Taper in pipe pier

[muletopia]

The post below is most informative on the subject of pier design.

http://www.iceinspace.com.au/forum/showthread.php?t=132544

The nett conclusion is that large diameter steel pipes with gussets as described by Avandonk and bolted down as described by him are best.

With this in mind my pier will be 9 inch diameter steel pipe with 10 mm wall thickness and will be 4 feet tall, the gussets are planned to be 3 feet tall.

The main unresolved design question is the reflection of vertical vibrations from the mount back into the mount at the top of the massive pier.

My neighbour, who is fabricating the pier has so far made the 25 mm thick base plate and drilled the holes, placed at Avandonk's suggested places.

He has volunteered that he can machine an internal taper in the pipe, starting at the top of the gussets and reducing to a wall thickness at the top of the pier of 5 mm.

My question is, is this likely to reduce the vertical vibration reflections?
Also, is it likely to introduce other unwelcome dynamic effects?

Chris

[AndrewJ]

Gday Chris

I think most people overthink/design piers
By machining an internal taper into the pier, you may spread out /disperse any internal resonances but at the end of the day, what stimulates the vibrations you are trying to kill????
Most random "vibrations" in piers come from ground resonances, and they have complex sources. A 9" dia pier with 10mm walls will be massively stronger than required for most situations, and simply filling it with a wet sand slurry will kill off most vibrations.
I find it funny that people will spend hours trying to make a "solid" pier, and then use a plate supported by 4 freestanding bolts on top to mount the scope. Its sort of like gaffa taping a micrometer onto the end of a 12" wooden ruler.

Andrew

[louie_the_fly]

Good points there AndrewJ.

I ran some simulations on my pier based on my locale's design wind speed (of 33m/sec, which equates to 118.8kph, and 700 pascals). My pier design is 1200mm high x 165mm dia PVC pipe with concrete fill & 3 x 12mm reinforcing, integrated into a 750x750x650 reinforced footing in M class soil.

At the top mounting plate I had maximum deflection of <0.25mm. Now this is assuming that the foundation is not moving in the ground, and it's blowing a gale, and everything is rigid. It was from here that I concluded that a lot of what I've read about pier design is over the top.

The "weak" points are going to be the footing in the ground, the pier to footing interface (in my case it's integral), the mount to pier interface, the mount itself, & the scope to mount interface. You need to know what might induce vibrations & where they will emanate from, and counteract them specifically.

The #1 culprit (IMO) will be intermittent wind gusts - A wind speed of say 40 kph gives you a wind pressure of around 70 pascals, which is nothing. That's pretty windy though. I'll be packed up and inside watching telly if it's up at that.

Fast slewing of an out of balance scope and mount can induce some movement which can resonate for a moment when the mount stops.

Chris, I did a sim on your design, assuming a base plate dimension of 16" square, and came up with nearly 0mm deflection at the top of the tube with 1 Mpa wind pressure. I wouldn't stress (so to speak).

:thumbsup:

[CharlesW]

I would add that if you are just doing AP you can make your pier as tall as you want but if visual is in the mix, four feet is a little high. Mine is 42", I'm 6'2" and I wouldn't have it an inch taller.

[AndrewJ]

Gday Stewart

Imaging in 40kph winds would be "interesting":lol:
That said, there is also a strength vs damping equation
that needs to be taken into account at the same time,
and as you noted, that depends on what is creating the vibrations,
and how the system reacts to the stimulus.
A tuning fork made of "tuning fork material" is tremendously strong
but vibrates horribly:shrug:
A tuning fork made of mild steel is also very strong
but wont vibrate for very long at all.
Thats one of the reasons properly designed wooden tripods
are so good. The fibres in the wood give great strength
but can also absorb energy when flexing and as such can self damp.
One interesting mechanism i read about was to use a std steel tube pier
and fill it full of very coarse sand or lead shot and used engine oil.
The engine oil supposedly transmits energy into the fill material
very efficiently and hence damps quite quickly,
but it still cant help totally if the vibration is started by the mount.
( This thread is bringing back horrible memories
of vibration analysis theory during my uni days )

Andrew

[muletopia]

Thank you Gentlemen,

Steve your link is correct,ta.
Charles, I sometimes find my 43'' high tripod painfully low when using my cassegrain telescope, I am likewise 6' 2" tall so what sort of telescope do you use?

Stewart, thanks for your investigations, my base plate is a circle of 25 mm thick steel, 500 mm in diameter with a pair of bolts one each side of each web near the outside of the plate. I spent some time today welding the bolts, 16 mm threaded rod, into a frame to go into my cubic metre of concrete, 1 m *1 m *1m hole dug and an arcmesh box welded up to sit in it and around the bolt frame.
With this plate and the four gussets flexure at the base of the pier should me minimised.

All this may seem like overkill as Andrew suggests, but do it once and do it properly.

Chris

[AndrewJ]

Gday Chris

As Stewart mentioned, Pier design is only part of the equation.
The ground fixing and scope connection mechanism are just as important.
I personally prefer a narrow but deep foundation for a pier,
vs a big mass of concrete.
I would just get a post hole auger and go down about 2 metres and use say
100mm of concrete/grout slurry around it.
Once set, its not going anywhere ;)

Andrew