Structures are designed to resist vertical loading, it's why the only buildings standing in Hiroshima and Nagasaki following the bombings were directly under the detonation point. They withstood the shockwave when loaded vertically, other buildings experienced side loading and collapsed. The comment about delayed fusing is right, but it's actually tough with these comniblock buildings, there's probably 30-40cm of reinforced concrete between each floor. Brick shit houses.