Twenty years later: why the 9/11 twin towers collapsed

The video explains the structural integrity of the World Trade Center, focusing on its vertical and horizontal load management systems. It describes how the core of reinforced concrete and steel pillars worked together to support the building. The National Institute of Standards and Technology (NIST) report is discussed, detailing how the impact of the planes and subsequent fires led to the collapse of the towers. The report highlights the role of structural steel weakening and the inability of lower floors to resist the energy of the falling structure.

What was the primary function of the reinforced concrete core in the World Trade Center?

• To convey vertical loads

• To support the outer steel pillars

• To absorb horizontal loads caused by wind

• To prevent the perimeter pillars from falling outward

How did the horizontal trusses contribute to the structural integrity of the World Trade Center?

• They prevented the outer walls from deforming

• They absorbed vertical loads

• They supported cement slabs and connected vertical systems

• They were part of the reinforced concrete core

According to the NIST report, what initiated the collapse of the World Trade Center?

• The rupture of fuel tanks and subsequent fire

• The impact of the planes

• The failure of the reinforced concrete core

• The deformation of the outer walls

What was the effect of the weakened structural steel on the World Trade Center?

• It absorbed the energy of the falling building

• It prevented the floors from collapsing

• It caused the outer walls to deform

• It strengthened the building's core

Why did the floors below the impact zones disintegrate quickly?

• They absorbed too much horizontal load

• They were not connected to the vertical systems

• They provided little resistance to the energy of the falling building

• They were made of weaker materials