Why did the North Tower collapsed on 9/11?

To understand why the WTC towers collapsed you have to first understand how they were built. Usually the structure of the high rise buildings consists of a structural steel beams and columns. The columns intrude on the interior space, often making internal partitioning of the floors difficult. To create a column-free floor plan the WTC structural designer shifted the vertical loads to the perimeter columns which carried 40% of the dead weight, while the concrete core carried 60%. To reduce the weight of the building the designer used lightweight horizontal trusses that span from the building's core to the perimeter columns to support the floors and to provide lateral bracing for the columns.

Every civil and mechanical engineering student must take a course called Strength of Materials. At about week 2 of the course they learn about how vertical columns fail in compression. It turns out that the load a column can carry depends on its free length: the longer the free length, the less load the column can carry. Also, in high school physics class students learn that objects expand when they are heated.

When the plane struck the building it immoderately cut a number of perimeter columns, shifting the loads they carried onto other, still intact, columns. The fire that followed heated the floor trusses causing them to expand. These trusses were rigidly bolted at both ends so they had no room to expand laterally; the only way they could move was to buckle sideways.

When the trusses buckled the lateral support they gave to the columns was gone. The columns that already carried more load than they were designed to carry suddenly found their free length doubled or even tripled so they started to buckle. When enough columns buckled the floors above the fire fell onto the floor below, causing it it to collapse to the floor below that, and so on and so on in a process called progressive collapse.

There was no melted steel. The was a lot of twisted steel, but it got twisted during the collapse. That came from the news reporters looking for a hyperbolas to embellish their reports; the word got picked up by public, and is now taken as an article of faith, even though it isn't true. The only "melted" steel people saw came from cutting the steel with oxy-acetylene torches during the clean up operation.

It takes two minutes for a firefighter in full gear and carrying all the equipment to climb one floor. Using that metric, the firefighters would have been just reaching the fire when the collapse started. However, the firefighters in the North Tower would have been slowed by the people evacuating the building and by the shear exertion of climbing 80 stories in full turnout gear so they probably never even got there.

Both towers collapsed as a result of the load bearing properties of the central steel supports being reduced, when heated by burning fuel.

The steel framing could have withstood the impact, and did, initially. And the steel was clad in fire resistant material, so it could have withstood a 2,000 degree fuel fire, which it did initially. Steel fails at 1,200, so it's only a matter of time without that fire-resistant cladding. But the impact knocked a lot of that cladding off (it was concrete), exposing the steel to the too-high temperatures of the ensuing fire. When the steel failed (softened then melted) the weight of even one storey above would cause it to collapse. When the several storeys above fell ten feet, the weight and impact of the fall caused the next storey below to become shock-overloaded and fail too, and the next, and the next. The collapse was actually a rapid series of individual collapses one floor onto the next, an the next, etc. Had the fire department been able to cool the steel within the first 30 mins or so, they might have saved the building. But it took 30 mins or more just to get the first guys up that high. So, in retrospect, the collapse was inevitable.

Exothermic reaction between aluminium and steel

Ask a structural engineer. This is NOT the place to ask. You won't get expert answers on the subject on a site like YA.

In fact, you're better off staying away from online sites altogether, unless you can find one that's for serious, professional engineers.

Well the plane was travailing about 500+MPH. Went right through the building.