Now I have had a quick look at the WTC7-reports, e.g; volume 2 - pp 609. It seem NIST assume that when one primary part (column 79) fails, then other secondary parts, attached to the failed primary part fail and then, strangely, more primary parts (columns) fail and the whole structure collapses like a house of cards.
It has nothing to do with reality. The following is what should happen!
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.
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.
8. Normally you would expect a single local failure (1.) of a primary structural part (a column) to cause some further local failures A and B (or C and D) of secondary structural parts, i.e. the connections between the primary part and the secondary parts and that, after that, the energy released is absorbed by local failures, straining of intact parts and friction, loads are redistributed and that the destruction stops.
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.
The WTC7 destruction can be explained by Controlled Demolition of internal columns at the ground, i.e. multiple, intentional local failures.
I have seen some figures in the report (p. 592) where loose primary and secondary parts disconnected at both ends are flying around. It cannot happen in reality.
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). 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.
It has nothing to do with reality. The following is what should happen!
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.
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.
8. Normally you would expect a single local failure (1.) of a primary structural part (a column) to cause some further local failures A and B (or C and D) of secondary structural parts, i.e. the connections between the primary part and the secondary parts and that, after that, the energy released is absorbed by local failures, straining of intact parts and friction, loads are redistributed and that the destruction stops.
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.
The WTC7 destruction can be explained by Controlled Demolition of internal columns at the ground, i.e. multiple, intentional local failures.
I have seen some figures in the report (p. 592) where loose primary and secondary parts disconnected at both ends are flying around. It cannot happen in reality.
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). 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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