Re: Crollo Torri Gemelle: perché si parla di effetto pancake?

Inviato da  Ashoka il 20/7/2006 9:53:10

Leggiamo sto famoso pezzo che ti avevo linkato (traduzione in italiano qui) e che tratta proprio dell'onda di propagazione della “martellata”..

The falling upper section with a velocity of no more than 8.5 metres per second at impact
would meet resistance from the impacted columns and have as its first task the necessity to load
these columns through their elastic range and thereafter through the plastic shortening phase. We
shall firstly examine this incremental time period.

1/ The elastic phase shows a linear relationship between load and deflection up to the
elastic limit. The load at this point is the failure load and the deflection at the elastic
limit for steel is generally 0.2% of the initial length.

2/The shortening phase allows for the same failure load to be applied until the vertical
deformation reaches 3% at which point the column begins to form buckle points.

3/The third phase shows a rapid decrease in the load requirement to continue
deformation, this load necessarily being less than the failure load. This phase lasts until
the total vertical deformation equals the original length. In other words the column is
bent in two.

To shorten the columns of the first impacted storey by 3%, sufficient to complete the
plastic shortening phase, a distance of about 0.111 metres, and allowing a constant speed of 8.5
metres per second, would take a minimum of 0.013 seconds.


The propagation wave of the impact force would therefore travel a distance of 58.7 metres in a
time of 0.013 seconds. This means that during the time taken in the plastic shortening of the
impacted columns, the same force would be felt at a minimum distance of 58.7 metres, or
approximately 16 storeys, from the impact. These storeys would thus suffer an elastic deflection
in response to, and proportional to, the failure load applied at the impacted floor.


We can estimate the elastic deflection of these 16 storey columns as being in the range 0
to 7mm. The full elastic deflection of a 3.7m column, using the generally accepted figure of
0.2% of its original length is 7.4mm. The columns in the uppermost of these storeys would suffer
almost their full elastic deflection since their failure load is similar though slightly greater than
that of the first impacted storey. Those storey columns more distant from the impact would be of
a larger cross section, requiring higher loads to cause full elastic deflection.


L'articolo stima l'energia necessaria proprio per provocare l'full elastic deflection e la plastic shortening phase

Ma leggitelo che è interessante la conclusione ma ricordiamo che è comunque un modello, come tutti quelli del NIST...

Queste due immagini del crollo del Wtc1 (del video che ti avevo sollecitato a vedere frame per frame) sono invece significative perché mostrano quello che è stato il crollo vero e proprio:





Rappresentano in pratica la descrizione fatta dal Nist?

Failure of the south wall in WTC1 and east wall in WTC2 caused the portion of the building above to tilt in the direction of the failed wall. The tilting was accompanied by a downward movement. The story immediately below the stories in which the columns failed was not able to arrest this initial movement as evidenced by videos from several vantage points. The structure below the level of collapse initiation offered minimal resistance to the falling building mass at and above the impact zone. The potential energy released by downward movement of the large building mass far exceeded the capacity of the intact structure below to absorb that through energy of deformation.

No, ci raccontano tutta un'altra storia...

Ashoka