Given that there is great debate about using an acronym to summarize the collapses, ROOSD for eg., I wrote out a summary of my own. I invite correction and suggestion.
http://www.internationalskeptics.com...postcount=2065
Generic Description of the Collapse Progression of the Two WTC Towers:
Video evidence demonstrates aircraft impact destroyed or bent a number of perimeter columns. A lesser amount of core column severance or damage is expected. Some floor spans also directly damaged.
Aircraft fuel spread through several vertically adjacent floors of the structure and ignited. This created a condition of very large area office contents fire on those floors with various levels of rubblization of the contents. This is a fire condition rarely if ever, reached within seconds of fire initiation in large office structures.
Aircraft impact debrided spray on fire insulation in varying degree, on a significant number of structural components on impact floors. This reduced fire resistance of those structural steel components.
Structural response to impact was a redistribution of loads previously carried by missing or foreshortened ( due to being bent) columns. Load redistribution was not even across all remaining columns.
Fire spread and movement was similar, but not identical, on each involved level and also spread to higher levels. Heat began affecting steel structural components, both vertical and horizontal, and the connecting components between them on all fire involved levels.
Floor spans had long trusses with their lower chords exposed to the heat and the trusses in several areas began sagging. A few areas had already seen truss or truss to perimeter connection failure at impact.
Perimeter columns and truss to perimeter connecting components heated less than the core columns and truss to core structure components. The interface between long span trusses and core was a belt truss rather than direct connection to core columns.
Core columns are subject to heating basically from all sides, over the several floors fully involved in the fires, and over the course of fire spread around each level.
( that is to say a column at level x may heat more on one face than that same column at level x-1 or x+1 but that over the several levels involved some columns are heated on all sides and taking into account the changes through time)
The capability of the structure to resist any collapse was being eroded. One very obvious indication of this is the inward bowing of part of the perimeter. Each tier also twisted and tilted noticeably up to collapse. These demonstrate that the structure was responding to further weakening due to the heat. Loads were continually changing and redistributing during this time.
At some point a redistribution caused one or more columns to exceed its capacity. Column capacity in this case does not necessarily mean it original engineered load bearing ability. Loads were not necessarily on axis and columns were not necessarily at room temperature, both of which would tend to lower any individual column's strength.
At this point a rapid redistribution the remains structure occurred, rapidly causing other columns to exceed capacity, causing another rapid load shift and column failures progressed to all columns over one or more levels.
A large section of the upper building then falls, accelerating to next contact. A combination of factors result in few if any buckled columns impact on lower parts of themselves. Upper falling section column ends are the first contacts with lower structure. That contact would be proximate to the lower section columns and thus the greatest impact occurs near or directly on lower floor truss to column connecting components.
This is occurring both to long span truss to column connecting components, at perimeter and belt truss, as well as with the core's beams running between core columns.
As upper mass continues its lowest floor begins contacting lower floors. This first contact is between floor systems that are already severely compromised. They are among those that have been affected by impact and heat and one has suffered loss of connections to columns as per last paragraph.
Both upper structure and lower structure now have major connection failures, both in the vertical/horizontal connections as well as connections between column sections. The later is due to both the destruction of Lateral Support and the buffeting collisions from the falling mass.
Perimeter column "trees" peel away, the first to do so are on the order of 1000 feet above ground as they pivot away.
Collapse continues with a relatively small amount of falling mass being ejected at the collision interface. Since initial contact was an order of magnitude greater than that which the long span open floors could absorb, little effect is seen on the falling mass. This means that the still intact portion of upper mass will continue to accelerate.
Again first contact is the lower column ends, though some movement of those column ends might not see them directly affecting column\truss(or beam) connections , they will spear through that next lower floor.
Next long span floors will now be impacted by more mass , albeit more rubblized, that is necessarily moving faster than first contact. Those floor spans must also be hit by an order of magnitude greater load thanthey can withstand, and indeed greater than at that earlier, first contact impact.
There existed no mechanism by which this floor truss and perimeter peel destruction could be arrested as this continues.
The core area beam destruction lags that of the long span destruction since those beams are heavier and shorter. Perimeter peel necessarily lags long span destruction as it is a direct result of that long span truss destruction.
Core columns stripped of inter-core Lateral Support succumb to a combination of slender column buckling and heavy buffeting from falling debris. A relatively small, heavily damaged, portion of the core remains after long span floor and perimeter destruction ( no longer being buffeted by debris impact) but cannot remain intact and it comes apart as well
http://www.internationalskeptics.com...postcount=2065
Generic Description of the Collapse Progression of the Two WTC Towers:
Video evidence demonstrates aircraft impact destroyed or bent a number of perimeter columns. A lesser amount of core column severance or damage is expected. Some floor spans also directly damaged.
Aircraft fuel spread through several vertically adjacent floors of the structure and ignited. This created a condition of very large area office contents fire on those floors with various levels of rubblization of the contents. This is a fire condition rarely if ever, reached within seconds of fire initiation in large office structures.
Aircraft impact debrided spray on fire insulation in varying degree, on a significant number of structural components on impact floors. This reduced fire resistance of those structural steel components.
Structural response to impact was a redistribution of loads previously carried by missing or foreshortened ( due to being bent) columns. Load redistribution was not even across all remaining columns.
Fire spread and movement was similar, but not identical, on each involved level and also spread to higher levels. Heat began affecting steel structural components, both vertical and horizontal, and the connecting components between them on all fire involved levels.
Floor spans had long trusses with their lower chords exposed to the heat and the trusses in several areas began sagging. A few areas had already seen truss or truss to perimeter connection failure at impact.
Perimeter columns and truss to perimeter connecting components heated less than the core columns and truss to core structure components. The interface between long span trusses and core was a belt truss rather than direct connection to core columns.
Core columns are subject to heating basically from all sides, over the several floors fully involved in the fires, and over the course of fire spread around each level.
( that is to say a column at level x may heat more on one face than that same column at level x-1 or x+1 but that over the several levels involved some columns are heated on all sides and taking into account the changes through time)
The capability of the structure to resist any collapse was being eroded. One very obvious indication of this is the inward bowing of part of the perimeter. Each tier also twisted and tilted noticeably up to collapse. These demonstrate that the structure was responding to further weakening due to the heat. Loads were continually changing and redistributing during this time.
At some point a redistribution caused one or more columns to exceed its capacity. Column capacity in this case does not necessarily mean it original engineered load bearing ability. Loads were not necessarily on axis and columns were not necessarily at room temperature, both of which would tend to lower any individual column's strength.
At this point a rapid redistribution the remains structure occurred, rapidly causing other columns to exceed capacity, causing another rapid load shift and column failures progressed to all columns over one or more levels.
A large section of the upper building then falls, accelerating to next contact. A combination of factors result in few if any buckled columns impact on lower parts of themselves. Upper falling section column ends are the first contacts with lower structure. That contact would be proximate to the lower section columns and thus the greatest impact occurs near or directly on lower floor truss to column connecting components.
This is occurring both to long span truss to column connecting components, at perimeter and belt truss, as well as with the core's beams running between core columns.
As upper mass continues its lowest floor begins contacting lower floors. This first contact is between floor systems that are already severely compromised. They are among those that have been affected by impact and heat and one has suffered loss of connections to columns as per last paragraph.
Both upper structure and lower structure now have major connection failures, both in the vertical/horizontal connections as well as connections between column sections. The later is due to both the destruction of Lateral Support and the buffeting collisions from the falling mass.
Perimeter column "trees" peel away, the first to do so are on the order of 1000 feet above ground as they pivot away.
Collapse continues with a relatively small amount of falling mass being ejected at the collision interface. Since initial contact was an order of magnitude greater than that which the long span open floors could absorb, little effect is seen on the falling mass. This means that the still intact portion of upper mass will continue to accelerate.
Again first contact is the lower column ends, though some movement of those column ends might not see them directly affecting column\truss(or beam) connections , they will spear through that next lower floor.
Next long span floors will now be impacted by more mass , albeit more rubblized, that is necessarily moving faster than first contact. Those floor spans must also be hit by an order of magnitude greater load thanthey can withstand, and indeed greater than at that earlier, first contact impact.
There existed no mechanism by which this floor truss and perimeter peel destruction could be arrested as this continues.
The core area beam destruction lags that of the long span destruction since those beams are heavier and shorter. Perimeter peel necessarily lags long span destruction as it is a direct result of that long span truss destruction.
Core columns stripped of inter-core Lateral Support succumb to a combination of slender column buckling and heavy buffeting from falling debris. A relatively small, heavily damaged, portion of the core remains after long span floor and perimeter destruction ( no longer being buffeted by debris impact) but cannot remain intact and it comes apart as well
via International Skeptics Forum http://ift.tt/1SrGdS8
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