Critical Structural Issue Contributing to WTC Collapse.

[Halophyte]

(I posted this info on another thread but it’s worth a separate post.)



Critical Structural Issue Contributing to WTC Collapse.

Weak link in design.



Viscoelastic (VE) dampers are dependent on both relative velocity and displacement to dissipate energy. VE damping system in Twin Towers is a double-layer shear damper using a 3M material, which is a rubber derivative, glued to steel plate and angle irons. This material will carry some load (which is temperature-dependent and would be less than the two-bolt connection) as it displaces. As installed it has several functions:

1. It develops continuity moment at the end of joist girder, that is, the joist girders will behave as partial continuous members under Dead and Live load. It is partially restrained under Wind load.

2. It restrained the lower chord of the joist girder (in the direction perpendicular to the plane of the joist). Therefore it stabilizes the concrete diaphragm. Note that for a 4-inch thick concrete slab spanning 60 feet, it would buckle if there were no joist girder. It also transfers compression load through bottom chord.

3. Joist girder-column connection is a moment connection.

4. It reduces the energy to be absorbed by the joist girder and the columns under Wind load.

As the temperature rises, 3M materials would loose its load carrying capacity, i.e. its energy-dissipating capacity. This is equivalent as loosing the two-bolt connection because it will act as though there is gaps between the steel plate and the angle irons. As a result, several things would occur:

1. The joist girder is no longer a continuous member. Therefore, even under Dead and Live load, its top chord would rotate more relatively to its supporting column.

2. All the compression or tension force to the diaphragm would go through the top chords only.

3. More rotation between the top chord and its respective column under Wind load.

4. No more lateral restraint for the bottom chord and the joist girder could buckle laterally and the slab diaphragm would follow. And the result is a tremendous demand on the connection between the top chord and its supporting column.

The VE damping system is a novelty design. First of its kind in the World. First of its kind implemented in a skyscraper. The reason I still think it is a design flaw is that:

In the 60’s and beginning 70’s there are many literatures about plastic design in steel including ASCE manual No 41. From J Heyman to Beedle, they all emphasize the importance of collapse mechanism in Limit Analysis. And in dealing with inelastic behavior such as VE damping system is engaging in Limit Analysis whether you want it or not because you have to think what will happen beyond the device limit.

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[Ponce Cuba]

Halo, good buddy, you are now repeating yourself and getting boring...how about something news for a change? :chat:

[Halophyte]

You mean, sort of like the WTC explosives posters ?

How many of them have you asked to quit posting ?

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[Ponce Cuba]

Hehehehehhe Halo, you have about 20 guys talking about explosives and only you on the other side.....you are persistent and I like that but all I said was to "change" it, not to quit

[Halophyte]

But ponce, this is the SMOKING GUN for the Elastic Dynamic Analyisis of the pancake collapse of the towers.

This is too important to dismiss !

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[Halophyte]

I repositioned figure 1 to orientate it to its true vertical mounting position.

See how the glued surfaces are all parallel to vertical loads ?

All loads on these plastic dampers are shear loads !

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[Ponce Cuba]

Hahahahahah good buddy, I give up.....I wont be answering any more of your posting about the Twin Towers.....be happy.

[Halophyte]

I know, it requires folks to think.

Not as exciting as CTs either ...


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[Large Sarge]

Originally Posted by Ponce Cuba

Hahahahahah good buddy, I give up.....I wont be answering any more of your posting about the Twin Towers.....be happy.

Did the troll scare you off Ponce?

[bigjon]

UL tested the flooring system. That means that Halo's weak link was included in the test.

The fire had no materials and definitely not enough oxygen that enabled the fire to approach the temperature of the test.

More claptrap in an effort that has a predetermined conclusion as the basis for investigation. As in this is a phony "investigation".

Halo our red herring man.

[Halophyte]

Each towewr was fitted with 10,000 Viscoelastic Dampers connecting all floor trusses to the outer vertical support columns. These damper connectors are almost impossible to fireproof due to their composite platic design, VE dampers do not use bolts or pins incorporated in its shear surfaces.

The maximum mid-span deflections of the long-span and short-span zones under the original WTC Design Criteria loads were approximately 1.55 in. and 0.70 in., respectively. That's all.

Failure of the VE damper shifts all vertical load limits to the top bolt/pins of the trusses. VE failure allows the trusses to hinge (rotate) on their top cord connertor bolt/pins. This allows the trusses to then have unlimited mid span deflection, well beyond design limits. What follows is floor sag and eventual buckling of floor trusses.

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