9/11 Physics from Non-Experts

[Furcifer]

The floors in the core areas were made of normal weight concrete, density 1760 kg/m3

2300 kg/m3 is closer to "normal concrete"

 

[Augustine]

One last thing, the weight difference is not significant but it annoys me: the floors were 4" concrete on 1-1/2" metal deck - 5-1/2" total thickness. (Generally you can estimate the additional concrete as being half the deck rib height if you don't know the deck profile, say 4-3/4" equivalent weight of concrete.) Just bugging me every time I see it.

 

[Architect]

Woah. That would be awfully thick permanent formwork. Where did you get the information?

 

[Furcifer]

Assuming a 10m rebar mesh in the floors, how much additional weight for C4 coating?
That's the green stuff right?

 

[beachnut]

Woah. That would be awfully thick permanent formwork. Where did you get the information?

Now you need a detail of the steel plate surface you can almost see it in other WTC floor photos and pictures. They pour the floor on the floor. How do you do that? If I did that all the cement would leak to the lower floor.

The concrete floor is 4 inches. Not sure if there is some extra concrete due to the steel plate being corrugated.

Floor estimates could be off 10 percent.

The metal decking was the support to the concrete floor poured on it. You have 4 inches of concrete poured on what looks like a 1.5 inch metal floor. They were like a unit.

http://www.chapelhill.indymedia.org/uploads/fig-2-9-big.gif Not sure if there was more or less concrete due to the floor form.

 

[Architect]

The basic build-up of in-situ concrete over profiled permanent shuttering (which may or may not have a significant reinforcing effect) is quite common. But perhaps I misunderstood. What are you suggesting that the thickness of steel itself is in the formwork/decking? Normally we're looking at only a few millimetres.

 

[GregoryUrich]

One last thing, the weight difference is not significant but it annoys me: the floors were 4" concrete on 1-1/2" metal deck - 5-1/2" total thickness. (Generally you can estimate the additional concrete as being half the deck rib height if you don't know the deck profile, say 4-3/4" equivalent weight of concrete.) Just bugging me every time I see it.

I have interpreted the metal deck as others have, as 22 guage thickness with the depth of the ribs as 1 1/2". 22-gauge steel is 0.0299 inches thick. I have assumed the thickness of the concrete to be 4" on average as given by NIST (i.e. 3.25" at the peak of the rib and 4.75" at the trough.

 

[rwguinn]

I have interpreted the metal deck as others have, as 22 guage thickness with the depth of the ribs as 1 1/2". 22-gauge steel is 0.0299 inches thick. I have assumed the thickness of the concrete to be 4" on average as given by NIST (i.e. 3.25" at the peak of the rib and 4.75" at the trough.

I can accept those values. 1.5" thick (36.75 mm) is extremely thick for decking material--you wouldn't need the da@n concrete if that were the case!

 

[GregoryUrich]

I can accept those values. 1.5" thick (36.75 mm) is extremely thick for decking material--you wouldn't need the da@n concrete if that were the case!

You being a professional, I'd like to ask you a question regarding comparing the wtc tower to the Empire State Building. We have all seen statements such as: the world trade center tower design was so innovative that it was much lighter relatively speaking to conventional structures.

For example a quote from wikipedia:

"A very light, economical structure was built by keeping the wind bracing in the most efficient area..."

How would you interpret "very light" in percent relative to the ESB?

 

[Augustine]

I have interpreted the metal deck as others have, as 22 guage thickness with the depth of the ribs as 1 1/2". 22-gauge steel is 0.0299 inches thick. I have assumed the thickness of the concrete to be 4" on average as given by NIST (i.e. 3.25" at the peak of the rib and 4.75" at the trough.

The deck was 1-1/2" Type B, 22 gauge. Your 3.25" and 4.75" have no basis in reality. It was 4" of concrete (nominal, meaning minimal) ON 1-1/2" metal deck. Depending on the corrugation profile, the equivalent depth could be 5 (B-INV), 4.75 (LOK-FLOOR type deck), 4.56 (B profile), or 4.35 (NIST value). Trust me on this, I am a structural engineer and spec this stuff all the time. Check out the NIST models for the bridging trusses. (Also, they pour minimum 4" concrete over the ribs. That is MINIMUM. As the trusses deflect, deck deflects, etc, they may pour MORE THAN 4" to attain floor levelness. Typically we ignore this unless we have long spans and anticipate large deflections.)

 

[Newtons Bit]

Don't forget camber Augustine.

 

[GregoryUrich]

If you were to actually calculate all of the mass, instead of saying "oh well this is really small, and my lack of this cancels out overestimating this" then he might be more amiable to what you're trying to prove. Remember, rwguinn does this for a living. Though he's probably at that point of his career where he has his underlings figure out the loads and he just does the fun stuff.

I appreciate you people taking the time to discuss this. Try to keep in mind that my paper is essentially a first draft and has been published as a letter (not peer reviewed) to help get the kind of criticism many are providing here.

I know I have alot of work to do. I agree that it won't stand up to say "this is really small", etc. I'm trying to find the level of detail that most people will accept. I just hope I don't have to go down to the nut and bolt level.

 

[Augustine]

Don't forget camber Augustine.

First rule of thumb is only camber for 3/4 of the dead load deflection. Trying to balance it precisely will leave your floor looking like rolling hills...can't get the damn beams to go down!

 

[GregoryUrich]

The deck was 1-1/2" Type B, 22 gauge. Your 3.25" and 4.75" have no basis in reality. It was 4" of concrete (nominal, meaning minimal) ON 1-1/2" metal deck. Depending on the corrugation profile, the equivalent depth could be 5 (B-INV), 4.75 (LOK-FLOOR type deck), 4.56 (B profile), or 4.35 (NIST value). Trust me on this, I am a structural engineer and spec this stuff all the time. Check out the NIST models for the bridging trusses. (Also, they pour minimum 4" concrete over the ribs. That is MINIMUM. As the trusses deflect, deck deflects, etc, they may pour MORE THAN 4" to attain floor levelness. Typically we ignore this unless we have long spans and anticipate large deflections.)

rwguinn has already stated that a 1.5" thick steel deck would require no concrete. Think about it. They use 1" thick steel plate during road construction over trenches wider that the joist spacing and semis can drive over them. I'll try to find a reference for the 22 guage deck.

http://www.m-w.com/dictionary/nominal

Nominal means approximately.

 

[Augustine]

I appreciate you people taking the time to discuss this. Try to keep in mind that my paper is essentially a first draft and has been published as a letter (not peer reviewed) to help get the kind of criticism many are providing here.

I know I have alot of work to do. I agree that it won't stand up to say "this is really small", etc. I'm trying to find the level of detail that most people will accept. I just hope I don't have to go down to the nut and bolt level.

Why don't you actually talk to Robert Fowler and Leslie Robertson? This isn't exactly original research, all the loads were calculated 30+ years ago. Maybe actually get an informed opinion on what percentage you should be using for sustained live load...That would actually be research, to talk shop with the guys who designed the towers.

 

[Augustine]

rwguinn has already stated that a 1.5" thick steel deck would require no concrete. Think about it. They use 1" thick steel plate during road construction over trenches wider that the joist spacing and semis can drive over them. I'll try to find a reference for the 22 guage deck.

I said

The deck was 1-1/2" Type B, 22 gauge.

That is not steel plate. Get your USD catalog from Nick Bouras and look it up.

P.S. When I say nominal means minimal I am speaking in what nominal means when it shows up on construction plans for composite deck. So go talk to a contractor, especially with that harebrained "4" concrete on 1-1/2" metal deck means 3.25 over the ribs and 4.75 over the troughs" insanity.

 

[GregoryUrich]

I said

That is not steel plate. Get your USD catalog from Nick Bouras and look it up.

P.S. When I say nominal means minimal I am speaking in what nominal means when it shows up on construction plans for composite deck. So go talk to a contractor, especially with that harebrained "4" concrete on 1-1/2" metal deck means 3.25 over the ribs and 4.75 over the troughs" insanity.

Sorry I misread your previous statement. I wasn't the only one who misinterpreted. We agree it's 22 guage. Would 4.75" average thickness for the concrete satisfy you?

 

[rwguinn]

I said

That is not steel plate. Get your USD catalog from Nick Bouras and look it up.

P.S. When I say nominal means minimal I am speaking in what nominal means when it shows up on construction plans for composite deck. So go talk to a contractor, especially with that harebrained "4" concrete on 1-1/2" metal deck means 3.25 over the ribs and 4.75 over the troughs" insanity.

I'm going to reiterate here:
I am NOT, repeat NOT, a structural designer I am a structural analyst. I don't typically deal with construction specifications--that's design's job. I do the joints, loads, stress, strain, and failure modes--but generally, in my business, buckling is not an issue, so I don't deal a lot with it, other than to avoid it.
When someone states, without other qualification, a 1.5" thick deck, I am reading material thickness. I can readily understand a 1.5" space claim for 22 gage corrigated steel, but will read 1.5" thick as material thickness, not space claim.
Sorry for the misundersatnding there. I'll go back to letting the construction design experts go at it again.

 

[Furcifer]

NIST reported a 1.5" deflection in the longer trusses, the height of a slab is usually measured on the outside edge. So it was probably bowed slighty down in the middle. Go with 5" and you should be pretty good.

 

[Architect]

I have interpreted the metal deck as others have, as 22 guage thickness with the depth of the ribs as 1 1/2". 22-gauge steel is 0.0299 inches thick. I have assumed the thickness of the concrete to be 4" on average as given by NIST (i.e. 3.25" at the peak of the rib and 4.75" at the trough.

Sorted. Terminology difference. In the UK the thickness of the decking would be the actual steel, and the depth of trough or profile would be the overall size.