Structural integrity of a welded steel stand for two condenser units 8

[Parker87]

I've recently bought two mini split heat pumps and want to build a stand to hold the outdoor units. I would like to obtain an E-shaped cantilever structure but am unable to determine whether the beams will hold the units safely in place without adding some form of additional support (e.g. diagonal braces).

I plan to use 50mm x 50mm x 2mm (2" x 2" x 14ga) square steel tubes which are going to be welded together as shown in the drawing above. The small cylindrical shapes are rubber-metal mounts that will go underneath the condenser units to prevent vibrations from being transferred to the steel frame. I might also use some larger rubber-metal buffers to decouple the stand from the concrete foundation on which it will be mounted, so any unmitigated vibrations don't find their way into the nearby walls of the house.

The weight of the outdoor units is 43kg (95lb) each, and the way this weight is distributed on each corner is shown on page 11 of LG's product data book (i.e. page 13 of the PDF) linked below.

https://www.lgbusiness.es/wp-content/uploads/2019/11/PDB-Dec.06.2018-10.pdf  

Since this stand is only going to be secured to the ground (and not to a wall), my main concern is stability, i.e., supporting the weight and stress from the two condensers without bending, breaking, or tipping over, while enduring the fluctuating weather conditions.

Any thoughts or advice would be greatly appreciated. 

 

[BAretired]

With the weld, you can develop about 80% of a CJP if properly done... about 3x what is needed, if prelim calcs are OK

When I said you can't develop the full strength of the 1.5" HSS outriggers, I was not thinking about the weld. No matter how good the weld is, each outrigger is fastened to the thin wall of the vertical HSS which is incapable of resisting the maximum fibre stress in the attached member. The deflection of each outrigger will be almost impossible to predict because of the flexible support. One thing for sure, deflections will not be equal because the loads are not equal on each side of the unit.

The triangular gussets suggested by the OP are not much use as noted by 3DDave.

BA

 

[dik]

concurr with your weld comment, and I wouldn't use gussets, either...

Dik

 

[dik]

I didn't realise they were true vibration isolators. I thought they were 'rubber bushings'

Dik

 

[Parker87]

So it's be better to use HSS with thicker walls (e.g. 2" x 2" x 11ga)?

The reason I want to use 2" x 2" for the horizontal arms is because the condensers' mounting legs are 2" wide and I can get rubber buffers with the same diameter so everything fits neatly on the 2" square tube.

One more question: is it better to weld the upper and lower (base) horizontal members like in my drawing or have the verticals stretch to full height? Consider that I will most likely be using rubber pads like in the picture below and the vertical members will not be touching the ground if they extend to the bottom.

I can get ones locally that look a lot like these:

https://www.polymax.co.uk/anti-vibration-rubber-mount/pedestal-mount/kpm-vibration-isolation-mount

 

[LittleInch]

Why don't you just copy this design and make it a bit taller?

Even now, all your weight is going through one small fillet weld on the top angle with a huge stress concentration. All your other welds are there for effect only.

Why weld it? Use bolted angle brackets.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Tmoose]

Unless those rubber isolators/mounts are gummi-bear soft AND selected based on static deflection and the units' frequencies of interest I think there is high likelihood that welded frame is going along for the ride, as if the mounts were aluminum biscuits.

In addition, all the electrical connections and "refrigerant" ping must be very flexible in 3 dimensions, and anchored to the building, lest the units' vibrations sneak into the building.

https://qph.fs.quoracdn.net/main-qimg-6c734999ac532364d7b895b8efee6e6c

 

[kingnero]

For what it's worth, if you make that using SHS 3mm thick (11ga), it won't go anywhere. Those are airco units ffs.
I'd have the bottom (horizontal) members full length for ease of mounting purposes, the rest doesn't really matter in a practical way. Whatever you can weld easiest/best is good enough.

 

[BAretired]

Extend two verticals as shown below and delete the triangular gussets and you should be okay. If you want to use 2x2 HSS for all members, that should be okay too. Make sure you have a skilled welder.

BA

 

[Parker87]

Thanks, BA. The reason I don't want to have those verticals on the front is because: (1) It will look ugly and restrict airflow to the bottom unit (minor concern) and (2) It will make it difficult to install (and service?) the bottom unit, which will have to be "slid in" from the side.

And to address LittleInch's post, I initially designed it a similar way with the units to be mounted on the front and back horizontals (as opposed to the right and left) but there is an obstacle on the ground right around 10cm inward from where the right verticals should be (the solid concrete basically ends there [dotted line in the drawing below]). A stand with a narrower footprint solves this issue.

P.S. Now that I am thinking of using anti-vibration mounts underneath the stand, the above might not be an actual issue anymore. A remaining issue would be leveling, since the ground is slightly sloped downward towards the right and front.

 

[r13]

I understand your operation concern to not to restrict the way to set the units in, but with merely a few inches wider than you detail shown above, you can done the BA's way, which is preferred. Then, add a few surface mount triangular brackets, you might have eliminate the stability concerns and stand free by its own.

 

[Parker87]

By the way, the only floor/ground anti-vibration mounts I can easily obtain are of the following kind:

https://4.imimg.com/data4/JY/UG/MY-2836580/kpn-type-pedestal-anti-vibration-mount-500x500.jpg.

Makes me wonder how long they would last outdoors before the rubber starts degrading and falling apart due to sun exposure and other environmental factors.


I want to do my best to eliminate vibration transference, but these things don't look like a permanent solution to me. The condenser mounts could easily be swapped for new ones if need be, these might be harder to replace.

 

[BUGGAR]

If you are using 14 ga, doesn't that put you into AISI design land? Different set of rules.

 

[r13]

For modern heat pump, the vibration is rarely a concern, if it is on solid ground, such as concrete. You can add a raised concrete pad, if necessary, you can sandwich a vibration absorbing pad between the concrete elements.

 

[BAretired]

The lower level is not a problem now. It's only the upper shelf that needs a little help. I suggest a couple of knee braces as shown in the sketch below. If it's a bit wobbly, add diagonal braces in the bottom tier. The whole structure must be fastened down to something solid to prevent overturning.

BA

 

[Parker87]

The whole structure must be fastened down to something solid to prevent overturning

Are you suggesting I don't use the anti-vibration mounts and secure the stand directly to the concrete or something else?

I was thinking of doing something like this:

 

[BAretired]

If you use this arrangement, the bottom shelf needs support. Mitering the corners of the base frame is one option. If they are butt welded, end plates would be required.

If the structure is just sitting on rubber mounts, with no attachment to a foundation, it cannot resist serious lateral forces such as strong winds or seismic events.

BA

 

[BAretired]

Parker87,

Our posts crossed in cyberspace. I don't know how much vibration condensers have but you are getting vibration isolation under each unit. Do you also need it under the structural frame? I was thinking of just bolting down to a concrete base to prevent overturning. It would have been my preference to extend the vertical members down to a base plate anchored in the concrete. In that case, the bottom frame would not be required and we would not have the issue of columns partially bearing on 14 ga. HSS walls.



BA

 

[r13]

The unit should be connected on top, if possible. It will be more stable than mounting on the ground alone. Also, up to this point, the frame looks great, but will it stands against wind and earthquake, it's head heavy laterally.

 

[Parker87]

With all these options and details to consider, I feel like I keep getting further and further away from making any final decisions.

If I do the boxed bottom and diagonal braces for the top shelf, do I still need to miter the joints?

I would refrain from messing with the concrete for various reasons... and I can't really know if the rubber buffers under the units will do a good enough job of isolating the vibrations and keeping them away from entering the house. I've had bad experiences with mini-split ACs in the past and it can be extremely annoying when you feel the walls or floor underneath you trembling ever so subtly. In other words, I don't want to let it get to this and then regret not having taken sufficient measures against it. Once the stand is mounted, it will be harder to tackle any vibration/noise issues or do any revisions with regards to the way the stand was initially constructed and mounted.

 

[r13]

With all these options and details to consider, I feel like I keep getting further and further away from making any final decisions.

That's the punishment for asking expert opinion/advice on the open space. I think from this point on, you already have adequate idea to talk to a local structural engineer to develop your project. If you willing to risk the investment on the equipment and the framing material, you can experiment on your own, but at least talk to a metal fabricator/welder to be realistic. After all, you might change your mind, and finding it is better just to purchase a preassembled rack/shelf instead of the troubles. Sorry to pour cold water on an enthusiastic mind.