I'm sure you have a rigourous NDE plan, including UT of all moment connections.  If you can't resolve the question regarding the change in pressure's impact on weld quality, perhaps you could increase the NDE frequency for a period of time and compare the results (as you reach higher levels).  If an increase in defect rate is noted, it would not take much to run additional test coupons at the highest possible level and have them destructively analyzed.  If you note defects that are attributable to the welding conditions (ie - clearly not welding outside established parameters, or poor technique by the welder), it would be worth digging deeper.

Another avenue to pursue would be to look into some of the FEMA standards.  I know they publish specifications related to seismic influences and how to mitigate their influence during construction.  Maybe they have something that addresses this.

If your search doesn't turn up any literature, maybe you should undertake the study as the construction progresses and publish your results.

My personal opinion is that the most significant influence will be on the shielding event.  I believe the reduced pressure's impact on the relatively violent shielding introduced by the SMAW process will be negligible.  Still, it would be interesting to have the same welder do identical test coupons at the most extreme height difference; and have them tested.

Charles.

If I may add a little perspective here. I know 520m or 600m or 700m seems high because its a building. But elevation wise, its nothing. Much of the North American continent (or Europe or Asia) is that high or higher. But I can certainly sympathize with the idea that perhaps standing on top of this monster (if you could even get me up there) would perhaps cause one to think.

Dave,

I got an answer to your question at a welding expo presentation about 10 years ago.

A College/NASA group were doing experiments in zero gravity vacuum welding.   They took a vacuum chamber and put it on a C130 and did eliptical dives to make Zero Gravity.

First they discoverd an arc requires plasma to transfer and that means an atmosphere of some sort.  So they actually milled out the center of the tungsten electrodes and ran a puff of inert through the hole to get the arc to strike.

They seemed happy with the results but the pictures of the welds were not all that impressive :)

Dave,

very first of all a heartily "Thanks" for asking this pretty interesting question.

Although Hugh has already replied and has clarified what kind of interesting welding application he has to inspect (really enviable), and also Lawrence has made a great comment I would like to (try) to answer you.

Honestly I have considered how to answer before I started writing since you have to know that I could read a great statement in the July's AWS Welding Journal. It has been a part of an advertisement where an appreciated welder said:

"... There's a thousand ways to do a job wrong, and only one to do it right."

And this - I guess - should also count for an explanation and in particular twice when it is planned as a contribution for this outstanding good forum!

Well, your question is more than eligible and to answer it directly and without any detouring:

I am nearly sure that you are right! Under consideration of my humble knowledge in physics I guess that in a complete vacuum - e.g. in aerospace - it should not be possible to generate a welding arc without having a medium which is ionisable just as a "shielding" gas. On the other hand it seems like a contradiction, since when I have no kind of "detrimental" constituents like existing within the surrounding atmosphere on earth (nitrogen, oxygen,...) one does not need a shielding to protect the weld pool against them.

But now, if you allow, as an addition please let me try to describe what the physical background of the Japanese experiments was or what their current considerations are, respectively.

Due to the Japanese surveys are based on Gas Tungsten Arc Welding I am going to deal now - in opposition to what we have spoken about in the Ukrainian experiments - with the GTA Welding under atmospheric pressures and - to compare - under low pressures (no complete vacuum as in space). This is what the Japanese researchers have conducted when they used the hollow Tungsten electrode for welding in a vacuum chamber and using very low gas flow rates. Without wanting to debase the work of the Japanese colleagues in any way, their investigations would have never been carried out when not many other researchers over a very long period in time would have conducted physical surveys on the nature of welding arcs. Among them were brilliantly US- (as Lawrence mentioned); Russian-; Australian-; German-; Japanese- and Researchers of many other countries in the world. As you can imagine, the quantitative (i.e. mathematical) details behind all the surveys are tremendous.

Therefore I request your understanding when I repeat myself, saying once again that I would like to avoid the usage of any intricate mathematical formalism. Strictly following the wise hint one of the greatest US-physicists - Richard Feynman - who should have said that one should firstly try to find a theoretical solution of a specific problem and the cohering mathematics are early enough to be formulated afterwards.

Although, what in particular Henry (ssbn727) and Jeff (js55) have discussed and posted about NEWTON and LEIBNIZ and the Greek philosophers was truly an enjoyment :-). Hurry up Stephan..!

Basically one perhaps should start with the predication that any arc is existing as a plasma which again is a kind of electrical conductor. This again means that I need anything which contains carriers for the electric charge and that again means that I need electrons which are a part of any atomic nucleus. When I have a - as you mentioned - complete vacuum - whereas I set the aerospace vacuum as the basis for being "complete" - I presume that within this "emptiness" are no - or better said - nearly no and thus definitely too less atoms, molecules or what kind of particles ever contained which could be used to being ionised.

But, by the way, here we have - from my personal point of view - the first surprise to be observed.

When we are going to weld under (earth) atmospheric conditions, the very first action in "igniting" a welding arc is the acceleration of a non neutral particle - just as a free electron - been generated by a stochastical reaction between e.g. the cosmic radiation with a gas atom or molecule in the atmosphere and thus being a result of this reaction. Presume you're using arc welding equipment and thus creating a weld circuit you're having an open circuit voltage existing between the anode (in GMAW normally the wire electrode) and the cathode (workpiece). When I am now feeding the wire electrode towards the workpiece to start the welding process, in between both there is also a - sure very less, but however - number of the mentioned non neutral particles. Due to approaching anode and cathode and decreasing the distance between them, the electrical field force is being increased. The lower the distance between anode and cathode, the higher the field force and the higher the probability to accelerate one (or more) of these non neutral particles - let's stay at the electron, which is a negative charge carrier - toward the electrical opposite pole. In this case toward the anode. On the other hand it is important that the distance between both electrical poles is not too low due to the electron must have a sufficient path length to be accelerated highly enough to receive enough kinetic energy for the collision with the gas atoms or molecules. The physicists talk about a specific cross section which was firstly surveyed and stated by a German physicist named CARL RAMSAUER. This cross section, which is a way for describing the interaction between an accelerated electron and the gas particles of the surrounding atmosphere is thus called the "RAMSAUER Cross Section" (in German: "Ramsauer-Querschnitt"). This is more or less important for the following explanations. But I would like to come to that a bit later on.

Once accelerated and on its way to the opposite direction the electron starts the first ionisation activities. This - so at least the physicists models and calculations - is the very first action in any kind of welding arc ignition! OK, this tiny bit of ionisation is surely much to less to initiate the charge carrier avalanche and thus ignite the arc finally, but however, here is it where everything begins. The second step, and this is the major mechanism in consumable electrode arc welding is by creating a short circuit between anode and cathode, to have a vaporisation of metallic material and this metallic vapour is lowering again the ionisation potential threshold value and thus improving the conductivity of the gap atmosphere between anode and cathode. The plasma is formed and the arc is being ignited. Oh, not to forget, in non consumable electrode arc welding (e.g. GTAW) the bridging of the "non conductivity" of the gaseous atmosphere between anode (normally the workpiece) and cathode (normally the tungsten electrode) is often accomplished by using "High Frequency". This is of course well-known but causes the same sequences (improving or initiating the ionisation conditions) as mentioned above.

Let's now presume furthermore that we are using - just comparable to the Japanese people who have used a hollow tungsten electrode in a vacuum chamber in their experiments - a conventional Direct Current Electrode Negative TIG-arc under atmospheric conditions. And let's also assume that the arc has been ignited under a "normal" shielding gas as Argon having a "normal" welding purity of 99.996%. I would like to neglect all the very specific details - from my point of view - being the true reasons, responsible for the "infinity" of welding-physics investigations just as tungsten electrode composition, base material composition (very important but however), different or changing welding parameters,.... Rather I would like to deal with the specific basics considered as being in charge for the outer shape of a TIG-arc burning under (earth) atmospheric pressure. Due to I won't like to overloading the response also neglecting the (interesting) background of energy-, current-, and thermal density distribution within such an arc I would like only to deal with the major aspect for making the TIG-arc - as we commonly know it - "bell-" or conical shaped. Specific investigations proved that the thermodynamical behaviour of a TIG-arc as mentioned, is being mainly influenced by the tungsten cathode area behaviour (mainly constant under constant cathode conical angle), it is strongly assumed that the relatively stiff bell shape is being created by a phenomenon called "cathode jet". Simplified this can be seen as a kind of "mass flow" being induced by arc current and its electromagnetical field. By varying e.g. the arc length one can influence the "inner" plasma properties as electron density or temperature, respectively on the anode surface and thus change e.g. fusion profile or penetration depth. Beside this - of course - the physical shielding gas properties have a strong influence on the outer shape of the arc, the shape does differ when using different shielding gases. Argon compared with Helium has a more bell shaped form due to its lower thermal conductivity which is again among other specific physical properties a function of its temperature. Therefore the arc has a strong and high energy conducting core and is loosing thermal energy in radial direction. This is, by the way, also the reason for having a strong kind of "finger"-penetration in using pure Argon e.g. in MIG-welding of aluminum and its alloys, or Argon rich shielding gases in welding steel materials.

This arc shape however is of course to be observed under "normal" atmospheric conditions, i.e. 1.013,25 hPa, or 101.325,00 Pa.

And now I would like to come to the conditions of reduced surrounding pressure. Since the atmospheric pressure has a strong influence on the density distribution of the shielding gas, the gas physical functional conditions like the effective "RAMSAUER Cross Section" and others being responsible for the ionisation behaviour, cathode jet velocity, arc stability,..., and whose values are being fixed normally under atmospheric pressure conditions but of course varying temperature ranges.

Presuming a definite surrounding pressure we can thus assume a definite gas particle density or concentration, respectively. Hereby we can assume to achieve a definite gaseous effective cross section wherein the electrons can be accelerated, interact with other particles and thus inducing and maintaining the ionisation. The lower the surrounding pressure, the higher the gas partial pressure and lower the density of (normally shielding-) gas particles. Hereby the effective cross section of electron to gas particle interaction is decreased and thus the probability to ionising the gas particles is reduced since - the probability of an electron to gas atom or molecule collision is reduced.  Hereby I assume one would no more achieve a "well" bell or conical shaped outer arc appearance (under presuming to use Argon as a shielding gas) but no more controllable and "conventional" known TIG-arc is possible to create.

What have now the Japanese people done?

They have created a hollow tungsten electrode (used as the cathode) which was - as far as I know - also been known earlier to use for experiments referring to welding in space, i.e. "complete" vacuum. But what they wanted to achieve was the practical usage of this technology as well for "earth" applications. As far as I could find out by the presentations they held on this topic, by using the hollow tungsten cathode in a vacuum chamber generating no "complete" vacuum but strongly reduced surrounding pressures (>= 3 Pascal) combined with very low gas flow rates (e.g. 0.17... 1.7 ml/s) they could create welding conditions, or let me better say "TIG-arcs" having had the appearance of a "beam". And also the electron density they could achieve - using the lowest gas flow rates of 17 tenths of a thousandths of a litre - and which is an indirect indicator for the energy transfer between the cathode and the anode and thus an indicator of the welding efficiency again, was proven to be as far as possible comparable with the appearance of an electron beam(!), please see also the attached "electron_density.pdf". As you know, when using an electron beam in a vacuum (i.e. reduced surrounding pressure) it is possible to achieve extremely high energy densities by achieving free electron path lengths of several meters. Hereby the probability for an electron - to gas atom or molecule collision is drastically reduced and the amounts of "lost" efficiency are stringently low.

The very interesting investigations - conducted by using different "shielding" gases (Argon - Helium - Neon - Xenon) and different anode materials (copper and aluminum) - could show that intricate - but controllable - physical coherences between the aspects as mentioned above (free electron path length,...), geometrical factors (inner/outer diameter of hollow cathode) combined with density and gas flow rate of the used gas can increase the penetration depth in a remarkable amount, please see also the attached "penetration_profile.pdf".

Therefore and concluding now, once again, Dave, you are certainly right when you suppose that without any kind of ionisable medium ("shielding" gas) one is unable to create a welding arc in a complete vacuum as it is observable in space. The Japanese people - and other enlightened researchers before them - however have found a way - from my very own and humble standpoint - to create a highly effective and stable arc welding process which could - normally - also being used in a "complete" vacuum, which has of course to be approved. But the Japanese do still stand - as far as I guess - with both feet on the earth and having ideas to use this "universal" process under "earth-conditions".

So far my humble try of an explanation..

My very best regards to you,
Stephan