Some people think fires and structural damage were enough to bring down the towers. Others, clearly, do not. You can’t properly decide for yourself until you’ve seen both sides of the argument, so here are a few sites you might like to explore (more general links are here and here).
Core still standing - According to some WTC controlled demolition theories, the building cores were weakened by explosives, which initiated the collapse. But if that’s what happened, then why does this new video seem to show a substantial portion of the core still standing after the collapse has begun?
The official story
Some people condemn the official reports as lies without even reading them. Don’t make that mistake.
FEMA Report - The World Trade Center Building Performance Study was the first official account of why the towers collapsed
NIST - News, updates, and access to all the various NIST WTC reports
The collapse of the WTC as a result of the attacks doesn’t seem to have come as a surprise to everyone. In fact many engineers had no great problem with it at all, and within weeks were publishing their suggested explanations.
MIT researchers - “...contains eight articles that deal with the September 11, 2001 World Trade Center (WTC) disaster and its consequences, written by researchers at the Massachusetts Institute of Technology”
Cad Digest - “Gathered here is a collection of articles about the architectural, engineering and related technical aspects of the terrorist attacks on the United States on September 11, 2001 and the ensuing recovery”
G Charles Clifton, HERA Structural Engineer - “Elaboration on Aspects of the Postulated Collapse of the World Trade Centre Twin Towers”
Tim Wilkinson, lecturer in Civil Engineering - “World Trade Center - Some Engineering Aspects of the WTC” (and a brief controlled demolition rebuttal)
Dr Thomas Eager - “The Collapse: An Engineer's Perspective”
Bazant - Zhou - “Why did the World Trade Center Collapse? Simple analysis”
Arthur Scheurman - “The Towers, Fire-Induced Collapse and the Building Codes”. A retired FDNY Battalion Chief offers his thoughts in a 2003 document first made available via Fire Engineering
Controlled demolition brought down the towers
Others suggest the buildings fell too quickly for a gravity-driven collapse, that the observed pulverisation of the concrete was impossible without an additional enery source, that videos and pictures of the collapse reveal the signs of demolition.
Scholars for 9/11 Truth - Links to various articles on controlled demolition and other topics
911Research - "A rational look at the Twin Tower collapses reveals that the official story contradicts the laws of physics and the most basic knowledge of the behavior of steel structures, and matter itself"
9-11 Review - “The official explanation that the Twin Towers collapsed due to collision damage and fire stress is blatantly incompatable with basic common sense in light of the surviving evidence”
911Eyewitness - “see and hear the massive explosions at the base of the towers causing clouds of dust to rise up from street level before each tower fell”
PlaguePuppy - “How strong is the evidence for controlled demolition?”
World Trade Centre Demolition - "This blog will deal with the the collapse of the world trade center buildings 1, 2 and 7 on 9/11/01, and in particular will focus on the idea that the towers were brought down by controlled demolition"
ImplosionWorld - “A Critical Analysis of the Collapse of WTC Towers 1, 2 and 7 from an Explosives and Conventional Demolition Industry Viewpoint”
NIST FAQ - An August 2006 page addresses 14 claims, some common and important, some not. Worth a look, although short so will ultimately satisfy no-one.
911 was an inside job, but the WTC may not have been demolished
Oilempire - “Evidence for the demolition of the towers is largely based on inference (the physical evidence has all been recycled). The demolition theories are not needed to prove complicity”
Comments on the WTC Collapse - Some interesting early thoughts on the causes of the collapse, primary by John Young, who "practices archtitecture in New York City" and also runs the intelligence site Cryptome
MIT Essay - “The World Trade Center Disaster: Analysis and Recommendations”. Useful as a quick review of various collapse theories.
The Collapse of the World Trade Center Towers: Theory, Expectation and Reality - A "Civil Engineering Panel Discussion" with Charles H Dowding (Sequence and Geometry of Devastation", Zdenek Bazant (Collapse Mechanisms and Fire), Raymond J Krizek (Underground Damage and Recovery) and David Schulz (Urban Security and Future Considerations). This is a video, but we’ve archived the slides here, so check them out to see if you’ll be interested (there’s a lot of them, so be patient if you have a slow Internet connection).
A Tale of Fire & Steel - Engineers, demolition experts and others discuss why the towers collapsed. Includes some comments on controlled demolition theories.
University of Manchester - “One stop shop in Structural Fire Engineering” sums up the WTC collapses.
University of Manchester - The same site also has a page discussing the Madrid Windsor fire and its building performance.
Fire Induced Collapse of Tall Buildings - a thesis from a student at the University of Edinburgh. Discusses issues of relevance to the WTC.
Fire and Structures: The Implications of the World Trade Centre Disaster - Report of a conference held at the Royal Society of Edinburgh on Wednesday 21st April 2004.
Innovativate Structural Engineering for Tall Buildings in Fire - A Powerpoint presentation looking at general issues, what might contribute to a collapse, and how these might be presented in future (not WTC-specific).
Beware obsolete articles!
We still commonly see people quoting from Jerry Russell's “Proof of Controlled Demolition at the WTC”. While Russell has in no sense become a believer of the complete official story (see http://www.911-strike.com for his current position), only a few weeks later he said this about his own article:
The central argument in my essay was that the process of collapse should have involved enough friction that the fall of the building should at least have been braked significantly compared to the acceleration of an object in free fall. The argument seemed perfectly reasonable if not obvious to me, and I managed to trick some pretty smart people with it. But the truth is that it is possible for a building to collapse in a process which concentrates high leverage at certain joints in the structure. The result is a nearly frictionless collapse. This was very counter-intuitive to me, but people who work with structures seem quite aware of it. This technical article by Bazant & Zhou explains this in some detail, and although I believe their presentation is oversimplified, the basic message seems to be correct.
Another reason to be careful, especially with old articles -- they may no longer represent the authors true views.