letter to NIST

Originally posted by Griff
reply to post by Newtons.Bit

 

Welcome to ATS. You obviously know what you are talking about. I can't disagree with anything in your post. We'd also have to take into account the p-delta affect. I look forward to learning from each other.

I'm not sure what effect p-delta would have in this case. It would be necessary to include the loss of some stiffness in the strong axis of the beam. This will result in some moment magnification in the strong axis. But the beam isn't going to buckle in compression buckling in the strong axis, it will buckle in the weak-axis. The only moment present in the weak axis would be due to an initial out-of-plumbness of maybe a 1/4".

Originally posted by Griff

The only difference is I believe it would happen in a progressive manner the same as the interior.

I envision what you're trying to relay to me is that you expect to see the exterior "crumbling" from east to west then. Is this correct?

Every video I've seen has the ext leaning a little bit to the east. Not by much, maybe 10-15 degrees or so. Now, I don't know what significance this has, but it seems to me that since it wasn't a "grid" design, that this is what would be expected during a collapse.

You stated that the ext columns were moment framed. Wouldn't this be the difference, compared to a "grid" framed building?

reply to post by Newtons.Bit


I'm talking about the p-delta affect that having the beams pushing on the girders causing the girder to misalign. Wouldn't this cause an added moment on the connection from the eccentrically located compression force?

Originally posted by Seymour Butz
I envision what you're trying to relay to me is that you expect to see the exterior "crumbling" from east to west then. Is this correct?

Yes, as the exterior columns become unbraced and buckle.

You stated that the ext columns were moment framed. Wouldn't this be the difference, compared to a "grid" framed building?

I'm not sure what the connection type would have to do with it as when the exterior columns become unbraced, then there is no connection. Correct?

reply to post by Griff


It'd probably be pretty minor. The main reason that short-span beams don't play a role in structural collapse is that they're usually in short-slotted holes. There's enough slip (even in a normal sized hole) to reduce a great deal of the thermal expansion.

You'd also need to consider the stiffness of the slab and the metal deck restraining the beam (and girders). That's a less simple problem.

Originally posted by Newtons.Bit
You'd also need to consider the stiffness of the slab and the metal deck restraining the beam (and girders). That's a less simple problem.

Exactly. It's nice to see you around these parts. I get a feeling you are structural of some sort. Do you mind if I ask your credentials? Just so I know whom I'm speaking with that's all. Cheers.

Originally posted by Griff

Originally posted by Newtons.Bit
You'd also need to consider the stiffness of the slab and the metal deck restraining the beam (and girders). That's a less simple problem.

Exactly. It's nice to see you around these parts. I get a feeling you are structural of some sort. Do you mind if I ask your credentials? Just so I know whom I'm speaking with that's all. Cheers.

I make a living doing structural engineering. That's all I really like to say about it. I prefer my arguments to stand or fall on their own, not as an appeal to authority.

Originally posted by Newtons.Bit
I make a living doing structural engineering. That's all I really like to say about it. I prefer my arguments to stand or fall on their own, not as an appeal to authority.

Good philosophy. I wasn't aware we were arguing about anything though. Unless you mean just in general. I was just curious. I'm glad you're around to challenge my ideas.

Originally posted by Seymour Butz

Originally posted by Griff

The only difference is I believe it would happen in a progressive manner the same as the interior.

I envision what you're trying to relay to me is that you expect to see the exterior "crumbling" from east to west then. Is this correct?

Every video I've seen has the ext leaning a little bit to the east. Not by much, maybe 10-15 degrees or so. Now, I don't know what significance this has, but it seems to me that since it wasn't a "grid" design, that this is what would be expected during a collapse.

You stated that the ext columns were moment framed. Wouldn't this be the difference, compared to a "grid" framed building?

Well the significance could be that the tower would look slightly worse than the leaning Tower in Pisa, that only lean about 5 to 6 degrees from the perpendicular!

That's pretty much all!

[edit on 5-9-2008 by djeminy]

Originally posted by Griff

Originally posted by Newtons.Bit
I make a living doing structural engineering. That's all I really like to say about it. I prefer my arguments to stand or fall on their own, not as an appeal to authority.

Good philosophy. I wasn't aware we were arguing about anything though. Unless you mean just in general. I was just curious. I'm glad you're around to challenge my ideas.

I've made a number of arguments from an engineering perspective against what I feel are the more... unethical... comments from truthers who are also mechanical engineers.

Originally posted by Newtons.Bit
I've made a number of arguments from an engineering perspective against what I feel are the more... unethical... comments from truthers who are also mechanical engineers.

For us non-engineer folk, could you be a little more specific? Could you list exactly what those mechanical engineer truthers state and what you arguments against them are? What are their specific claims, and why do you disagree backed up with proof, if possible(even if the majority don't understand your arguments against, Griff would).

Thanks for the clarity.

Originally posted by PplVSNWO

Originally posted by Newtons.Bit
I've made a number of arguments from an engineering perspective against what I feel are the more... unethical... comments from truthers who are also mechanical engineers.

For us non-engineer folk, could you be a little more specific? Could you list exactly what those mechanical engineer truthers state and what you arguments against them are? What are their specific claims, and why do you disagree backed up with proof, if possible(even if the majority don't understand your arguments against, Griff would).

Thanks for the clarity.

Most of my stuff is here:
newtonsbit.blogspot.com...

and/or here:
forums.randi.org...

Originally posted by Newtons.Bit
Most of my stuff is here:
newtonsbit.blogspot.com...

I'm curious about this part of your blog.

Mu/Mn + Pu/Pn < style=""> The other limit state is P-delta. When the exterior column is pulled inwards, it deflects. This deflection(Δ) generates a moment, specifically P2*Δ. This moment, creates more deflection, which further magnifies the moment, creating more moment, and so forth. P-delta has two outcomes: the moment reaches equilibrium at some point, or becomes unstable and continues to grow. This phenomenon can be easily shown with a simple experiment. Take a straw and try to compress it between your fingers. It has a surprising amount of strength. Now push the middle in slightly. This is p-delta.

I'm curious because you poo-pooed my comment about P-delta effects. Wouldn't a girder in compression (due to thermal expansion) and deflected from beam expansion into it also cause a P-delta effect? The only difference is the columns that you describe are vertical while the girder is horizontal?

Edit: Also, I see you use AISC LRFD 3rd Edition. I recently bought the new AISC LRFD/ASD combined manual 13th edition. I'm curious if you have used this edition and what your thoughts are on it. So far, my only gripe is that A36 steel isn't listed like in older editions, only A50. Other than that, I like it. It shows the two methods side by side.

Edit 2: The more I read your blog, the more curious I become as to why you dismissed my comments about P-delta.

For all compression members with eccentric loads or externally produced bending moments, the bending moment is magnified as the axial force approaches the ultimate compression capacity of the member. This is what is known as P-delta effects.

Why does this not apply to a girder in compression? BTW, wouldn't the P-delta effect actually strengthen NIST's findings? Pun not intended.

Damn, I wish ATS had spell check.

[edit on 9/6/2008 by Griff]

[edit on 9/6/2008 by Griff]

[edit on 9/6/2008 by Griff]

[edit on 9/6/2008 by Griff]

Originally posted by Griff

Originally posted by Newtons.Bit
Most of my stuff is here:
newtonsbit.blogspot.com...

I'm curious about this part of your blog.

Mu/Mn + Pu/Pn < style=""> The other limit state is P-delta. When the exterior column is pulled inwards, it deflects. This deflection(Δ) generates a moment, specifically P2*Δ. This moment, creates more deflection, which further magnifies the moment, creating more moment, and so forth. P-delta has two outcomes: the moment reaches equilibrium at some point, or becomes unstable and continues to grow. This phenomenon can be easily shown with a simple experiment. Take a straw and try to compress it between your fingers. It has a surprising amount of strength. Now push the middle in slightly. This is p-delta.

I'm curious because you poo-pooed my comment about P-delta effects. Wouldn't a girder in compression (due to thermal expansion) and deflected from beam expansion into it also cause a P-delta effect? The only difference is the columns that you describe are vertical while the girder is horizontal?

Edit: Also, I see you use AISC LRFD 3rd Edition. I recently bought the new AISC LRFD/ASD combined manual 13th edition. I'm curious if you have used this edition and what your thoughts are on it. So far, my only gripe is that A36 steel isn't listed like in older editions, only A50. Other than that, I like it. It shows the two methods side by side.

Edit 2: The more I read your blog, the more curious I become as to why you dismissed my comments about P-delta.

For all compression members with eccentric loads or externally produced bending moments, the bending moment is magnified as the axial force approaches the ultimate compression capacity of the member. This is what is known as P-delta effects.

Why does this not apply to a girder in compression? BTW, wouldn't the P-delta effect actually strengthen NIST's findings? Pun not intended.

Damn, I wish ATS had spell check.

[edit on 9/6/2008 by Griff]

[edit on 9/6/2008 by Griff]

[edit on 9/6/2008 by Griff]

[edit on 9/6/2008 by Griff]

There actually needs to be a bit of deflection for p-delta to have an affect. For the condition that beams framing into girders aren't going to expand much, and a good deal of that expansion will be removed from allowable movement in the bolt holes themselves (short slotted or normal). In the vertical deflection, 80% of the dead-load is cambered out of the beam by design, so there's only going to be what, maybe 10psf DL and 20psf LL? If we assume that design follows modern live load recommendations and the beam is designed for 1" absolute deflection (it may also be L/240, some designers differ on limit states of deflection), then there's perhaps 0.5" total deflection? P-delta may play a role, but it's going to be minor.

reply to post by Newtons.Bit


I see what you're saying. So, in your professional opinion, how do you see this happening? I'm not a fire engineer, nor do I design steel skyscrapers, so I will admit, connections aren't my forte. I always hated doing block shear calculations. Cheers.

How do I see what happening?

reply to post by Newtons.Bit


Whether it was tension, shear, block shear, compression of the connectons. In the Cardington tests, the connections failed in tension once the buckled beams/girders contracted. Do you think this could have played a roll in the connection's failure in WTC 7?

Originally posted by Griff
reply to post by Newtons.Bit

 

Whether it was tension, shear, block shear, compression of the connectons. In the Cardington tests, the connections failed in tension once the buckled beams/girders contracted. Do you think this could have played a roll in the connection's failure in WTC 7?

Check out the wtc NIST report. They do an analysis to see what the failure modes of all the connections are in the area. They even tabulated what the failure loads are.