Several of the resident truthers in this forum make a habit of asking the same question over and over, primarily in the effort to distract from the other huge holes in their pet theories of what happened on September 11, 2001. In a recently closed thread, this was the one they had fastened on: Of course, the question assumes several facts which are not in evidence. Let's dispatch 2 of them first before looking at an analysis of the core issue. 1. The failure was not symmetrical. As all existing videos show, both WTC towers 1 & 2 experienced tilting of the upper sections as they began their collapse. By definition, a tilt is not "symmetrical." Such a tilt can only have taken place if columns on some sides failed before columns on others did, allowing the last intact columns to act as the fulcrum around which the building section could pivot. This failure of symmetry extended throughout the entire rest of the collapses, producing completely asymmetrical debris field. We know this from the work of professional engineers who had no interest one way or the other in either proving or disproving any particular theory of what happened that day. The engineering firm Weidlinger Associates Inc. (WAI) was hired by the owners of the WTC to determine if the tower collapses were one or two insurable "events." If Tower 1 was toppled over by Tower 2, for example, the insurance company would pay for only one event. If the collapses were independent, however, two events would have to be covered. WAI, in their detailed analysis titled "World Trade Center - Structural Engineering Investigation," produced detailed maps of the debris distribution. Here is the distribution map for WTC 2 which failed first: As you can see, while there is a very rough clover leaf shape created by the fact that each of the four faces peeled off in their respective directions, there is essentially no symmetry present in the debris field. The lack of symmetry also is clearly not explainable by the presence of nearby buildings.The map shows that they had essentially no effect on the distribution. This is of course because most of the debris fell from heights above the tops of those surrounding buildings. Here is the distribution map with WTC 1 subsequently added: Note again... there is virtually no symmetry in the debris field, proving that there was no symmetry to the collapse of either tower. 2. The columns did not all fail simultaneously, or within milliseconds of each other. As already noted, the initial tilt of the falling buildings prove that the supporting columns cannot have all failed at or near the same moment. Additional photographic evidence proves that the failure of the load bearing perimeter columns in the WTC towers was a gradual process that took most of an hour rather than just a few milliseconds. For example, the progressive sagging of the floor slabs (which ultimately caused at least some of the perimeter columns to fail) can be proved to have been gradually underway for more than 50 minutes. We also know that many of the columns had been severed by the initial impact which occurred about one and two hours before collapse respectively. The truther error here seems to derive from the false belief that the initiation of the catastrophic phase of the collapses records the moment at which all the columns failed. But in fact (and as this analysis will show) that only records the point at which the proverbial last straw broke the proverbial camel's back. The Analysis: The WTC towers were a unique design for steel framed, highrise office buildings. It has been described as a "tube within a tube" design in which the load bearing structures were distributed across two tubes of structural steel columns. These were the inner tube consisting of 47 core columns designed to carry roughly 60% of the vertical load, and the outer tube of 240 perimeter columns designed to carry the remaining 40%. At the top of the towers was a "hat truss" which served both to help absorb the lateral loads caused by wind and earth tremors, and to redistribute vertical loads between the core and perimeter columns during periods of high lateral stress. Finally, each floor served to distribute the lateral loads along the entire height of the towers through their own complexes of steel truss work. With this as our starting point, we can calculate the designed distribution of the vertical loads (on a windless day) across every individual column as follows: The core columns carried 60% of the load, distributed over 47 columns. This means that each of the core columns was designed to carry 1.277% of the entire vertical load of the WTC towers (60 / 47 = 1.277). The perimeter columns carried 40% of the load, distributed over 240 columns. This means that each of the perimeter columns was designed to carry 0.167% of the entire vertical load of the WTC towers (40 / 240 = 0.167). We also know that the designed safety margin of the WTC anticipated the normal static load to be at .6 of maximum capacity. So we can calculate the maximum (i.e. the point beyond which the columns should be expected to fail) at 1.667 X the designed loads. Thus... we can expect any individual core column to fail at 2.128% of the total load, and any individual perimeter column to fail at 0.278% of the total load. Conversely, we can expect any column to fail when it has been weakened (by damage, heat, whatever) to below 60% of its designed carrying capacity. With all of this back ground understood, let's look at the state of WTC 1 immediately after impact, but before fires could begin to affect the remaining load bearing structures. Immediately after impact: At 8:46:30 AM, American Airlines Flight 11 crashed at roughly 466 mph (790 km/h or 219m/s or 425 knots) into the north face of the North Tower of the World Trade Center, between floors 93 and 99. As it passed through the perimeter it severed between 31 an 36 external columns (for the sake of this analysis we will conservatively use the lower number and consider those columns that are damages to be essentially intact). A more direct impact than would later occur in the South Tower, the wreckage of the plane preceded into the core, severing at least 6 (some models give as many as 11 or 12) of the critical core columns. At this moment, the complete load bearing capacity of the tower had been reduced by at least 12.826% (31 X 0.167% for the perimeter columns, 6 X 1.277% for the core columns). When the designed static load becomes disturbed in this manner, the hat truss is designed to redistribute the load across the remaining columns. At its most efficient, it will restore a 60-40 balance between the core and perimeter columns. The new post-impact static distribution now has the remaining perimeter columns bearing 0.192% (from the original 0.167%) of the total, and the core columns now carrying 1.465% (from the original 1.277%). This is well below the failure points of 1.667% and 2.128% respectively. At this point the towers are wounded, but still standing. Fires: The slow subsequent death of an engineering marvel Now... let's consider the effect of the fires. The fires can cause any specific individual column to fail one of two ways. The most obvious is for it to heat the column itself to the point the strength of the steel is reduced to 60% and the entire safety margin is gone. This occurs at around 520 degrees Celsius (970 degrees Fahrenheit). Even the most aggressive truther efforts to downplay the effect of fire acknowledge fires of at least that intensity were burning in the WTC towers after impact. At that point the individual column will no longer be able to bear the experienced load and it will fail, usually by buckling. Its load will then be redistributed via the hat truss to the remaining structure. The second way a column can be compromised is for the columns to be pulled out of position by floor trusses that have sagged in the heat of fires on the floors; i.e. fires that are not necessary engaging the columns themselves. Designed to distribute the lateral loads between the perimeter and core, a sagging floor would place more of its lateral strain on the lighter, thinner perimeter columns than those of the core. And of course, there is no shortage of photographic evidence of the perimeter columns bowing and eventually buckling in exactly this way on 9/11. Either way, the failure of any specific individual column is not dependent on fire engulfing an entire floor or any other columns. Each individual column will fail or not essentially on its own during this phase between the initial impact and the eventual catastrophic collapse. But the failure of any specific individual column will always immediately result in a redistribution of the load, through the hat truss to those columns that remain. In this way, the individual failure of any single column pushes all remaining columns some distance closer to their own breaking points. It is not actually possible to determine exactly which specific columns, and what combination of core and perimeter columns failed in the WTC Towers, or in what order. But we can still model exactly how many columns would have to fail for a catastrophic collapse to initiate. To simplify the demonstration let's look first at just the core columns (which would have been most vulnerable to direct heating), and then afterwards consider just the perimeter columns (which would have been most vulnerable to floor sagging). Keep in mind that in actuality, both were taking place at the same time. And remember: Any value above 0.278% for the perimeter columns, or 2.128% for the core columns means that we have exceeded their load and they are now in failure. [table="width: 800, class: grid"] [tr] [td][/td] [td]Perimeter Column Load[/td] [td]Core Column Load[/td] [td]Remaining Core Columns[/td] [/tr] [tr] [td]Baseline Loads[/td] [td]0.192%[/td] [td]1.465%[/td] [td]41[/td] [/tr] [tr] [td]1 Core Column Failure[/td] [td]0.194%[/td] [td]1.524%[/td] [td]40[/td] [/tr][tr] [td]2 Core Column Failures[/td] [td]0.197%[/td] [td]1.586%[/td] [td]39[/td] [/tr][tr] [td]3 Core Column Failures[/td] [td]0.200%[/td] [td]1.653%[/td] [td]38[/td] [/tr][tr] [td]4 Core Column Failures[/td] [td]0.204%[/td] [td]1.725%[/td] [td]37[/td] [/tr][tr] [td]5 Core Column Failures[/td] [td]0.207%[/td] [td]1.801%[/td] [td]36[/td] [/tr][tr] [td]6 Core Column Failures[/td] [td]0.210%[/td] [td]1.884%[/td] [td]35[/td] [/tr][tr] [td]7 Core Column Failures[/td] [td]0.214%[/td] [td]1.972%[/td] [td]34[/td] [/tr][tr] [td]8 Core Column Failures[/td] [td]0.218%[/td] [td]2.068%[/td] [td]33[/td] [/tr][tr] [td]9 Core Column Failures[/td] [td]0.222%[/td] [td]2.171%[/td] [td]32[/td] [/tr][/table] It takes only 9 additional core columns to incrementally fail before all the remaining core columns are pushed beyond their design capacity. It may take seconds to get there, it may take minutes, it may take hours. But once it gets there, we are at the moment of catastrophic failure. When the rest of the core columns have exceeded their carrying capacity and buckled, the hat truss will immediately try to transfer 100% of the load to the perimeter columns. This is the final blow. [table="width: 800, class: grid"] [tr] [td][/td] [td]Perimeter Column Load[/td] [td]Core Column Load[/td] [td]Remaining Core Columns[/td] [/tr] [tr] [td]Complete Core Failure[/td] [td]0.766%[/td] [td][/td] [td]0[/td] [/tr][/table] The failure of the core immediately leads to the failure of the remaining perimeter columns. At this point there is nothing left to hold the upper section of the WTC Tower up, and total catastrophic collapse is underway. The failures did not all occur "in milliseconds." Only nine additional columns (out of 41) had to be exposed to any fire at all. And still... a point of catastrophic failure is reached at which all the remaining elements are overwhelmed, and complete collapse begins in a moment. We can perform the same analysis for the perimeter columns instead of the core columns. Again, keeping it simple and only considering perimeter failures, and leaving out some of the interim steps just to save bandwidth, the results look like this. [table="width: 800, class: grid"] [tr] [td][/td] [td]Perimeter Column Load[/td] [td]Core Column Load[/td] [td]Remaining Perimeter Columns[/td] [/tr] [tr] [td]Baseline Loads[/td] [td]0.192%[/td] [td]1.465%[/td] [td]209[/td] [/tr][tr] [td]1 Perimeter Column Failure[/td] [td]0.193%[/td] [td]1.468%[/td] [td]208[/td] [/tr][tr] [td]2 Perimeter Column Failures[/td] [td]0.194%[/td] [td]1.471%[/td] [td]207[/td] [/tr][tr] [td]3 Perimeter Column Failures[/td] [td]0.196%[/td] [td]1.474%[/td] [td]206[/td] [/tr][tr] [td]43 interim failures[/td] [/tr][tr] [td]47 Perimeter Column Failures[/td] [td]0.274%[/td] [td]1.622%[/td] [td]162[/td] [/tr][tr] [td]48 Perimeter Column Failures[/td] [td]0.276%[/td] [td]1.626%[/td] [td]161[/td] [/tr][tr] [td]49 Perimeter Column Failures[/td] [td]0.278%[/td] [td]1.630%[/td] [td]161[/td] [/tr][/table] At the 49th individual perimeter column failure, all the remaining perimeter columns have been pushed to their failure point. In the reverse of what we saw in the core column analysis, the hat truss then attempts to transfer 100% of the vertical load to the core columns, which then in turn are driven to catastrophic failure. [table="width: 800, class: grid"] [tr] [td][/td] [td]Perimeter Column Load[/td] [td]Core Column Load[/td] [td]Remaining Perimeter Columns[/td] [/tr] [tr] [td]Complete Perimeter Failure[/td] [td][/td] [td]3.415%[/td] [td]0[/td] [/tr][/table] The failure of the perimeter immediately leads to the failure of the remaining core columns. At this point, again, there is nothing left to hold the upper section of the WTC Tower up, and total catastrophic collapse is underway. And again, The failures did not all occur "in milliseconds." Only 49 additional columns (out of 209) had to buckle as the result of floor sag. This does not even require all the columns on a single side, even though the way the WTC was constructed they would all tend to fail as a unit. But still... a point of catastrophic failure is reached at which all the remaining elements are overwhelmed, and complete collapse begins in a moment. Summary: The collapse of the WTC Towers was not symmetrical, and the model shows there was no expectation that it be so. The truther claim to that effect is a fabrication with neither evidence nor reason behind it, and is contradicted by both the video evidence from that day as well as the physics involved. The columns of the WTC Towers did not fail "in milliseconds." The model shows that there is no timing requirement whatsoever for such a catastrophic failure to take place. It could have taken minutes, it could have taken hours. It actually took 56 minutes for the South Tower, and 1 hour, 42 minutes for the North. And the total failure of the Towers does not require fires to range uniformly "across a 200 x 200 ft building." Only 9 core columns (roughly one out of five) or 49 perimeter columns (roughly one out of four), or some equally limited combination needed to be affected by fire or by floor sag for the entire structure to be driven past the point of failure.