1. Start condition: support removed for all columns on one floor (97th floor) Collapse between 97-98th floor Upper part = 13 floors + roof and antenna Core contents begins to crush immediately (~200 tons)

2. Collision phase 1: outer floor truss matrix impacts and begins to crush office contents (~250 tons)

3. Floor slab (4.35”height Floor truss matrix (height = 29 in; width 60 ft; depth 206 ft) Office contents (height approx. 4 ft; width 60 ft; depth 206 ft; long span mass ~90 tons) Open (height = 5’9”) Portion of cross section outside of the core

4. Collision phase 2: core floor horizontal members impact and begin to crush core office contents (~50 tons)

5. Collision phase 3: outer floor truss matrix impacts upper-most floor of lower part Floor truss connections are designed for gravity loads not resistance to upward impacts. Upper floor truss connections are broken and floor truss matrix is dissociated from structure. (This is corroborated by NIST’s observations that truss seats on upper floors were bent upward and had bolt tear out.) Concrete floor of the lower-most floor of the upper part probably fractures due to strains with no support. Lowest upper-part floor trusses probably collapse to some extent Top-most lower-part floor remains mostly intact

6. Collsion phase 4: core horizontal members impact upper-most floor of lower part Core horizontal members are most likely bolted and welded with no bias toward upward or downward forces An even number of upper and lower horizontal members fracture Equal amount of floor fracture for upper and lower floors within core Horizontal member fractures probably lead to some bending of core columns

7. Collision phase 5: first possibility of end to end impact of columns End to end impact in occurs in a low percentage of cases due to tilting of the upper part and bending of columns during phase 4 Predominately floor punch through

8. Potential conclusions Floor dissociation, core horizontal member fracture, and column-floor punch through lead to a high level of concrete comminution Inelastic collision energy considerations –Energy for fracturing at least an entire floor of horizontal members needs to be accounted for –Strain energy for truss seats and trusses needs to be accounted for –Strain energy for concrete needs to be accounted for –Strain energy for crushing >500 tons of building contents Basic mass on spring concept probably still valid except there are two springs and there is time for rebound before the next collision