The Total Collapse of the Twin Towers: What It Would Have Taken to Prevent It Once Collapse Was Initiated

Nikolay Lalkovski,Uwe Starossek

doi:10.1061/(asce)st.1943-541x.0003244

Abstract

It is generally taken as a given that there is no reasonable design concept that could have prevented the collapse of the Twin Towers, once it was initiated, from progressing all the way down to the ground. This view is rooted in the idea that the force generated during the inevitable impact between what may be called the intact upper section (IUS) and the intact lower section (ILS)—meaning the building sections above and below the initially lost columns, respectively—will exceed by at least one order of magnitude the capacity of the latter. On closer inspection, this turns out to be only partially correct—it is correct with regard to the topmost floor plate of the ILS but not with regard to the columns below this floor plate. This paper shows that if the ILS in the Twin Towers had been topped by a stronger-than-ordinary floor plate allowing the columns below to respond properly, rather than be bypassed, these columns—and with them the ILS—would likely have survived. The paper subsequently proposes a building design concept consisting in the insertion of strengthened floor plates in intervals of 10–20 stories.

Highlights

The relatively young research field of progressive collapse, naturally receiving a thrust each time a prominent structural collapse occurs, has undoubtedly received its strongest thrust yet due to the terrorist attacks on September 11, 2001
Two are the main questions that can be asked from an engineering standpoint with regard to the Twin Towers’ collapse:
Once the intact upper section (IUS) was set in motion, how did it manage to destroy the entire intact lower section (ILS) of the building? This paper provides an answer to the second question, as well as to the more important question that naturally follows: how could the ILS have been saved? Making the distinction between two possible and mutually exclusive types of downward collapse progression, referred to as column-failure-driven and floor-platefailure-driven, the paper has shown that the total collapse of the ILS in the World Trade Center (WTC) Twin Towers was clearly the result of F-P-Fdriven downward collapse progression

Summary

Introduction

The relatively young research field of progressive collapse, naturally receiving a thrust each time a prominent structural collapse occurs, has undoubtedly received its strongest thrust yet due to the terrorist attacks on September 11, 2001. With most of the attention falling on the design methods of providing increased local resistance and alternative load paths, the research efforts since 2001 have been mainly concentrated on what may be called the outermost defense lines. Very little attention has been given to the question of what can be done if these outermost defense lines are broken and collapse is initiated after all. If the explanation for the total collapse of the World Trade Center (WTC) Twin Towers proposed by Bažant and Zhou (2002) is correct, as currently widely accepted among engineering professionals, it really would seem that given the various practical restrictions in the design of high-rise buildings, the endeavor of developing measures to arrest a collapse once initiated is very challenging, if not hopeless. The “if” at the beginning of this last sentence, must not be forgotten; it is among the issues addressed in this paper

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Conclusion