Building Collapses
1. B-25 Empire state building crash, 1945
Event
July 28, 1945, was a Saturday morning and a ten-ton (having a wingspan of 67 ft 6 in, length of 52 ft 11 in) B-25 bomber en route to Newark airport flew through a dense fog and crashed at an estimated 400 kph into the 79th floor of the Empire State Building, tearing a hole in the 78th and 79th floors and killing ten people in the building.
- Figure: Impact of B-25 plane
Impact
To estimate the force applied to the building, we can use;
1. The deceleration of B-25 :- use the force-acceleration method
2. The distance it took the B-25 to come to rest :- use work-energy
3. The time it took the B-25 to come to rest :- use impulse-momentum
Plane did not pass through the building, so a distance of 10 to 20 meters for the plane to come to rest could be used for calculations. If the bomber came to rest in 20 meters with a constant deceleration (assumed), the force would be about 4440kN exerted over about 0.25 seconds.
The force of the impact sheared off the wings of the plane and propelled one of the two motors across the width of the building, through the opposite wall and the other motor crashed into an elevator shaft and fell all the way down to the sub-cellar.
The center of impact aligned almost exactly with a column on the face of the tower. The column itself was barely damaged, although steel beam supporting the masonry wall torn out.
Lessons learnt
- Lack of damage to the building was a consequence of the redundancy inherent in a frame structure. The connections in the frame structure were executed with rivets. The building, with columns spaced 5.8 m on center in both directions compensated for the loss of a leg by redistributing its weight to the remaining legs.
- The building was designed to resist a wind load momentum two hundred times the moment of B-25 plane. The tower arrested its movement as if a brake, a characteristic called “damping”. Because of the friction between the elements of its structure in this older heavy masonry-clad tower, it exhibits strong damping. It would not cause seasickness as in modern lighter sky scrapers which would vibrate for long.
- Kemper arena roof collapse, 1979
The Crosby Kemper Memorial Arena opened in 1973, consisted a large flat roof, 97 by 108 m which was suspended on hangers from 3 large space frame cantilever trusses.
- Roof structure – concrete reinforced by a corrugated steel deck
- Open web joists – steel angle chords and bent rod diagonals, 16m long and 2.7m spaced
- Steel truss – Joists rested on the system of trusses each consisting 2, 30m length trusses.
- Hanger – Truss system was hung from the lower chords of the 3 portals
Collapse of the Kemper Arena could not be explained by a single cause, but a collection of causes which includes rain downpour, drain deficiencies, fatigue of bolts, wind load effects, and lack of redundancies.
The roof was designed not to dispose immediately of sudden downpours of rain but to use roofs as temporary reservoirs and to limit the flow rates. 12000m2 area roof had been provided only with 8, 130 mm diameter drains to discharge when water reached 50 mm of depth. The water accumulation was severely developed 70 mph gusts that pushed the accumulated water from North to South and by the upward suction. A roof structure’s flexibility and limited stiffness, due to large spanning, can cause ponding. Ponding depends on stiffness and unit weight g of the liquid accumulating. The critical value of g was 0.627 which makes roof unstable (for stability usually g =1) when including the deformations of the deck, long span portals, the joists &trusses.
42 hangers were installed to transfer the entire roof load (dead, rain, mechanical, and live loads) to the 3 large space frames and to withstand horizontal wind forces. The hangers experienced many oscillations due to wind loads throughout the 6 year life of the structure which affected the bolt connections of the hangers. An extreme amount of oscillations, over time, can cause fatigue in steel elements. The collapse was triggered by fracture of the high strength bolts A490 which steel codes warn against their use under variable loads, which may not have been considered in arena design. Analysis demonstrated that failure of just one of four bolts per hanger resulted in a large prying load on the three remaining bolts.
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