Catastrophic Design Failure of WTC; Angle Clip Joint System
One important factor of structural design discussed in this course and alluded to in Chapter 2 of Hamburger’s et al. (2002) report is the integration of structural joints into reinforced concrete structures. Theoretical knowledge from Structural Mechanics course unit elaborates that structural joints have two main functions: To allow vertical, rotational and horizontal displacements right-angled to the joint surface and the transfer of floor truss flexural loads to support columns (World Trade Center). However, optimum joint functionality and design is affected or dictated by three factors: Thermal-induced displacement, Wind loads and Seismic events. WTC architects used a special viscoelastic angle clip joint damper that allowed the exterior wall to sway during increased wind loads whilst the floor truss remained stationary.
WTC structural construction as reported by Hamburger et al. (2002, p. 2-3) explains that the central core and the exterior wall were linked by a complex series of composite floor trusses that supported the entire weight of the floor decks. The entire floor system (comprising trusses, decks and angle clip dampers) acted as a grillage able to effectively distribute floor loads to the various perimeter columns through angle clip damper attachments as depicted on Figure 1 below. When the two planes struck the WTC the following combination of events led to catastrophic collapse: First, the planes hit the exterior wall weakening it by causing a biased redistribution of stress-strain loads entirely to one side (mostly the impact side). Secondly, jet fuel burning caused the generation of intense heat that distorted the floor trusses making them pull from their angle clip dampers. Upon detachment another force came into effect: Gravity. As the upper floors affected by heat detached from the exterior wall and fell, their entire potential weigh was quickly converted to kinetic energy increasing the stress-strain on consequent lower floor angle clip dampers. This was intensive especially as the upper floors collapse pancaked into lower floors causing significant load increases that the angle clips were not designed to withhold; this phenomenon is schematically illustrated on Figures 2 and 3 below (Bažant et al., 2008). This forces were:
- Stress-strain variations causing increased shear and angular momentum leading to angle clip translational rotation as depicted on Figure 2 and 3.
- Strain increases for each compaction front per front causing a domino-like effect as each floor fell upon another.
Figure 1: The positioning of the angle clip dampers in relation to the floor/roof joists and the inner core (Eagar & Musso, 2001).
Figure 2: Heat-induced floor detachment causing translational rotation of floor joists resulting in shear and angular momentum effects/forces on angle clip dampers (Wolberger, 2009).
Figure 3: A schematic illustration of the strain redistribution after each floor joist collapse (progressive floor-upon-floor collapse caused significant localized strain-stress load forces on the angle clip dampers) (Wolberger, 2009)
Bažant, Z. P., Le, J., Greening, F. R., & Benson, D. B. (2008). What Did and Did Not Cause Collapse of World Trade Center Twin Towers in New York? Journal of Engineering Mechanics, 134(10), 892-906. Doi: 10.1061/ (asce) 0733-9399(2008)134:10(892)
Eagar, T. W., & Musso, C. (2001). Why did the world trade center collapse? Science, engineering, and speculation. JOM, 53(12), 8-11. Doi: 10.1007/s11837-001-0003-1. Retrieved on April 28, 2017 from http://www.tms.org/pubs/journals/jom/0112/eagar/eagar-0112.html
Hamburger, R.; Baker, W.; Barnett, J.; et al. (2002), “Chapter 2. WTC 1 and WTC 2”, in McAllister, T., World Trade Center Building Performance Study: Data Collection, Preliminary Observations, and Recommendations. Retrieved on April 26, 2017 from www.fema.gov/media-library-data/20130726-1512-20490-1698/fema403_ch2.pdf
Wolberger, L. (2009). WTC Collapse: Slide 59 Generalization of Progressive Collapse. Retrieved April 28, 2017 from https://www.slideshare.net/lwolberg/world-trade-center-collapse