The NIST report, start to finish

Originally posted by Azp420
Because they cancel each other out.

When tension in the diagonal connected to the columns remain about the same, there will be a net pull in force.

If you want to have compression in the middle and tension at the ends like you described above, the bottom chord cannot just take over the internal forces which have been reduced in the top chord. The internal forces in the bottom chord would decrease also. The overall reduced bending capacity would have to be carried by catenary action in the top chord but at the deflections estimated is not possible.

A tensile axial load applied to the top chord would have the effect of also reducing the amount of tension in the bottom chord.

I am not very sure about this, it does sound a bit counterintuitive that additional tension on the truss results in a lower tension in the lower chord. But if we assume its like that, why isn't that possible when the forces come from catenary action?

To summarize, I am not convinced that there can not be both forces from catenary action and bending at the same time, resulting in a net pull in force. I don't see why that is impossible. We both can't really prove it either way (at least not without spending some significant time on it), so that makes it, at least for me, a matter of who is more creditable. The people who wrote that paper or you. And then it is and easy choice. Those people have written several papers on that specific subject over the years, making them experts on the subject. I don't believe they have been wrong all that time, I think the problem lies with your interpretation.

Anyway, the only way I can be convinced is by showing actual prove. But since you made it clear you are not ambitioning to prove the theory is wrong, I think we can close the subject. If you want I can email the authors containing your case (although I still think it should be you sending that mail as it is your case, I am not sure why you don't want to do that) and I can post the reply here.

reply to post by -PLB-

I am not very sure about this, it does sound a bit counterintuitive that additional tension on the truss results in a lower tension in the lower chord.

I suppose it is counter intuitive, which is why certain imaginings regarding truss behavior must be treated with caution.

The reason this happens is because all the internal forces in the bottom chord get there by being transferred via the top chord into the diagonals. All adding tension to the top chord does is cancel out some of the compression it is carrying. This sends less compressive force to the compressive diagonals and less tensile force to the tensile diagonals. The diagonals then pull the bottom chord apart by a smaller amount, resulting in smaller tensile forces in the bottom chord.

But if we assume its like that, why isn't that possible when the forces come from catenary action?

What I described above also applies to any top chord tension resulting from catenary action. Catenary action reduces the tension in the bottom chord and overall bending capacity of the truss. This is why the paper describes the top chord taking the majority of load.

To summarize, I am not convinced that there can not be both forces from catenary action and bending at the same time, resulting in a net pull in force. I don't see why that is impossible.

If you believe what I've described above to be true, then I've shown why the claimed amount of pull in at the deflection claimed is not possible.

We both can't really prove it either way (at least not without spending some significant time on it), so that makes it, at least for me, a matter of who is more creditable.

I already proved it, and if you want to get clarification or the actual internal forces from your "credible" source I'd be happy to prove it again.

The people who wrote that paper or you. And then it is and easy choice. Those people have written several papers on that specific subject over the years, making them experts on the subject. I don't believe they have been wrong all that time, I think the problem lies with your interpretation.

I can see where you're coming from. Why would you trust some guy from the interwebs with 420 in his username over a couple of guys who belong to a real life university? Appeal to authority is still a logical fallacy however.

Also, when have I ever made a technical claim in this thread I couldn't back up?

And I have years of experience interpreting technical documents (although I'm only offering my anonymous word on that for proof). I'm confident in my interpretation. Is there anyone else from either side of the debate with an opinion on the interpretation?

If you want I can email the authors containing your case (although I still think it should be you sending that mail as it is your case, I am not sure why you don't want to do that) and I can post the reply here.

I don't wish for these authors to have my email address. Just asking for the internal forces at the point of max pull in would be sufficient. If they have no rebuttal to my case don't expect a reply.

Originally posted by Azp420
I suppose it is counter intuitive, which is why certain imaginings regarding truss behavior must be treated with caution.

Agreed, which is why I try to avoid to base my position on intuition.

The reason this happens is because all the internal forces in the bottom chord get there by being transferred via the top chord into the diagonals. All adding tension to the top chord does is cancel out some of the compression it is carrying. This sends less compressive force to the compressive diagonals and less tensile force to the tensile diagonals. The diagonals then pull the bottom chord apart by a smaller amount, resulting in smaller tensile forces in the bottom chord.

But isn't part of the tensile forces transfered to the lower chord directly by the diagonal attached to the column, without even touching the upper chord? It seems to me the forces in this diagonal do not change drastically, and it will still carry the majority of the vertical load. Meaning the tension as result of bending forces in the upper chord is minimal.

What I described above also applies to any top chord tension resulting from catenary action. Catenary action reduces the tension in the bottom chord and overall bending capacity of the truss. This is why the paper describes the top chord taking the majority of load.

I just don't see why this can't be happening gradually. If I get you correctly you are saying there is no pull in force as result of catenary action until a certain threshold or moment. If so, what would this threshold or moment be exactly? Failure of the diagonals?

If you believe what I've described above to be true, then I've shown why the claimed amount of pull in at the deflection claimed is not possible.

I already proved it, and if you want to get clarification or the actual internal forces from your "credible" source I'd be happy to prove it again.

I think a diagram of it would help a lot in convincing me.

I can see where you're coming from. Why would you trust some guy from the interwebs with 420 in his username over a couple of guys who belong to a real life university? Appeal to authority is still a logical fallacy however.

Appeal to authority is not a logical fallacy. Not in the sense I am using it at least.

arguments from authority are an important part of informal logic. Since we cannot have expert knowledge of many subjects, we often rely on the judgments of those who do. There is no fallacy involved in simply arguing that the assertion made by an authority is true. The fallacy only arises when it is claimed or implied that the authority is infallible in principle and can hence be exempted from criticism.

Also, when have I ever made a technical claim in this thread I couldn't back up?

I haven't really seen proof that the chord must carry all load before any pull in can happen.

And I have years of experience interpreting technical documents (although I'm only offering my anonymous word on that for proof). I'm confident in my interpretation. Is there anyone else from either side of the debate with an opinion on the interpretation?

Argument from autoriry . I don't have years of experience. In fact, I have none, structural engineering isn't my area.

I don't wish for these authors to have my email address. Just asking for the internal forces at the point of max pull in would be sufficient. If they have no rebuttal to my case don't expect a reply.

It seems to me that would require them to do a lot of work so I don't expect them to give that. But I will send a mail if nobody has anything to add.

reply to post by -PLB-

But isn't part of the tensile forces transfered to the lower chord directly by the diagonal attached to the column, without even touching the upper chord?

I should have specified that I was talking about a uniform tensile force applied to the upper chord (as would be the case under catenary action). If you apply a tensile force at the end(s) of the truss only (which is what it sounds like you are describing), some of the force would indeed be transferred straight into the bottom chord increasing its tension. However, the change in tension is very, very small compared with the change in compression in the upper chord (pretty much negligible).

It seems to me the forces in this diagonal do not change drastically, and it will still carry the majority of the vertical load.

At no point do they carry the majority of load. The top and bottom chords are always carrying the majority of load.

Meaning the tension as result of bending forces in the upper chord is minimal.

Tension as a result of bending forces in the upper chord remains zero at all times, rather, compressive forces are induced.

I just don't see why this can't be happening gradually. If I get you correctly you are saying there is no pull in force as result of catenary action until a certain threshold or moment. If so, what would this threshold or moment be exactly? Failure of the diagonals?

I haven't been making many claims with regard to this, because without a model it is hard to say for sure (and be able to prove it). All I am claiming is that the claimed pull in amount is not possible under the specified deflection and loading mechanism.

I haven't really seen proof that the chord must carry all load before any pull in can happen.

I don't recall making that claim. The reason my calculations have been reflecting the top chord taking the majority of load is because that is the claim made by the report.

It seems to me that would require them to do a lot of work so I don't expect them to give that.

If their model is worth the paper their report is written on a diagram showing the internal forces (similar to fig. 4) should only be a mouse click away.

Originally posted by Azp420
I should have specified that I was talking about a uniform tensile force applied to the upper chord (as would be the case under catenary action). If you apply a tensile force at the end(s) of the truss only (which is what it sounds like you are describing), some of the force would indeed be transferred straight into the bottom chord increasing its tension. However, the change in tension is very, very small compared with the change in compression in the upper chord (pretty much negligible).

Indeed. So the net horizontal force at the columns connection is a pull in force.

At no point do they carry the majority of load. The top and bottom chords are always carrying the majority of load.

I don't get this. Figure 4 shows the vertical force in this diagonal to be 6.4P and the vertical force on the column to be 7.9P. Seems to me this vertical diagonal is carrying the majority of the vertical load? (note that I am not talking about bending forces).

Tension as a result of bending forces in the upper chord remains zero at all times, rather, compressive forces are induced.

I don't get this also. It seems to me that the members also suffer from local bending forces. Hasn't been your argument that the tension in the upper chord as result of bending becomes too large?

I haven't been making many claims with regard to this, because without a model it is hard to say for sure (and be able to prove it). All I am claiming is that the claimed pull in amount is not possible under the specified deflection and loading mechanism.

Yes, and that was my point, we can't prove it either way without making a model. Its basically your intuition versus detailed models and simulations from people who published several papers on the subject.

I don't recall making that claim. The reason my calculations have been reflecting the top chord taking the majority of load is because that is the claim made by the report.

In that case, how do you come to the conclusion there can not be any significant pull in force from catenary?

If their model is worth the paper their report is written on a diagram showing the internal forces (similar to fig. 4) should only be a mouse click away.

I don't see why. I don't know what kind of visual representation their software offers. It also seems to me they will have to run the simulations again.

reply to post by -PLB-

Indeed. So the net horizontal force at the columns connection is a pull in force.

Catenary action induces tension across the entire top chord. To achieve pull in force the bottom chord would only be taking a very small amount of load.

I don't get this. Figure 4 shows the vertical force in this diagonal to be 6.4P and the vertical force on the column to be 7.9P. Seems to me this vertical diagonal is carrying the majority of the vertical load? (note that I am not talking about bending forces).

I was referring to the typical diagonal members. This long first diagonal outside of the bottom chord you are referring to acts more like a bottom chord and transfers load from the bottom chord to the support (as opposed to the loading description of the typical diagonals I gave in my previous post). Therefore as the bottom chord loads decrease as a result of tension in the top chord, the load in this member also decreases.

I don't get this also. It seems to me that the members also suffer from local bending forces. Hasn't been your argument that the tension in the upper chord as result of bending becomes too large?

No. No bending forces of any significance are induced in any of the members because the loading is applied where the diagonals meet the top chord only.

Yes, and that was my point, we can't prove it either way without making a model. Its basically your intuition versus detailed models and simulations from people who published several papers on the subject.

You are reading too much into my response. I'm not going to commit to a great deal of time and effort to: at what point, if any, pull in forces as a result of catenary action are able to be induced. The reason for this is because it's besides the point. The point is that under the claimed deflections catenary action inducing a pull in force enough to collapse the entire structure is not possible. To me that is the only thing that needs proving or disproving with regards to catenary action in the trusses.

In that case, how do you come to the conclusion there can not be any significant pull in force from catenary?

See my numerous calculations presented in previous posts.

I don't see why. I don't know what kind of visual representation their software offers. It also seems to me they will have to run the simulations again.

It seems to me they created fig. 4 using their software before putting it through a fire simulation.

If their software does not offer these results (I would be very surprised and almost horrified at the level of incompetency), then you should trust it even less, because the first thing they should have checked was that none of the internal forces were greater than the capacities of the corresponding members.

I would also highly doubt they did not save the results of the simulation...(again would be horrified with the level of incompetency)




Now that I am somewhat finished scrutinizing the University of Sheffield report, is anyone able to point me in the direction of where I can find NIST's conclusions and methods with regards to the truss pull in forces?

Originally posted by Azp420
Catenary action induces tension across the entire top chord. To achieve pull in force the bottom chord would only be taking a very small amount of load.

I was referring to the typical diagonal members. This long first diagonal outside of the bottom chord you are referring to acts more like a bottom chord and transfers load from the bottom chord to the support (as opposed to the loading description of the typical diagonals I gave in my previous post). Therefore as the bottom chord loads decrease as a result of tension in the top chord, the load in this member also decreases.

You are saying here tension in that diagonal will reduce, but won't that result a major issue:

No. No bending forces of any significance are induced in any of the members because the loading is applied where the diagonals meet the top chord only.

If the tension in the diagonal connected to the column reduces, the vertical load it puts on the column also reduces. This means that the top chord takes over this vertical load. How can the top chord both put a vertical load on the column, but not suffer from internal bending forces? That does not seem consistent.

It seems to me the tension in the diagonal connected to the column does not change much. And with the compression in the top chord decreasing, the result is a net pull in force.

You are reading too much into my response. I'm not going to commit to a great deal of time and effort to: at what point, if any, pull in forces as a result of catenary action are able to be induced. The reason for this is because it's besides the point. The point is that under the claimed deflections catenary action inducing a pull in force enough to collapse the entire structure is not possible. To me that is the only thing that needs proving or disproving with regards to catenary action in the trusses.

See my numerous calculations presented in previous posts.

But those calculations applied to the model in that paper and were to show it is wrong. It doesn't show why forces as result of catenary action are insignificant in general. I haven't seen you provide a good reason why you think that (or I have missed it).

It seems to me they created fig. 4 using their software before putting it through a fire simulation.

If their software does not offer these results (I would be very surprised and almost horrified at the level of incompetency), then you should trust it even less, because the first thing they should have checked was that none of the internal forces were greater than the capacities of the corresponding members.

I would also highly doubt they did not save the results of the simulation...(again would be horrified with the level of incompetency)

Let me put it like this: if I had done that research I would not put that kind of effort in a response to an email of this kind.

reply to post by -PLB-

If the tension in the diagonal connected to the column reduces, the vertical load it puts on the column also reduces. This means that the top chord takes over this vertical load. How can the top chord both put a vertical load on the column, but not suffer from internal bending forces? That does not seem consistent.

We are talking about catenary action developing. The vertical component of the decrease in tension in the diagonal will be picked up by the vertical component of the top chord in catenary action.

In pure catenary action there is no internal bending capacity.

It seems to me the tension in the diagonal connected to the column does not change much. And with the compression in the top chord decreasing, the result is a net pull in force.

Imaginings without proper analysis should be treated with caution.

But those calculations applied to the model in that paper and were to show it is wrong.

I thought that's what we were still discussing? Are you ditching their model as being feasible now?

It doesn't show why forces as result of catenary action are insignificant in general. I haven't seen you provide a good reason why you think that (or I have missed it).

I'm not about to provide an elaborate mathematical proof showing what the maximum pull in force this truss would be capable of generating given perfect conditions. The day to day role of most structural engineers is checking. Checking certain member specifications under certain loadings to see if they are sufficient (or over designed and over-costly). Show me a model or set of internal forces that you think are feasible and I will check if they are. Starting from scratch and trying to find the maximum pull in force possible is much too time consuming.

Let me put it like this: if I had done that research I would not put that kind of effort in a response to an email of this kind.

Which is another reason why I'm not enthusiastic about emailing them myself. I doubt they will appreciate the criticism from yet another truther. If they excluded the internal forces from the report IMO they must have done so for good reason so would not be willing to give them up to an email request. It's seriously only a couple of clicks away on any decent software.

Are you still going to clarify the interpretation? (ie was the bottom chord yielding which is said to have happened shortly after 11.7 minutes part of the simulation?)

Originally posted by Azp420
We are talking about catenary action developing. The vertical component of the decrease in tension in the diagonal will be picked up by the vertical component of the top chord in catenary action.

But since the angle of the top chord is a lot smaller, you still get a net pull in force. For example, when the vertical load of the diagonal decreases by 1P, it results in a decrease in horizontal pull in of 2.6P. On the other hand, when the vertical load of the top chord is increased by 1P, and it is sagged for for example by 5 degrees, you get a horizontal component of 11.4P. The net horizontal force is a pull in force of 8.8P.

I thought that's what we were still discussing? Are you ditching their model as being feasible now?

I think we more or less agreed that we interpret it differently, so there isn't that much more to say about it. That is why I moved to the more general question if significant pull in force from catenary is possible at all.

I'm not about to provide an elaborate mathematical proof showing what the maximum pull in force this truss would be capable of generating given perfect conditions. The day to day role of most structural engineers is checking. Checking certain member specifications under certain loadings to see if they are sufficient (or over designed and over-costly). Show me a model or set of internal forces that you think are feasible and I will check if they are. Starting from scratch and trying to find the maximum pull in force possible is much too time consuming.

You have been making the claim that no significant pull in force would occur. But it seems that this claim isn't based on anything substantial. Do you still stand by that claim? In any case, I see no reason to believe it.

Which is another reason why I'm not enthusiastic about emailing them myself. I doubt they will appreciate the criticism from yet another truther. If they excluded the internal forces from the report IMO they must have done so for good reason so would not be willing to give them up to an email request. It's seriously only a couple of clicks away on any decent software.

Are you still going to clarify the interpretation? (ie was the bottom chord yielding which is said to have happened shortly after 11.7 minutes part of the simulation?)

Well I am glad we can agree on that . As for asking for clarification, I am still planning to do so yes.

reply to post by -PLB-

But since the angle of the top chord is a lot smaller, you still get a net pull in force.

Yes. I was describing how the loads would be distributed under hypothetical catenary action, but did not say to what extent, if any, that was achievable.

You have been making the claim that no significant pull in force would occur. But it seems that this claim isn't based on anything substantial. Do you still stand by that claim? In any case, I see no reason to believe it.

No significant pull in is able to occur based on the loading mechanism described in the Sheffield report. I stand by that claim. If you would like to present the results of an alternative model or loading mechanism I would me more than happy to check if it is viable.

■WTC workstations would readily burn and produce temperatures significant enough to damage steel

I do not agree with this.

■These temperatures were maintained for a long enough period to affect the steel

I do not agree with this

■Steel with damaged insulation would heat up very quickly

I do not agree with this.

■Insulation damage was very likely

I agree with this.

reply to post by Azp420


So you have no major issue with the collapse initiation mechanism proposed by NIST in general, just with the way it is described in the Sheffield report?

There is another paper out there I know of, but I don't really feel like putting more time in this without there being any substantial objection to the collapse initiation mechanism in general. It may be an interesting discussion among structural engineers, but has not much to do with conspiracies.

reply to post by -PLB-

So you have no major issue with the collapse initiation mechanism proposed by NIST in general, just with the way it is described in the Sheffield report?

I've just spent the past little while searching for a detailed description by NIST as to how the pull in force was achieved, but was unable to find anything (I'm useless in that department). I don't suppose it would greatly differ from the Sheffield report claims, and so am likely to have a problem with it.

There is another paper out there I know of, but I don't really feel like putting more time in this without there being any substantial objection to the collapse initiation mechanism in general. It may be an interesting discussion among structural engineers, but has not much to do with conspiracies.

I highly doubt catenary action would be able to provide around an 85kN pull in force, so would like to see any modelling on this.

Originally posted by Azp420
I highly doubt catenary action would be able to provide around an 85kN pull in force, so would like to see any modelling on this.

I think the main problem here is that the exact forces will always remain guesswork. There are too many uncertainties involved. The 85kN is just a maximal value that was published based on one model. In reality, the sagging happened on several floors. On some floors the connections may have failed, leaving the columns unsupported. The exact weight on the floors is also a bit hard to determine when a plane crashed onto it. However, one fact remains, inward bowing was observed, and I have yet to hear any alternative explanation that could explain that. Even if sagging trusses are not responsible, there must have been something causing it.

reply to post by -PLB-

I think the main problem here is that the exact forces will always remain guesswork. There are too many uncertainties involved.

If they were modeled the models will produce exact forces. If the model is of high quality these forces will in the neighborhood of the actual forces.

If the forces were mere guesswork it speaks pretty loudly about the quality of the theory of collapse initiation.

The exact weight on the floors is also a bit hard to determine when a plane crashed onto it.

The models should have no problem with this. They know the mass of the plane and claim to know how it behaved upon impact.

However, one fact remains, inward bowing was observed, and I have yet to hear any alternative explanation that could explain that. Even if sagging trusses are not responsible, there must have been something causing it.

So far the only alternative explanation I've heard (not saying I subscribe to it) is that the appearance of bowing could have been caused by light refraction from the heat being expelled. This could be debunked by comparing similar photographs before and after bowing while the heat remains the same (I have not had the time to investigate this yet).

If the trusses are not responsible I would assume the bowing is caused by unrestrained lengths of columns or columns having to carry extra load due to neighboring columns being severed.

Originally posted by Azp420
If they were modeled the models will produce exact forces. If the model is of high quality these forces will in the neighborhood of the actual forces.

If the forces were mere guesswork it speaks pretty loudly about the quality of the theory of collapse initiation. [/

The models should have no problem with this. They know the mass of the plane and claim to know how it behaved upon impact.

They can model in the uncertainties, but an argument I often hear is that the NIST does not give the exact collapse conditions and it is too vague. While this may be true, without the exact actual data available it is the best anyone can ever do.

If the trusses are not responsible I would assume the bowing is caused by unrestrained lengths of columns or columns having to carry extra load due to neighboring columns being severed.

This was also investigated by NIST, and I think it was concluded that these effects alone were not enough to initiate collapse.

reply to post by esdad71

 

You have proven nothing. You are stating thoughts and not facts. How about a private debate on 9/11? Just me and you strawman...any subject you want...

OK I'll debate you on the collapse of the towers, LETS GO! Set it up. Or how about a debate on Newtonian physics? Maybe we could include materials science as well?

Hey esdad what happened? It's been a week. I excepted your challenge, and you disappear from the thread?

reply to post by ANOK


Since you excepted it we can't have one. If you would have accepted it we could...



Tell you what, if you would like, please send a note to a mod and set it up. I have tried this before and it never pans out. Maybe if someone from the other side suggests it somehow it may work out. How about we debate

"why was NIST chosen for the project and did they provide the results that were expected with the information provided to investigate what happened on 9/11?"

That would be a good sub argument from this thread....

Originally posted by esdad71

Tell you what, if you would like, please send a note to a mod and set it up. I have tried this before and it never pans out. Maybe if someone from the other side suggests it somehow it may work out.

You made the challenge, why do that if you can't set it up? An empty challenge you never really planned on carrying out in the first place.

If I debate you it's going to be on the possibility of the towers collapsing from your gravity and KE, no matter what you call it, and why you are so wrong in your understanding of Newtonian physics. I will be able to prove once and for all you are wrong when it comes to the physics of collapsing buildings. You won't be able to run and hide, or change the subject, hide behind the onslaught of other posters, post links and say 'read this' acting as a proxy for others work. I will never let you forget anything you say, it will always be handy for reference to your claims in other threads.

Set it up or back down esdad...

reply to post by ANOK


I meant I have sent it to mods before and nothing happens or I have been told it cannot be done. I made the challenge. If you want it, take it. If not, I could care less. No hiding, no running.



Set it up or back down???