The complete collapse of the Twin Towers has more to do with
the construction than anything else.  Yes, it was the loss of
fireproofing and subsequent fires that collapsed the
building but if it were not for the construction, a total
collapse may not have occurred.  Ironically, the construction
also saved many lives by letting the towers stand as long as
they did.  These pages explain the construction and provide
evidence for collapse by fire. 

"Tube in a
Tube"

The
designers coined the term "Tube in a Tube" to describe the
buildings’ construction.  The design was an innovation of its
day.  Thick steel columns ran up the middle of the building. 
This housed the elevators, stairwell, electrical conduits,
water, sewer and other services which ran up and down the
length of the buildings. 

For
you to understand the collapse, you will have to remember
four main elements. 

Core
columns

Perimeter columns

Floor Trusses

Fire
proofing

The
columns of the building normally found evenly spaced out on
a given floor became the outer wall of the building.  This
left large open areas for renting.  A good explanation of
this can be found on
PBS. 

Instead of encasing each column in heavy concrete, (normal
fire proofing) the designers relied on 'sprayed on' fire
proofing with a 2 hour rating to protect the load bearing
columns and trusses from fire. 

Each
WTC tower was essentially bolted together like an erector
set.  Each average floor had 376 5/8" bolts, 188 Truss seats,
120 ViscoElastic pieces and 120 gusset plates. 

Below is a diagram of the truss system with connections. 

Trusses connected
Perimeter columns to the core.  Without this connection
neither one could stand alone.  Note how the columns are
constructed in threes known as column trees.  Each column is
bolted onto the column beneath and to the tree beside it. 

There was a 3 to 4 inch layer of concrete on the average
floor.  There were reinforced concrete floors in the core as
well.  Contrary to what some conspiracy theorists say, the
core walls were NOT concrete reinforced.  The columns in the
core were also not incased in concrete.  This was an error
made by the BBC which grew a life of its own.  Here is the
article.  Note the date it was created, Sept. 13, 2001, just
2 das after the collapse.