steel tube Strength of the structure lay in its external steel framework of columns and Vierendeel trusses These supported light steel girder floors

Progressive Collapse Each tower remained standing immediately after it was hit Immediate damage to the structure was not disproportionate in the circumstances On impact the aviation fuel caught fire Intense heat weakened structure in the crash location to such an extent that it was unable to support the building above

Progressive Collapse A progressive collapse of the whole building followed, the increasing kinetic energy being sufficient to cause catastrophic damage to propagate downwards through the previously undamaged lower stories Under intense heat the structure above failed, allowing the building to fall under gravity onto the section below This descending section of building which gained momentum as potential energy was converted to kinetic energy

Structural Engineering Implications ‘Structural engineering is the science and art of designing and making, with economy and elegance, buildings, bridges, frameworks, and other similar structures so that they can safely resist the forces to which they may be subjected’ The Institute of Structural Engineers, U.K.

Structural Engineering Implications Clearly WTC towers did not ‘safely resist’ aircraft impacts Structural engineers readily recognise that extreme events will cause structural damage and place people in jeopardy Damage cannot be avoided entirely by design

Structural Engineering Implications The term ‘safely resist’ must be qualified Safety is not an absolute as perceived by the general public Virtually always some risk of loss in structures Perhaps the definition of risk should also be included in the definition of structural engineering The scenario for aircraft needs to include not only the impact event but also subsequent explosion and fire Aim is to design the structure, its fire resistance, means of escape and in-use management systems so that lives are saved and damage minimised Loss of life and damage should not be disproportionate to the cause of collapse

Human and Managerial Factors Aircraft impact Fire deformation DESIGN Stored energy Air space monitoring Tannoy broadcastHUMAN Security measures

Conclusions The actual performance of a building depends on... The passage of time The behaviour of users The natural elements Unnatural events Is it possible for a tall building to... Withstand the impact of an aircraft Survive a catastrophic fire Have adequate fire protection Have adequate modes of evacuation

Lessons to be learnt from the disaster “We learn more from buildings that fall down than from buildings that stand up.”