What size channel to use to support AC compressor

[hipfisher]

I need to replace an air conditioner compressor unit that sits on the 2nd floor walk way on my stilt home in the Florida Keys. The walk way is 24" deep and the original AC unit fit that dimension, however, the new unit is 31" square. When I install the new unit I would like to leave a 18" space between the unit and wall to provide space to walk by. The plan is to mount a pair of aluminum channels on the deck extending out 48". The replacement AC is no greater than 175 pounds. There will be 24" of the AC unit supported by the channel and 7" supported by the reinforced concrete walk around.
Question: What size channel should I use? My AC contractor is proposing 4" x 1" x 1/4" thick aluminum channel. That seems light to me. I am used to over sizing materials to afford me greater piece of mind. Also what fastener technology would be best to secure the channel to the deck.

 

[Q_Goest]

Hi hipfisher. A lot of people post things like this, but it's difficult to help you without a picture or drawing. Can you provide something that shows what your AC support looks like? The strength of the support is highly dependent on the geometry.

 

[Studiot]

7"+18" makes 25", not 24"

Who measured this, you or your contractor?
Why do the aluminium sections need to extend out twice the overhanging width of the compressor? Are you ever going to push the compresor or anything out that far for servicing?

Why aluminium? won't this corrode through in no time in the marine atmosphere? If you are going to use stainless steel fixings you could have a chemical action problem which will also attack the aluminium.

Whatever you choose, 7" bearing for the cantilever sounds inadequate. Could you not undersling the sections?

As to fixings I always advise against chemical anchors (and contractors always propose them) in this type of situation as they are not failsafe and likely to deteriorate rapidly in the marine atmosphere.

Expansion anchors are safer, but still not failsafe - I prefer through bolting which is.

Then there is the issue of the concrete in the walkway itself.
If it is just reinforced then everything will be fine for drilling, but if it is fashioned form prestressed planks then you must avoid damaging the prestressing wires whne drilling or there could be a catastrphic failure. ( they should be in the bottom half of the plank like in a house lintel)

 

[hipfisher]

Goest;

Thanks for the reply. The walkway support is 24" wide reinforced concrete 6" thick.

Studiot;

Thank you also. Your are correct my math stinks! The space between the wall and AC unit would indeed be 17".

Are there not multiple aluminum allows which provide adequate protection in a marine environment? 6061 comes to mind. I have several pieces of aluminum trim on my 17 year old boat held in place with stainless hardware showing no significant signs of corrosion.

I'm confused as to why you feel there is only 7" of cantilever, 24" of the channel will be bolted to the walkway with 24" overhanging. If I were to through bolt, which I concur would be a superior attachment, I probably could not drill any closer than 3" to 4" from the wall. So wouldn't the cantilever be at least 20" from this attachment point (I checked my math on that one).

 

[Studiot]

The 7 inches is under the part of AC unit that is on the walkway.
If you extend the supports beyond that, will they not form a serious trip hazard on the open part of the walkway?
The cantilever is the 24" (?48") part that sticks out.

Yes the materials problems can be overcome, just make sure your contractor knows how!

 

[hipfisher]

Studiot,

The original point of my question concerns the size of the channel needed to support the proposed load under the stated conditions. Is 4" x 1" x 1/4" channel adequate?
Your concerns about the trip hazard poised are noted. The walkway is only used infrequently for lowering awnings, washing windows or other maintenance issues and there is a safety railing.

 

[nvn]

hipfisher: I think we did not know why you said, in post 1, the cantilever extends out from the walkway 1219.2 mm, which is twice as far as the air conditioning unit extends out. Nonetheless, we now see from post 4, your cantilevers extend out from the walkway 609.6 mm.

By the way, you did not state the channel cross section orientation. There are two ways to orient the channel cross section: Does your channel web lie in a vertical plane, or a horizontal plane?

 

[Studiot]

You only talk about a single channel.

I can't imagine you only using one and balancing the unit on it as it would be very unstable and soon blow off in the wind.

So I am going to assume 2 channels.

In order to make a practical job I would not go for 4 x 1 channel I would go for 4 x 2.
I am saying this because I assume that they will be bolted to the underside of the AC unit offsite. Presumably you will mount the channel with the 4" dimension vertical and the 2" flanges horizontal - by far its strongest orientation (by a factor of about 8 to 1 over the other way). This will allow easy access to the bottom flange to bolt to the deck.

These practical reasons are why I suggest 2 off 4 x 2 channels. With a 4 x 1 any misalignment becomes critical.

As regards to strength there is no problem there - the max stress will be under 500 psi giving a good safety margin with any form of aluminium alloy. I estimate the deflection as about 0.025" with two channels.

 

[nvn]

There are two cantilevers. This is stated in post 1.

 

[Studiot]

There are two cantilevers. This is stated in post 1.

Looking again I see it. Thanks