Steel is a wonderful building material, but has a fault that designers protect against, but there's never a guarantee it will work long enough.
The fault? Heat. At high temperatures, steel changes. The material becomes plastic, loses strength and fails as a structural support. In building fires, you can see the damage in the twisted girders and columns after the fire is extinguished. Intense heat caused the steel to loose rigidity, so the weight of the structure caused sagging, with an eventual failure.
With skyscrapers, this fault is known, so exposed steel is exposed to fire proofing. The fireproofing has a design problem, too. It's usually brittle, so impacts can lead to spalling of the material and exposing the steel. That, and poor workmanship. Without fireproofing, even low heat fires can cause damage to steel.
Even the rebar in a concrete foundation will distort in fire. After a containment pit was struck by lightning, the oil in the pit burned for over an hour, before it was extinguished. The damage was something I'd never seen before. Large concrete girders holding equipment lost the outside layer of concrete, when the concrete spalled due to the intense heat. The rebar between the stirrups was sagged like spaghetti, which I found surprising. I thought only an oxy/acetylene torch could provide such heat, but my thoughts were proven otherwise. Seeing one inch rebar sagging in two foot span was something to see.
The World Trade Center Towers were built of steel, but the structures were innovative. The core of the building provided most of the strength, and the floor were supported by trusses, with the external structure much lighter.
A truss is an interesting building component, since it's composed of small steel rods, and angles, which are constructed in a lattice work to gain strength through cross members and bracing. They work,are lightweight, can handle substantial loads, but are more susceptible to heat. Heat that takes a long time to cause a large steel member to start distorting, can cause distortion in a short period of time on a truss. I've read that firefighters are aware of this, and detest working a fire in buildings with trusses for supports.
When the jets hit the towers, they immediately caught fire. With almost full fuel tanks, the fires were destined to burn for a long time. Jet fuel, due to it's composition, burns at high BTU's and the temperature increases, when aided by strong drafts, such as those found on a tall building.
It's theorized the initial crash removed much of the fireproofing on the trusses, which exposed them to the heat. Since the trusses were supporting light weight concrete, their failure would lead to the materials on the floor they supported to fall to the floor below. If the floor held, the event would have ended, but there were multiple floors above the fire.
When the floors started falling, they slammed into the floor below the fire, which caused it to fail. The added weight sheared the other floor, and the debris fell to the next floor below. Within seconds, the banging of the floors was a constant noise included in the rumbling. Firefighters that survived described the banging as the floors collapsed and the buildings fell.
I've read of conspiracy theories about the destruction of the World Trade centers, but find they don't satisfy the known physics involved. Some may find it comforting to think something more sinister was the cause of the destruction, but it doesn't fit with what I've read about, and experienced.
The World Trade Towers were a marvel of modern architecture, and engineering, but the biggest fault was always there. While planes caused the fire, a conflagration of the same proportion, without any intention would have caused the same disaster. All that was needed was fire, and time.