Balcony or Patio Mount

[yorkpf]

I live in a unit block in Brisbane, three floors up, and my only access to the sky is from my balconies / patios - one faces north, the other south. My latitude is approximately 27.5 degrees south.

At the moment, on each observing night, I have to set up my telescope on a tripod (HEQ5 Pro mount + Celestron 8-inch SCT) and then tear it down.

Using a tripod precludes seeing a significant part of the sky. Because of the spread of the tripod legs, I can’t get the central axis of the mount any closer than about 55 cm (20 inches) to the patio railing; also, there is the roof overhang to consider (about 50 cm or 18 inches). Bottom line is that from my south-facing balcony, for example, I can see only from about -60 degrees south, down to the south celestial pole, then to the horizon.

Anyway, I wanted to try a more permanent setup - one where I don’t have to tear down the mount each night and one where I can see all the way to the zenith.

What I had in mind was to fix a steel beam onto an upright concrete pier on the building and have it project outward about 66cm (or 26 inches). I would weld a solid vertical pier (100 mm (4 inch) diameter steel pipe on the far end on which to place the HEQ5 + telescope + counterweights. The pier would be about 200 mm (8 inches) high to allow clearance for the polar axis and counterweights.

The steel beam would need to carry a point load of about 30 kg at its far end (scope 10 kg + counterweights 10 kg + mount 10 kg). I would fix the horizontal steel beam to the vertical concrete upright with maybe three bolts that run right through the concrete pier to the other side. These bolts would need to be a bit over about 300 mm (12 inches) in length.

What I had in mind for the steel horizontal beam is an SHS (Square Hollow Section) of 100 mm x 100mm. I can obtain this in 3, 4, 5 or 6 mm gauge.

See my attached diagram, shown in elevation (from the side) …

What I would like to know is:

1. Would this broadly work? That is, will it carry the load OK, with capacity to spare?
2. What gauge of steel should I use for the beam? 3, 4, 5 or 6 mm?
3. Should I use 3 bolts or 2 to attach the steel beam to the concrete post?
4. What gauge of bolts should I use?

I am aware that this is a problem in calculating loads etc for a cantilever beam, as talked about on sites such as: http://www.efunda.com/formulae/solid_mechanics/beams/casestudy_display.cfm?case=cantilev er_endload#target.

Trouble is, I don’t really understand the calculations. For example, how can I find the value of Young’s Modulus for my steel beam?

Thanks in advance, for any help you might be able to give …

Paul

[billdan]

Gooday Paul,

The figure for Youngs Modulus (or Elasticity) should be provided by the manufacturer of the Steel, but is approx 210,000 Mega pascals depending on the alloys that are in the steel.

The max deflection of a cantilevered beam is given by

Force x Length^3 / 3 x Elasticity x I section modulus

Force is in Newtons ( 1Kg = 9.81 Newtons)
I section Modulus = pi x (OD^4 - ID^4 ) / 64 ( this is for round tube but should be close for a square tube)
Length in mm
Elasticity in Mpa

Its been decades since I did these, so I am not sure whether the length is in mm or Metres.

Set yourself up with a spreadsheet and play with different wall thicknesses.

I would say 3 bolts would be better, also put a steel plate on the other side of the concrete for the nuts to grip onto.

Cheers
Bill

[julianh72]

Paul,

I'm a Structural Engineer (based in Brisbane), so I'll have a look at the sums for you. I'll take a look tonight and post back separately.

At first glance, it would be easy enough to make a bracket which is strong enough to carry the load, but the challenge will be vibrations. With an angle bracket as you have sketched, there are lots of vibration modes to worry about - vertical bounce, horizontal sway, and twisting of the whole bracket, for example. I would guess you'll need to tighten the bolts very tight, and maybe put in a pad of rubber or similar as a damping layer between the bracket and the concrete column. You may also need more bolts and / or bigger section tubing to get the vibrations manageable.

In principle, your arrangement can be made to work, but a few things to consider:

1. The Body Corporate will probably take a dim view of drilling holes through the concrete column.

2. Assuming they let you drill the holes, you'll probably have to take the bracket down whenever you are not using it, as most Bodies Corporate have by-laws preventing any modification which alters the external presentation of the building.

3. How will you (safely) get the scope and mount on and off the bracket? Most of us have enough trouble lifting 30 kg loads onto a waist-high pier or tripod right in front of us - the thought of handling those loads at a reach of 660 mm from the handrail is a bit worrying. (I'd be worried about both the risks of back strain, and of course, the risk of dropping the scope or mount). Put it this way - we wouldn't allow manual handling of such loads on my work sites, we would require it to be done by "cherry picker" from the ground below.

4. Is this for visual or photography? If it's for visual, have you thought about how much sky you can actually access, with your eye at the eyepiece, and you not leaning over the balcony handrail too much?

[yorkpf]

Hi Julian

I was hoping there would be a structural engineer on the forum, because that is precisely the sort of advice I need.

Thanks for your suggestions so far. In reply to your 4 points (all very relevant) -

1 & 2: Body Corporate: I would seek permission - it is a pretty small unit block and a pretty small Body Corporate. My main concern is: Would weakening the column be an issue with drilling 2 or 3 holes? We are on the top floor (only 3 levels) and the column has done nearly all of its work by the time it gets to us - it's really just holding up that edge of the roof. Tell me, please, if you think I am wrong about this.

3: You are quite right about 30 kg loads - manhandling that mass would be tough. However, it is only 3 separate sets of 10 kg. First, I would instal the mount (10 kg), then the counterweights (10 kg) and finally the scope (10 kg). I think lifts of 10 kg are probably manageable?

4. It is for photography only. The scope will carry a DSLR which I will use to capture images.

Anyway, thanks for your comments so far and I look forward to your posting.

Thanks: Paul

[yorkpf]

Hi Bill

Thanks for your suggestions. I will have a go at doing the calculations with some spreadsheets.

Thanks: paul

[RobF]

No body corporate in its right mind would approve or could ever get insurance for you to do this. Sounds like you're fortunate to have both S and N facing balconies. If security allows why not just leave set up and covered against weather in between uses. Put away if very bad weather or expecting to be away for extended period.

[julianh72]

Paul,

The short answer is that (ignoring the Body Corporate / Safe Handling issues) your design concept could in theory be made to work, but it could be problematic for photography work, due to the likely vibrations that you would get from building vibrations, gusts of wind, etc. The member sizes are OK to carry the loads, but vibrations for a bracket of this type (especially twisting of the arm, leading to sway at the top plate) could be a problem, to keep the deflections of the scope down to arc-second accuracy and stability that you want for astro-photography.

1 & 2: Body Corporate: I would seek permission - it is a pretty small unit block and a pretty small Body Corporate. My main concern is: Would weakening the column be an issue with drilling 2 or 3 holes? We are on the top floor (only 3 levels) and the column has done nearly all of its work by the time it gets to us - it's really just holding up that edge of the roof. Tell me, please, if you think I am wrong about this.
Drilling into concrete is fine in principle, as long as you try to miss all of the rebar - but only if the Owner / Body Corporate approves.

3: You are quite right about 30 kg loads - manhandling that mass would be tough. However, it is only 3 separate sets of 10 kg. First, I would instal the mount (10 kg), then the counterweights (10 kg) and finally the scope (10 kg). I think lifts of 10 kg are probably manageable?
While you or I could probably manage the task if we had to do it occasionally, safe work practices would suggest that manual handling of 10 kg loads at an outreach of 600 mm or so beyond a parapet is a non-no - risk of back strain, risk of dropping the load, etc. (The Body Corporate's concern will be the risk to other occupants and property if the load falls - they won't care about the cost of the dropped scope or mount!)

4. It is for photography only. The scope will carry a DSLR which I will use to capture images.
Astro-photography demands an exceptionally stable mount; cantilever brackets are notorious for being prone to sway and twisting deflections and vibrations.

To have any chance of success, the fixing to the concrete wall needs to be REALLY rigid. I would suggest a side-plate connection using Masonry Anchors (Ramset or Chemset type) into the concrete as per the attached sketch, rather than the "Through Bolts" in your sketch - but the Body Corporate would almost certainly object to these sorts of fixings.

As for how you would go about getting Body Corporate approval to install something like this, and how you would address the personal injury risk to yourself and others - well, I'm afraid I don't have any suggestions there!

[dannat]

would the semi-pier mounted az eq5 alleviate some of your problems? or even the pier mount of an astrotrac [though might be too light for you]

I presume your hand rail is way too flimsy to mount anything? could you reinforce it behind what already there & direct mount the head on top of it?

[yorkpf]

Hi Julian (and Rob)

Thanks for your thoughts on the mount and the Body Corporate issues. Some of these things occurred to me anyway, others didn't. But as well as information, I was looking for a reality check and you both provided that. I definitely won't be proceeding with the idea.

I will either put up with what I have or move house.

Cheers

[yorkpf]

Thanks for those suggestions Daniel, regarding pier-mounting or reinforcing the rail. I think I will just stick with what I have for the moment.
Paul