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  1. econ41

    econ41 Active Member

    I've been following this debate but probably wont participate in any depth.

    However - since I seem to have been the first member to refer to the issues of underlying physics without concern for either Mick's creditable and innovative model OR whatever NIST may have said I think I should put the following comments on record:

    This statement by Tony is wrong as Mick correctly identifies in a couple of later posts.
    The base point of disagreement is that the two effects are NOT mutually exclusive. They co-exist and can be treated as separate factors in analysis THEN added. There is no benefit in me speculating as to how the error has arisen.

    This statement by Mick identifies the key issues of the true situation:
    Mick correctly identifies the key point when he says "...must be acting less like a rigid structure, and more like a chain" i.e. it is somewhere between the two and each "mode" contributes to the overall situation.

    The key factors are:
    1) There is a range of possibilities between the extremes of "fully rigid beam" and "fully flexible rope";
    2) Any one of those "grey area" scenarios involves a mix of rigid beam effects and flexible rope effects;
    3) The proportion contributed may vary depending on situational specifics.

    Whilst the two effects are NOT mutually exclusive as Tony suggests the interaction is not as simple as it may appear - and Tony's references to bending moment go partially to explaining the true situation.

    The following suggestion by Mick MAY help.
    ....as long as participants dont lose sight of the reality that both factors co-exist and that the pull in can be validly separated as if it was "the idealized loads from an idealized infinitely flexible rope".
     
  2. Tony Szamboti

    Tony Szamboti Active Member

    Mick, what you are doing so far looks good, although like you I am not sure grout can be used as a substitute for concrete as far as compressive strength.

    Don't forget you should also use something to reinforce the grout/concrete with something like 1/16" diameter steel/stainless steel wire to mimic the welded wire fabric in the concrete.
     
  3. Mick West

    Mick West Administrator Staff Member

    Yeah, I thought of that after the grout was setting. I've got some 1/2" mesh wire cloth.
     
  4. Oystein

    Oystein Active Member

    No reinforcing within the grout necessary at all - that's a wild goose chase and unseemly distraction as long as scaling issues have not been carefully addressed and evaluated.

    I get why Tony is keenly interested in this silliness, but hope Mick will see it for what it is.
     
  5. Tony Szamboti

    Tony Szamboti Active Member

    Jeffrey, while looking over the truss connection details in the NIST report yesterday I found that the floor trusses were actually welded to their seats at the perimeter and the 5/8" diameter bolts there were considered construction bolts which remained in place after welding. This would make the truss to perimeter spandrel connection even stronger than I said earlier.

    See NIST NCSTAR 1-3 section 4.2.4 (Connections, Bolts, and Welds) at the bottom.
     
    Last edited: Mar 10, 2018
  6. Tony Szamboti

    Tony Szamboti Active Member

    The concrete in floors in buildings is generally reinforced with rebar and welded wire fabric to prevent it from cracking underneath due to tensile forces from bending. What Mick is doing will bring his test a little closer to reality.

    I would agree that the relationship between span, depth of the trusses/beams, and concrete thickness have to be scaled properly and it is somewhat complicated due to the exponential factors involved with moment of inertia and deflection, as well as two different materials with different moduli of elasticity.
     
    Last edited: Mar 10, 2018
  7. Oystein

    Oystein Active Member

    You say this without having assessed and evaluated scaling issues.

    You could be easily wrong, and the exact opposite could be true: That the grout alone, if built into the model with random dimensions, is too STRONG in tension already in true scale with the original. In that case, adding wires or whatever as tensile reinforcement would remove the model even further from reality.

    And yet, without evaluating any of this, you make unfounded claims.
     
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  8. Oystein

    Oystein Active Member

    Also, I doubt that the rebar in the WTC floors plays much of a role in making the floor assemblies stiffer. The trusses are composite with the (only 4 inches thin!) lightweight (!) concrete by having the bends (knuckles) of the diagonal web bars poking into the slabs. The system will lose that composition by having those knuckels tear out - laterally or vertically. The rebar would do little to prevent this, I think. It's then down to friction between top chord and slab.
     
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  9. Mick West

    Mick West Administrator Staff Member

    I'm fully aware that making a scale model of the truss+slabs+connections that's 18" (1.33 feet) long is an impossible task. Even getting the linear dimensions of the components is difficult enough. For the 36 foot slabs Reducing by 1.33/36 is a 1/27 reduction, taking the concrete from 4" to 4/27", or about 4mm. The material itself does not scale, with the granule size being much larger. It's impossible that spalling or cracks can be anywhere near as localized as they would be on a larger model.

    And of course there's the issue of loading, 1/27 linear reduction means I need to load the "truss" with 27*27 = 729x its actual weight. The aluminum bar weighs 120g, so that's 87kg, a bit more than my weight. The bar cannot support that laying flat, but can support much more than that on its side.

    There's many more complications of scale, materials, and structural accuracy. The intent of the original experiment is simply to demonstrate that a sagging horizontal structure (or a structure with reduced bending resistance) pulls in.

    Unfortunate when I went to check on the grout this morning the test blob I left on the side was still pliable. Reading the instructions it says it takes 7 days to fully cure. So I'll set that aside for a while.
     
  10. Oystein

    Oystein Active Member

    It's good to stick with that and not get lost in irrelevant details, and false claims about such.

    For starters: Any solid, strictly horizontal structural member (truss, beam, anything), no matter how supported, will sag under its own weight. Perhaps by an imperceptibly little amount, but sag it will.

    I you either increase load, or decrease capacity (i.e. by heating), it will sag more.
    This changes the geometry of the assembly: If the length of the horizontal element stays the same, but it sags through deeper (cuvrves more), then its ends will (try to) move closer to each other. This results in a pull-in force.

    Tony's claim that such a pull-in force does not occur as long as the member can resist moments is patently FALSE, and he needs to retract that claim, if he wants to be taken seriously. No adding of intricate details to the model can undo the falseness of his claim. Let's not get confused by bad engineering thinking!

    Perhaps he imagines that making a truss composite with a 4-inch lightweight concrete slab will make a floor infinitely stiff no matter how hot fires get underneath - but that would of course be a dangerously unsound imagination.

    (That pull-in force may of course be counter-acted by other effects, e.g. by expansion if it is heated. But the mere fact that a horizontal member sags adds a pull-in force.)
     
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  11. Mick West

    Mick West Administrator Staff Member

    Here's an interesting illustration of this:
    Metabunk 2018-03-10 10-25-26.
    That's a bungee cord that I soaked in water and froze overnight. It is by no means rigid, but it has more moment resistance than an unfrozen cord.

    The setup just after initial placement.
    Metabunk 2018-03-10 10-48-53.

    Result:


    Source: https://www.youtube.com/watch?v=ikZlvl_vAVY


    The cord sags as it defrosts. The right columns is pulled in (left column is fixed).

    The important point here is that this is a small change in the moment resisting capacity of the cord. It's not going from stiff to flexible. It's going from somewhat flexible to slightly more flexible. It had a small moment resisting capacity, which was then reduced slightly. This small change was sufficient to increase the pull-in force at the end connections and move the 4x4 column measurably.
     
    Last edited: Mar 10, 2018
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  12. qed

    qed Senior Member

  13. Mick West

    Mick West Administrator Staff Member

    Ask Tony. I don't think he's made a specific prediction. He's aware there's too many variable to predict what will happen. The experiment with grout bonded to aluminum is not going to prove anything, it an illustration at best, an a possibly misleading exploratory test at worse.
     
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  14. qed

    qed Senior Member

    @Tony Szamboti
    • @Tony Szamboti What difference will we see with the addition of the concrete layer?
     
  15. Tony Szamboti

    Tony Szamboti Active Member

    To those like Oystein,

    I want to make it clear that I certainly understand that any horizontal member will sag under its own weight and load. The point is that usually in structures it is imperceptible and the amount of tensile load from a horizontal member on a vertical support is negligible when the horizontal member retains stiffness in the vertical direction, which is a function of moment or bending resistance. Most of the load is transferred to the supports and handled in shear on the mounting brackets attaching the horizontal member to the vertical and then in compression by the vertical member.

    The ability to take moments caused by a vertical load affects how much tensile force will develop in the horizontal direction in a significant way, as Mick showed above with the frozen bungee cord. The concrete will allow the composite floor to retain some moment resistance even when heated and thus will affect how much tensile force is developed by the composite floors. It will be different from the trusses themselves and I don't think anyone can simply say it would be negligible in a credible way.

    The amount of concrete involved in the composite floor situation is also more than just that on top of the truss chords. The composite floor is essentially a "tee" shape. Structural engineers generally use a value for width of the slab involved to be the width of the truss + 16 x the slab thickness. In the case of the Towers the width of the slab used for calculating its contribution to moment of inertia would be 4 x 2 + 2 x 1.09 + 16 x 4 = 74.18 inches, with the first two terms multiplied being the calculation for width of the double truss which was 10.18 inches.

    Additionally, everyone should be asking why the NIST FEA program did not realize the tensile force that would have been necessary to pull the columns inward.
     
    Last edited: Mar 10, 2018
  16. qed

    qed Senior Member

    @Tony Szamboti
    You state that the addition of your concrete layer "should take the compressive resistance away from the metal enough."
    • In this simulation, will the addition of the concrete layer you propose prevent the aluminium bar from significantly sagging when the composite concrete/aluminium bar is heated?
    Yes or no? In your expert opinion.
     
    Last edited: Mar 10, 2018
  17. Ray Von Geezer

    Ray Von Geezer Senior Member

    As another civvy, I think Tony is referring to how the concrete and steel components effectively prevent each other from sagging by the use of components like shear studs attached to the beam and embedded in the concrete.

    I could be wrong, but if so then I think it’s going far beyond the intention of Mick’s model (to show simply how the sagging/pull in occurs) into modelling things like when did the studs fail due to thermal expansion, how many failed and which ones - that would be a whole different level of complexity.

    It seems to me that once the studs do start to fail then the contribution of the slabs to preventing sagging is lost and they’re simply supported weight which increase it. Is there any point modelling the post-failure sagging by adding the cement/concrete/grout? Intuitively if the “beam” sags under its own weight it’ll sag more when supporting extra, but again I could be wrong about that too.

    Ray Von
     
  18. qed

    qed Senior Member

    Yes. Since this was suggested by @Tony Szamboti.

    What Mick has here (I think) is a "minimal viable product" which can be honed to cater for objections, etc. For example adding shear studs from the aluminium bar into the concrete.
     
    Last edited: Mar 11, 2018
  19. Ray Von Geezer

    Ray Von Geezer Senior Member

    Tony, I think, wants Mick to model whether the floors (beams & slabs) would sag, so far as I can see the purpose of this demonstration was to show what would happen when they did, and to this layman it meets that purpose.

    Mick already pointed out several issues of scale, differences in materials and such and how the demonstration as envisaged by Tony is unlikely to be of much use. The layer of “concrete” seems particularly odd - weren’t the beams supporting slabs much wider than the beams themselves? Mick’s use of aluminium makes sense in the context of the original experiment, but for Tony’s suggestion it’s just another difference between the model and reality.

    I guess my pont is; if this is Tony’s suggestion, and Mick is happy to accommodate it, then shouldn’t Tony define some parameters and predict outcomes to show why it’s worthwhile?

    To me it just looks like make-work.

    Ray Von
     
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  20. Oystein

    Oystein Active Member

    But earlier you had claimed the opposite:
    That was clearly, unequivocally FALSE, and you should come out admitting that you made an utterly FALSE claim tight there, if credibility is important to you.

    Well yes, "usually".
    You are kinda forgetting the unusual circumstances like planes crashing into the towers, fire protection getting stripped from the trusses, and huge, unfought, multi-story fires raging through those floors.

    Obviously, trusses did not retain stiffness and sagged when subjected to intense fires. Sagged much more than they did while cold.

    That, obviously, created a catenary force, which you previously denied.

    And I don't think anyone can simply say it would be negligible in a credible way.

    10.18 inches.
    Can you explain what relevance that number with 4 significant digits has towards the topic of this thread? What that the hell are you talking about??
     
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  21. cloudspotter

    cloudspotter Senior Member

    Would something like this do the job?

    2678.

    https://www.roofingsuperstore.co.uk/product/roofers-single-furnace.html


    (also useful for roofing jobs and clearing weeds)
     
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  22. econ41

    econ41 Active Member

    Ray Von Geezer raises a couple of significant issues as shown by parts of his two comments:
    I have always admired Mick's efforts at physical modelling and have commended them and linked to them on other forums on numerous appropriate occasions.

    BUT as the record of recent years shows I have regularly questioned the purpose of the modelling and the implicit target which I will loosely describe as "helping lay persons understand the physics". My concerns usually about extending the modelling into areas more complex than the layperson needs or can benefit from. And where the modelling becomes less appropriate. I think that Ray Von Geezer is correct when he suggests that the "inform the lay person" purpose has been met - at least in Ray's case:
    Tony may have a different objective BUT Ray's lay person need is met.

    Now there have been two strands of discussion in the thread:
    1) The OP topic involving modelling; AND
    2) The relevant issues of underlying physics.
    ...and I am conscious of the preference of this forum to stay strictly with one topic. So I will make a brief comment on each of the two topics and leave it for guidance as to whether the second should be pursued here in this thread.

    Topic #1 As far as the modelling is concerned the modelling process and discussion is heading down the path of more rigorous modelling of detail. The same path was followed a year or more back with the several threads discusing modelling of the "progression stage". My question at that time - the same point I make here - has the modelling gone far enough for the likely laypersons who would benefit? Because pursuing details is not likely to increase benefit to such lay persons.

    Topic #2 - The underlying physics. Tony Szamboti address some issues "To those like Oystein,...."

    That post and others preceding identify many of the key aspects of physics but there are two issues of principle which Tony relies on which I would dispute. They are:
    (a) The first - which Oystein has already addressed - is the false reliance on "ordinary" conditions and their misapplication to the physics of the WTC real situation and of Mick's modelling - neither of which are "ordinary". I agree Tony's analogous reliance on "ordinary" is wrong - my reasons more tightly focussed than Oystein's but no need to go into details here.

    (b) The second is the actual physics involving a balance of "beam" and "catenary" actions. Tony correctly identifies in this recent post the relevant factors but does not state how they "share the load". In fact in earlier posts he has stated both explicitly and by implication that the two are mutually exclusive. That is not so. Both are present concurrently.

    The "missing bit of physics" as far as I can see is an explanation of how the loads share between residual "stiffness" as a beam allowing the "normal" style of force distributions and the transfer of part loads to the catenary mechanism with resulting "pull-in" forces.

    What would happen can be explained - initially in broad outline - and sort of "first order" like this:

    Assume for simplicity the beam has a single central point load "2W" - therefore vertical reaction at each end = "W"
    As the temperature of the beam rises it weakens and starts to sag. The weakened beam cannot support 2W - with W at each end. So "Wba" (Residual load carried by beam action) is less than W and the lost portion is taken up by "Wca" load due catenary action. Where somewhat obviously Wba + Wca = W - we have the same vertical load irrespective of which mechanism carries what part of it.

    And this is where some of the complexities alluded to by Tony start to complicate the analysis. (Don't "prevent" - just "complicate" :rolleyes:) The "inwards pull" results simply from the vectors of the downwards angle of the tension pull in the sagging beam. Which - if we refer back to what got us to this point - is why such high "pull in forces" can be developed - and which will probably show up as Mick starts measuring. BUT it is not simply the angle of the sag in this "heat weakened" beam scenario. It would be if it was only a rope. Or Mick's chain with all the ice melted.
    But let's leave this preliminary explanation there - see if there is any interest and whether it should be a separate thread.
     
    Last edited: Mar 12, 2018
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  23. James W Wing

    James W Wing New Member

    Yes, I guess so. Maybe I missed the point of your experiment, but it seems like the heating of the beam from the top would cause the beam to extend slightly on the top and actually bend upwards as shown in B of my diagram, assuming that the beams are pinned top bottom or both, the lengthening of the material would cause a momentary outward motion. Your experiment did not pin them so there is definitely an outward motion.

    As the beam heats up the elastic properties become such that it no longer holds its shape and the beam will sag due to weight, if heated from the bottom (C) the sag will occur sooner, but whether from top or bottom the heating will cause a momentary outward motion of an inch or two on both sides until the beam begins to fail.

    As the beam begins to fail further outward push will occur until the chord of the sagging beam becomes less than that of the nominal beam length and the whole thing will begin to pull the walls inward until the pins break and the whole floor crashes to the floor below. It will hit in the center, overloading the beams again and breaking the pins and the two will fall together to the next floor bam bam bam bam - all the way to the point where a floor has enough underpinning to withstand the weight of the onslaught. Heated beam.
     
  24. Mick West

    Mick West Administrator Staff Member

    The point is simply to demonstrate that a sagging floor pulls the sides in.

    Thermal expansion of the beam breaking connections is another issue. NIST did discuss this as I remember, but it was much more of an issue in WTC7.
     
  25. James W Wing

    James W Wing New Member

    Sorry, I was only trying to explain why the momentary outward movement of the vertical poles occurred in your earlier experiment, not that they might have broken some bolts. BTW I doubt that it could have - not purely thermal expansion, maybe along with the crash or explosions, but who knows?

    There is no doubt in my mind that the walls will come inward if the beams are connected to them. I am at a loss as to how your myriads of experiments show that they will or won't - the closest is that of the melting ice on the chain - that should have made it perfectly clear. You must be having way too much fun! :)

    I might have missed something though, was the question also posing that a different scenario than that of falling inward might have occurred - if so, what and how were they suggesting that it might have happened? For a few milliseconds I thought it was strange that it was all maintained within the confines of the building walls, but then it was clear to me - how else could it fall - sideways? That could happen only if the underpinnings were removed - so all of this discussion appears to be quite meaningless unless it is for the pure scientific aspects of understanding what happened. In other words - it is debunked already, isn't it? You have proved your point as far as I can see - who is disputing it further?
     
  26. Mick West

    Mick West Administrator Staff Member

    Cascading floors is off-topic. Feel free to discuss elsewhere.
     
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  27. Seymour

    Seymour New Member


    For what it’s worth NIST predicted thermal expansion of the trusses that would push on the columns, followed by sagging when heated further that results in pulling.

    If Mick’s thing shows that, then it’s confirmation
     
  28. Mick West

    Mick West Administrator Staff Member

    I'm not sure about "confirmation" however if you look back at this post:
    There is distinct thermal expansion. It's not very significant compared to the sagging though.
     
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  29. econ41

    econ41 Active Member

    Confirmation of what? That NIST was right? The physics is straight forward and independent of whether NIST was right. (And NIST was right...)

    A beam (joist if you prefer but the terminology is not important) subjected to heating will expand along its length while still maintaining "rigidity" (stiffness if we want to be more pedantic).

    THEN - if it gets hot enough - it will lose stiffness and start to sag. It will STILL continue to expand in length but that aspect is swamped by the other factors which are:
    1) The "beam" will lose beam strength and start to sag because it can no longer carry all the applied load. AND
    2) The "sagging" will put the "length" into a curve which means the horizontal vector of the now curved length will start to reduce; AND
    3) Because it sags catenary action will start to pull inwards.

    (And - subject of my previous comment and some errors in earlier posts - the catenary action applies concurrently with the now reduced strength of "beam" action - the heated beam can no longer carry the full load so catenary action takes up the shortfall.)

    And the inwards pull from catenary will be more that the outwards push from linear expansion. Simple geometry of the relevant vectors.

    So there are two topics implicit in your contribution to this discussion - (1) Was NIST right >> yes and (2) is the physics understood >> yes .. and the physics of the situation is straightforward AND independent of whether NIST was right or wrong.
     
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  30. econ41

    econ41 Active Member

    ...it certainly DEMONSTRATES - and I suggest it "confirms" the physics for those who need such confirmation.

    Yes. And "not very significant" is guaranteed by the geometry of the vectors.
     
    Last edited: Mar 27, 2018
  31. Hierophant

    Hierophant Member

    Could you also do an experiment that shows the behavior or 2 attached steel beams instead of one? I'm asking because the floors were held up by a sort of girder that basically looks like 2 steel beams connected by diagonal beams.

    If the lower horizontal beam is heated more than the upper one, thermal expansion will cause the 2 beam system to bend, because one becomes longer than the other. A bit like a bi-metal thermometer/thermostat, except that its the same kind of metal with one having a higher temperature (and therefor higher length).

    Even if the lower beam expands, it will make the (non expanding) upper beam bend, which will make it effectively pull in. It's and additional effect to the one of bending by weakening that you showed. Bending by differential heating/expansion. I'm curious how that would look, even though it's probably harder to reproduce.
     
  32. Mick West

    Mick West Administrator Staff Member

    A truss.
    [​IMG]

    I'm not sure if the differential expansion of the lower part (the lower chord) would do anything, as it's not attached to the inner columns.
     
  33. econ41

    econ41 Active Member

    It would affect the overall performance/effect of the joist as a sub system Mick. The "fight" between bottom chord and top chord defines how the joist overall acts in the bigger picture. Let me try to put it in context.....it is a "systems" v "sub-systems" issue.

    I read Hierophant's post and thought of how I could meaningfully respond. It raises some potentially complex issues which may hinder rather than help understanding. Let's see if we can explore the issues at a simple level....."(first??:rolleyes:).,

    The simplest way I can address it - which may not communicate well - is that is is an issue of "systems level".

    The top and bottom chords of the floor joist truss are within the system of "floor joist". The issue if we think of the whole building structure is "what effect do the mechanism of floor joists" have on the overall structure.

    The issue as raised by Hierophant is WITHIN the sub-system of "joist". It affects how the joist overall interacts with the"bigger picture" of the whole structure.

    So think about what a "joist" does within the whole system of the building. It - the "joist" including its competing bottom and top chords - initially expands due to thermal expansion thereby pushing the perimeter columns outwards. (Yes - and pushing the core columns inwards but the perimeter ones would lose the "tug of war" AKA "push of war". So initial "push out" of the perimeter would be the effect.) (I won't derail to explore why that is so.....)

    Now in the context of Hierophant's questions that "macro" effect is the result of the action of the whole truss.....which is the NET result of the "top chord" fighting the "bottom chord" conflict he identifies. The OVERALL result in the early stages is "Joist pushes OUT".

    Later - as the whole joist gets hotter - it - the whole joist - starts to sag and the force on the perimeter becomes "pull in" as per all the previous discussion and explanations. Once again it is NET pull in - independent of the "state" of the battle between the top and bottom chords.

    Let's see if we can discuss it further - or even if we need to go further.
     
  34. Hierophant

    Hierophant Member

    What I was trying to say was that the expansion of the lower part would force the truss (sorry for using the wrong words) to bend.

    The bending is what will make the truss pull inwards. The weakening through fire combined with the weight of the truss would make the truss bend (thereby pulling inwards), which is what you showed in your experiment.

    But differential expansion of upper and lower parts of the truss alone would also force it to bend, even without weight and weakening. Or, if the diagonal rod at the end of the truss is able to give way in some way when pushed (its intended to pull not push), the lower part of truss would not fulfill its function of supporting the upper part anymore.

    (sorry not a native speaker)
     
    Last edited: May 18, 2018
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  35. econ41

    econ41 Active Member

    << Yes.

    << Yes. the key word is "also" - two different causes but the same end result.... the truss bends causing inwards pulling.
     
  36. Mick West

    Mick West Administrator Staff Member

    That was badly put on my part. Of course it will do something - I was thinking more along the lines of something that would also show up in a model. I don't think it's really plausible to model the effects of the geometry of the trusses on a very small scale.
     
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  37. econ41

    econ41 Active Member

    ** Understood - my interest was in getting the "parts of system" versus "whole system" point posted to see if it would help Hierophant. And taking a risk of using "system language" tho his later post shows he "gets" the key points. Realised them for himself without my risky higher level conceptual explanation.

    *** Understood that - as we know you and I tend to take partially different approaches to the use of models. Differences aside I doubt it would be possible to model that aspect in a meaningful way - easy to demonstrate the "sub-system" effects BUT not in a model of a more extended part of the building.

    **** Probably true. And the main point is independent of the "truss" setup with discrete upper and lower chords spaced by diagonals. The issue of temperature differential between top and bottom would apply to a rolled section "I" beam, a boxed fabricated beam - even a solid bar. The macro effect on the "joist" would behave analogously. Initial linear expansion pushing the ends outwards followed by inwards pull from sagging as the beam became heat softened.