Pier: Concrete or Steel?

[rally]

I'll add - in fear that I have provided fuel to the fire !

Wall thickness does increase the strength of a pipe for a given diameter but it is the increase in outside diameter that provides the greatest increase in strength/rigidity (or rather lack of flexure).

Vibration at the frequencies we can hear is almost completely irrelevant.
Its only very, very low frequencies that might cause a problem.

Yes, I know, I have read all the various posts and threads on astro forums over the years by would be engineers who say it matters or are just repeating what they heard.
It does not matter if the pier is otherwise rigid.

The real issue is the purpose of a pier in astronomy ?
In partiocular astrophotography because for visual astronomy these things are not so important.

The piers purpose is to provide a rigid support base that is mostly free of LOW frequency vibration that has a high enough amplitude to affect the image on a CCD (these are not audible sounds in the 1-5Khz range, they are very low frequencies usually well below 50Hz.

But more specifically and most importantly it is designed to ensure that there is the least amount of deflection when lateral loads of the order of a few Kgs are applied to the top of the pier/mount etc - either as a result of wind on the OTA, imbalance of the Mount and OTA as it moves throughout its range and to resist the opposing forces when fast slewing.
Being rigid means it transfers these forces into the ground where they will do the least harm and allow the system to settle very quickly.

Since we are asking the pier to resist deflecting by arc seconds or less - the pier has to be substantially stronger and more rigid than ordinary engineering would dictate on the normal basis of material limits and stengths for what are in effect quite trivial loads.

We want the pier to deflect less than the image scale of the system or maximum resolving capability of the system - preferably half or better.

Filling your pier with sand may make you feel happy that you have done something about it and it will change and subdue the resonant pitch of your pier at frequencies that have nothing to do with a pier deflecting arc seconds under a lateral mechanical load.

If you fill a hollow steel pier with concrete you will increase its strength (improve the modulus of elasticity) and that will help, but the preference would be going an extra couple of inches larger in diameter.
Loose sand doesnt add any strength !

I would start at 10.75" and look to 12" with 1/2" wall thickness or greater, but if you dont have a suitable mount (without backlash) and very solidly built and an optical system that is already rigid and without flexure all the way through - eg threaded adapters, solid focussers and rotators etc, solid attachment to mount etc then it may be a moot point what your pier construction is.

Calculating pier deflection (cylindrical beam) is a trivial engineering exercise for those who have studied it or wish to repeat it - the relevant function for calculating the moment of inertia OD (Outside diameter) in inches and ID in inches - pi = 3.142

i = pi * (OD^4-ID^4) / 64
deflection angle in radians is
(W * L^2) / (2 * E * I)

Where
L is the length of the pier in inches
W is the lateral load in lbs
i is the value from the first equation
E is the modulus of elasticity of steel = 3.00E+7 (Concrete without reo is approx 3.60E+4)

As you can see the function shows that it is proportional to the diameter to the fourth power ie (OD * OD * OD * OD) that means that a small increase in OD yields a large improvement.

Hope that helps

[mill (Martin)]

Ok what are we doing here i will ask.
Do we want a pier for professional astronomy with a 100" scope or are we just amateur astronomers making pictures for ourselfs?
The bottom line is that when we make a pier strong enough to hold a car, what more do we want?
And who is stupid enough to hit a pier with a hammer when imaging?
Whether we make pictures at a star party on a say EQ6 with the tripod or at home with a pier made out of steel or concrete, there is no difference.
And i am personally sick and tired of people saying a steel pier is better than a concrete pier and vice versa, they are both good.
PS: i wouldnt image in 20Km/H winds even when i would have a pier that can withstand winds of 100Km/H.
Just my two cents worth

[h0ughy (David)]

Originally Posted by mill

Ok what are we doing here i will ask.
Do we want a pier for professional astronomy with a 100" scope or are we just amateur astronomers making pictures for ourselfs?
The bottom line is that when we make a pier strong enough to hold a car, what more do we want?
And who is stupid enough to hit a pier with a hammer when imaging?
Whether we make pictures at a star party on a say EQ6 with the tripod or at home with a pier made out of steel or concrete, there is no difference.
And i am personally sick and tired of people saying a steel pier is better than a concrete pier and vice versa, they are both good.
PS: i wouldnt image in 20Km/H winds even when i would have a pier that can withstand winds of 100Km/H.
Just my two cents worth

OK i will step in here - at starparties when you are imaging and you get people walking around it does cause vibration into your mount, and sometimes it can be horrendous. The pier at home should be very sturdy and stable as possible - and separate as much as possible from walking traffic - even if it is you walking around - and i am no tooth fairy with wings so i need all the help possible. that's why i went for a very large mass pour of concrete - but instead of a steel pipe 8-12" diam i have a square section pier. it does vibrate ( especially when extended) but this is shielded very much by the pier base i have. It really becomes an individual thing - and very much dictated by the terrain. We all have different ideas - some work some don't. for the record i did one night as an experiment to see what moved the image - a light tap, rap and bump and to see how long it took to settle

[allan gould]

I must admit I don't get a lot of these arguements. In the field I've used an HEQ5 and an EQ6 for imaging. As long as you don't bump the tripod then they are adequate for imaging. Now setting up an 8" PVC pipe with reo And concrete has to be better than the Chinese tripods and after making several of them I've found them to be excellent with no observable flexture or resonance. Why? Probably because I don't run into it or hit it with a hammer while imaging. All your arguements have some reason and rationale to them but practically ..........?
Just my 2 cents worth

[bojan]

Practically, in those arguments I do not see the concrete numbers for specific case, only formulas - which are of course just indication of what is better and what is worse..

So, bigger diameter is better than smaller one (that is not a surprise for me)..

But what would be good to see are some absolute numbers, not comparisons: what kind of flexure (in arcsec) we have at a specific case, for example: 16cm dia concrete pier, 1 meter high, with 10N force applied to the side.
Otherwise some people will always think that their pier is not *quite* rigid enough.. and most likely it is already 10x overkill.

I will try to talk to a friend who is civil engineer, and I will give you guys some specific numbers when I get them.

[troypiggo (Troy)]

I'm a structural engineer. I wasn't going to say anything, but I feel there's some facts, figures, and equations getting thrown around here that are all getting quoted out of context. I'm not saying they're entirely incorrect, just that you need to look at the big picture rather than get caught up in detailed formulae[1].

Yes, Young's Modulus (or the Modulus of Elasticity "E") of steel is higher than concrete. More like 5 times higher, not 10, but that's neither here nor there. However it's almost irrelevant unless you're comparing a steel hollow tube with a concrete hollow tube, or a concrete solid column with a solid steel column, and both are the same size and height. Reason is that shape, size, and height also affects stiffness, and to a much higher degree than Young's Modulus alone.

Yes, wall thickness is a factor too. A thicker walled tube will be stiffer than one of the same outside diameter, same shape, same material, same height, but thinner walls. But change any one of those other properties and things change and you're comparing apples and oranges all of a sudden.

Concrete versus steel? I reckon it's more about what you have readily available or have more experience with rather than it's structural performance. Each will work adequately.

Other factors to consider would be what are your ground conditions - sand, clay, gravel? If loose sand or gravel, you'd be best going with a pad footing. If any clay and the sides will stand up, I'd go with a piled footing.

If it was me building a permanent pier, I think I'd go with casting hold-down bolts into a piled footing, then bolt down a steel pier. Reason is you can adjust any out of vertical in the pier, and remove the pier if you ever move so you can just lay a new footing at the next house. But then, I have access to local steel fabricators and would know exactly what I want and how to specify it. If it's exposed to the weather, make sure bolts and pier is galvanised or you use a very, very good paint.

You might prefer to just buy some PVC tube as formwork for the pier, some stock lengths of reinforcement, and pour concrete yourself.

What sized pier would I use? Depends on what you're going to throw on it. Now and in the future. It's too hard to calculate exactly what sized pier to use. Too many variables for precise equations to be worth it. I'd just pick a size by feel, and then maybe double it. Sorry if it sounds vague.

[1] Take care if you decide to go against my advice and try to use that formula rally posted above. The formula itself is correct, although what it calculates is the deflection of a cantilever under a point load - like how much a diving board would deflect down from horizontal if you were standing on the end of it. Not sure it's relevant for what you're trying to achieve. I suspect rally was quoting it to point out the effects of size and shape rather than use it to help you size your pier. I also suspect there's something off in the numbers he quoted for E of conc and steel - looks like one is metric and one is empirical and you shouldn't try to mix and match those.

[Moon (James)]

Bojan

Have a look in the second link I posted. Here it is again:
http://www.cloudynights.com/item.php?item_id=1275


Extract:
72 Inch Height, 8.6" Diameter, Deflection 3.4 arc sec
72 Inch Height, 12.75" Diameter, Deflection 0.1 arc sec


James

[troypiggo (Troy)]

Originally Posted by Moon

Bojan

Have a look in the second link I posted. Here it is again:
http://www.cloudynights.com/item.php?item_id=1275


Extract:
72 Inch Height, 8.6" Diameter, Deflection 3.4 arc sec
72 Inch Height, 12.75" Diameter, Deflection 0.1 arc sec


James

From an engineering point of view, that article looks pretty good. Shame he skims over some parts, but all in all it's pretty good article.

I'll have to check some of the figures he's quoted. As per rally's post above, some of the Young's Mod values don't look right, but they're empirical not metric. I live in Australia in the year 2010, I'm all metric, baby! Haven't got a feel for empirical values off the top of my head.

[spinnaker]

OK, interesting stuff being posted here! Can I pose another questions? Assuming I were to build a steel pier (with diameter of say 200-250mm), is there any advanatge or disadvantage or using a circular steel pipe v a square-section pipe?

I suspect the answer may lie somewhere in the formulae quoted above, but I'm no engineer ;-)

[bojan]

Thanks mate, for pointing this out.
However the problem is specifying the lateral push, I am assuming it will come solely from imbalance (centre of gravity of the mount with telescope not aligned with vertical axis of the pier).
Also, the pier itself does not have to have constant diameter - if it is larger at the base and slimmer at the top (conical) , it will have almost the same deflection for the same load but it will be much lighter.

Originally Posted by Moon

Bojan

Have a look in the second link I posted. Here it is again:
http://www.cloudynights.com/item.php?item_id=1275


Extract:
72 Inch Height, 8.6" Diameter, Deflection 3.4 arc sec
72 Inch Height, 12.75" Diameter, Deflection 0.1 arc sec


James

[troypiggo (Troy)]

OK, interesting stuff being posted here! Can I pose another questions? Assuming I were to build a steel pier (with diameter of say 200-250mm), is there any advanatge or disadvantage or using a circular steel pipe v a square-section pipe?

I suspect the answer may lie somewhere in the formulae quoted above, but I'm no engineer ;-)

Assuming same wall thickness, same material, and the sides of the square section are the same as the diameter of the circular hollow section, the square is stiffer by some 75-80%. It's a more structurally efficient shape in bending.

But it's also slightly heavier by some 25-30%, therefore uses a bit more steel, therefore is probably slightly more expensive. Steel is usually priced kg/m give or take.

[bojan]

Originally Posted by spinnaker

OK, interesting stuff being posted here! Can I pose another questions? Assuming I were to build a steel pier (with diameter of say 200-250mm), is there any advanatge or disadvantage or using a circular steel pipe v a square-section pipe?

I suspect the answer may lie somewhere in the formulae quoted above, but I'm no engineer ;-)

I think it it will be better (in terms of deflection due to a variable load), especially if the diagonal of the cross section is aligned with S-N line.
And if the diagonal of the square pipe is bigger than diameter of the round pipe.

[hatman]

I simply can't believe the rubbish I am reading.

I have seen several piers made of 4-6mm wall thickness and there is no scientific evidence of lateral deflection at all.

You really have to ask the question: You have a 900 mm high (average) pier that is anchored by 4 anchor bolts that are each designed to hold several tons of weight/force.

How much force is it going to take to laterally move it at all? Lets get in touch with reality.

In all cases I have seen deflection in steel piers it has been because of the foundation moving or even more common, the "stretching" of the threaded anchoring bolts!! Yes threaded rod does stretch and will do so long before a metre long tube welded to 12 - 16 mm plate ever will.

I don't beleive that this has been looked at at all in the research and thus creates flawed conclusions.

Lets deal with another fact. we all spend lots of time with lots of astrophotographers who produce images of an incredible optical quality (many which are published)and they all seem to use steel piers, usually filled with sand and usually around 700 - 1000 mm high and generally have wall thicknesses on the tubes of 4 - 6 mm.

There is no evidence at all in any of their images to date to support any argument for lateral deflection! If this was the case then why are we not seeing that movement come out it in these images.

Think about for a second.

As a person that works with working and manipulating steel daily, I can say with great confidence that it is absolute overkill to construct a steel pier under a metre long of any steel thicker than 6mm wall thickness unless you are constructing taller pier supports.

To the contrary, to go for a thicker wall thickness in an attempt to try and make the shorter pier more rigid can actually introduce extra vibration problems - Physics 101

In other words, you have to be careful of being too unforgiving.

High rise buildings in eathquake zones are not built with rigid foundations but rather with dampeners at their base. Why? Scale it down to a pier and you get the picture.

Piers need to have a minute bit of give to absorb any external vibrations, This minute deflection usually occurs with the fittings "absorbing" the deflection, not the pier itself. If the pier moves, it is because the vibration is being transmitted through the anchors or not sufficiently dissipated through the anchors to the ground!

The question poses itself, If the theory of thicker and more rigid is better was true, then why is it that we place dampening pads under field tripods to reduce vibration? Bigger is not always the best solution.

Any person with basic engineering knowledge would also know that concrete fractures under the smallest lateral force compared to steel. More importantly, concrete and is a better conductor of vibration than steel.

If you really want the low down on piers then check out Brendon Downs' latest pier with a Paramount ME mount and Meade 14" on it.

Better still, go and have a look first hand google at the Pegasus piers in at Sirius Optics. These ones have 4 gussets top and bottom fully welded on 12 mm plate. I think Brendans is 16 mm thick plate from the same maker.

I would love to see what size truck could cause those bad boys to deflect.

[rally]

Hatman,

What deflection calculations are you using to justify all of this ?

With only 2.5kgs of lateral load applied - this will cause a 1m high 5" dia, 5mm wall thickness steel pier to deflect approx 2.3 arc seconds - that is a function of the laws of physics.

Now for someone with an image scale of say 0.4 arc secs per pixel - eg Your example - Meade 14" f8 scope with a KAF8300 chip
Thats 6 pixels in just one direction.

So if we have some quite light breeze gusting on a 14" scope and the pier and mount etc that is going to cause it move in both directions maybe yielding up to twice that movement.

So your small stars just got huge.
You just lost ALL the detail of the pillar nebulosity in the Rosette nebula.

But wind is not the only thing that provides a small lateral load to the top of a pier - cable drag at one end, load imbalance, just the mount with OTA.
Ever undone an EQ5 on a pier and watched it flop to one side - they are naturally unbalanced.

So its a question of what sort of imaging you are doing and what sort of results you are striving to get with what sort of gear.
The one thing you don't want is the pier (a very cheap item in the whole scheme of things) to be the thing that lets you down. Just overengineer it and know that it will not be the factor causing you grief.

This may not matter to you, but it does to some.

Astrophotography pier design calculation is not an engineering exercise in strength - piers could easily support a small building - its an issue of deflection and that requires by all accounts quite massive construction.

Very small lateral loads do occur and thus deflections do occur and they will shift your image.
If it reduces your FWHM by 2-3 arc seconds or more then you may as well pack up and go home.


In answer to all your various points in order.

- You shouldnt discount all the rubbish you read without at least verifiying some of it and ensuring that you havent in fact contributed to the rubbish yourself.

- The scientific evidence was the formula used by every engineer and engineering graduate for the last 100 years listed earlier in the thread.

- The force required to move such a pier enough to seriously degrade an image is actually trivial as calculated herein.

- Maybe the bolts will stretch ! but not as much as the pier will bend. The bolts are already under considerable tension.

- Most piers are held down by numerous bolts all appropriately tightened - so it has been considered otherwise we would be using contact adhesive or staples.
2-10kg lateral load at the OTA will not cause multiple bolts each having tonnes of clamping force to measurely deform enough to affect imagin.

- Astrophotographers tend publish their good results not their bad ones as a rule. In any case its difficult for anyone to be able to accurately identify (with all the parameters involved) exactly which problem caused them grief - they will simply take another sub and reject the bad ones.
If you look at what most of the Pros" and commercial institutions have installed for a pier its usually of a substantial nature and often that is inside a perfect environment like a large dome.
Its a question of improving your odds - your choice by how much.

- It is clear you state this with great confidence - that does not mean you are correct - you are applying incorrect logic to the problem at the very least.

- Since your additional vibration problems statement is so vaugue and generic - its impossible to offer a comment on that. The idea is to make a pier substantial enough that all vibrations are carried to ground and are hence dampened as quickly as possible.
The vibrations you can hear that are dampened by sand have nothing at all to do with pier deflection and the problems that causes. Such frequencies have such small amplitudes that they do not (themselves) affect our imaging.

- Earthquake proof construction is another science altogether (and I dont have any expertise in that field) - we are talking about building a pier for astroimaging in this thread.
We dont want that (pier) to be able to move on a flexible interface and . . if an earthquake occurs then I will reject that particular frame as part of my normal post processing but since it is only likely to last a second or two in a 1-10 minute exposure - chances are it may not even matter.
I don't get that picture at all - I don't want a wobbly pier on a non rigid foundation !

A pier is not designed to absorb vibration at all - it is designed to transmit all it can to the ground.
Its principal design function is to defeat deflection.


A field tripod is not a rigidly designed contruction that is anchored to the ground, it has a different purpose - that being lightweight and portable - these are not the ideal platform choice for astro imaging, but if the conditions are ideal they can still perform, but never as well as a pier under less than perfect conditions and I recall reading a good essay about the comparisons made between a pier and a tripod - but I cant prove that !

Any person that knows concrete knows that a 2.5kg lateral load on reinforced concete is not going to fracture it at all - it will just deflect a little bit, and that little bit is more than a steel pier of equivalent diameter with a suitable wall thickness. But it is cheap - works very well, easily accessible by most amaterurs and is usually solid verses a steel pier
Is concrete really a better conductor of vibation than steel ? - But that matters not - it is not the vibration that is the main issue - if the pier is so flexible that it cant support a a small lateral load without bending, then I guess it will vibrate a lot - but thats because its simply inadequate for the purpose.

Please provide engineering and structural details of this pier and the results of its performance or how it its better or worse than alternatives discussed here - that would be constructively useful to members.

I looked at Pegasus Piers in at Sirius Optics - I didnt see any technical specifications on pier deflection listed
But I can say that 1000mm high 165mm OD 6.5mm wall thickness pier (without gussets) will have a deflection of
just under 1 arc sec with only a 2.5kg lateral load !
Flexure is not eliminated as they say - that is impossible.
The sand will not change the flexure unless its is mixed with cement and water - then it will add a small improvement.

I would estimate that a standard sized Tonka Toy truck used by most 4 year olds without a load of sand on board will in fact deflect a pier by some amount if it was hung off the end of the average OTA assembly causing an imbalance.

Hope that helps address some of the issues you raised.

[PeterM]

This is a very interesting thread. Thanks to all of you for the informative posts.
In my case I am doing multiple short movement, scripted slews to many galaxies for 25 second exposures with an alt az mounted 12inch LX200, I allow an 8 second damping time once I arrive at the object. I was using a 850mm high homemade pier of 165mm od with no gussets and dampening seemed quite ok. I have recently bought a Pegasus Pier as replacement (basically bought on looks, construction quality AND that it has the gussets). I can't give a report yet as only went in on the weekend and skies are crap, but will do as a comparison to what I had, don't know what value that will be given my needs over say those doing longer exposures.
After reading carefully the above I am not sure if we are comparing apples with apples here when it comes to steel piers, that is a section of pipe with plates welded top and bottom versus one with gussets (as I originally had).
Rally notes that the deflection on a pier example of 1000mm high /165 od /6.5mm wall as about say 1 arc sec (2.5kg lateral load) and notes this is without gussets (again, pretty much what I had).
As an example all the Pegasus piers I have seen (and most others I have seen) are gusseted (4 top and bottom, extending to about 50mm across the bottom 12mm plate and about 140mm high and about 6mm thick) with substantial welding, so in order to compare apples with apples does anyone know what affect does this have on lessening deflection as compared to a pier with no gussets? ie does it effectively act as an increase in the tube diameter (footprint at ground where the major issue is) and how would this be taken into account in measuring deflection?

PS where's Mark Bolton when you need him, he teaches this stuff.

Thanks

PeterM.

[rally]

Peter,

Yes you are right - the original comparisons were between ungussetted mild steel piers of a cylindrical shape.
Using higher tensile steel - eg Gas pipeline improves the bottom line.

I am not sure what effect these gussets will have, that is why I calculated it without and stated accordingly.
A structural engineer would need to model that one.

But if we assumed they were perfectly rigid (which they are not) and completely shortened the pier by their approx length becasue of this perfect rigidty - I still calculate a deflection of 0.51 arc sec, so I am guessing its somewhere between the two measurements for 2.5kg lateral load

Remember this is with only a 2.5kg load applied, the reality is that if the load is applied at the OTA (which is most likely) the increased leverage will increase the amount of deflection.
It is extremely difficult if not impossible in practical reality to properly balance eccentric loads on larger systems - which are a normal occurrence.
The imbalance is usually at the very ends of the OTA - eg a STL11000m with 8 filter wheel and cables is quite eccentric and it hangs out nearly a metre rearward from the centre of Dec rotation on a large refractor.
Focuser motors, Rotation motors and other items such as guide scopes and guide cameras hang off the sides or tops of the system in most cases.
The entire camera system is rotating about the end of the telescope too - so the eccentricity is a dynamic problem.
Thus as the telescope swings through the full movement of travel it goes in and out of ideal balance in many different directions - although this is happening slowly and guiding can take care of that, it does cause problems with Polar Alignment and pointing accuracy and its variable.

People can go to a lot of trouble with getting their Polar Alignment below 1 arc minute in RA and Alt in a pemanent observatory situation and then create a many point star model to get the pointing and tracking accurate and apply PEC correction.
A pier that deflects through its travel simply adds to your woes - I would argue unnecessarily.
Spend $100+ extra on extra steel or concrete on a system worth potentially $10,000's and this problem is for the most part nearly eliminated.

Rally

PS
If you want to get fancy - as is done in many military pier designs they use a cone shaped pier and I am aware of at least one amateur astronomer who has applied this engineering design to his pier.
It means it takes up a larger footprint but the increase in rigidity is enormous and the wall thickness can then be reduced down from 10-15mm.

[bojan]

Guys,
This lateral load you are talking about is more or less constant.
So what if there is a constant deflection present ? Most of us are not in the positional astrometry business.
The only things that really matters are fast variable loads (cable pulls, wind, walking around the pier etc).
I have a feeling we are grossly oversizing things here..
Not that this is bad per se, but it all adds the cost and time to the whole operation.

What about people that use tripods in the field?
Surely any tripod is inferior compared to 180mm dia concrete pier.

[rally]

Bojan,

I am only offering the calculations for those that are interested, nobody is wrong if they dont use them !
Rigid piers are definitely not needed by everyone.

But the problem is the loads are changing and this means the flexure is different at different camera rotation angles and at different positions in the sky

Wind tends to gust - even a light breeze will push a large OTA around quite a bit.

One set of deflection affects Pointing and Polar Alignment, the other set affects the position of stars in the image frame during the image sequence

When exposure times can be very long this can be an issue.

I think the people to whom this matters will know who they are, for the rest its not as relevant.
Planetary imaging is unaffected by the pier design for the most part

At the end of the day it boils down to degrees - to what degree do you want to reduce bloating your stars.
For many, gaining an extra 0.5FWHM does matter if it can be achieved relatively easily and cost effectively.
For them that may represent an increase of 25% in image quality from a tripod setup (I dont have calculations to show that).

Big scopes with long focal lengths are obvously more affected than widefield - but the final measure is image scale - I guess if you are at 3.5arc secs per pixel or greater you can afford to relax a little, if you are at 0.5 arc secs per pixel or less you need all you can get !

I agree a 180mm concrete pier is superior to a tripod in the field. But that doesnt mean that on occasion you couldnt get a good image from the tripod - under the right circumstances you can.

I just say that you should get the largest diameter heaviest chunk of pipe you can manage (physically an within your own budget) at the local scrap yard and use that !

Most people end up with a relatively longer pier than 800-1000mm though - because they need to allow for the extra height that their Obs is above ground level.
2700, 2800, 1900 and 1600mm are some that I have checked.

I think I have said more than enough toward the OP's original thread so I'll sign off unless someone wants to start a new thread about pier deflection and how it affects your FWHM and image quality.
It would be nice to have two comparable systems setup side by side - one on a solid pier and one on a less rigid support - eg tripod or small pier and measure the results across a range of tests.
At least there would some tangible results to argue over !

Rally

[troypiggo (Troy)]

As a structural engineer, it's very difficult to model and analyse these piers with any high degree of precision. I have all the relevant software right here on my computer, but would need to know the ground conditions, site location, size and weight of scopes, height of pier, preferred construction materials and methods, what is your acceptable deflection, is the pier in an observatory or out in the open, and more. There's just too many variables to make it worthwhile. What you'd do is make assumptions here and there, and in doing so all precision is lost, but you end up with a slightly conservative solution in the right ballpark. That's what I was trying to point out in my posts above.

Peter - not sure if this answers your question or not. It's not strictly correct to say that adding gussets to a pier is effectively increasing the diameter of the pier. Yes, it increases the stiffness because the gussets increase its Moment of Inertia, which is directly proportional to stiffness.

If you've got any specific questions, happy to answer them - maybe PM so this thread doesn't get too off-topic. I do have a little bit of structural engineering experience after 18 years of working as one, and now a partner and director of a structural engineering consultancy.

[koputai (Jason)]

I'm not an engineer, but I did listen at school when spelling and grammar were being taught!

Dampened = wet
Damped = reduced

Dampening = the process of wetting
Damping = the process of reducing

Just taking my pet ant for a walk.

Cheers,
Jason.