After the horizontal steel girders the curtain wall is made by glass and concrete which is attached to the steel structure, and covers and finishes up the building.

What you have got through these 4 pages of How are skyscrapers built? is how these giant buildings are built. When the Twin Towers collapsed people sought after answers to how it crumbled. One of the major reasons is the high temperature which was developed by the explosion of the plane and the fuel which lit up. It made the steel structure melt, and when the steel melts the structure (especially the glass and concrete cover) is left with nothing to stand on, so it collapsed downwards to the sub-structure.

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