Bundled steel posts as pier?

[AstroApprentice]

Hi all,
I’ve been trawling through some great old discussions & debates on concrete v steel piers in this forum. In particular there have been some great points on the importance of steel OD & wall thickness. One idea I haven’t come across is the use of multiple steel posts bound together to form a single pier. Do you think this could work?
The reason i bring it up is I have access to four old 50 mm square steel posts with end plates welded on. If I cut away one side of end plates on each post I could then bolt posts together to form a 100 mm square bundle and still make use of remaining end plates for fastening. The end plates are 10 mm thick, but I won’t know the post wall thickness until I drill through. Posts are 2 m long, which is a good length to go from footing up through deck.
Anyway, do you think four narrow posts bundled in this manner would be more rigid than a single 100 mm post and ok for a light mount & imaging setup? Managing four single posts would also be much easier than a single thicker one.
I’m hoping for an engineering lesson, if you have the time ;)
Thanks

[AndrewJ]

Gday Jason
Using 4 small pieces to make one big piece would certainly work without problems. ( Its essentially what laminated wood beams are )
As to strength vs a single 100x100.
Given the same wall thickness, as long as the pieces were joined properly, it would be approx 80-100% stronger, as there will be 2x more metal in there.
Rather than bolt them together, if you can get the joints stitch welded, it will be very rigid for not much weight, and no bolt holes to weaken anything

Andrew

[OzEclipse]

Yes providing you can join them at multiple points up the posts. All bolt would be fine. If you have a welder, I would weld two together with short seam welds at several points along the length. Probably not necessary to weld the whole length. Do the same to the other two. Then bolt the two resulting sections together with 1/4" bolts at intervals along the lengths.

Whether it's stronger or weaker would depend on the wall thickness of your 50mm posts compared to the wall thickness of a 100mm post. A 100mm RHS would probably have thicker wall. It's a moot point. The construction should be more than strong enough to hold a moderately large mount.

Joe

[Imme]

....and to reduce vibration fill with concrete..... Too be honest I'd say 1x50mm pole filled with concrete would prob do.....plenty of people take amazing pictures on a tripod.....I think we all get carried away sometimes thinking we need something to hold up massive loads...... For the majority of us well never have more than 30kg on it.
Just got to make sure vibration is dampened

[traveller]

Yes, bundle them into a 2x2 configuration and weld them into place. Also get some nice thick steel plates and weld them to the joints as gussets/fins to provide lateral stability.
Then get the lot powder coated and finally fill the tubes with sand and old engine oil to dampen vibration.
Dig a hole 400x400x600 as your footing and you will have a niceness sturdy pier.
Bo

[traveller]

Links to my pier build
http://www.iceinspace.com.au/forum/showthread.php?t=136618

[Kunama]

I would tack weld the top of the four together but then splay the four out so the bottom makes a 300x300 square and then weld mild steel flat bar at several levels around the four tubes .......

[Wilso]

I don't think it's large enough.
But if you go ahead I would recommend stitch welding 50mm in 150 on every face including inside and welding gussets at the bottom for extra support.
Including surface mounting as it easier to shim the pier at the bottom if the ground moves.

Good luck!

[AndrewJ]

Gday Darren

He mentioned having a light mount, so 100x100 should be plenty strong enough.
I agree with Matt that creating a pyramid would be better
but again, it depends on the load, and what it has to resist.
ie Most piers are massively over designed for strength,
what you need to look at most times is vibration damping.

Andrew

[Wilso]

I think if your going to the trouble of making your own pier why not go slightly larger, (just in case) as most of us upgrade our equipment at some stage.
At your local steel supplier the cost of my steel was only just $120
If you want to dampen the pier, partially add sand. Wall thickness is also very important. Solid isn't good.
Surface mounting the pier also if you want to move location or sell.

[Boozlefoot]

Hi Jason, I am currently going through the transition stage from tripod / to astropark (a series or piers/plinths in a dedicated garden for various scopes)/ to a full observatory. Lots of experimentation, and your 4 posts intermittently welded and then filled will be a fantastic change for you, (if this is for your Borgs) Vibration will diminish significantly to what you are accustomed to.

Might I suggest that a major factor for these is the actual plinth the pier is to be mounted into/onto. Size really does matter when it comes to this, and an extra hour or two of digging will produce substantially significant results, and allow you much more confidence in your project. Depending on your soil type will determine your plinth size. Always try to "elephant's foot" the base of your plinth in the ground. Ensure you have sufficient isolation "gap" between deck and pier when observing, and a good packing piece when finished observing to prevent vermin/insects getting into your ob. When setting your plinth, you must decide to set the posts actually in the concrete when you pour the plinth, or provide a "flange" held into the plinth with fasteners. I use a 6" pipe flange held in with threaded rods, this allows me to alter the configuration or upgrade relatively inexpensively with off the shelf industrial items, however there are also many chemset systems also available.

[AstroApprentice]

Thanks for all the great advice.
I’m now thinking of separating the posts by ~200-300 mm & welding diagonals to hold the four posts together in a frame - inspired by some rail towers near me. I can then bolt the post end plates to larger plates top & bottom, and then the large plates to footing & mount plate, respectively. I’ll bolt it all up and if it looks ok, I’ll weld together.
Damping is another story. I can fill the posts with sand, but it will be fiddly as I want to keep post end plates intact, so will have to do it through drill holes with a funnel! Maybe there’s an easier/better option? I know some street lights have a chain hanging down their core that mitigates vibration somehow...

[Kunama]

If you splay the tubes out at least to 300x300 at the bottom and weld some plates around them then there is no need to fill with sand or anything. Just remember to not hit the pier with a metal hammer during imaging runs.... ;)

[sil]

Wouldn't you run into issues with expansion/contraction being uneven across each pillar in the bundle? Like a bimetallic strip would a single pier try to bend when made up of many smaller poles?

[Tinderboxsky]

Not if the individual sections have the same coefficient of expansion. Bimetallic strips work with two metals with different coefficients of expansion.

[jenchris]

2metres.
Hmm
Cut off 600 from each cut into 150mm lengths. Separate the remaining 1400 with the cut off sections and weld. You now have 300x300 space beam.