Faster than Freefall, Proof of Demolition

Originally posted by billybob

yeah.

well, here's the thing.

say you have two bowling balls,

Say, you drop a bowling ball from a 100 story height.. when it passes a bowling ball on the 50th floor, it knocks that one and it starts to fall too..

now, from that moment on.. which is what the video shows (the first ball being the building above the filmed area coming down) which goes faster: the 1st ball, or the second.. after 1 second, ball 2 has a speed of 9,81 meters/second, ball 1 is allready falling for, say 5 seconds, and has a speed of about 49 meters per second.



[edit on 31-3-2006 by phiniks]

EDIT: Not worth it.



[edit on 2006-3-31 by wecomeinpeace]

WCIP...what's not worth it? I was only pointing out that Howard was right in his equation. If he and I aren't, please let us know. I would rather be corrected than have the wrong information out there.

BTW...I still haven't made up my mind yet on the whole thing, but still lean towards something other than plane damage and fires bringing the buildings down. Take care.

I am pretty sure WCIP was referring to the revised version of Newton member phlinks has created above.

WooooT!

Springer...

[edit on 3-31-2006 by Springer]

Originally posted by phiniks

Originally posted by billybob

yeah.

well, here's the thing.

say you have two bowling balls,

Say, you drop a bowling ball from a 100 story height.. when it passes a bowling ball on the 50th floor, it knocks that one and it starts to fall too..

now, from that moment on.. which is what the video shows (the first ball being the building above the filmed area coming down) which goes faster: the 1st ball, or the second.. after 1 second, ball 2 has a speed of 9,81 meters/second, ball 1 is allready falling for, say 5 seconds, and has a speed of about 49 meters per second.



[edit on 31-3-2006 by phiniks]

As soon as the first ball hits the one on the 50th it will lose a lot if not all of the acceleration it had built up, possibly even imparting some of the enery to the second ball to hasten it's descent.

Now if the second ball was dropped right when the 1st ball gets to it's level, the second ball could never catch up to the first (assuming both balls are in freefall) since the 1st ball's speed will always be higher.

[edit on 3/31/2006 by yadboy]

Originally posted by yadboy

Originally posted by HowardRoark
Then you are probably familiar with supply and return air systems, air shafts, and air fresh air intakes.

What would happen to the air in an air shaft as the floor is feeds collapses?

Not sure what your trying to allude to, but more likely than not the air in the ductwork would simply be forced back out the supply diffusers and return air grilles. Some would be trapped momentarily by the fire dampers, but that wouldn't last. Commercial HVAC is usually sheet metal for the most part, not really that sturdy, if anything it probably wouldn't have made much difference one way or the other.

No, I am talking about the type of air handling system typically found in a large high rise. You have two air shafts in the core. A supply and a return shaft. These shafts, or plenums) are usually not ducted, the drywall enclosure is the duct. At each floor the supply ductwork branches out from the supply shaft to the floors. The return is through the open ceiling plenum and is not ducted.

The ducts lead down to the fan room on the mechanical floor where a certain percentage of the return air is remixed with the fresh air intake and sent back to the floors.

If the collapse forces the air out between tow floors, then the air will have to go somewhere. If will travel down the duct and out the fan room intakes (in a giant squib like puff).

Originally posted by yadboy
There WOULD be more resistance to the collapse from the structural steel in that building.

Originally posted by HowardRoark
Can you do the calculations, or are you just a CAD operator?

Aw that hurt, no I am not "just a CAD operator" I am currently helping design a centrifuge balancing stand and the HVAC/piping for a shaped explosives manufacturing facility. Your job seems to be to make assumptions about people you know nothing about. If you wanna come watch my newborn for me tonight I'll get those calcs for you, make sure to bring the bldg plans with you.

Sorry, ‘bout that. But you said you were a drafter, so I assumed you meant that you were a mouse jockey.

Originally posted by yadboy
Yeah, see there this thing called "standards", it's a bunch of rule books dreamed up by guys in a lab that are trying to simulate real world conditions to make things work better or safer. Fire protection standards are WAY overdone. But you gotta follow their rules or you don't pass inspection. Overreaching standards add quite a bit to the cost of many construction projects, but you gotta do what the man says.

Really, WAY overdone? I have NEVER heard a structural engineer state that.

Have you read this yet?
fire-research.group.shef.ac.uk...

Originally posted by Springer
I am pretty sure WCIP was referring to the revised version of Newton member phlinks has created above.

WooooT!

Springer...

[edit on 3-31-2006 by Springer]

Ah, ok. NVM WCIP. Keep up the good work btw. I've already voted for you this month from one of your other posts I think.

Yeah, I didn't think that sounded correct to me either but wasn't sure.

Originally posted by HowardRoark
Then you are probably familiar with supply and return air systems, air shafts, and air fresh air intakes.

What would happen to the air in an air shaft as the floor is feeds collapses?

Originally posted by yadboy
Not sure what your trying to allude to, but more likely than not the air in the ductwork would simply be forced back out the supply diffusers and return air grilles. Some would be trapped momentarily by the fire dampers, but that wouldn't last. Commercial HVAC is usually sheet metal for the most part, not really that sturdy, if anything it probably wouldn't have made much difference one way or the other.

Originally posted by HowardRoark
No, I am talking about the type of air handling system typically found in a large high rise. You have two air shafts in the core. A supply and a return shaft. These shafts, or plenums) are usually not ducted, the drywall enclosure is the duct. At each floor the supply ductwork branches out from the supply shaft to the floors. The return is through the open ceiling plenum and is not ducted.

The ducts lead down to the fan room on the mechanical floor where a certain percentage of the return air is remixed with the fresh air intake and sent back to the floors.

If the collapse forces the air out between tow floors, then the air will have to go somewhere. If will travel down the duct and out the fan room intakes (in a giant squib like puff).

I have to admit that my knowledge of high rise HVAC may be lacking (we mostly do gov. facilities & comercial manufacturing facilities here). Your explanation sounds feasible, but only solves a small part of the problem. You explained the squibs, but what about the the actual collapse? The squibs aren't really very telling to me, they could be explained away any number of ways.

Originally posted by yadboy
There WOULD be more resistance to the collapse from the structural steel in that building.

Originally posted by HowardRoark

Really, WAY overdone? I have NEVER heard a structural engineer state that.

Have you read this yet?
fire-research.group.shef.ac.uk...

Yes actually I hear that on a weekly basis from structural engineers, mechanical engineers & civil engineers. It is better to be safe than sorry, so of course we follow all standards, but those standards are written to give a lot more protection than is needed in the real world in many cases.

Your pdf is apples to the WTC oranges. Floor trusses are not meant to support buildings. They are just there to keep the floor in place. Comparing a floor truss to a structural beam is like comparing a chopstick to a steel pipe.

[edit on 3/31/2006 by yadboy]

Griff wrote:
WCIP...what's not worth it? I was only pointing out that Howard was right in his equation.

Sorry for the confusion, as Springer said, I was actually referring to another member's post, not yours and Howard's.

But anyway while I'm here, force is not equal to momentum over time, it is equal to the change in momentum over the change in time. A force acting on an object causes a change in its momentum. The shorter the time it takes to create the same change in momentum, the larger the force. Or put another way, changing a large momentum in a very short time takes a very large force.

Acceleration is the change in velocity over the change in time. So (where mass is constant), F = ma = m*dv/dt = dp/dt

HowardRoark wrote:
Force equals momentum divided by time.

A 10kg object flying through space at a constant velocity of 10ms^-1 has a constant momentum of 100kgms^-1. If you observe it for 5 seconds and the object's momentum stays constant, using Howard's physics you get 100/5 = 20N of force acted on the object for the time you observed it, which is complete poppycock. The obvious fact is that zero force acted on the object.

It's very intelligent people like Howard speaking about things they don't understand with mock authority and in an intimidatory manner which has everybody convinced that gargantuan steel buildings can disintegrate into dust in a matter of seconds under gravity alone. Physics is not my area of expertise by a long, loooong shot, so perhaps someone can correct my Newtonian ramblings (Val..? U there...?) and I'll have a big, smelly egg on my face. But if HR doesn't want to get the sack, he should stick to HVAC systems and thermal load graphs methinks...

Anyways, "OMG the huge force of the plane striking the building OMG wow" hand-waving is moot, since we all know that the buildings withstood the impacts easily. On any other day, folks will swear that it's only the fires that matter, until the lack of physical evidence showing any steel temperature excursions over 250C is raised, at which stage the argument goes back to the impacts, and around and around it goes.

A little on the same vein below are some momentum calculations I did for the pancake theory which show WTC1 should have taken 14+ seconds to collapse in a hypothetical world where the structure provided ZERO resistance. Which leaves about 1-4 seconds at most added to the collapse time by the resistance of the structure. Divide that by 98 floors and you get a ridiculously negligible amount of resistance which simply doesn't add up. Someone please correct me if I'm wrong in my calcs =>

Assumptions:

* For distance purposes (except for the cap striking the ground), each floor is an infinitely thin slab with all of its mass concentrated into that slab.

* Collapse initiates at the 98th floor.

* Air resistance is zero.

* Resistance from the structure is zero.

* Collisions are "flush" and perfectly inelastic, meaning the falling mass of the cap "acquires" each floor it collides with.

* 30% of the mass of each lower floor is lost over the side of the building (note this only adds 1s to the total time).

* None of the mass of the original cap is lost.

* When the cap strikes the ground, it collapses on itself in free fall.

(What may seem like minor errors are in fact because the xls file calculates using all of the decimal places, but I only set it to display four.)





Wow, I get de ja vu every time I post in 9-11 forum these days, we've been around the merry-go-round so many times. I wonder what's happening in chat...?

[edit on 2006-3-31 by wecomeinpeace]

Originally posted by wecomeinpeace
But anyway while I'm here, force is not equal to momentum over time, it is equal to the change in momentum over the change in time. A force acting on an object causes a change in its momentum. The shorter the time it takes to create the same change in momentum, the larger the force. Or put another way, changing a large momentum in a very short time takes a very large force.

Acceleration is the change in velocity over the change in time. So (where mass is constant), F = ma = m*dv/dt = dp/dt

Ah, I see where you're coming from. And actually, you are the one who wins the prize as this is the correct way of putting it. Because acceleration and velocity are derivatives of time, you are correct in saying that they are the differences in time, acceleration, velocity and momentum. Sorry for the confusion.

Edit: I never was an expert on dynamics. Statics is what a structural engineer studies. Thanks for the enlightenment. I actually remember my dynamics courses now

[edit on 31-3-2006 by Griff]

Originally posted by wecomeinpeace
But anyway while I'm here, force is not equal to momentum over time, it is equal to the change in momentum over the change in time.

Well if you want to get picky about it, time doesn’t “change,” it just is a straight value. 1, sec, 3 sec, ect.)

The delta is implied.

In any case, the problem as I originally stated it is correct.

Originally posted by HowardRoark
If we assume that the impact caused the planes momentum to go from 400 mph to 0 mph in ½ a second (totally arbitrary numbers), then we can calculate the force involved.

Originally posted by yadboy
we mostly do gov. facilities & comercial manufacturing facilities here

Originally posted by yadboy
Yes actually I hear that on a weekly basis from structural engineers, mechanical engineers & civil engineers. It is better to be safe than sorry, so of course we follow all standards, but those standards are written to give a lot more protection than is needed in the real world in many cases.

If you do government work, I can understand that attitude. An elephant is a mouse built to government specs.

Originally posted by HowardRoark
Well if you want to get picky about it, time doesn’t “change,” it just is a straight value. 1, sec, 3 sec, ect.) The delta is implied.

Only in the case of the example in question where you already stated the parameters, not in general theory nor application. However the crux of my post was not regarding time, but rather regarding the change in momentum, without which essential factor your physics are completely wrong. You didn't imply any change in momentum, you merely implied by your hand-waving that you googled for the equation - my knowledge of physics is pitiful, but I can tell a mile away.

I also missed this one:

Originally posted by HowardRoark
...the impact caused the planes momentum to go from 400 mph to 0 mph

Since when is momentum expressed in mph?

But anyway, moving on...

Originally posted by HowardRoark
If we assume that the impact... then we can calculate the force involved.

So what's your point? Where is this leading exactly?

[edit on 2006-3-31 by wecomeinpeace]

WCIP, you are right, this is going no where.

You seem to be rather antagonistic latley, so . . .

I'll just pay no attention to you for a while.



[edit on 31-3-2006 by HowardRoark]

Originally posted by yadboy
Your pdf is apples to the WTC oranges. Floor trusses are not meant to support buildings. They are just there to keep the floor in place. Comparing a floor truss to a structural beam is like comparing a chopstick to a steel pipe.

Not really. The floor trusses WERE structural beams in the WTC towers.

I think the heart of the problem is that people fail to appreciate the unique aspects of the WTC structural design.

What you had was essentially a hollow tube filled with floor slabs and an internal core structure.

The floor slabs served a vital purpose in the structure besides supporting the live loads, they also provided the exterior column walls with the necessary resistance to buckling.

One thing that I see is that many people tend to think of each of the floors as a separate structural system. This is not the case.

It would have been possible for a floor slab to collapse entirely down onto the floor below without affecting the overall height of the building (In fact, photograph evidence indicates that this indeed did in happen in several areas).

Either the outright failure of the floor or the sagging of the floor due to the buckling of the floor trusses would have reduced the ability of the exterior wall to resist buckling. If the floor was sagging, you also have to account for the inward forces on the exterior walls from that sagging.

It is an inverse square law. If you double the length of a column, then your critical buckling load is reduced by 1/4. If you increase the column length by a factor of 3, you reduce your critical buckling load to 1/9th of the original value.

Thus the exterior walls will start to buckle inward, as is shown in several photographs of both buildings just prior to the collapse.

Buckling failure

Originally posted by HowardRoark
You seem to be rather antagonistic latley, so . . .

Pot...meet kettle. Kettle...pot. But I honestly fail to see how proving you wrong is construed as being "antagonistic".

Aww shucks, now don't be like that, you know I luvs ya, Howie! It wouldn't be the same around here without you, seriously.



[edit on 2006-3-31 by wecomeinpeace]

Whatever.. if you simply refuse to read what I write, we won't have a reason to discuss further..

If you guys judge my knowledge about the simplest mechanics by simply not being able to read then sorry, but go do something else.. my words are bent in your minds even before you start your replies.. buncha disinformation agents.



[edit on 31-3-2006 by phiniks]

Not to want to be a dead horse, but exactly how many demolitions charges would have had to been placed and when were they placed? I'm sure it's been talked about here but my search skills leave much to be desired.

What if one floor of the WTC below the pancaking gave way early due to being unable to sustain the stress more than the rest of the floors? Wouldn't that mess with all the calculations being tossed around here.

Originally posted by phiniks
If you guys judge my knowledge about the simplest mechanics by simply not being able to read then sorry, but go do something else.. my words are bent even before you start you replies.. buncha disinformation agents. burn in hell.

In my experience here (which hasn't been as long as others) I've noticed it's more productive to have a civil conversation. Plus, you don't want to get warned...or even worse, banned. Take care and don't take things so seriously on here....it is only an on-line forum after all.

Originally posted by pavil
What if one floor of the WTC below the pancaking gave way early due to being unable to sustain the stress more than the rest of the floors? Wouldn't that mess with all the calculations being tossed around here.

Certainly. I think that is what Howard is getting at when he says about the outer columns buckling (Euler's Equations). If one or two floors collapsed while the structure was standing, that would significantly decrease the buckling resistance of the outer columns. IMHO, the inner columns wouldn't be affected though because they weren't designed to take any lateral forces to begin with. Also, the inner columns had their own lateral bracing.