It seems NIST is not following this method to explain the collapse of WTC7. NIST suggests that further primary structure (columns) fail, when secondary parts connected to them are ripped off but there are no loads that can do that.
Normal building loads were not the
only factors involved in the collapse of WTC 7. It's naive to arbitrarily single out one factor and justify the entirety of the end result off of an individual contributor. First time or not the causes of the failures that led to collapse are well studied phenomena and are all taken into consideration in the design process regardless of how they manifest. If the designs do not consider factors that contribute to wear and tear as well as overall deterioration over time they become even more pronounce and eventually to the point at which occupant safety is placed in jeopardy. Measures must be taken because if the structure is not adequately maintained, even normal climactic changes and weathering can deteriorate the structure given enough time.
And remember, the floor spans in between columns inside the building were longer than average. Such existing factors should never be ignored when considering design factors also affect how any sort of failure spreads throughout the building.
The WTC7 destruction can be explained by Controlled Demolition of internal columns at the ground, i.e. multiple, intentional local failures.
I crossed out the word
"intentional" for you. Multiple local failures is like a slang definition of progressive collapse. I know I'm pulling at strings to pound this in your logic, but that statement doesn't seem well thought out. You seem to agree that some kind of progressive collapse is possible but
only via human intervention?
A local failure of a primary part (a vertical column) can only cause further local failures of one of the end connections of secondary parts (horizontal beams).
Based on what standard design method? It's apparent that since you're making the same case for this building as you did for WTC 1 & 2, is there some standard that I should be aware of where the buildings structural frame has no effect on the performance or redundancy of the overall building? What references do you derive this generalization from?
The
WTC 7 floor plan shows that column #79 held a significant section of floor span. A failure of one column and the weakening of others by fire exasperates the problem. Your argument comes across as if the fires had no effect on other columns?
The latter remains attached to undamaged primary parts and the structure should remain standing with only one failed primary part detached from its secondary structure. Only a serious local structural failure should have developed if, e.g. column 79 failed.
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Again on what grounds? Does this take into account secondary effects from structural damage on one side? Does this take into consideration plastic creep of some portions of the structure and/or connections? Does it consider shear forces applied by thermal expansion and contraction cause by structural members being heated by the fires? Does this consider WTC 7's own design for the structural frame?
I'm not sure I follow what you base this conclusion on, or if it considers any underlying factors for that matter.
1. One primary structural part fails due heat, e.g. column 79.
2. Other, secondary structural parts connected to the failing primary part thus lose their end connections to the failing primary part = further local failures (interface between failed primary part and attached secondary parts is modified). Let's call the further local failures A. What happens then?
3. The (2.) secondary structural parts evidently try to find a new equilibrium; e.g. the lose ends drop down (energy is released) to rest on intact, secondary structural parts (we now have a partially damaged structure and a new situation). It is very unlikely that the secondary structure (2.) will detach from other primary structural parts/columns, which NIST suggests.
According to the draft report Column #79 by the time of collapse initiation spanned 9 floors without any lateral bracing whatsoever because of partially floor failures that took place throughout the day. Such a long unbraced section, in addition to thermal strain from fires would have made it especially vulnerable to buckling. Once the column failed, all remaining loads would have been transferred to the the nearest intact structural members. Look at the typical floor diagram of WTC 7, and look at how much of the interior would have had the loads redistributed. The interior had very significant sections that were supported by individual structural members. If those intact members are unable to sustain added loads then they too fail, and the loss of any single member had significantly more impact than a similar failure might have in say, a steel frame structure with a traditional skeleton design.
4. Thus we have to study the structure in (3.). If the intact parts can carry the load of the lose parts hinging down (good redundancy due to excess strain energy in the intact parts) further destruction is arrested.
5. If not, these intact parts may also fail (further local failures) and drop down (more energy is released) and drop down on other intact structural members. Let's call the further local failures B.
6. Now we have to study a structure with one failed primary part (column 79) and local failures A and B and a fair amount of partially loose structural members/parts and a lot of intact structure.
7. It is now friction comes into play. Many of the partially lose members are in contact with each other and intact members and rub against each other and intact parts and more energy is absorbed or wasted. Also loads are redistributed.
You say we should study the structure but your proposed scenario does not do any of it. It's a generic interpretation that simply doesn't consider any of WTC 7's unique design features. You simply cannot apply a generic assumption to it. Not to mention this essentially attempts to rule progressive collapse as an impossibility. I honestly don't know what you intend to convey with your friction argument however the structure of WTC 7 is a lot more complex than you seem to give credit for. The floors are not going to simply "hinge downward". If such rotation is applied to connection points they will completely fail just from the moment forces, there are so many flaws with that friction argument you want to convince us with it's not even funny.
I think I finally understand the sort of principal you are applying to both WTC 7 and WTC 1 + 2, however applying a standard assertion to these structures simply flat out ignores design differences which greatly affect how the structural failures occur. Seeing as you cannot seem to consider such difference while summing up your model I can't see this discussion moving any further, nor can I ever consider your position all that credible.